control & instrumentation-1
TRANSCRIPT
(Restricted Circulation Only)
CONTROL AND INSTRUMENTATION VOLUME I
Power Management Institute NOIDA
CONTENTS
S.No.
Description Page Nos.
1. Introduction 1
2. Power Station Instrumentation 2
3. Interpretation of Instrument Readings 18
4. Pressure Management and Measuring Instruments 25
5. Level Measurement and Measuring Instruments 41
6. Flow Management and Measuring Instruments 69
7. Temperature Measurement and Measuring Instruments 89
8. Pneumatic Instruments 104
© PMI, NTPC 1
1. Introduction
Control and instrumentation in any process industry, can be compared to the nerve system in
the human being. The way the nerve system controlling the operation of various limbs of human
being, C & I in the same way controlling and operating various motors, pumps, dampers, valves
etc. and helping us to achieve our targets.
Control and instrumentation, as the name indicates, is a branch in engineering which deals with
various measurement, indication, transmission and control in different technical fields. The latest
development made in the area of instrumentation are so wide that it has become humanly
impossible to master over all the system individually. Even in instrumentation there are further
sub groups now. The term instrument means “ A device or combination of devices used directly
or indirectly to measure and display a variable.”
Instrumentation is a measurement if various parameters with comparison to set standards. We
have been using for ages different instruments suck as weights, yard stick, scales, measuring
tapes, standard container for liquid measurement e.g. Litre, gallons etc. Each of these
equipments is an instrument. Similarly, in industries and Process plants, Instrumentation makes
use of various measuring components designed to suit the process and the purpose. As some
of the big industries and process plants needs to control different process variable from a
remote distance control room, the further measuring, transmitting indicating, Recording,
abnormality alarm system and innovated. The process of innovation is marching ahead in fast
rate. In the near future, we are certainly to enter in towards more and more sophistication n C&I
stream.
© PMI, NTPC 2
2. Power Station Instrumentation
TYPES OF INSTRUMENTS
This discussion is only on the process instrumentations measuring the physical quantities such
as temperature, pressure, level flow etc. The other type of instruments are the electrical
instruments measuring electrical quantities such as current, voltage etc. The different type of
instruments normally in use are given below:
Indicators
Indicators are of two categories local and remote. Local indicators are self contained and self
operative and are mounted on the site. The remote indicators are used for telemeter purposes
and mounted in the centralized control room or control panel. The indicators both local and
remote are sometimes provided with signaling contacts where ever required. The remote
indicators depends on electricity, electronics, pneumatic or hydraulic system for their operation
and accordingly they are named. The indicators can be classified as analogue or digital on the
basis of final display of the reading.
Recorders
Recorders are necessary wherever the operating history is required for analyzing the trends and
for any future case studies or efficiency purposes. Recorders can be of single point measuring a
single parameter or multipoint measuring a number of parameters by single instrument.
Multipoint Recorders are again categorized as multipoint continuous or multipoint dot recorders.
The multipoint dot recorders select the point one after the other in sequence where as the
continuous recorders measure simultaneously all the points.
© PMI, NTPC 3
PRESENTATION OF INFORMATIONS
Enormous amount of information measured and received from the various parts of the
plants/process are to be presented to the operators giving appropriate importance to each one.
In order to have an easy and effective presentation, the information are generally grouped in to
the following three groups:
Vital information which are required by operators at all times for the safe operation of the plant.
These information are presented through single point indicator/recorder, placed on the front
panels. Main steam pressure, temperature, condenser level, vacuum, drum level, furnace
pressure etc. are some such parameters.
The second group of information are generally not vital under the normal operation of the plant.
But they become vital whenever some sections of the plant start malfunctioning. Such needs
are met through multipoint indicators/recorders placed in the front panels. Temperature and
draft across the flue gas path bearing temperature of the motors of fans etc are some such
examples.
The last group of informations are not required by the operators but for the efficiency engineers.
These informations are given by recorders mounted on back panels or local Panels. D.M. make
up quantity, fuel oil flow quantity etc. are some examples.
CODING OF INSTRUMENTS
In order to distinguish the parameters required from the other instantly, a shape coding of
instrument face is being adopted in some advanced countries. This is an useful practice and
may find place in other power stations also shortly. Coding may vary as per the practices of the
organizations. A general approach could be as below:
Level Instruments - Horizontal edgewise
Temperature instruments - Horizontal edgewise
Pressure instruments - Circular
© PMI, NTPC 4
ARRANGEMENT OF INSTRUMENTS
Another welcome feature of instrumentation in advanced countries is the ‘Master Panel’
arrangement. In this arrangement, instruments measuring important parameters are provided in
one panel. All instruments in this panel will be of circular shape with normal rated values will be
quickly realized by the operators.
SELECTION OF INSTRUMENTS
Instruments engineers are required to work in close association with the system design as well
as the equipment design engineers in selecting instruments and sensing system. After deciding
the capacity of Thermal Power Station the designs of Boiler turbine and auxiliary equipments
such as mills, pumps, fans, deaerator, feed heaters etc. are taken up.
Based on the design of the main and the auxiliary equipments, the parameter values for fficient
and economic operation determined load are specified. The instrument and system design
engineers decide the location for the measurement of various parameters such as level,
pressure, flow, differential pressure, temperature and other parameters based on the system
design and layout conditions.
Then the instrument engineers select the appropriate instruments influenced by following
factors:
i) Required accuracy of measurement
ii) Range of Measurement
iii) The form of final data display required
iv) Process media
v) Cost
vi) Calibration and repair facilities required/ available
vii) Layout restriction
viii) Maintenance requirement/ availability
© PMI, NTPC 5
CONCEPT OF INSTRUMENTATION IN THERMAL POWER STATION
The concept of instrumentation are that:
i) Instruments should be independent for their working
ii) The total instrumentation should be independent to each other in assessing the process
conditions.
iii) Instrumentations should be sufficient to provide adequate information to the operators
for:
a) Cold start of the unit
b) Warm/hot start of the unit
c) Shut down both planned and emergency shut down.
POWER STATION INSTRUMENTION
The process conditions and the equipment conditions are to be assessed by the operators from
the informations received from the various instruments. The instruments and range vary very
widely as per the process media. The following section deals with these instruments. The inter
dependence and inter relation of these instrument readings play very significant role in the
stability and the efficiency of the heat balance.
TEMPERATURE MEASURING INSTRUMENTS
Accurate measurement of temperature is required to assess the material fatigue, heat balance,
heat transfer etc. The measurement ranges from ambient temperature where air inlet to F.D. fan
is measured to 13000C to 14000C inside the furnace zone. Temperature measurement is to
made in many medias such as water/stream, oil (fuel oil and lubricating oil), air fuel gases,
hydrogen gas, metal temperatures of bearing babbits, turbine top and bottom, generator winding
and cores, S.H. tube metal etc.
© PMI, NTPC 6
Filled system thermometry such as mercury in glass, mercury in steel, vapour filled or gas filled
are used for local indication. The selection of thermometer depends upon the range of the
temperatures to be measured. These instruments are available with electrical contacts for
setting up annunciation and protection system wherever required.
Resistance thermometer or thermocouples are used as primary sensors in remote
measurement of temperatures depending upon the range. Resistance thermometer are of
platinum and copper resistance type. Platinum resistance thermometers are calibrated to
have46 ohms or 100 ohms at 00 C. the secondary instruments used in conjunction are cross coil
indicators or electronic bridges. These instruments indicate temperature by measuring the
nature of resistance which changes with the change in temperature. Resistance thermometers
are generally used up to 3000 C.
Above 3000 C, thermocouples are used as primary sensor. The common type of thermocouples
used in thermal power station are chromel-alumel or chromel-copel depending upon the
temperature. Iron constantan is another thermocouple in use. The secondary instruments for
thermocouple sensor are pyrometriv millivolt meters or electronic potentiometers. Null balance
method is used for the very accurate measurement of millivolts generated by thermocouples
sensing the process temperatures.
The electronic bridges and potentiometers can be either indicators, or indicator cum recorders
with alarm/protection contracts and with remote transmission facilities.
PRESSURE MEASURING INSTRUMENTS
The pressure measurement in thermal power station ranges from 1 Kg/cm2 (nearly) at
condenser to hydraulic test pressure of boiler. Here again many medias exist such as
steam/water, lubricating oil, fuel oil, air, fuel gases, hydrogen etc.
For local indication of pressure and differential pressure, bourdon tube, type and diaphragm
type gauges or liquid monometers either electronic or pneumatic coupled with a secondary
instrument indicator/recorder. Many varieties of transmitters are in use. In these transmitters the
mechanical movement of sensing elements such as bourdon, bellows, diaphragm etc. due to
the pressure causes an electrical property change such as current, voltage, resistance,
© PMI, NTPC 7
capacitance, reluctance, inductance etc. which is utilized as a measure of pressure in the
secondary instruments. The secondary instruments are either indicators or recorders. Which
may incorporate signaling contacts.
LEVEL MEASUREMENT
Level measurement is generally carried out as differential pressure measurements. In power
stations, level measurement in open tanks such as DM storage Tank and Fuel Oil and Lub Oil
tanks and in closed tanks such as dearator, condenser hot well, boiler drum and L.P. & H.P.
heaters are to be made. Gauge glasses and floats are used for local indication of levels and the
transmitters used for measuring the differential pressure are used along with the secondary
instruments for remote level measurements.
The measurement of the boiler drum poses many problem because of varying pressure and
temperature and many computations and corrections are to be made in order to get correct
levels. A recent development in this area is the ‘Hydra step’. Though it is very costly it improves
the accuracy and the reliability of this measurement.
Other problem area is the solid level measurement where the coal bunker levels and dust
collector hopper level are required. In both these cases continuous level measurement is not
possible. However fairly reliable and accurate provisions are available to indicate the extreme
level on either directions (low or high). The nucleonic level gauges or the capacitance and
resistance type sensors serve in these areas very well.
FLOW MEASUREMENT
Flow measurement of solids, liquids and gases are required in Thermal Power Stations. Though
the liquid flow measurements are made very accurately, the gas flow measurement cannot be
so, water flow measurements are done fairly easily and accurately whereas steam flow
measurement requires density correction under varying pressures. The air and flue gas flow
measurements suffer accuracy and reliability due to variation in pressure, temperature, duct
leakage, dust accumulation etc. The solid flow measurement is very difficult and only on a
rough area is arrived at about the P.F. flow through inferential means.
© PMI, NTPC 8
In Power stations flow measurement are based on inferential principles. Differential pressures
are created by placing suitable throttling devices in the flow path of the fluids in the pipes/ducts.
The throttling devices are suitably selected depending upon the media, flow quantity etc. from
among office, venture, flow nozzle dall tube etc. the differential pressure developed across such
sensing devices is proportional to the square of the flow quantity. The differential pressure is
measured by the devices discussed in 9 with additional square root extraction facilities.
ANALYTICAL INSTRUMENTS
Apart from the above there are few quality measurements necessary in thermal power
generation plants of high capacities. These include feed water quality measuring instruments
such as conductivity, pH, dissolved oxygen, and sodium instruments, steam quality measuring
instruments, such as conductivity, silica and pH analyzers. The combustion quality is accessed
by the measurements of the percentage of oxygen, carbon monoxide or carbon dioxide in the
fuel gases. The purity of the oxygen inside the generator housing is measured by utilizing the
thermal conducting capacity of the hydrogen gas.
The water and steam purity is measured as the electrolytic conductivity by electronic bridge
method in which one arm form the electrodes of conductivity cell dipped into the medium.
The volume percentage of oxygen in combustion gases are made utilizing the paramagnetic
properties of oxygen. The carbon monoxide percentage is measured by the ‘Absorption of
Electromagnetic radiation’ principle.
Both these gas analyzers require elaborate sampling and sample conditioning system resulting
in poor reliability and availability of these measurements. Recent developments in these fields
have brought out on line ‘in-situ’ instruments for these two parameters where the problem of
sampling is dispensed with.
The ‘Analytical Instruments’ as the above instruments had been the neglected lot so far in the
power stations. But now the authorities seems to think their importance for the process.
© PMI, NTPC 9
TURBOVIBORY INSTRUMENTS
The turbovisory instruments have become very important in modern day turbines where the
materials have been stressed nearer to the yield points and the internal clearance have become
the minimum. Shaft eccentricity, vibration (both shaft and bearing pedestal) differential
expansion of shaft and cylinders, over all some of the turbovisory measurements. These all
measurements are interrelated and interdependent.
LIST OF INSTRUMENTS
All these measurements discussed above and their correct interpretation enables the operators
to check and watch the behavior of the process and the equipments and take necessary
corrective actions in time.
A typical list of important measurements carried out in Thermal Power Stations are given below:
Temperature a) Steam temperature at boiler outlet, super heater stages, steam legs before ESVS, CVS
after ESVS, IVS and at turbine curtis wheel-indicators/indicator-cum recorders with
alarm and protection facilities in control room.
b) Steam temperature at turbine HP cylinder outlet, hot reheat and exhaust hood
temperatures.
c) Metal temperature of turbine casing and metal temperature of super heaters and
reheaters-indicators, indicator cum recorder in U.C.B. with multipoint selection.
Other temperature measurement in various zones of boiler indicator
a) Flue gas temperature measurement in various zones of boiler indicator and indicator
cum recorder in control room.
© PMI, NTPC 10
b) Air temperature at inlet and outlet of air preheater.
c) Turbine bearing oil drain temperature-indicator cum recorder in U.C.B.
d) Generator winding and core temperature-indicator cum recorders in control room.
e) Temperature of auxiliary equipments bearing such as mill ID, FD and P.A. fans etc
indicator cum recorder in U.C.B.
Pressure
a) Condensate pressure after condensate pumps and before the ejectors-indicator in
U.C.B.
b) Deaerator pressure-indicator cum recorder in U.C.B with electrical contacts for
interlocking facilities.
c) Feed water pressure after feed pumps-individual indicators for each pump.
d) Feed water pressure before and after feed regulating stations-indicators in U.C.B.
e) Drum pressure indicator cum recorders in U.C.B. with alarm signaling facilities.
f) Super heater steam pressure at boiler outlet 2 Nos. indicators one for each side in
U.C.B. and at local with alarm protection facilities. Measurement is done at the outlet of
superheater and before boiler stop valves.
g) Steam pressure – 1 No. indicator cum recorder, one of the lines before turbine stop
valves in U.C.B.
h) Steam pressure at emergency stop valves and TVS.
i) Steam pressure after control valves indicators in local panel for Pressure of each valve.
© PMI, NTPC 11
j) Steam pressure at Curtis wheel indicator cum recorder in U.C.B. with alarm contacts.
k) Steam pressure in H.P. turbine exhaust indicator in U.C.B. for cold reheat steam.
l) Vacuum in condenser indicator cum recorder in U.C.B. with alarm facilities and separate
vacuum relay for protection.
m) Hot reheat pressure indicator in U.C.B. with signaling contacts.
n) Steam pressure at the exhaust of I.P. cylinders-indicators in local panel.
Pressure : Fuel And Lubricating Oil
a) Heavy oil pressure indicators in U.C.B. with signaling contacts. Measurement is made
before and after pressure regulating valves.
b) Light warm up oil pressure indicators in U.C.B. with signaling contacts. Measurement is
made before and after the flow control valves.
c) Ignitor oil pressure indicator in U.C.B.
d) Governing oil pressure-indicator in U.C.B. with signaling contacts.
e) Lubricating oil pressure-indicator in U.C.B. Measurement is made after oil coolers.
Presssure : Air Flue Gas
a) Air pressure indicators in U.C.B. before and after air heater for secondary air.
b) Indicators in U.C.B. before and after air heater for primary air.
c) Wind box pressure indicators in U.C.B.
© PMI, NTPC 12
d) Furnace draft-indicators and recorders in U.C.B. Measurement is made averaging left
and right side drafts.
e) Flue gas draft before and after economizer-indicators in U.C.B.
f) Draft after air heaters two indicators in U.C.B. one for each air heater.
g) ID fan suction – 2 Nos. indicators in U.C.B. one for each fan.
