Chapter 11

Control System Instrumentation

Measuring Instrumentations

The typical process measuring instrument consists of sensing

elements and transmitters (driving elements) . This combination

of sensor and transmitter is called transducer .

Transducers and Transmitters

Sensor: is a phenomenon that detects process variable and

produces a signal that can be measured like mV, current,

pressure difference, etc.

Transmitter: converts this phenomenon into a signal that can be

transmitted such as current to air (I/P), volt to

current (V/I), volt to pressure (V/P), etc.

Sensor Systems

• Sensor

– temperature sensors

– flow sensors

– level sensors

– pressure sensors

– composition analyzers

• Transmitter

The Control Relevant Aspects of

Sensors

• The time constant/deadtime of the sensor

• The repeatability of the sensor

Sensor Terminology

• Span

• Zero

• Accuracy

• Repeatability

• Process measurement dynamics

• Calibration

Span and Zero Example

• Consider the maximum temperature that is

to be measured is 350ºF and the minimum

temperature is 100ºF.

• The zero is 100ºF and the span is 250ºF

• If the measured temperature is known at

two different sensor output levels (i.e.,

ma’s), the span and zero can be calculated

directly.

Smart Sensors

• Sensors with onboard microprocessors that offer a number of

diagnostic capabilities.

• Smart pH sensors determine when it is necessary to trigger a

wash cycle due to buildup on the electrode surface.

• Smart flow meters use statistical techniques to check for

plugging of the lines to the DP cell.

• Smart temperature sensors use redundant sensors to identify

drift and estimate expected life before failure.

Transmitters

• A transmitter usually converts the sensor output to a signal level

appropriate for input to a controller, such as current signal of

range 4 to 20 mA or pneumatic signal of range 3-15 psig.

• Transmitters are generally designed to be direct acting.

• In addition, most commercial transmitters have an adjustable

input range (or span).

• For example, a temperature transmitter might be adjusted so that

the input range of a platinum resistance element (the sensor) is

50 to 150 °C.

Transmitter/Controller

May need additional transducers for Gm if its output is in

mA or psi. In the above case, Gc is dimensionless (volts/volts).

Measurement / Transmission Lags

• Temperature sensor

make as small as possible (location, materials for

thermowell)

• Current or Pneumatic Transmission lines for

Control Loop

Pneumatic, usually, produces a pure time delay (no

time delays for electronic lines); less common today

compared to electronic transmissions; but it is useful

for place of hot or inductive conditions.

ss

ssM

AU

Cm=

1+s

1

)s(T

)s(T

• Flow Measuring Device:

1. Differential Pressure Meter

- Orifice, Venturi, Nozzle

2. Electromagnetic Flowmeter

3. Vortex Flowmeter

4. Turbine Flowmeter

5. Ultrasonic Flowmeter

FLOW MEASUREMENT

• What is flowrate?? Amount of material passing one point for certain time

How To Choose the Right Flowmeter ??

There are key questions you can ask yourself

when trying to determine which flowmeter is the

best choice for you. The purpose of the

measurement and the physical characteristics of

the fluid being measured are the two main

considerations.

Some specifics to consider are:

• What is the fluid being measured (air, water, gas, etc?)

• If it's not water, what are the properties of the fluid you are measuring?

• What construction materials are acceptable?

• What are the track records of the various technologies you are considering?

• What are the minimum and maximum process pressures and temperatures?

Flow Nozzle

• Consist of an elliptical converging section and cylindrical

throat section.

• Suitable for high-velocity, non-viscous, erosive flows.

• Flow Nozzles have a smooth elliptical inlet leading to a

throat section with a sharp outlet. This restriction in the fluid

flow causes a pressure drop.

• The flow can be calculated from the measured pressure drop

• This device has a greater overall pressure loss or operating

cost in terms of head pressure than a Venturi but offers lower

installation costs.

Orifice