LEVEL MAESUREMENT a) Drum level indicators and indicators cum recorders (total 3 Nos from different tapping) in
U.C.B. with alarm and protection facilities. Normally 3 types of measurement are
adopted:
i) Local gauge glass
ii) Remote gauge glass and
iii) Remote indirect measurement
b) Drip level in H.P. and L.P heaters-indicators in U.C.B. with alarm and protection
facilities.
c) Condensate level-indicator in condenser-indicator in U.C.B. with alarm and protection
facilities.
d) Dearator level-indicator in U.C.B. with signaling contacts for alarm.
e) The various storage tank level such as D.M. water, fuel oil, lubricating oil etc. are
measured by the local direct gauge glasses.
Flow
a) Condensate flow to dearator-indicator/recorder in U.C.B. with integrator unit for totalizing
in two locations (i) between air ejectors and L.P. heater No. 1 and (ii) between the final
L.P. heater and dearator.
© PMI, NTPC 13
b) Feed water flow indicator/recorder in U.C.B. with integrator unit. Measurement is made
between final H.P. heater and feed regulating valves.
c) Super heated steam flow – 2 Nos. indicators cum recorders one for each pipe with
integrator unit in U.C.B.
d) Re-heater steam flow –2 Nos. indicators cum recorders one foor each side of the boiler.
Measurement is made at the inlet to rehaeater.
e) Air flow-2 Nos. indicators cum recorders one for each FD fan in U.C.B. and
measurement is made at the discharge of th FD fans.
f) Fuel Flow
The fuel oil flow to the unit is given by two indicators cum recorders in U.C.B., one
measuring the oil in the incoming line and the other in the return line. Normally the coal
flow is measured for the whole station by the belt conveyor weighers.
AUTOMATIC CONTROL
The importance of maintaining a balance in the process was discussed under section 1
whenever the process gets disturbed due to the deviation of process elements behavior; they
are to be brought back to the balance condition. Since a lot of process elements are involved
and disturbances are very frequent, the correction can be carried out efficiently and quickly only
by the introduction of automatic control system eliminating any possible human error. The
following are the important automatic control loops in the thermal power station.
Automatic Boiler Control
i) Steam pressure always called as Boiler Master
ii) Combustion control
iii) Furnace draft control
iv) Boiler feed regulation or drum level control
v) Super heater/ reheater steam temperature control
© PMI, NTPC 14
vi) Auxiliary steam pressure control
vii) Mill group control
Turbine Automatics
i) Condenser hot well level regulation
ii) Drip level control in L.P. and H.P. heaters.
iii) Feed pump speed control
SEQUENCE CONTROL AND INTERLOCKING SYSTEM A power station is a combination of many individual equipments and systems and for better
performance it relies upon the performance of these individual equipments. These equipments
are interdependent and interrelated with each other, and therefore they are to operate in
coordination with each other. Electrical interlock systems connect these individual equipments
and operate then with required sequences. For example Boiler is a system comprising milling
plant, ID fans, FD fans, PA fans etc. these equipments are interlocked in such a way that they
are started / shut down in specific sequences in order to avoid damage to equipments and men.
For example in a milling system the coal feeder is interlock such a way that it will not start
unless it’s succeeding system to crush and discharge the coal into the furnace such as
exhauster / P.A fan and mill are in operation. These schemes may very little with different
manufacturers but generally all P.F and oil fired boilers have common sequences.
Also equipment is so interlocked that in case the failure of the running equipment to deliver the
good, automatically the reserve one is put in to service. For example in case a feeds pump
which is running fails to meet the demand of the boiler, the interlock system will put the reserve
pump into service to meet the demand. As the unit size increases the number of
interdependency of operations increases. A system of automatic sequence control simplifying
the operator’s duty has come into existence.
© PMI, NTPC 15
REMOTE CONTROL AND OPERATION OF EQUIPMENTS
As discussed earlier that power station comprises many types of equipment, it become
necessary to operate them from a centralized room. Moreover as the capacity of plant increases
its operating electrical supply potential, also increases which is very dangerous on safety point
of view. As a result the indirect way to remote operations came into practice. A very low voltage
level such as 110 V or 240 V AC/DC is used to close a breaker of the electrical motor of 3.3 or
6.6 KV voltage level. The low voltage switches are usually provided on the operating desks in
the control room. Where D.C is used for station batteries are provided as standby.
DATA ACQUISITION AND DATA LOGGING
The conventional central control room is rather a cumbersome system. Large number of
instruments must be observed to know what is happening inside the plant. The data acquisition
simplifies this job by collecting all the measurements transmitted from the process, converting
them into digital term and storing in the memory bank. The periodic loggings of parameter by
the operators are dispensed with after the introduction of data acquisition system, which prints
out the periodic conditions on predetermined time intervals. All the important measurements at
one time are printed along a row. Data loggers thus reduce the use of graphical recorders.
Since data logging gives too many measurements at a time, it cannot be easily digested by the
control staff. Now data reduction systems are finding their use where only the process quantity
deviated from normal value is shown.
VISUAL DISPLAY UNIT (V.D.U) Visual display units go along with the data acquisition system. In V.D.U pre selected schemes,
flow paths with parameters, running alarm conditions etc. can be brought on colour television
tubes on demand. This gives the life picture of the happening inside the plant making the
operation easy and effective.
© PMI, NTPC 16
AUTOMATIC TURBINE RUN UP Turbine start up is the most difficult operation. Each start up will be different. many parameters
and procedures should be scrupulously adhered to where an error in human decision will result
in heavier damage to the unit. Therefore modern day machines of higher capacity or machines
which are to be frequently started are to be provided with automatic run up, synchronizing
loading gears.
Under normal condition this gear will accelerate the turbine from having gear speed to full speed
uninterrupted at a rate determined by the initial turbine temperature conditions. Under hot start
conditions, this run up period may be of the order of 5 to 10 minutes and comprise two zones,
the first half is relatively slow rate upto 1000 rpm, the second half is a fast rate from 1000 to
3000 rpm (a range which includes critical speed). In the event of abnormal conditions this
program of acceleration is temporarily held (except in the regions of critical speed), or in more
severe conditions, reserved or tripped.
The abnormal conditions are monitored by the turbo-visory instruments discussed earlier.
Automatic synchronizing is also effected by the gear if selected for the function. This scheme
matches the frequency voltage and time phase of the generators out put to that of existing
busbar and close the circuit breaker.
Automatic loading gear enables the machine to be loaded automatically at the selected rate
through the control of governor speed motor. The supervisory gear also will be in use during this
function. The rate of loading various widely from 5% per minute during the initial block loading to
20% M.C.R.
SCANNING SYSTEMS In the complex process extending over a considerable area lot of massages are transmitted to
and from the process. These transmission channels are quit expensive and there may be
danger of loss of data owing to confusion of signals by extraneous electrical noises. In cases of
coded transmission less sensitive to such noise is found useful. Hence all the process signals in
© PMI, NTPC 17
analogue from scanned in sequence one at a time converted into digital form and transmitted to
the central information system for display or control purposes.
BURNER MANAGEMENT For higher capacity boiler, fuel firing rate is also higher. Explosion occurs within 1 to 2 secs of
fuel accumulation. Therefore leaving the management of fuel firing to the operators will lead to
explosion because human reflexes will be little slower. A complete automatic burner
management system called furnace safeguard supervisory system ‘FSSS’ in short has been
introduced to manage the present day boilers.
This system takes care that every increment of fuel input corresponds to the available ignition
energy inside the furnace.
The following functions are entrusted to such an automatic burner management system:
i) Furnace purge supervision
ii) Ignitor control
iii) Warm up oil control
iv) Pulverize control
v) Secondary air damper control
vi) Flame scanner intelligence
vii) Boiler trip protection
The above discussion gives some synopsis of the instrumentation in thermal power station.
© PMI, NTPC 18
3. Interpretation of Instrument Readings
INTRODUCTION To give the operation engineers a correct picture of the happenings inside the plants enormous
numbers of instruments are required to be installed. Also those instruments are to be mounted
in panels in centralized locations to avoid many personals watching the readings and
interpreting on their own way. Also these instruments are to be arranged in such a way that they
give plant behaviours in a systematic way and with minimum complexity. Here comes the
aesthetic and ergonomics view of the installation.
TELEMETERING For the centralized instrumentation remote indication facility is required and consequently
telemetering was introduced. The method of placing the instruments at a distance from the
measuring point is called telemetering. This type of metering is very common in power stations
as nearly all the instruments for measuring and controlling the power flow are centrally mounted
on a panel.
Electrical instruments are now widely used for this purpose because they are convenient to
install, reliable and reasonably accurate. Also it is cheaper to transmit an electrical signal by a
cable than a pipe lines in case of pneumatic. Transmission lag is very negligible. However if the
telemetering is required for a short distance, pneumatic system is used.
INTERLOCKING SYSTEM A power station is a combination of many individual equipments and systems and for better
performance is relies upon the performance of these individual equipments. The equipments are
interdependent and interrelated with each other, and therefore they are to operate in
coordination with each other. Electrical interlock systems connect these individual equipments
and operate them with a required sequence. For example boiler is a system comprising milling
© PMI, NTPC 19
plant, ID fans, FD fans, PA fans etc. These equipments are interlocked in such a way that they
are started / shut down in specific sequences in order to avoid damage to equipments and men.
For example in a milling system the coal feeder is interlock such a way that it will not start
unless its succeeding system to crush and discharge the coal into the furnace such as
exhauster and mill are in operation. These schemes may very little with different manufacturers
but generally all P.F and oil fired boilers have common sequences.
Also equipment is so interlocked that in case the failure of the running equipment to deliver the
good, automatically the reserve one is put in to service. For example in case a feeds pump
which is running at time, fails to meet the demand of the boiler, the interlock system will put an
idle/reserve pump into service to meet the demand.
ANNUNCIATORS The operation Engineer’s attention should be drawn towards a parameter which deviates much
from the desired value. This is done by the annunciators installed in the control panels in front of
him by audio, visual or both means. Whenever the system deviation occurs, a relay gets
energised by the signal received from the deviated system which sets up a flashing light, an
audible alarm. These are to be received by the operator till then they continue to be on. It
becomes necessary for him to take necessary remedial action to correct the deviation.
REMOTE OPERATION OF EQUIPMENTS As discussed earlier that power station comprises many types of equipment, it become
necessary to operate them from a centralised control room. Moreover as the capacity of plant
increases its operating electrical supply potential also increases which is very dangerous on
safety point of view. As a result the indirect way of remote operations came into practice. A very
low voltage level such as 110 V or 240 V AC/DC is used to close a breaker of the electrical
motor of 3.3 or 6.6 KV voltage level. The low voltage switches are usually provided on the
operating desks in the control room. Where D.C is used for station batteries are provided as
standby. Circuit breakers are provided with protection relays.
© PMI, NTPC 20
INTERRELATION OF INSTRUMENTS In order to have stable generating conditions the heat energy supplied through the fuel must
balance the electrical energy output of generator plus the normal losses. But often this balance
is disturbed due to fluctuations in temperature, pressure, steam flow or electrical output. A large
number and variety of instruments are required to measure and indicate the cause and amount
of disturbance so that steps can be taken to keep the energy flow in balance. Each instrument
has its own function to perform but the value of its measurement often depends on the accuracy
of the other instruments associated with it. The interdependent and interrelations of these
instrument readings play very significance roll in the stability and efficiency of the heat energy
balance.
Furnace Draught
The balance between the induced and forced draught fans is produced by measuring and
controlling the furnace suction. Balance draught usually occurs when there is slight suction
inside the combustion chamber. This is achieved by properly adjusting the speed or the
dampers of the fans. Disturbances in the draught can cause unstable combustion and this in
turn will affect the readings on many of the other instruments associated with the boiler.
CO2, CO, and O2 Measurement
These instruments are valuable guides to know:-
1) The quantity of air supplied.
2) The variation in the quality of the fuel being burnt.
3) The performance of the automatic control if in circuit.
The percentage of CO2 may not necessarily be an indication of efficient combustion. It may be
showing an optimum value yet the combustion must be incomplete due to the variation in non-
carbon combustibles such as hydrogen, sulpher and chlorine.
Therefore the reading of the O2 may be a correct percentage according to the reading yet, it
may be found still in excess. Therefore in modern day practice the CO measurement is taken
© PMI, NTPC 21
as valuable information regarding the combustion. Air is adjusted till we get some traces of ‘CO’
in the flue gases.
Temperature
Accurate measurement of steam temperature is very important because of the high degree of
superheat used and boilers are operated with critical temperature margins. Steam temperature
measurement itself is comparatively easy but its control is more complicated due to the time log
and thermal inertia inherent in the system.
There will be wide variation in the moisture content of the coal flowing through the mill. Air with
varying temperature is to be sent to dry out that moisture. But the primary air temperature
variation affects the stability of the combustion, steam temperature, exit gas temperature etc.
Fuel Measurement
It is easy to measure the liquid fuel by the conventional instruments in volume quantities. In
case of solid fuel such as P.F. the measurement is not possible with conventional type of
instruments. Therefore their quantities are determined indirectly by measuring the quantities of
primary air. But a certain volume of fuel do not always have the same amount of heat units due
to the variation in calorific value moisture and ash contents. Variations in quality and quantity of
fuel affected the steam, air and gas flow as well as temperature pressure, CO2, O2 and power
output. However the variations can be easily adjusted in the fuel flow system.
Measurement of Air And Gas Flow
Since boiler handles large volume of air/gas, it is difficult to measure the quantity correctly due
to the variation in pressure, Temperature, casing leakages, dust ladens etc. The measurement
of the air/gas is used as a means of establishing the correct fuel/air ratio.
Normally measurement of air flow is very easy as compared to gas flow because of
accumulation dust, slag and varing temperature etc.
© PMI, NTPC 22
Turbovisory Measurements
The turbovisory readings such as differential expansion eccentricity, vibration, and temperature
differentials give a fairly clear picture of the behaviours and clearances and also the eccentricity.
Eccentricity in turn give a picture of vibrational level. All these parameters are interdependent
and interrelated.
AUTOMATIC CONTROL
Whenever the balance gets disturbed due to the deviation of process elements behaviour they
are to be brought back to the balance condition. Since lot of process elements are involved and
disturbances are very frequent, the correction can be carried out efficiently and quickly on by
introducing automatic system for elimination any possible human error. Thus automatic control
was established to maintain the system balance.
For example when an operator has to fire the boiler by regulating the fuel to the burners and at
the same time to adjust the position of dampers or the speed of the fans for the control of air
supply, haw well he does this depends of the type of fuel and his own ability. His mistakes in
assessing the things in a correct proportion will aggravate the disturbance. But an automatic
combustion control does this job, more quickly, efficiently and smoothly. Automatic control
system detects the changes signal and direct the regulator accordingly to correct the deviation.
Advantages Of Automatic Control System Are:
a) The values of the process elements such as steam pressure, temperature flow are kept
close to the desired value.
b) Combustion efficiency is improved resulting in:
i) Fuel Economics
ii) Reduction in boiler fouling
iii) Less atmospheric pollution
iv) Less carbon in ash and grit
c) Metal fatigue is reduced by maintaining stable metal temperatures.
© PMI, NTPC 23
d) The operator is relieved of continuous tedious operations.
e) Increased safety both to men and equipment since human error is eliminated.
f) Operator has more time to spend in regular operation and routine inspection.
Disadvantages
a) The equipment has to be much reliable.
b) The standard setting should have to be watched and adjusted to suit the varying
characteristics of fuel etc.
c) Sometimes control action goes on the reverse direction due to the time lag in the
measurement.
d) Control equipments are very expensive and require periodic maintenance.
COMPUTERS
With the increasing size of modern plant, the distances between items of plant run of four to five
hundred yards away from the centralized control rooms. Also large number of equipments
necessitate large number of instruments. It becomes very difficult to watch these many
instruments and supervise the operation of such a large number of equipments. A computer
relieves the operator from routine tasks leaving him free to concentrate on the overall
inspection.
The operator cannot always watch every instrument and at the same time make the necessary
adjustments to suit the constantly varying conditions. However a computer can be programmed
to make all the necessary adjustment as and when required.
The starting up operations of a large unit involves somewhere around 1000 separate steps
including nearly 500 switching operations to bring on load. All those steps are to be carried out
in short time and in correct sequence. These all activities become cumbersome and any
mistake will lead to disastrous end. Computer fed with correct programme, performs these
duties will.
© PMI, NTPC 24
It is very difficult by the operation engineer to keep a constant watch on these temperature
gradients. For a computer it is very easy job. In many cases computers are exclusively used to
run up a turbine. It allows the steam into the set at the appropriate temperature and accelerates
the set after monitoring the internal clearances temperature differentials and other mechanical
aspects, runs upto speed and synchronise the set. In the computers facilities are available to
check the efficiency of the plan then and there. The evolution of computer can be compared to
teaching animals to do tricks. Every trick has to be acquired by much study and
experimentation on a slow progressive basis.
DATA LOGGING
The conventional central control room is rather a cumbersome system. Large number of
instruments must be observed to know what is happening inside the plant. The data logging
simplifies this job by collecting all the measurements transmitted from the process, converting
them into digital form and printing them on the log sheets. All the important measurement at
one times are printed along a row. Data loggess thus reduce the use of graphical recorders.
Since data logging gives too many measurements at a time, it cannot be easily digested by the
control staff. Now data-reduction systems are finding their use where only the process quantity
deviated form normal value is shown.
SCANNING SYSTEMS
In a complex process extending over a considerable area, lot of messages are transmitted to
and from the process. Theses transmission channels are quite expensive and there may be
danger of loss of data owing to confusion of signals by extraneous electrical noise. In such
case some coded transmission, less sensitive to such noise is found useful. Hence all the
process signals in analogue form scanned in sequence one at a time converted into digital form
and transmitted to the central information system for display control purposes.
© PMI, NTPC 25
4. Pressure Measurement and Measuring Instruments
PRESSURE MEASURMENT
Pressure measurements are one of the most common measurements taken and recorded in the
Power Station ranging from very low, i.e. condenser vacuum to very high i.e. hydraulic
pressures in some actuator systems. Between these two limits of say 30-40 millibar absolute to
300 bar are to be the measurements of different process media-steam, water, oil, air, gas etc.
and each with varying degree of accuracy and reliability.
PRESSURE MEASURING DEVICES
The common pressure measuring devices are-
1. Manometers using water, mercury and other liquids of known density for low pressure
measurement.
2. Diaphragm, Capsule bellows for measuring medium pressures.
3. Bourdon tube gauges for measuring medium and high pressures.
4. Transducers of different types for measuring pressures of all ranges for telemetering
purposes.
Of these above, the manometers are mainly used in laboratories for calibration purposes and as
such, the diaphragms, Capsule bellows have taken its place for site use.
Manometer Elements
The manometer are mostly used in laboratory for calibration purposes as these are the
fundamental type of instruments. At site they are mainly used for test purposes, in the low
ranges 0-1000 mm with mercury as manometers liquid maximum being.
© PMI, NTPC 26
If one end is sealed, then the manometer can be used for absolute pressure measurement. If
the area of one of the limb is made considerably greater then the other, then the measurement
of the differential pressure is represented by the height of the liquid column in the smaller tube
with negligible error. Such system is called the single limb manometer or cistern manometer
since the larger area pipe is in the form of a metal cistern. The manometer liquid normally used
is water. Sometime coloured water is used to distinguish the column. The other liquids used are
i) Transformer oil having specific gravity 0.864
ii) Mercury having specific gravity 13.56
iii) Blended Paraffin liquid
Industrial type high pressure ‘U’ tube manometer are available having metallic tubing. These
manometers employ a secondary system of linkages / liverages for indication purposes. Inclined
tube manometer are the special development to give increased length of column for less
differential pressure. The inclined tube carries the scale. Manometers are available with
adjustable inclination depending upon the range required. Fig. 1 to 2 shows a system of
manometers.
© PMI, NTPC 27
© PMI, NTPC 28
Diaphragm, Capsule and Bellows
The present days low pressure to medium pressure applications are met with diaphragms. Also
the introduction of these elements as greatly helped in remote measurement and control of
pressures even of very low range (0-4 mm wcL).
Material and Range of Measurement
The various types of diaphragm and below elements are made of steel of special composition,
phosphor bronze, nickel silver and beryllium copper etc.
Bellows and multistack are made from 80-20 brass, phosphor bronze, stainless steel and
beryllium copper.
For very low pressures, the diaphragms are required to be extremely flexible. For these
applications materials like colon leather, gold beater skin, nylon rubberized fabric etc. are used.
These groups of sensors are used for the measurement of very low pressure upto 20-25 kg/cm2.
© PMI, NTPC 29
Bourdon Tube Gauges
This is the oldest instrument introduced initially to the measure pressures from medium to high
ranges. But present days these are used almost for every range of pressure measurement.
However their application is limited to measure “Gauge Pressures” only.
It consist of a metal tube approximately elliptical in cross section formed into a ‘C’ shape, a long
spiral (helical) or to a flat spiral by special machines one end of the tube is closed and sealed
and the opposite end is left open and terminated to a block where the process pressure is
applied. If the pressure inside the tube is more than that existing outside, the elliptical section
changes its shape and it begins to straighten out, with the result that the free end deflects is an
arc. The deflection is proportional to the pressure difference between inside and outside
pressures since the outside pressure is atmospheric. These gauges measure ‘Gauge pressure’
as shown in Figure 3.
© PMI, NTPC 30
Ranges of Pressure
Burdon tube gauges are in use from the range 0-0.5 kg/cm2 to 6,000 kg/ cm2 and even higher
ranges occasionally. The practical range for each type of listed below.
Helical boundon - 0-0.5 Kg/cm2 upto 0-6000 kg/cm2
‘C’ Type boundon - upto 700 kg/cm2
Flat spiral bourdon - low ranges upto 0-70 kg/cm2
© PMI, NTPC 31
Materials
Materials like phosphor bronze, steel, berrylium copper etc. are used depending upon the
pressure range and the media’s corrosiveness. The chart given in Table I give more details of
bounden tube materials and their pressure ranges.
TABLE - I : BOURDON TUBE MATERIAL
Material Percentage Composition Joints
Treatment Heat range Pressure kg/cm2
Phosphor Bronze (Drawn)
Copper 95 Tin 5 Phosphorus Trace
Soft Soldered
None 1-70
Beryllium Copper (drawn)
Beryllium 1.8 Cobalt 0.3
Brazed Precipitation hardened
03-350
Alloy Steel (machined)
Carbon 0.26/ 0.32, Chromoum 0.8/1.1, Molybdenum 0.15/0.25
Screwed
Quenched and tempered
650-5500
K. Monel (Machined)
Nickel 66 Copper 29 Aluminium 2.75 Iron 0.9
Screwed
Precipitation hardened
70-1350
Stainless Steel (machined)
Chromium 16/18 Nickel 10/14 Molybdenum 2/3
Welded
Stress relieved
2-70
The simplicity and ruggedness of a Bourdon gauge makes it the most frequently used pressure
gauge. The reference pressure in a Bourdon gauge is atmospheric pressure. Hence, the dial
reading gives gauge pressure.
ERRORS IN BOURDON TUBE GAUGES
Errors that may occur in Bourdon gauge are zero error, range error, angularity error, and
hysterisis. For zero error the pointer is adjusted, for range errors the quadrant screw is adjusted,
for angularity error the linkage screw is adjusted. In case of error due to hysterisis the tube
should be replaced if the error goes beyond the specified value.
© PMI, NTPC 32
Hystersis is the difference in the indicated value of the gauge for an applied pressure during the
increasing cycle and during the decreasing cycle of pressure.
TESTING A BOURDON PRESSURE GAUGE 1. Gauge is tested at 5 points up and down before adjusting anything. Divisions
corresponding to about 10%, 30%, 50%, 70%, 90% are chosen.
2. About 1% pressure is applied then zero is set by removing and replacing the pointer to
read the pressure applied.
3. About 90% scale pressure is applied if necessary the range is adjusted by loosening the
shoulder screw and moving the linkage along the slot in the quadrant (towards the pivot
to increase the range, away from pivot to decrease the range).
4. Step 1 and 2 are repeated until gauge is correct at both points.
5. When zero and range are correct then angularity is adjusted if necessary. Half full scale
pressure is applied to the gauge, angularity adjustment screw is loosened and
adjustable linkage is slided until the angle formed by the quadrant and linkage is right
angle.
6. Approximately five points of the scale are checked with pressure increasing, the
readings are tabulated.
7. The same five points with pressure decreasing are checked and result tabulated.
8. Result from 6 and 7 are used to check for hysteresis.
© PMI, NTPC 33
9. Gauge is assembled. Pointer should not foul the glass over any part of its travel.
10. Result sheet is made the final condition of the gauge as a % of full scale. Gauge should
be within 1% of full scale.
Zero Error A zero error can be observed easily by quickly testing at the cardinal points. A zero error will
have exactly the same amount of deviation at all points. in this type of error the pointer is reset.
Applied Pressure kg/Cm2 10 30 50 70 90
UP 9 29 49 69 89 Gauge Reading
DOWN 9 29 49 69 89
CALIBRATION OF BOURDON TUBE GAUGES
Pressure gauges in industrial process must be accurate so that any time the process pressure
is known. This helps to achieve accurate control of the industrial process. The pressure gauges
described till now required regular calibration. The calibration is possible if one is sure what
pressure is being applied to the gauge.
Hydraulic calibrator is one such device which used in calibrating the pressure gauges. The
hydraulic units dealt with in this chapter use oil for application of pressure.
The principle of operation, setting up and maintenance of two calibrators’ viz. comarator and
deadweight tester is given here. In both the devices the pressure that is being applied is known.
Ina comparator the applied pressure is indicated in a standard weight placed on the instrument.
DIFFERENTIAL PRESSURE GAUGE
Fig. 5 is a differential pressure gauge with a dual bourdon system. The two tubes are connected
to the single pointer. Tube 1 forms port 1 and tube 2 forms port 2.
© PMI, NTPC 34
Ports 1 and 2 are connected to the process whose differential pressure is to be measured. The
deformation of tube 1 causes the pointer to rotate anticlockwise via link cradle and quadrant.
The deformation of tube 2 causes the pointer to rotate clockwise via link and quadrant. The
movement of the pointer is opposite for the individual pressures and hence the gauge reads the
differential pressure.
A typical application of this unit is the measure of differential across filters to indicate blockages
or end of life of filter.
During installation of this unit due consideration must be given to overloads. A differential unit
must always be accompanied by an equalizing unit shown in Fig No. 6.
© PMI, NTPC 35
To remove the gauge, first valves A and B and closed, valve C is opened to equalized pressure
in both the part P1 and P2. The gauge is removed now. To check zero on plant the same
procedure is followed, but the gauge is now removed.
To restart the unit is installed with valves A, B and C losed. After installation value C and B are
opened. Then C is closed and A is opened.
SNUBBER This is a protection device for pressure measuring instrument from violent pressure surges and
pulsation. Snubbers also known as deadners reduce the effect of pulsating pressure. They
result in the instrument indicating or recording an average pressure, instead of recording each
individual surge or pulse. Snubbers are used in pipe lines leading to the instrument.
In general, these snubbers reduce the velocity of fluid to the instrument and thus prevent
sudden extreme change in pressure from reaching the measuring element too rapidly. The
reduction in velocity can be achieved by several methods. The body consist of two parts, the
lower part and the upper part lower part is connected to the pipe line. It contains a piston. The
pin piston assembly rises and falls with the pressure impulses and absorbs the effect of shock
and surge. Owning to the rise and fall of the piston the snubber is self cleaning. The upper part
of snubber is screwed to the lower part, on one side and to the pressure instrument on the
© PMI, NTPC 36
other. The upper part has a stop for piston. The stop has a hole in the centre for the process
fluid to pass to reach the instrument from the pipe line and vice versa.
GAUGES WITH ALARM CONTACTS
These types of gauges give alarm when the pressure reaches a set level. The working of gauge
in which the alarm contact is to be made when the pressure reaches a set higher limit is given
here.
The gauge has a pressure setting needle. This needle is set via a knob through the centre of the
glass of the gauge. The glass is generally of acrylic. The needle has a projection where contact
will be made. The needle is connected to a wire.
The pointer is also connected to a wire. It has a projection when it touches the projection on the
setting indicator a contact is made. When the pressure reaches the set value then the gauge
pointer touches the projection on the setting needle. The pointer and setting needle behave like
an open switch till the set limit is reached. The gauge can be connected to relay, hooter or lamp
for alarm or control. Sometimes a magnet is provided on the needle to enable quick closing of
contact Thereby avoiding chances of sparking.
In a similar manner alarm controls can be made at two points one for a low pressure limit and
the other for a higher pressure limit. If the working pressure reaches the low set limit or the high
set limit alarm contact is made.
DIGITAL PRESSURE GAUGES
Digital pressure gauges working with integral or remote pressure sensing transducers are now
becoming more easily available and these are usually of a very high accuracy. This means of
course that they can be used for calibration purposes or for efficiency monitoring. The following
pages describe two such devices and they have proved in practice to be very valuable
calibration standards for the Eggbrough commercial instrumentation In particular the device is
used for checking the transmitters which measure the condenser absolute pressure.
© PMI, NTPC 37
The system for measuring condenser vacuum has had to be investigated at great length initially,
one tapping into the condenser steam space was used. Following test it was found necessary to
use one from each LP T/A exhaust in to the condenser and average the pressure via a common
manifold. This system now gives a representative average absolute pressure in the condenser.
And all readings which originally disagreed because they tapped in to different points on the
condenser now agree.
Calibration over the full range 0-100 mbar absolute is achieved by use of the condenser’s own
vacuum when the T/A is on load and the use of a vacuum pump in series with the gauge
tapping.
DRAUGHT GAUGES Draught gauges are used extensively throughout the Power Station to measure air and gas
pressures through the boiler and mills. The draught gauge is basically a diaphragm pressure
gauge with an elongated scale.
The readings are all transmitted as (0-10 mA) standard signals, if the process medium is not
allowed in the control room.
The foregoing comments on pressure gauge installation is appropriate to draught gauges also,
and since they are measuring relatively low pressures it is important that the pipe work is
installed very carefully with the added provision of a blow down facility to clear the lines of dust.
For suction gauges it has been found that the drilling of a small hole in a draught gauge line
near the tapping point can give an automatic cleaning of the lines without loss of reading or
sensitivity.
Calibration of draught gauges is best achieved with a manometer. Manometers of reasonable
accuracy and the correct range can be obtained from various manufacturers. It is important
when calibrating a draught gauge in this way that the correct type of manometer is used.
© PMI, NTPC 38
TRANSMITTERS FOR PRESSURE AND DIFFERENTIAL PRESSURE MEASURMENT A transmitter has a process signal such as pressure, flow, level or temperature as its input and
an electric or pneumatic signal as its output.
PR FLOW LEVEL TRANSMITTER ELECTRIC/PNEUMATIC TEMP OUTPUT SIGNAL
Thus the basic function of a transmitter is to proportionally modulate an electric / pneumatic
signal in response to the process parameters. We shall confine our discussion to electronic
transmitters. These transmitters sense the change in process variable within a certain range
and produce an output current within a range. The output range is standardised to bring
uniformity in the construction of secondary instruments as well as to facilitate the test and
calibration work. The prevent output signal ranges are:
A. 4 – 20 MA DC
B. 0 - 20 MA DC
C. 10 - 50 MA DC
Transmitters are generally connected in a measurement loop according to one of the following
methods:
a. Four Wire Transmitters In this method four conductors are led to the transmitter. One pair is used to carry the power
supply, which may be 220 VAC or 24 VDC. The other pair is used for signal transmission as
shown in Fig. No.7.
© PMI, NTPC 39
b. Two Wire Transmitters
This is presently the most widely used method for transmitter connections. There are three
basic elements in this loop, namely a/c power supply, transmitter and the receiving instrument.
They are connected in series and the transmitter acts as a current regulator in the series circuit.
The current in the series circuit changes with respect to change in process parameters as
shown in Fig. No.8.
Thus only two wires are needed for connecting one element to another. This simplifies cabling
and reduces erection and cable costs.
Being a series circuit, the input resistance of the receiving instrument plays an important role, as
higher input resistance will generally limit the loop current. For this purpose transmitter
manufacturers generally provide a load drive capacity curve for the transmitter. Referring to Fig.
© PMI, NTPC 40
9 we find that, this curve gives the maximum value of input resistance that can be connected at
the operating power supply voltage, without effecting the output current of the transmitter.
COMPARISON OF DIFFERENT TRANSMITTERS
The force balance transmitter is one of the early designs of two wire transmitters and is still
used widely. It has inherently rugged construction and is of simple design. However it has a very
large number of mechanical linkages and moving parts. This inherently reduces its accuracy
and makes it prone to errors due to hysterises, deadband. It also make it very bulky and heavy.
The zero and span adjustments are also mechanical and as such achieving good calibration
accuracy is not very easy.
The capacitance type and strain gauge type are definitely of superior design. They have virtually
no moving parts and hence are very accurate and have a good repeatability. They are
lightweight in construction and much smaller in size. Also al the adjustments such as zero,
span, damping are electronic therefore calibration becomes very easy.
© PMI, NTPC 41
5. Level Measurement and Measuring Instruments
INTRODUCTION In the most power station applications level can be defined as ‘the height of a liquid or solid
above a reference line’.
If the dimensions of a vessel are known then the volume or mass of its contents can be
determined by measuring the level. Hence the vessel contents can be directly displayed in units
of level (meters), volumes (litters) or mass (Kilograms).
There are three main reasons for making measurements of solid particles or liquid level; of the
three, safety of personnel and plant is the most important.
LEVEL MEASURMENT – METHODS There are many methods of measuring level, the selection of a particular system is largely
determined by the practical consideration already mentioned together with capital cost
(equipment and installation), reliability, maintenance cost and degree of expertise required by
maintenance personnel.
The method to be considered can be classified as follows:
1. Floats and liquid displacers
2. Head pressure measurement
3. Electrical / electronic
4. Ultrasonic
5. Nucleonic
6. Direct viewing
© PMI, NTPC 42
Floats and Liquid Displacement The use of a float enables the level of liquid to be measured when direct viewing is
impracticable. The float material may be of hollow metal, a plastic material, or molded rubber.
Float and Counterweight Type This method consist simply of a large area float connected by a chain type or cable to a
counterweight which passes in front of a scale and acts as an index.
The float should have the largest possible area in order to reduce the errors owing to friction
and out of balance forces of the cable or chain. If the surface of the liquid under measurement is
turbulent, a guide will have to be set up to stop the float moving around in the tank and causing
errors.
Rigid Arm Float
With the further development of attaching the float to a pivoted arm, the total energy available
can be increased due to the moment of force of the buoyance factor acting on the float at a
distance from pivot point. With this arrangement the total force can therefore be regulated by
changing the size of the float and also the length of the float arm as shown in Figure 10.
A rigid arm float has the advantage of being completely self contained and may be fitted to open
or close tanks in virtually any position, though its range is much smaller than that of the cable-
and-pulley type. Remote indication may be built in as in the other type.
© PMI, NTPC 43
The float movement is limited to about 120o as a maximum, the motion being transmitted to the
pointer by a worm drive or similar arrangement. If the gauge is mounted below the liquid level,
there must be some seal between the gauge and the tank. Some gauges use a magnetic
method or pointer transmission.
Float Operated Switches
Where it is required to initiate an alarm, start or stop a pump or open or shut a valve at a high or
love level the magnetically operated switch or air pilot may be used. The float assembly carried
© PMI, NTPC 44
with a permanent magnet which is opposed by a similar magnet which operates the switch, or
air pilot valve. This adjacent poles of the two magnets are of the same polarity so that they repel
each other, thus giving the mechanism a snap action. In the level switch mechanism the
contracts change over with snap action when the float passes the mid position. In the air pilot
valve, a compressed air supply is led into the unit, and when the float is in its highest position
the air valve permits the passage of air to the diaphragm or piston-operated value causing the
valve to close. A fall in liquids level causes the air valve to change over, shutting off the air
supply and venting the air in the diaphragm valve to atmosphere permitting the valve to open.
LIQUID DISPLACER SYSTEMS
This gauge, embodying a displacer, relies on Archimedes principle for its operation. According
to this principle if an object is weighed in air and then in a liquid there is apparent loss of weight
which is equal to weight of the displaced liquid. The displacer is a long hollow cylinder loaded to
remain partially submerged, and is suspended in the liquid in the vessel or in an adjacent small
© PMI, NTPC 45
diameter chamber connected to the vessel. The apparent weight of the displacer will decrease
as the level of the liquid rises.
Head Pressure Measurement Systems
These system use the principle that a column of liquid will exert pressure whose value depends
only on the weight of liquid, density of liquid and acceleration due to gravity and is totally
independent of the cross-sectional area of the column as shown in Fig. 12.
If the density of the liquid remains constant then the height of liquid above a datum (tapping)
point is directly proportional to the pressure measured at that datum point. Thus a pressure
measuring device can be used scaled in units of level.
Measurement of Liquid Level In Open Vessels
Since the static pressure at a chosen point of measurement (datum line) will very directly with
the head of liquid above it, it can be seen this pressure can be measured and the gauge
calibrated directly in head of liquid. The tapping point is always taken above the sediment level.
The gauge will read directly the total depth of liquid in the tank.
The gauge can be pressure measuring device, for example, bourdon tube, bellows, U tube,
enlarged leg manometer etc.
© PMI, NTPC 46
Measurement of Liquid Level In Closed Vessels
With closed vessels in most cases the vessel is closed because the system is to be pressurised,
or to operate conditions other then atmospheric as per Fig. 13.
FIGURE NO 13
© PMI, NTPC 47
In these cases it is necessary to see that same the same conditions exist on the reference side
of reference side of the indicator as inside the container, so the reference limb is fed back into
the top of the vessel.
Closed Vessel With Condensable Vapour
With closed vessels a further condition that may produce errors is when the pressure in the tank
contains vapour and these vapours then to condense on top of metering fluid in reference limb,
again causing the pressure factor which must be taken into account.
To offset this condition condensing chambers are used, these are chambers with a considerably
greater area than the meter chambering areas, so that the level of liquid in it does not change
much when the metering liquid moves in the manometer. The whole line will thus be filled with
condensate, thus forming a pressure head of relatively constant value, any additional
condensation now overflowing back into the vessel.
© PMI, NTPC 48
© PMI, NTPC 49
Liquid Seals
When there is a danger that the liquid whose level is being measured will, due to its nature,
adversely affect, the manometer fluid or transmitter diaphragm material then liquid seals should
be used.
The sealing liquid must not mix with the vessel liquid, be attacked by it, absorb corrosive
elements from it. Of course it also must have no adverse effects on the manometer fluid or
diaphragm material.
Gas Purge System of Level Measurement
Basically this method consists of a tube which is inserted into a liquid whose depth is to be
measured. An air pressure is applied to the tube and the air pressure is built up until bubbles
just begin to escape from the bottom of the tube. Bubbles will only form only when the pressure
in the tube is negligibly higher than the pressure exerted on the bottom of the tube by the height
of the liquid above the bottom of the tube as per Fig. 15.
When bubbles from the pressure in the pipe P = gh. When the density of the liquid is known the
pressure will be proportional to the height of the liquid above the bottom of the tube. Therefore if
the pressure in the tube is measured by the pressure gauge or U-tube the scale can be
calibrated in terms of depth of liquid on into any units required such as the volume or weight of
the liquid in the tank.
Air Trap System
In some cases where measurement of level is required, such as strong corrosives or at working
temperatures unsuitable for diaphragm, the air trap system can be used.
© PMI, NTPC 50
The box is covered by a plate with a small hole just large enough to allow liquid to enter. As the
level of liquid in the tank rises, the pressure on the air trap increases, liquid flows into the trap
and compresses the gas in the trap. When the air pressure plus the head of the liquid in the trap
is equal to the head of liquid above the trap no more liquid above the trap, no more liquid enters
the trap. The air pressure set up can be measured by a suitable indicator or recorder which can
be calibrated directly in terms of level.
Bellows Type
In the above type of diaphragm box is replaced by a box containing a bellows of synthetic
material. Changes of pressure within the bellows due to changing levels are communicated to
the measuring bellows by copper tubing having a fine bore. The bellows are filled with air at a
pressure slightly above atmospheric pressure. As the level increases the measuring bellows is
compressed – this increases the pressure in the system and the detecting bellows detects the
© PMI, NTPC 51
change in pressure and indicates it on a gauge, calibrated directly in units of level. In common
with other instruments, its reading of depth will be in error due to the change in density of the
tank contents with change in temperature.
Diaphragm Stack System
As the level increases the diaphragm stack is compressed, this compresses the air in the
system which creates which creates an increase in pressure. This increase in detected by
suitable indicator which is calibrated in terms of liquid level, volume or weight.
Electrical/electronic Methods Of Level Measurement And Control
Electrical methods for level measurement are very useful as generally where is the minimum
limitation on transmission distances between transducer and display or control devices. Their
speed of response is often better than pneumatic systems and they are very useful when
measuring the levels of vessels containing the solids.
© PMI, NTPC 52
Conductivity Methods-Levels Measurement
The system consists of number of conductors of different lengths connected together by a
series of resistors. As the level increases more and more conductors are shorted together, so
shorting out the resistors joining them, thus the overall resistance will decrease. See Fig. 18.
If a constant voltage is applied across the terminals, then as level increase, resistance
decrease, hence the current flowing in the circuit will increase. Therefore current will be
proportional to the level. If an ammeter is placed in series with the circuit, then it will indicate the
current flowing in the circuit. Since the current is proportional to level the ammeter can be
calibrated directly in terms of level.
This method can be adapted for use in manometer level measurement system by locating the
electrodes in the mercury of one of the limbs.
Conductivity’ Methods – Hydrastep System
The hydrastep system is probably the most common system used for boiler drum level
measurement. It has three main advantages over traditional gauge type system:
© PMI, NTPC 53
a. Smaller errors incurred due to change in liquid densities.
b. The output supplied to a conventional analogue level controller can be easily checked
for error using level indicating lamps.
c. Digital outputs for computer or microprocessor control/logging can be readily provided.
Density Errors In the sight gauge and head pressure manometer it has been assumed that the density of the
liquid remains constant throughout, but this is not necessarily true. If the temperature of the
liquid in the limbs varies then its density varies thus errors in level indication will occur.
The hydrastep vessel uses a side-arm method of attachment to the drum, and carries a number
of separate electrodes spaced vertically at intervals, usually of 25 – 50 mm (1-2 in), each of
which is associated with a separate channel of the electronic indicating system. The design of
the vessel, however, gives a very much reduced density error. The conventional visual gauge
body has a small cross – sectional area and a small bore, with only a small flow of condensate.
A by pass tube is often fitted so that condensate from the stemp, pipework is diverted from the
gauge. Because of the small cross section, the heat flow in to the gauge body occurs more or
less equally from both the steam and the water, and because of the slow flow, the temperature
gradient of the water column is large.
The hydrastep vessel has a metallic cross-section some four times that the visual gauge, and a
bore cross-section of about 10 times. The reduced thermal resistance vertically permits a
substantial quantity of heat required by the lower half of the vessel to be supplied from the
steam space, which is of course maintained at saturation temperature. In addition to providing a
larger surface area for heat exchange purpose in the steam space, the large boar reduces the
turbulence of high condensate flow and encourages the formation of a significant boundary
layer on the inside of vessel well below the water/steam interface, and this layer acts as a partial
thermal insulator. Instead of the mean water column temperature for a half-full gauge being
about 90oC below saturation temperature, as in the visual gauge the hydrastep vessel exhibits
only about 8oC mean drop, which results in a density error of only one-sixth of the visual gauge.
© PMI, NTPC 54
Basic Principle of the Hydrastep System
The principle upon which the Hydrastep is based is that of the differing electrical resistivities of
water and steam. Accepting for the moment that this difference exists, the vertical arrangement
of electrodes in the vessel, each of which is connected to a separate detector vessel, each of
which is connected to a separate detector circuit, enables the level to be determined at which
the transition occurs between water and steam values.
Each electrode, with its associated portion of the vessel, forms a cell in which the resistance
measured is a function of the contents of the cell. Except at the interface, each cell is filled
either with steam, (high resistance) or with water (low resistance); at the interface for a
continually falling water level (gradually increasing cell resistance) the corresponding channel of
the detector follows the curve shown. As the output voltage reaches about +4.5 VDC, the
corresponding logic circuit changes to indicate steam. A small amount of hysteresis is built in so
that in the reverse direction the change back to water occurs at about +3.5 V DC, to avoid
excessive operation due to insignificance level fluctuations.
At the time that the use of a direct electrical measurement was first considered for the
determination of drum level, little was known of the resistivities of water and steam at elevated
temperatures as exhibited in a dynamic system with continuous condensate flow through a side-
arm vessel from the boiler drum. A series of measurements was made, therefore, using the
vessels designed for the field trial equipments, on boiler in the 110 and 183 bar groups. These
measurements are presented and show that up to 360oC, the water resistivty was always less
then 10 6ohm.cm and steam resistivity always greater then 5 x 107 ohm.cm (and almost always
greater then 108 i.e. a differential of two orders of magnitude from cold to 360oC ).
© PMI, NTPC 55
FIGURE NO 19
Other work shows that an adequate differential for Hydrastep exist between the resistivities of
the water and the stream in a side arm–gauge at boiler pressures up to about 216.5 bar (3140
lb. f / in2, Tsat 372.8oC).
The switching band for the Hydrastep electronic circuits also shows superimposed at
approximately midway between the water and steam resistivities. The anomaly shown at 140–
180oC concerned the related water and steam readings taken during a severe steam valve leak
which resulted in steam entrainment in the vessel water column and water droplets in the steam
space.
To ensure absolute safety to personal the maximum voltage which appears at an electronic
terminal is 10v rms, and its maximum short circuit current is 10 uA, 50 Hz. Each electrode circuit
© PMI, NTPC 56
therefore meets the requirements for intrinsically safe apparatus with a margin of safety of five
orders of magnitude in respect of current. At 10 uA, the maximum electrode current is only one-
fortieth of the 0.55% human perception current at 50 Hz. The mechanical design of the
electrode is such that its centre be ejected from the vessel in the event of the failure of the
ceramic insulation, and a guard is fitted to deflect any steam jet which may arise from a faulty
electrode or seal.
The potential on each electrode is applied to its own individual discriminator channel to control
an output electromagnetic relay carrying six sets of changeover contacts.
One set of contacts is used for display purpose in the control room of either water or steam, as
appropriate, for each channel or the electronics, each display module being arraigned in the
order corresponding to the disposition of the associated electrodes in the vessel. Two further
sets of contacts on each relay are used in a logic matric to raise an alarm should a fault. Occur
such that any channel is ‘out-of-step’, i.e. that it gives an indication which is physical
impossibility such as “water above steam” or “steam below water”.
The three remaining contact sets are available for high or low level alarm purposes, or for
additional logic configurations to provide validated control signals, alarms and / or emergency
tripping of the generator and its auxiliaries.
THERMAL CHARACTERISTICS
FIGURE NO. 20
© PMI, NTPC 57
Further ‘fail–operative’ safe guards are provided by the connection of alternate channel of
electrodes on any one vessel to electrically separate power supplies, providing an interleaved
system. The loss of one power supply will still allow even a single Hydrastep to operate within
the terms of the Factory Inspectors’ Certificate of Approval, and the instrument can be repaired
with the generator still on load.
Failure characteristics The appearance of the display under normal conditions is shown in the left hand column of the
drawing. A colour change principle is used for each display module to avoid the ambiguity
possible between a true fault and a burnt out lamp where a simple on–off arrangement is used.
A power supply failure appears in either column 2 or 3, and the failure of a signal channel as in
the column 4 to 11. In no case of an electronic fault does the indication error exceed one step. It
is simple matter to include an automatic comparison between adjacent steps, on the premise
that water cannot exist about steam in the vessel in sufficient quantity to cause such an
indication (column 4 and 8). This comparison may be performed quit easily by means of
additional contacts on the relays controlling the display lamps. The usual station annunciator
operates when a connection is made between the alarm bus–bars. In the logic matrix, wherever
© PMI, NTPC 58
the water/steam interface may be, all channels above it should show steam with their contacts
in the upper position and all below should show water. If a fault should indicate water more then
one channel above the interface - for example, as shown doted on channel 11 – the bars are
shorted through 8 and 7, causing an alarm. Other logic systems, and techniques other than
relay circuitry could be used, including station computer if spare capacity is available.
Although electrically separate, the odd and even logic drive circuits are physically adjacent for
ease of inter-connection and are mounted with the power supplies close to the display unit in
the control room.
A design of colour change module using sub-mioiature long-life filament lamps has enabled a
small graphic display to be used, which could be mounted directly into the control console.
Two-Gauge Hydrastep
In the simplest multi-gauge instrument vessel A Carries the odd numbered electrodes and
vessel B the even, when the whole of the electronic system may be identical to that already
described for the single Hydrastep. Separate pipework for the two vessels is essential so that
pressure variations caused by a fault on one vessel will not affect the performance of the other,
and so that either may be shut off independently. Since the water steam interface within the
drum is not a plain surface under operating conditions, the inner ends of the waterside pipes
must terminate at substantially the same point in the drum so that the same head of drum water
is applied to both manometers. This prediction is not required for steam connection.
It is worth noting that a leak or blockage on either the steam or the water side of a vessel or its
pipe-work will result in a fractional pressure drop in the vessel concerned, the manometer will
then rebalance the vessel showing a higher level than for the non-faulty one. This means that
the faulty half of a two-gauge Hydrastep arrangement may be identified and switched out, so
that the gauge still remains operative using the sound vessel. The only exception to this
condition occurs with a leak of such proportions that water cannot remain in the vessel. Such a
leak would normally have developed comparatively slowly from a minor leak which should
already have been recognised; but in any case, the operation of a low level alarm with one half
of the Hydrastep showing a level within normal tolerance will identify the fault.
© PMI, NTPC 59
Twin Hydrastep The standard twin Hydarstep gives additional security by the provision of identical Hydrastep
units, A and B operating from both ends of the drum.
Not only does this extra redundancy permit the shut down of a complete end (e.g. to exchange
a faulty electrode or valve packaging), with the generator on load , but under normal conditions
the adjacent arrangements of the two electrically separate displays from the drum ends gives
the operator valuable information concerning end–to-end level variations either cyclic or static,
which can occur under certain plant running conditions.
© PMI, NTPC 60
Four–gauge Hydrastep (Twin Two–gauge) All Hydrastep System are self validating by the continuous comparison between adjacent
channels and the “water–above–steam” logic matric alarm, which gives an immediate alert to
the operator should a gauge fault cause a reading error in excess of ±1 step (normally ±50 mm).
Considerably higher security can be provided, even against double concident Hydrastep faults,
where a Hydrastep installation is to be used for control purposes, and in particular in those
installation which are required to provide an automatic emergency trip for the complete
generator set, in that event a minimum water level is reached at either end of the drum. It should
be appreciated that on a typical 500MW unit at full load, approximately 14 seconds are available
for shut down to be initiated after the minimum level is reached before internal pipe work is
subjected to damaging conditions. Because of the possible end–to-end level difference, each
drum end must be separately protected and reliance can not be placed upon cross-validation
between Hydrastep at opposite ends of the drum. Further more, because of the very close
timing sequence for the start up of standby feeds pumps or the opening of alternative feed water
valves, it is necessary that a trip must not be initiated before the minimum level is reached. This
is basically an economic consideration: if a trip should be initiated while the situation could still
be saved by the stand by plant, a loss of revenue could result from the need to use lower
efficiency plant to meet demand during the period (i.e. .. the penalty for an “unwanted action”
occurring). On the other hand if a trip is not initiated at the required minimum level widespread
damage could be caused to the boiler, furnace, turbine and generator (i.e. the penalty for failure
of a “wanted action”). It is essential in this application that the security of the Hydrastep must be
maintained and that the ±one step tolerance must be eliminated even in the presence of the
gauge fault, since there is no permissible deviation from the “wanter” trip level. Such
characteristics can be achieved using a four vessel Hydrastep in which the water side pipe work
for both vessels at either drum end terminate at substantially the same point (about 100 mm
apart) within the drum.
© PMI, NTPC 61
The relay logic matrix is connected to give a ‘three-out-of–four’ system, that is three out of four
gauges have to indicate a low level before a trip is initiated.
Hydrastep – Display Unit Instead of a sliding box as in the rest of equipment, the display unit uses a hinged door from
construction, with a removable rear cover. Sufficient clear space around the unit must be left to
allow access. The philosophy of continuous comparison between adjacent channels by the logic
matrix ensures that the indication presented to the operator has been fully verified. The
channels at either end of one vessel (i.e. channels 1 and 12 of both drum and indicators in the
case of the standard Twin Hydrastep) can each be verified on one side only, since channels 0
and 13 do not exist. If, for example, the case of steady falling water level is considered in
conjunction with a fault on channel 1 such that water is permanently indicated, it would appear
to the operator that some water still existed in the gauge although, in fact, the fall in level had
continued past this point. Had a channel 0 existed below channel 1, as the level continued to
fall, a “water above steam” condition would have appeared and the fault on channel 1 would
have been recognised. Without channel 0, therefore, channel 1 cannot be fully verified
and is not presented to the operator since it could be misleading; similarly channel 12 could be
suspect without a channel 13 for verification. To ensure that no misinterpretation can occur,
therefore, only channels 2 to 11 inclusive is displayed in normal operation, channel 1 and 12
being covered by the hinged outer panel. This outer panel, however, may be opened by the
Instrument Engineer to gain access to the remote test switches and telephone socket, when the
indication from these channels can also be observed.
© PMI, NTPC 62
Because of the level differences between drum ends which can occur in normal operation, it is
impractical in a Twin Hydrastep to cross validate any channel on one vessel and the
corresponding channel on the other, and the restriction in the previous paragraph must apply to
both columns of the duel display. However, in the special case of the four gauge system,
whether or not the automatic tripping facility is connected, cross-validation may be incorporated
at all the available corresponding levels between the two vessels at the same end of the drum
and all indication including the extremes may be then presented to the operator.
Capacitance Methods A capacitor is a device for storing electrical energy. In its simplest form it consists of two plates
of area, separated by a distance. The air between the plates is called the dielectric.
When a voltage is applied across the plates on electrical charge is stored proportional to the
applied voltage.
© PMI, NTPC 63
Capacitance level measurement involves the use of an electrode which extends the full length
of the tank and form a capacitance between itself and earth where earth may be the vessel, the
contents or a concentric cylinder around the electrode, depending on the type of electrode
involved.
A variation of capacitance will occur when the depth of the medium in the vessel alters therefore
the capacitance change will be proportional to level.
By this method the level of liquids, powders or granular solids may be measured.
Conducting Mediums When the medium is a good conductor of electricity then the system works as a variable area
capacitance transducer. The electrode is one ‘plate’ of the capacitor and is insulated with a
material that is compatible with the medium, the insulation forming the dielectric. The medium in
© PMI, NTPC 64
the vessel from the other ‘plate’ of the capacitor. Thus, as the level changes, the area of the
capacitor ‘plates’ varies. If level falls then area decreases and capacitance decreases.
Non-Conducting Mediums When the medium is non-conducting the electrode is not insulated and the system works as a
variable – dielectric capacitance transducer. The dielectric is made up of, say, liquid in the tank
and the air or gas in the space above the liquid thus the electric constants will be different
(normally those of liquids are much greater then gases). As the liquid level varies then the
overall capacitance will change due to change in dielectric. A rise in level ill increase
capacitance and fall in level will decrease capacitance.
ULTRASONIC METHODS
Ultrasonic
Ultrasonic beams are a form of energy transmitted by means of mechanical vibrations and
carried through the transmitting converting one type of energy into mechanical vibrations which
are received by a device which detects the ultrasonic beams converting them into a more
readily usable form of energy. Above a certain frequency (20 kHz) it is known as ultra sound or
ultrasonic sound. For level switching a range between about 36 and 40 kHZ is used. Ambient
noises or their harmonics are ineffectual in this range.
Principle of Operation of the Sensors
When certain materials, mainly nickel, iron and cobalt, are placed within a magnetic field, their
lengths will very by an amount dependent on the strength of the magnetic field.
The fundamental generator is a nickel tube which carries the coil and bias magnet. The current
through the coil either weakens or strengthens the field, depending on the direction of the
current. Application of an alternating current causes the length of the tube to increase and
decrease at the supply frequency. owing to the mechanical properties of the tube it will tend to
oscillate longitudinally as a half-wave resonator.
© PMI, NTPC 65
Similarly with the receiver, a sound wave impinging on the diaphragm will cause a relatively
large amount of movement in the nickel tube, if within the band path’s frequency, virtually non if
outside. Changing the length of the tube will cause a change in the magnetic strength of the
bias magnet, thereby generating an e.m.f within the coil. Hence the same cant be used as either
a transmitter or a receiver.
The system is unaffected by dirt, vapour, moisture etc. The sensors are temperature-sensitive;
the resonant frequency falls as the temperature rises but there is no effect if both sensors are at
the same temperature. Very little maintenance is required.
NUCLEONIC METHODS
Nucleonic
Since the advent of nuclear reactors and the ready availability of radioactive materials, nuclear
techniques have been employed for the extension of some of the more conventional methods of
level measurement, as well as the invention of new methods. The special advantage of nuclear
gauges is that they can operate entirely from outside the containing vessel. They may be
designed to provide on/off control at the fixed level in the vessel, or to provide continuous
indication of level over a given range.
The nucleonic type level instruments involve in radioactive source, a radiation detector and
electronic measuring circuits.
DIRECT VEIWING
Sight Glass
The sight glass is a very useful as a simple arrangement whereby a section of the liquid is
brought outside the vessel and displayed alongside a main scale. If the diameter of the bore of
the sight glass is not small enough to introduce errors due to capillary action, the liquid will
stand at the same level in the sight glass and the vessel, provided the top of the sight glass is
© PMI, NTPC 66
subjected to the same pressure as the top of the vessel. It can be used for open or closed
vessels.
The system is analogous to a U-tube manometer where the vessel is one limb and the sight
glass the other limb.
Sight glasses are usually installed with two isolating valves and a blow down valve for cleaning
purposes. The tube material is generally pyrex or armoured glass. Reflex glass is sometimes
used to improve readability, the division between the liquid and gas region being made very
pronounced. Other sight glasses include a float market to improve readability especially if the
meniscus at the liquid/gas interface is obscured by scum or scale.
In high level installation such as boiler drum the gauge is usually fitted with an automatic cut-off-
so that if the sight glasses breaks, the danger of anyone being injured by the contents of the
vessel will be minimized. The cut-off-device usually consists of two ball bearings which normally
hang lose in the connecting pipes, but if the glass breaks the flow of steam and hot water forces
the balls against valve seats so cutting off the escaping flow.
Impurities in the liquid are one of the problems of sight glasses systems as the glasses
becomes discolored and obscures the liquid meniscus.
Regular cleaning of the gauge glass is the common maintenance task. Other problems are
broken glass tubes or leaks and / or blockages at the connections.
The range of sight glasses largely depends on the nature of the liquid, the static pressure and
the temperature involved. Ranges of 0.2 to 2 metres are typical.
If the density of the liquid is constant then sight glasses are simple, accurate devices for level
measurement they can be calibrated by comparison with a dipstick or the addition of a known
volume of liquid.
© PMI, NTPC 67
WATER GAUGE WITH CLOSED CIRCUIT
Television (C.C.T.V.) Remote Display
One of the obvious problems with a simple sight glass system for, say, boiler drum level
measurement is that local indication only is provided. The use of a special type of side glass
(water gauge) with an associated c.c.t.v. system allows level display to be remotely located in
the Control Room as shown in Figure No. 25.
The water gauge works on the principle that different substances have different refractive
indexes i.e. they bend light by different degrees.
© PMI, NTPC 68
The gauge is vertical tube of triangular wedge cross-section. Two faces of the three sides are
made up of glass and mica divided up into small compartments. Illumination is projected
through the gauge and the light is bent by the medium. The degree of bent depends on whether
the medium is water or steam. Because of the prismatic arrangement either the ‘water’ windows
or ‘steam’ windows are illuminated thus the level of water in the drum can be determined.
A C.C.T.V. camera is mounted a few feet away from the gauge and is carefully aligned with the
light path through it. The camera and lens system being fully protected against fuel dust and
ash. The C.C.T.V. monitor is located in the Control Room.
This system is more difficult to operate with high steam pressures (120 bars) because the
refractive index of water closely approaches that of steam thus angular deflection of the light
paths is very small making level indication difficult. At pressures of 166 bars it is almost
impossible to accurately determine water level by direct level viewing means.
The problem can be overcome by gauge rear illuminators using quartz iodine lamps and by
using an optical magnifier to enlarge the small differential in the refractive index of the two
mediums.
The main problem with this system are that of faulty alignment, hostile environmental conditions
which affect the reliability of the camera and high degree of technical expertise required for
C.C.T.V. maintenance.
© PMI, NTPC 69
6. Flow Measurement and Measuring Instruments
FLOW MEASUREMENT
Introduction Fluid flow in industrial undertakings occur in two general forms: either as a flow in a pipe or
conduit or, in the case of liquids only, as a flow in an open channel. In both cases, the rate of
flow is of primary importance, and, in a large number of plants, the totalized flow over a
specified period is required in addition. The rate of flow measuring instruments will be examined
first.
Rate Of Flow Measuring Instruments: This class may be broadly subdivided into:
a. Differential pressure flowmeters
i. Orifice pattern
ii. Venturi and nozzle pattern
iii. Pitot tube pattern
iv. Dall tube pattern
b. Variable area flowmeters
c. Displacement and interfrential flowmeters
d. Electromagnetic flowmeters
e. Ultrasonic flowmeters
© PMI, NTPC 70
CONCENTRIC ORIFICE PLATES
A universally used method of making an abrupt change in the cross-sectional area of fluid
stream flowing in a pipe is the concentric orifice plate. This involves a circulate metal plate with
a central hole or orifice centric with the circumstance of the place. It is fixed between the pipe
flanges and is located by the flange bolts. The orifice is then concentric with the internal bore of
the pipe.
It will be convenient before describing particulars to see what occurs when an orifice plate is
inserted in a fluid stream in a pipe, and a liquid flow is considered. Fig. 26 illustrates the action
in a simplified manner.
ILLUSTRATING THE VARIATION OF STATIC PRESSURE UPSTREAM AND
DOWNSTREAM OF THE ORIFICE FIGURE NO. 26
Suppose that tubes are inserted through the pipe wall at the position shown in the diagram, the
pipe liquid will rise in these until the pressure due to the column of liquid in each tube is equal to
the static pressure at that position. The column heights are then a measure of the pressure and
from observing the different values we may trace the pattern of the pressure changes as we
proceed along the pipe. At position 1 and 2 there is no pressure change worth specifying.
© PMI, NTPC 71
At 3 and 4, just before the orifice, we find a slight increase in pressure. The stream is then
constrained to flow through the smaller size of the orifice, from which it issues as a jet. At
position 5 and 6 there are lower pressures then at the up stream position due to the change in
the stream sectional area. Since this is similar the velocity has increased, and the pressure has
fallen. The stream or jet cross section decreases in area after leaving the orifice until it reaches
a point, indicated as 7 in the diagram, where it is a minimum and the velocity a maximum. This
is mainly due to the liquid being directed inwards as it approaches the orifice, and, through
inertia effects, persisting in this direction for a distance after it leaves the orifice. The static
pressure also reaches it minimum value at this position, which is known as the vena contracts.
The distance from the orifice varies with the ratio of orifice diameter to pipe diameter but an
average value be about one half the pipe diameter. From the vena contracts, the steam station
expends until it reaches the pipe diameter at 8. Two facts emerge from the study of Fig. 27. one
is that the downstream static pressure never recovers its upstream value. This would appear to
be caused by the velocity changes being accompanied by considerable turbulence with
resulting dissipation of energy involving a pressure loss. Taking a typical value of 0.6 for orifice
to pipe diameter ratio, the percentage loss works out a 65 percent of the differential pressure.
Where pressure loss is important this factor should be borne in mind.
The second point which emerges is that there appears to be a variety of positions at which to
take pressure trappings or connections for obtaining the differential pressure.
The following are the main tapping positions (shown diagrammatically in fig. 27 & 28).
© PMI, NTPC 72
D And D/2 Taps (Radius Or Throat Taps)
The upstream pressure tapping is taken at one pipe diameter D, upstream from the face of the
orifice face, and the downstream pressure tapping is taken at one half pipe diameter, D/2,
downstream from the orifice face, approximately the vena contracta position.
Corner Taps
Corner tapping are taken via holes cut obliquely through a flange or boss on pipe, bringing the
inside openings of the holes adjacent to the orifice positions.
Plate Taps
In this variety connecting holes are actually bored in the orifice plate itself each hole
communicating with one face.
Flange Taps
These are situated 1 inch from the upstream and 1 inch from the downstream face of the orifice
plate, with the tapping bored through the flanges.
© PMI, NTPC 73
Vena Contracta Taps
The upstream tapping is 1 pipe diameter from the upstream face, and the downstream tapping
is determined from the curve relating the required dimension to the radio of orifice to pipe
diameter. These are very similar to the D and D / 2 taps.
Pipe Taps
These may be 2.5 pipe diameter upstream and 8 diameters downstream from upstream face of
the orifice plate.
Carrier Ring
Where it is not desirable to drill or tap actual pipes, bosses, or flanges, a self contained orifice
assembly may be inserted between pipe flanges. It consists of a metal ring holding the orifice
plate, with tapping drilled through the ring to communicate with the upstream and downstream
sides of the orifice. Fig. 28 shows diagrammatically a carrier ring assembly. One advantages of
this type is that drillings etc. are carried out at the manufacturers works and the errors due to
site operations are eliminated.
Having established the possibilities of a definite constructive device, for fluid flow measurement
under ideal conditions, we must now examine what modifications are necessary in practice.
Turbulent Flow
In practically all cases of the flow in pipes for industrial purposes the flow is turbulent, that is the
particles of the fluid do not follow paths parallel to the direction of flow. Some, if not all, of the
particles have a transverse motion as well as longitudinal one and form little eddies or swirls
giving rise to turbulence. Stream line or laminar flow formulae will not apply here without
modification and a new set of equations must be derived.
© PMI, NTPC 74
Discharged Coefficient
Due to friction and velocity distribution, the practical flow figures do not line up with theoretical
ones. Observe that the stream area contracts after leaving the orifice to the vena contracts
position. The cross-sectional area there may only be about 0.6 that of the orifice.
Orifice materials Materials used for orifice plates include mild steel, stainless steel, monel, phosphor bronze,
gunmetal, depending on the application. A rough classification would be:
Water metering : Gunmetal, bronze, stainless steel
Air metering : Gunmetal, monel, mild steel
Steam metering : Stainless steel, monel,
Sewage, fuel oils, coal gas : Stainless steel
Venture Tube We have seen the effect of inserting an orifice plate in a fluid stream, causing a abrupt change
in stream area to produce a differential pressure. The operation can be accompanied by a fairly
high permanent pressure loss, and where pressure loss is important, it is necessary to turn to
other method of producing differential pressures. Let us consider devices with a gradual change
in area. The first of these is the venture tube.
© PMI, NTPC 75
Constructional Features To some extent, the construction of the Venturi tube depends on the application. For normal
uses, the section would be of gun-metal, cast iron, or Mechanite, and smoothly machined liners
of gunmetal or stainless steel inserted at the inlet and throat pressure tappings. The use of gun
metal or stainless steel reduces the risk of corrosion. To facilitated construction work a victualic
joint is sometimes inserted in the downstream cone. The extreme end of the cast section are
flanged to match with the pipe flanges, and with the adjacent section, and pressure tappings are
arranged for screw in or flanged connections depending upon the particular installation
condition.
For high pressure hot water flow as in boiler feed water in power station, the design is used, and
the gun metal lining is inserted. The lining is made in three sections: inlet cone, throat, and
outlet cone, profiled as for a standard Venture. This design is suitable for pressures up to 1400
lb/in2. Another pattern has a maximum working pressure of 2000 lb / in =2.
The Venturi tube possesses a big advantage over the orifice is that its section need not be
circular. Square or rectangular shapes have been used for measuring large volumes of fluid
flow. The non circular section lends itself to constructional materials other than metal, and
concrete has even been used for one or two very large flows. Note that the design renders the
tube useful for fluids containing suspended matter because of its gradual area changes.
Nozzles The nozzles falls between the venturi tube and the orifice plate as a means of flow
measurement. Its approximates to a venturi tube with the curved form of approach, giving a
gradual change of sectional area and has the same order of discharge Coefficient. But the
absence of a downstream expansion core brings the pressure loss in to the same region as that
for an orifice plate. It is cheaper than a venturi tube, and at high velocity flow it is use in place of
an orifice plate may be necessary. See Figure. 30:
© PMI, NTPC 76
PITOT TUBE
Let us study the effect of placing a blunt object in a fluid stream as an obstruction to the flow
(Fig. 31). As the fluid approaches the object, the velocity will decrease until it reaches zero at
the point where it impinges on it. From the previous, a declaration should mean an increase in
pressure. This would follow from Bernouilli’s Theorem.
It is very convenient to be able to measure the static pressure in the close neighborhood of the
tube and standard Pitot tubes. Both designs consist basically of inner and outer tubes. The inner
one leads from the impact hole to one construction of the differential measuring instrument. The
outer tube, referred to, sometime, as the static tube, has a series of holes bored into it so that its
interior connects to the out side surface to be in contact with the static pressure. This tube is
joined to the second connecting of the measuring instrument.
The pitiot tube can only measure velocity at one position in the cross-section of a pipe. Now a
velocity of a fluid in a pipe, taken across the section, is not uniform, varying from zero at the
pipe surface to a maximum at some point (not necessarily the centre) along the diameter. To
find the mean velocity it is necessary to make a traverse of the pipe with the tube, taking the
differential pressure at certain specified positions. An ideal distribution curve is shown in fig. 31.
For Reynolds numbers above 100 000, the ratio of average velocity to velocity at the centre of
© PMI, NTPC 77
the pipe is frequently specified as 0.82 or 0.83. Whereas this value would apply for ideal cases
for as curve of the type in Fig. 31, the actual curve may be different. The desirability of carrying
out a traverse, therefore, is obvious. Once having determined the ratio value, the Pitot tube may
be placed at the pipe centre and the instrument calibrated in terms of average velocity.
Another theoretically possible means of determining the average velocity is to select a position
where the velocity corresponds to the average value. This has some practical drawbacks. The
location may be near the wall of the pipe a very approximate value being 0.25 of the radius in
form the wall. It could be at a point where the velocity curve slope is fairly steep and any
misplacement could lead to significant errors in velocity determination. At the centre of the pipe,
by comparison, the curve is normally flatter and errors in location are not so serious.
© PMI, NTPC 78
Two other types of Pitiot tube deserve mention. One is the double, tip pattern shown in Fig.31.
in which there are two holes, one facing upstream and the other downstream, the former
measuring the impact head and the latter the section head. The differential pressure obtained is
greater than with the standard types, but is not double the value. Actually, the increase is
between 35 per cent and 40 per cent depending on the position of the tube in the pipe. The
other type is the Pitot-Ventury. It is a combination of two concentric venturi tubes, the out let
cone of the inner one terminating in the throat of the outer. The throat pressure of the inner tube
and the impact pressure on an impact hole in the supporting tube give 7–10 times the
differential produced with the normal types under the same conditions.
DALL TUBE The principle features of the Dall Tube are indicated in Fig. 32. It involves two truncated cones
separated by a narrow throat. The throat length is between 0.3d and 0.1d where d is the throat
diameter. The inlet cone has an included angle between 40o and 50o, the out let cone between
12o and 17o. The mouth diameter Dm, the inlet pipe diameter D and throat diameter d are
connected by the following relation
Dm4 – d4 = K (D4 – d4 )
Where K = 0.5 to 0.75
© PMI, NTPC 79
Observe the diameter of the inlet cone is less then that of the pipe, resulting in a sharp step.
This creates an impact pressure which is additional to the static pressure existing at the step.
The high or upstream connection is made just in front of the step. The other connection is made
at the throat where the relativity abrupt change in area results in a marked static pressure
depression. The original patent specification No. 689, 474 claims a pressure loss expressed as
5% or 6% of the differential pressure. This compares with a loss of between 2 and 3 times this
value with a normal venturi tube. In addition, the Dall tube has the advantage of being
considerably shorter than the normal venturi.
GLASS TYPE The basic feature of this type of meter is the conical section glass tube. For accuracy, the
diameter of this must be maintained at very close limits. Clear borosilicate glass is used which is
highly resistant to thermal shock and chemical action, and the method of its manufacture
enables tolerances of I/10 000 of an inch to be observed. The use of glass introduces the
question of a safe working pressure for the fluid being measured. At present this is about 500
lb/in2 and applies to the smaller diameter tubes. For larger sizes the safe working pressure falls
from this figure. (The normal diameters range from 2 mm to 60 mm depending on the flow to be
measured.) The tube is normally clamped in a metal frame, the inlet and outlet being sealed into
connections as required, e.g. flanged or screwed. Where danger may occur from flying glass
resulting from a fractured of the tube, “Armour Plate” glass protection windows encase the
instrument.
© PMI, NTPC 80
The standard float shape is indicated in Fig. 32, and is perfectly free. Viscosity immune floats,
however, may demand a guide, as the float disturbe the equilibrium of the liquid. In one pattern,
the guide is a central rod around which the float is made to rotate, so that visual evidence that
the float is moving freely is obtained.
The glass tube type measures from 2 cc/min up to 3000 liters/min of gas, and 0.5 cc to 225
liters/min of liquid.
The pressure drop will depend on the type of float being used and the nature of the fluid, but
varies between about 0.2 cm (0.078 in) w.g. for small gas flows and 3.5 cm (1.38 in) w.g. for
liquid flows.
METAL TUBE TYPES For larger flows than the glass type tubes can accommodate, a conical metal tube pattern is
introduced. Here, the metal body is of gun metal, cast iron or stainless steel with a stainless
steel float. The latter is carried on a rod which moves between two guides, one at the lower end
and the other at the upper end of the tube. The guide rod passes through the upper part of the
tube in to a compartment with a glass scale, the end of the rod acting as an indicator (Fig. 33).
This type of meter has typically ranges from 250 to I 20 000 liters/min of gas flow and 20 to
7000 liters/min of liquid. The maximum fluid working pressure is 500 lb/in2. When used with
opaque liquids, a compressed air supply may be connected to the top of the scale unit and the
level of liquid depressed, so that a clear view of the indicator is obtained. Opaque liquids may
also be metered by the high pressure version.
© PMI, NTPC 81
ELECTROMAGNATIC FLOWMETERS The principle of the electro magnetic flow meter may be understood better if we first consider a
very thin disc of an electrically conducting liquid moving with a velocity
V along a pipe of internal diameter d. An external magnet system directs a magnetic field of
strength H across the section of the pipe so that it acts at right angles to the direction of motion
of the disc. Now, by Faraday’s Law of indication, when an electrical conductor of length L moves
through a magnetic field of strength H at a velocity V in a direction at right angles both to the
magnetic field and its length, an e.m.f. is generated of value.
E=K H L V ……… (I)
WHERE K = a constant
Our disc liquid is a conductor obeying the general requirements of Faraday’s Law, and it can be
seen without much difficulty that L in equation (I) is replaced by d, the diameter of the disc. If,
now, there is an indefinite number of such moving disc continuous to one another, we have the
equivalent of a conducting liquid stream flowing continuously through the pipe (see Fig. 34.)
© PMI, NTPC 82
The stream will satisfy the following equation
E = K H V d . . . . . . . . (II)
In (II) d is constant and if H remains constant
E = K V
Where K = a general constant . . . . . . . . (III)
Alternatively, since Q = V A where A is the area of the pipe,
E = C Q . . . . . . . . (IV)
C being a general constant.
Thus, provided we can physically measure E, a very simple means of determining the flow rate
of liquids in pipes is available.
© PMI, NTPC 83
ADVANTAGES OF THE ELETRO-AGNETIC Flowmeter
1. Linear relation between flow rate and measuring signal as compared with the square law
relation of differential pressure devices. This results in a range ability of the order of
100/1.
2. The measuring instrument can be arranged with a centre zero for measuring flow in
either direction. Alternatively the electrode leads may be changed over to measure a
reverse flow.
3. The only pressure loss is that due to the length of tube, forming the meter. But a
pressure loss would be present with the same length of ordinary pipe so that the
introduction of the meter cannot be said to involve significant additional pressure losses.
4. There is no obstruction to flow which renders the meter suitable for liquids containing
suspended matter. Abrasion may be avoided by choosing a suitable lining material.
Wood pulp and paper mill stocks, cement slurries, sewage, food pulp are but a few
difficult fluids which may be metered.
5. The design lends itself to the metering of corrosive liquids since parts on contact with the
fluids may be made of corrosion-proof materials.
6. It is not affected by velocity profiles, since the e.m.f. is at all points proportional
to the velocity of flow across the diameter.
DISADVANTAGES OF THE ELCROMAGNETIC FLOWMETER
1. It is not suitable for measuring gas or vapour flows.
2. The normal design is not suitable for hazardous areas.
© PMI, NTPC 84
3. Liquids to be metered must be conductors of electricity.
4. There is minimum value of conductivity which is related to the lengths of the cable leads
to amplifiers and the size of the meter. The readings are unaffected by increases in
conductivity above the minimum value, but decreases cause the meter to read flow.
5. If concentric build up of deposit of much different conductivity to that of the liquid takes
place, significant errors may be introduced. Values will be found in the paper by B.W.
Balls and K.J. Brown. Note that it is possible for non conductive deposits to insulate the
electrodes. Where the concentric deposit is of the same conductivity as the metered
liquid the meter continuous to read correctly.
A meter has been constructed of 1/10 in. diameter with a flow range 0.002 to 0.2
gal/min. In contrast, a typical large diameter meter has been 42 in. covering a range 500
to 50,000 gal/min.
ULTRASONIC FLOWMETER 1. Consider Fig. 35 in which a fluid is flowing at a velocity V a transducer T1 transmits a
beam of sound to receiving transducer T2 situated at a distance d downstream. If C is
the speed of sound through still fluid, t the time for sound to travel from T1 to T2 is
d t = ----------- ………. (I) C+V
with no flow, to = d / c ………… (II) The difference between t and to, t, is given by
Vd t = ………….. …………. (III) C (C + V) C for most fluids is of the order of 1500 meters/sec. whilst V for most industrial application would
be a few metres/ sec equation (III) then reduce to
Vd t = …………. ………….. (IV)
C2
© PMI, NTPC 85
1. This suggest that t could provide a measurement of V. But it involves a knowledge of to,
not readily measurable, and C, which varies with temperature and pressure. To eliminate
to, the differential arrangement shown in Fig. 35 may be used.
2. Two sets of transducers, T1 and T2 and T3 and T4, are installed in the pipe, the distance
between T1 and T2 and T3 and T4 being d. A beam of sound is transmitted from T1 to T2
downstream and from T3 to T4 upstream, both being of the same frequency. The time for
the beam to travel from T1 to T2 is
© PMI, NTPC 86
d t1 = --------------- . . . . . . . . (V) C + V
Where C = the velocity of sound under the temperature and pressure conditions existing
in the pipe, and from T3 to T4.
d t2 = ------------- . . . . . . . . (VI) C – V
The difference between t1 and t2 is
2 Vd t1 = t2 = t = -------------------- . . . . . . . . (VII) C2 – V2
If (V) is small compared with C; (VII) can be reduced to
2 V d t = ----------------------- . . . . . . . . (VIII) C2
3. The measurement of t now involves some problems. It may be solved by pulse
techniques or a continuous wave beam may be used. In the latter case, the transmitting
transducers are driven from a common source and the phase difference between the
two received signals measured. The phase difference Ø is given by
2 wvd Ø = ---------------- . . . . . . . . (IX) C2 Where W = the angular frequency.
4. Observe that it all the methods considered, C the velocity of sound is present. This can
be eliminated if the methods of Fig. 35 is adopted. A short pulse is emitted from
transducer T1 and is received by T2. The arrival of the pulse from T2 triggers another one
from T1. The time between pulses is
d t1 = --------------- . . . . . . . . (X) C + V
The pulse repetition frequency is f1 and since f1 = I / t1
C + V f1 = ------------- . . . . . . . . (XI) d
© PMI, NTPC 87
A similar pulse is transmitted from T3 to T4 and calling the repetition frequency here f2,
C - V
f2 = -------------- . . . . . . . . (XII)
d
f1 – f2 = f = 2 V / d . . . . . . . .(XIII)
Equation (XIII) is independent of C.
5. A further techniques used has been a differential arrangement across the pipe. It can be
shown that a beam of sound can be deflected in the downstream direction in the
traversing a pipe from one to the other. The deflection x is approximately given by
x = V d / C . . . . . . . . (XIV)
Which of the methods outlines is most suitable for industrial applications?
There are several factors to consider.
1. In the phase difference method the phase difference is given by
Ø = 2 WVd / C . . . . . . . . (XV)
Ø is proportional to the operating frequency W and is suggests that W should be
as high as possible.
Measurements of Ø above 2 n are not desirable, from the point of view of
interpretation, but Ø should be as large as possible below this limit. But the
higher frequency the greater the attenuation since it is a function of the square of
the frequency. Thus, already we have two conflicting factors. There is yet a third
effect: that of the beam width. This is a function of the velocity of sound in the
liquid, the radiating area of the transducer and the operating frequency. There
may have to be a compromise between all the factors involved.
2. In the frequency difference method, care must be taken to avoid coupling
between neighboring circuits carrying frequencies relatively close to one another.
© PMI, NTPC 88
The frequency difference f is dependent on the flow rate V and may be of
extremely low value, e.g. 10 c/s or 20 c/s unless the flow rate is relatively high.
3. The beam deflection method suffers from the fact that the deflection is
proportional to flow rate and at low flow rates, may not be sufficient for accurate
measurement.
© PMI, NTPC 89
7. Temperature Measurement and Measuring Instruments
Several methods are available to measure the temperature. Appropriate method is to be
selected for any particular measurement. The selection of the type of measurements is based
on the following consideration.
i) The accuracy required
ii) The range of temperature
iii) The corrosive action of the process media on the sensing element
iv) The catalytic phenomena of the sensing element on measuring media.
v) The layout conditions and restrictions
vi) Facilities available for the calibration of the instrument
THEORY OF TEMPERATURE MEASUREMENT
Temperature rise in a substance is due to the resultant increase in modular activity of the
substance on application of heat which increases the internal energy of the material. Therefore
there exist some observable properties of the substance which change with its energy content.
The temperature measurement is based on this very fact. The changes may be observed in the
substance which itself or in a subsidiary system in thermodynamic equilibrium with it, and it is
called the testing body while the system itself is called the hot body.
THERMAL EXPANSION OF TESTING BODIES On application of heat, testing bodies either in the form of solids liquids or gases expand almost
proportional to the rise of temperature and this principle is utilized in various thermometers.
© PMI, NTPC 90
EXPANSION OF SOLIDS
The expansion of solids is utilized by means of bimetallic strip to measure temperatures. Two or
more layers of metallic alloys having different co-efficients of thermal expansions are coiled in
the form of (a) spiral (b) helical or (c) multiple helical depending upon the range temperature.
One end of the coil is fixed on to the bulb to be used as a test body and the other and free to
move carrying the pointer over a scale calibrated in degrees.
A simple bimetallic strip composed of a layer of brass (high expanding material) and a layer of
invar (low expanding material) will deflect when subjected to a change of temperature and if the
strip is coiled its angular motion will be given by CTL/t where C=length of strip in CMS and
t=thickness in CMs Fig .36 shows the typical bimetallic gauge.
© PMI, NTPC 91
SPIRAL (a) SINGLE HELIX (b)
PRINCIPAL TYPES OF ELEMENTS USED IN BIMETAL THERMOMETERS
(A) FLAT SPIRAL (B) SINGLE HELIX (C) MULTIPLE HELIX. FIGURE NO. 36
EXPANSION OF LIQUIDS
Changes in volume of a liquid by the application of heat enclosed in a test body is utilised to
measure the quantity of temperature. The liquids normally used are, mercury and hydro-carbons
such as ethylalcohol, for low temperature, metaxylene for medium range temperature,
tetrahydro naphthalene (tetralene) for higher temperature.
© PMI, NTPC 92
The test bodies (bulb) are either glass or of steel material.
The liquid filled system, consists of an element sensitive to temperature change (i.e. bulb), an
element sensitive to volume change (bourden, bellow of diaphragm), means of connecting these
two and a device for measuring and indicating.
The liquids filled in a bulb from which a capillary is drawn which ends in a bourden or bellow or a
diaphragm. The entire system is filled completely with the liquid at 0o C at high pressure of the
order of 1000 PSI. When the temperature rises, the volume of the liquid increases thereby
tending to enlarge the enclosure. As a result a mechanical motion is achieved which is
transmitted to the dial indication by lever arrangement or rack and pinion arrangement. Instead
of capillary connection, a short solid stem is also used. There is also a possibility that the metal
enclosed (bulb & capillary) also may increase in volume due to thermal expension which will
add to the error of the system. To remove this error, a compensation means is provided.
COMPENSATING LINK
This method used two metals with different co-efficients of expansions. Instead in the capillary
as a link-chamber. The chamber contains a core of Invar having negligible co-efficient of
expansion. The wall of the chamber is made of the steel material. The space between the core
and the wall is filled with the system liquid. If the size of the chamber and volume of the Invar
material are carefully proportioned, then on any change in ambient temperature, the volume of
the angular space, due to the expansion of the outer wall is sufficient to accommodate any
variation in volume of the liquid in the capillary and so prevent it exerting an effect on the
bourdon tube.
DOUBLE CAPILLARY
The second method used a second capillary of the same diameter as the first one filled with the
same liquid under the same condition. This second capillary and is sealed off without the bulb
and run along the first capillary and connected to a second bourdon. This bourdon is made to
act on the instrument points in an opposite sense to that of the main bourdon.
© PMI, NTPC 93
Since both capillaries and bourdon tubes are subjected to the same conditions it can be seen
that the ambient temperature effect in the main system is counteracted by that of the second
system.
Bulb Design
All manufacturers keep the change of bourdon volume for all ranges in their production a
constant for commercial reasons. This leads to varying bulb sizes for various ranges. Therefore
for higher range smaller bulb volume is required.
EXPANSION OF GASES
Here the changes in pressure of the gases filled in test bodies (bulbs) of constant volume on
changes of temperature is utilised as means of measurement of the temperature. The gas used
normally is nitrogen. The system works on the gas law PV= RT. Therefore the pressure of the
gas is proportional to the temperature. The bulb is evacuated and filled by the gas at a required
pressure and then the system is sealed. Rest of the system is the same as the liquid filled
system and here the bourdon becomes sensitive to pressure changes.
EXPANSION OF VAPOUR
This works on the basic principle that all enclosed liquids at a given temperature will create a
definite vapour pressure if the liquid is only partially filled. This vapour pressure will increase
with temperature and this property is utilized in measurement.
The advantages of the filled system are:
I. Simple and self contained system.
II. Sensitity, response time and accuracy are comparable with other methods, of
temperature measurements.
III. No auxiliary power needed for the operation of these instruments.
© PMI, NTPC 94
The system has the following shortcoming too :
I. Limited up to certain temperature say upto 500o C.
II. Bulb size is too large to be accommodated in similar space for example to measure the
bearing temperature winding temperature etc.
III. In case of failure the entire system are to be changed.
IV. Remote indication and other telemetering are impracticable.
THERMO ELECTRICITY
Thermocouples
Thermocouple consists of two wires of suitable materials which are joined together at the end by
twisting together and then joining the tipe by brazing or welding. The wires selected should have
the following characteristics.
i. They must physically withstand the temperature for which they are selected, rapid
changes in temperature and the effect of corrosive atmospheres.
ii. Their composition should not change at this temperature range.
iii. They should posses reasonably liner temperature e.m.f relation ships throughout the
range.
iv. They should develop an e.m.f per degree change of temperature that is detectable with
standard measuring equipment.
v. They should not change its characteristics by physical fatigue caused by some
materials.
Very few combination of wires are developed so far satisfying the above conditions. They are:
© PMI, NTPC 95
a. Iron-Constant Type J range up to 760 o C Composition of constantan (equivalent to that of copel) Cu. 56-57% Ni 43-44%
constantan has got bright appearance and non-magnetic.
b. Chromel Alumel Type K range upto 1260 o C composition:
Chromel : Ni; 89% Cr. 9.8% Fe 1% Cobalt 0.2% Dull appearance, non-magnetic Alumel :
Ni 94.5% Al 2% s 1.0%., Mn 2.5%
Glossy surface and slightly magnetic.
c. Copper Constantan Type T range - 180 o C to +370 o C
d. Chromel Copel Type E range 0-870 o C
e. Platanium –R Hodium – Platinum Range 0-1480 o C with 10% Rhodium type S and with 13% Rhodium type R.
Selection of Thermocouple Wire Size And Length
Though there is no general rule for the selection of wire size but, it is suggested to select a
smaller gauge wire where sensitivity is desired and heavier size wire is preferred for longer life
and higher temperature applications.
The length also should be sufficient to minimize the effect of condition. Insufficient insertion
causes low readings. Though there is no rule for length a conventional method is devised that
the length should be 4 times outer dia or protecting tube.
© PMI, NTPC 96
Protecting Tube
Maximum accuracy and sensitivity are obtained if we use bare THERMOCOUPLE wires. But
the possibilities of corrosive action and mechanical injuries call for a protecting tube
thermocouple both metallic and ceramic tubes are used.
Extension Wire
Since it becomes extremely costly to take the thermocouple wires up to the measuring
instrument located at far of places some substitute wires are used to connect the thermocouples
to the instruments. These wires posses the same characteristics as that of thermocouple wires
but up to a lesser temperature. These wires are termed as compensating leads.
Also ordinary copper wires are used as extension wires after compensating the e.f.m. at the
thermocouple terminals, the difference in temperature of the terminals and the cold junction
using a bridge circuit utilizing the law of intermediate temperatures.
Cold junction compensation circuit is a wheat stone bridge consisting of three arms with
constant resistance of mangan due to the temperature changes and the fourth arm with copper
wire wound resistance which is sensitive to temperature. This bridge is supplied with 4 volt D.C
and is balanced at the temperature of reference junction (say at 27oC) when the temperature
change unbalance occurs due to change in resistance copper wire across diagonal which adds
or substracts the thermocouple e.m.f. accordingly. Other method of providing cold junction
compensation are by means of constant temperature oven or ice box in which the reference
junctions are kept inside a constant temperature oven.
Resistance Thermometers
The material selected for resistance thermometers should have the following properties:
i) Stable temperature-resistance relationship.
ii) The specific resistance should be within the limit for easy construction.
iii) Little change in the resistance due to non- temperature methods such as strains etc.
iv) Change in resistance w.r.t temperature should be large.
© PMI, NTPC 97
v) Commercially available with consistent quality.
Three resistance thermometers are available having above properties. They are Nickle, Copper
and Platinum.
NICKEL RESISTANCE THERMOMETERS
Its characteristics are not linen throughout the range but is frequently used to its specific
resistance and less cost specific resistance 6.38 micro-cm., Temperature Co-efficient .0066
ohm/ohm (oC)
Copper Resistance Thermometers
It has got a linear characteristics; specific resistance of copper is very less of the order of 1.56
micro ohm-cm. Temperature co-efficient 53 ohm.
Platinum Resistance
Through costly platinum is more suitable than either copper or nickel. It’s usage is restricted to
jobs that cannot be properly handled by the other two types of thermometers.
Specific resistance 9.38 microohm-cm Temperature co-efficient 0.00385 ohm/ohm oC
© PMI, NTPC 98
Construction Details
The different types of wires are employed in different way depending upon the ranges and the
measuring media normally wire size 0.05 top 0.07 mm dia is used.
i. Platinum wire element wound on mica strip and protected by the mica strip.
ii. Coiled wires mounted on ceramic mendrals and casted in ceramic.
iii. Copper or Nickel element wound on an ebonite plate or on mental mendral.
iv. The coiled element wound on a mica cross.
The sensing elements wound as above are provided with porcelain beads, and inserted inside a
protecting sheath and the terminals brought to the porcelain blocks. The sheath is provided with
a head and cover where the block is kept.
Measuring Circuits
The change in resistance of the temperature sensitive resistance element can be measured by:
i. Cross coil indicators (CI) called ratiometer.
ii. Wheat stone bridge
a. Null balance method,
b. Deflection galvanometer type.
Ratiometer
The ratiometer consists of two crossed moving coiled placed in the field of permanency magnet
at an angle of 20oC approximately and connected detector of a wheat stone bridge with the
resistance thermometer constructing its own arm.
The current flowing through the crossed coils create deflecting and restoring moments. The
change in resistance of thermometer with temperature causes change in current through the
crossed coil producing deflecting moment to the pointers in such a way that the deflection is a
function of ratio of the two currents and measures the temperature on the scale.
© PMI, NTPC 99
To lead in and out the currents through the coils and to bring the pointer to zero in off conditions
– two hair springs are connected. See Fig. 39.
Wheat Stonebridge a. Null Balance Method:
In the null balance instruments, the unbalance voltage across the bridge which is proportional to
the change in resistance, is fed to a phase sensitive amplifier and after amplification feeds to the
control winding of the servomotor which operates and adjust the slider to balance the bridge.
At balance the pointer coupled to the servomotor indicate the temperature value. See Fig. 40
© PMI, NTPC 100
b. Deflection Galvanometer Types
Here the unbalance is directly fed to galvanometer whose deflection is proportional to the
change of temperature and calibrated in terms of temperature.
Compensation for Lead Wire Resistance
Since the resistance element is generally far away from the secondary instrument, the
resistance of the lead wire also will be included alongwith the element resistance to the arm.
Since the lead wire will be of copper which has high temperature co-efficient, it will cause an
appreciable error in the readings. In order to eliminate the error, 3 wire or 4 wire systems are
employed which include the lead wire resistance in the opposite arms and thus cancels the
effects.
© PMI, NTPC 101
Advantage of Resistance Thermometers Over Thermocouples
i. Thermocouples requires that the reverence junctions temperature should be maintained
constant or a suitable compensation is to be applied. It is very difficult to applied. It is
very difficult to maintain the reference junction temperature constant since it is in the
instruments which has many heat dissipation. Also compensation cannot be practically
provided perfectly. But in resistance thermometer the measurement of temperature is
absolute measurement, and no reference is required.
ii. Resistance thermometer have greater sensitivity because the change of resistance per oC is much larger hence is more easily measured than the microscopic change of
voltage per oC in thermocouples
Disadvantages
Thermistors are semi conductor material having resistance values which vary by a ratio of
10,000,000 from - 100oC to +450oC. thermistors due to their thermoresistive characteristics,
stability, and high sensitivity have become more versatile tool for temperature measurement.
Their temperature co-efficient varies from 1 to 5 per oC the semi-conducting materials of which
the thermistors made are metal oxides and their mixtures like oxides of cobalt copper, iron etc.
Optical Pyrometer The radiant energy is measured by photometric comparison of the relative brightness of the
object of unknown temperature with a source of standard brightness such as the tungsten
filament of an electric lamp.
Radiation Pyrometer
The radiation from the target a portion of the object whose temperature is being measured is
focussed by lens arrangement on thermopile (a number of small thermocouples connected in
series). This thermopile generates an e.m.f proportional to them amount of energy falling upon.
This e.m.f fed to a millivolmeter or poteniometer which indicates the temperature.
The radiation pyrometer is ideally suited for –
© PMI, NTPC 102
i) When very high temperature are involved, temperature beyond the practical beyond the
practical range for thermocouple measurement.
ii) Where furnace atmosphere is detrimental to thermocouples and cause erratic
measurement and short life.
iii) Where for other reasons, it is impractical to contact the material whose temperature is to
be measured.
CHANGE OF STATE OF TESTING BODIES
For pure chemical element or compounds change of state viz from solid to liquid to gaseous etc.
takes placed at a fixed temperature and this property thus gives a method to measure the
temperature.
Fusion Method
Fusion of different metals takes place at different temperature. Pyrometric comes are made for
different temperature and ar placed inside the furnace which will indicate the temperature when
the rated fusion temperature is attained.
Vapourisation Method
Vapourisation temperature of different volatile liquid are different. This property is utilised to
measure the temperature.
© PMI, NTPC 103
TEMPERATURE MEASUREMENT
Thermal Expansion of Test Bodies Thermal Electricity Thermo couple Expansion of solids
Expansion of liquids
Expansion of gases
Expansion of vapour
Type-E Chromel Copel 870oC
Type-I Iron Constanatan 760 oC
Type-K Chromel Alumel 1260 oC
Type-S Platinum1480
Type-R P1 87% Rho 13% Rohdium 10% Platinum1480 oC
Biometalic Thermometers - 65 oC to 540
oC
Ntrogen filled 130 oC to 670
oC
Mercucy filled Thermometer
Hydrocarbon filled
Methyle chloride, Ether, Butane, Hexane. Toluene etc.
-39 oC + 600
oC -85 oC to280
oC -85 oC to +340
oC
Change of electric Resistance Intensity of Total Radiation Other Types
Resistance Thermometers
Thermistors Radiation Pyrometers
Optical Pyrometer
Fusible Theromoters
Pyrometric Cones
Colour Change Indicator
500 oC to 6000 oC
-40 oC -1300 oC
600 oC 2000 oC
100 oC 800 oC
Platinum -182 oC to 630 oC
Copper - 200 oC to 150 oC
Nickle -70 oC to 150 oC
0-300 oC 0-300 oC
Lence Type 500 oC 2000 oC
Photocell Pyrometer 150-2000 oC
© PMI, NTPC 104
8. Pneumatic Instrument
INTRODUCTION
Pneumatic instrument systems were the main method of controlling and monitoring industrial
plant. Electrical instrument systems, with fast response times and ease of installation, have
already overtaken pneumatic systems and are now used for most applications previously
considered to be the duty of their pneumatic counterpart.
The slow response and costly installation problems of a pneumatic system are, however,
accepted when the prevailing conditions make electrical systems unacceptable.
Pneumatic instruments also find service in the smaller one off control system, where
transmission lags are small due to the size of the loop.
FLAPPER/NOZZLE
Pneumatic instruments relay on the accurate conversion of mechanical movement to a
proportional pneumatic signal. In most cases this conversion is achieved with the use of a
transducer known as flapper/nozzle.
Air is supplied at a pressure of 1.5 bar. Due to the fact that the nozzle orifice is three times
larger than that the restrictor orifice air can, in fact, exhaust faster than it can pass through the
restrictor. This will result in gauge reading zero. /see Fig. 41.
© PMI, NTPC 105
If the flapper is now positioned so as to seal off the nozzle, the pressure will build up to the
supply pressure and be indicated on the gauge. In actual practice the flapper would be
connected through some form of linkage to the measuring element and it would be the
movement of the measuring element that moved the flapper. It follows that movement of the
measuring element changes the flapper relative to the nozzle and will, therefore, change the air
output pressure in a similar manner to that shown in the graph (Fig. 41).
© PMI, NTPC 106
The flapper movement required to change the output from maximum to minimum is very small,
the actual movement/pressure change ratio will depend upon nozzle and restrictor sizes but is
usually about 0.02 mm.
Provided the flapper nozzle output is restricted to the straight line portion of the graph we can
say that output will be proportionately to measuring element movement.
While in principle the single flapper/nozzle is an effective transducer it does have some serious
drawbacks, for instance any change in supply pressure would affect the output pressure and
also since the amount of flapper movement is so small even the slightest amount of wear on
pivots or linkages would render the system useless. The difficulties may be overcome by the
use of negative feedback bellows. The feedback can be used to oppose the measuring element
force (force balance) or it can be used to change the position of the flapper relative to the nozzle
(position balance).
POSITION BALANCE – PRINCIPLE
The flapper now flat and can be moved by the feedback bellows are well as the measuring
element. Assume the measuring element moves the flapper towards the nozzle, the output
pressure will increase and the feedback bellows will expand. The upper end of the flapper will,
therefore, be moved away from the nozzle and the effective movement of the flapper about the
nozzle is reduced. This increases the amount of measuring element movement needed to give
the complete range of output pressure and gives a proportional relationship between measuring
element movement and the corresponding output pressure. Small changes in supply pressure
will not effect the output. If the measuring element is in the position where the out put pressure
should be 0.6 bar for example, and the air supply suddenly increased, the output pressure
would tend to increase, but the increase in pressure would expands the bellows, pushing the
flapper away from the nozzle until 0.6 bar is again obtained. This technique is used extensively
in pneumatic proportional control. As shown Fig. No. 42
© PMI, NTPC 107
FORCE BALANCE PRINCIPLE
The force balance principle also uses negative feedback but not, as in the position balance, to
move the flapper. The force created by the feedback bellows is used to oppose the force of the
measuring element. Consider a change in the measured variable that causes the flapper to
move closer to nozzle, this would result in an increased output. The increase in output would
also cause the feedback bellows to expand until the force created by it balances the force of the
measuring element, when at such time there will be no further increase in pressure and hence
no further increase in output. See Fig. No. 43
© PMI, NTPC 108
When the movement of the measuring element cause the flapper to move away from the nozzle
the drop in pressure cause the feedback bellows to reduce the force in opposition to the
measuring element force, until equilibrium is again established.
RELAY VALVES OR AMPLIFIERS (BOOSTER RELAYS)
All flapper and nozzle systems are usually operated together with a relay valve, this is intended
to reduce any lag or sensitivity which the introduction of the feedback bellows may produce, and
it is also necessary where a large volume of air is involved in connecting pipework to the
secondary element.
If the nozzle alone carries out the transmitting operation, all the air supply must come from it
and the inflation and deflation of the various volumes (receiving elements and pipe works) may
© PMI, NTPC 109
take some time, causing delay in the signal, getting to the receiver, therefore, again creating a
loss of sensitivity.
A relay valve is, therefore, used to improve the response factor and it is also a pneumatic
amplifier of volume as well as sometimes a pressure amplifier i.e. the variation in output
pressure and quantity of air may be greater but proportional to flapper and nozzle movement.
In general use there are two basic forms of relays.
a. continuous bleed type
b. non-bleed type
Continuous Bleed Repay (Reverse Acting)
The nozzle pressure enters the diaphragm chamber and adjusts the position of the valve in
relation to the valve seats of the valve chamber. The air continuously escapes via the vent and
the rate of leakage determines the back pressure in the output chamber and thus the output
pressure will increase as the nozzle pressure decreases. See Fig. 44
© PMI, NTPC 110
Continuous Bleed Replay (Direct Acting)
The nozzle pressure enters the diaphragm chamber and adjusts the position of the double seat
valve in relation to the valve seats of the valve chamber. If the nozzle pressure increases the
force on the diaphragm moves the double seat valve to the left. This results in the output
pressure increasing proportionately. A drop in nozzle pressure would cause the double seat
valve to move to the right, this seals off the supply and causes a drop in the output, excess
pressure vents through the vent hole. It therefore, follows that any increase in nozzle pressure
results in an increased output. See Fig. 45
Since, with both the reverse and direct acting continuous bleed relay, output pressure is
maintained by venting excess air to atmosphere there is a continuous consumption of air.
Typically this will be about 0.5 cubic feet/minute and can be overcome by the use of a non-bleed
type relay.
Non-Bleed Relay
The noozle pressure is applied to the exterior of the large outer bellows.
© PMI, NTPC 111
The control line pressure is exerted on the interior of the small bellows ‘B’ and when the forces
due to the two are equal, a balanced condition exists. The relay flapper ‘C’ the covers both the
exhaust nozzle ‘D’ and the supply nozzle ‘E’.See Fig. 46.
If the primary nozzle pressure increases the larger bellows are deflected downwards carrying
with it the smaller inner bellows. Exhaust nozzle ‘D’ forces the flapper ‘C’ away from month of
the supply port ‘E’, but remains closed itself. Air is admitted to the control line and the interior of
‘B’. The force supplied to ‘B’ increases and the bellows assembly is now moved upwards until
‘C’ is back to its original position and the support ‘E’ is closed.
With a decrease of primary pressure the outer bellows move upward taking below ‘B’ upwards.
The exhaust nozzle now comes into operation because of its mouth is uncovered in the action
of moving away from ‘C’ relay flapper. Air bleeds away from the interior of bellows ‘B’ and also
the control line via port ‘F’. Pressure is reduced and the bellow assembly begins to move
downwards until nozzle ‘D’ meets the relay flapper ‘C’ and the exhaust passage is closed.
With this type of relay it should be noted that a dead spot may occur if a spring type of relay
flapper ‘C’ is used to provide a positive closing force on the two ports ‘D’ and ‘E’. Any increase
in primary nozzles pressure must overcome this spring pressure and so tend to reduce the
response time of the relay. In practice this is reduced to minimum by using an outer bellows of
an area as large as possible.
© PMI, NTPC 112
ELECTRICAL/PNEUMATIC CONVERSION
Because of the modern trend towards electronic control and display equipment it is frequently
necessary to convert pneumatic signals to a proportional electrical signal or to convert an
electrical signal to a proportional pneumatic signal.
This is achieved by the use of a pneumatic/electrical converter as shown in fig. 47.
Electro pneumatic converts are also used where transmission signals cover great distances or
to improve response times of existing pneumatic equipment.
THE FIELDEN E/P CONVERTER
This Fielden E/P converter is a force balance device without feedback. Because of the lack of
feedback the setting up of the nozzle is critical. The device is supplied with air at 1.5 bar and
has restrictor and nozzle size ratios similar to be a conventional flapper nozzle system i.e. 3:1.
© PMI, NTPC 113
The beam is pivoted at one end whilst the other end is attached to a permanent magnet, the
plug of the primary valve is also connected to the bream. Zero adjustment is achieved by
varying the spring tension and positioning the primary valve plug relative to its seat (nozzle).
Current is applied to the coil and a magnetic field is set up, (the strength of which depends upon
the valve of current) the permanent magnet is forced down which brings the primary valve
closer to its seat, pressure builds up and forces the diaphragm down which seals off the exhaust
valve and opens the secondary valve, resulting in an increase in output pressure. If the value of
current falls the permanent magnet will rise relieving the pressure on top of the diaphragm
which closes the secondary valve in. Excess pressure is vented through the exhaust, resulting
in a drop in pressure. Oil damping is provided on the magnet to give smooth operation. See fig.,
48.
DIAPHRAGM AIR MOTORS
It consists of a motor unit which contains a flexible diaphragm. The diaphragm virtually seals the
chamber into two parts, the upper sections receiving the pneumatic signals from the controller
via the air input. The input signals deflect the diaphragm which is fixed to the thrust plate. The
spindle attached to the thrust Plate extends downwards into the body of the valve. The
deflection is opposed by the range spring whose, rating determines the extent of travel of the
spindle for a given pressure range and effective diaphragm area. Since the spindle is connected
to the valve plugs we have the means of automatically adjusting the orifice area in response to
© PMI, NTPC 114
action from the controller and thereby altering the flow of the medium through the valve. See
Fig. 49
PNEUMATIC CYLINDER MOTORS
Although position control of pneumatic cylinders is possible, in practice it is seldom done without
using some form of feedback. A theoretical type of cylinder positioning, without feedback as
shown in Fig. 50.
© PMI, NTPC 115
The output pressure of the regulator is fixed at say 3 bar. The air operated pressure regulator
will accept inputs between 0.2 and 1 bar, 1 bar input resulting in 3 bar output. The range of
output would be dependent upon piston specification and condition and would, therefore, be set
up in situ. It follows, therefore, that by varying the pressure on top of the piston we can
effectively position the piston rod anywhere within its length of travel.
A control valve must be capable of responding smoothly and rapidly to small changes in the
controller output signal. The quality of control will be impaired if any force; for example, that due
to friction of working parts, oppose the movement of the spindle and the valve plug. This can be
overcome by emplying mechanical feedback in the form of a positioner.
THE VALVE POSITIONER
The primary function of a valve positioner is to ensure that the control valve plug position is
always directly proportional to the value of the controller output pressure, regardless of glad
friction, actuator hysteresis, off-balance of forces on the valve plug etc. This is usually achieved
by incorporating a feed back lever that acts in opposition to the movement to the input bellows.
The system can be either a position or force balance system but in practice force balance
systems are more common. Positioners can be incorporated into diaphragm, or cylinder type
actuators. See fig. 51.
© PMI, NTPC 116
The controller output signal does not directly actuate the valve stem but is fed to a bellows unit.
Assume that the system is in equilibrium and then the controller output increases slightly. The
flapper is moved towards the nozzle and the relay output pressure beings to increase.
This output pressure continuous to increase until the valve spindle moves, mechanical feedback
then restores the equilibrium. Thus the force applied to move the valve spindle is sufficient to
overcome the effect of all forces, no matter what the origin, which tend to oppose the spindle
movement. Without the positioner the slight change in controller output signal may have been
too small to initiate any corrective action/ The matching of input signal range to valve travel
range is achieved by changing the ratio of bellows/nozzle distance to feedback arm/nozzle
distance.
Positioners incorporate into pneumatic cylinders generally operated on a pilot valve principle of
which two are shown in Fig. 52 and 53.
KENT MARK IV
The controller output acts upon the bellows, the pilot valve spool is attached to the bellows via a
connecting link. Assuming an increase in the controller output the bellows will expand,
unbalancing the spool of the pilot valve. Air is then admitted to the top of the piston and it begins
to move down. In doing so it takes the cam with it, as the cam moves down the bell crank level
turns about its pivot and through the spring opposes the movement of the bellows and restores
the spool of the pilot valve to its original position. The system is the back in equilibrium. An
equalizing valve is included to enable manual positioning of the piston.
© PMI, NTPC 117
BAILEY CONTROL DRIVE
The controller output acts on the bellows the spool of the pilot valve is attached to the pilot
beam at one end whilst the other end is anchored and pivoted. Assuming an increase in the
controller output the bellows will expand and push the pilot beam up against the restraining
force of the spring. This unbalances the pilot valve and causes ait to be admitted to the top of
the piston. The piston, therefore, begins to move down, this results in the positioner drive arm
turning the cam which puts more tension on the spring and so restores the pilot beam to its
original position.
© PMI, NTPC 118
PNEUMATIC SEQUENCE CONTROL
Sequence control is essentially the carrying out of a series events in a logical progressive
manner.
The actual even is usually the carrying out of some physical work utilising the movement of an
actuator. In pneumatic sequenced control compressed air can provide power through either
linear motion or rotary motion i.e. diaphragm valves, pneumatic cylinders or air motors. Since
the pneumatic cylinder is by far the simplest form actuator it shall be used as the basis for the
following notes, although the principle can be applied to any actuator.
TYPES OF CYLINDER
The simplest type is the single acting cylinder (Figure 54).With this type air is used to make the
unit out stroke or extend (+). Once the pressure has been removed, the return or in stroke (-) is
achieved by mechanical means, in this case a spring. The cylinder can be air to extend
© PMI, NTPC 119
(application of a signal will push the piston out) or air to retract (application of a signal will push
the piston in).
In the double acting cylinder, if air is applied to P1 (with P2 open to exhaust) the piston will
outstroke (+); and if air is applied to P2 (with P1 open to exhaust) the piston will in-stroke (-).
The symbols + and – are often used as a shorthand notation to indicate movement of the
cylinder, particularly when describing the sequence of operation of a circuit. For example, there
may be three cylinders A, B and C which operate in the sequence A+, B+, B-, A-, C+, C-.
PNEUMATIC CYLINDER CUSHIONING
On high pressure systems, piston speeds can be in the order of 450 mm/sec. and impact forces
at the ends of the stroke can be great. In order that damage may not be cased by sudden
contract between the fast moving piston and the cylinder end housing, some form of buffer or
cushioning can be used. This does not limit the piston travel but allows gradual declaration in
the last 25 mm or so of travel, this is achieved as shown in fig. 55.
© PMI, NTPC 120
As the cylinder outstrokes under the action of applied pressure air is displaced from the other
side of the piston to atmosphere through the main part and needle valve. When the cushioning
boss enters the cushioning seal, the main port is blocked off, air can, therefore, only escape
through the needle valve at a much slower rate thereby causing the piston to slow down for the
premium period of travel. This results in the cushioning effecting shown in Fig. 56
SEQUENCE CONTROL VALVES
An ordinary on off valve has one inlet and outlet. However, for sequence control applications
facility must be made to exhaust any unwanted signal. The two most common types of valve
used in sequence control are three port and five port types.
© PMI, NTPC 121
THREE PORT RELAYS
The basic three port valve consists of a two lobe spool running in a surface ground cylinder,
compressed air can be switched to the outlet by the application of a force to the spool.
The force can be removed and the outlet will remain connected to the air supply.
The application of a second force will return the spool to its original condition, main air will then
be isolated and the device connected to the outlet will exhaust from part 3.
FIVE PORT RELAYS
The basic construction of the five port relay is the same as the three port relay, the only
difference being the use of a three lobe spool. Compressed air can now be routed through the
valve whilst the same time a signal can be exhausted through it. The direction of force will
determine the routing of supply and exhaust.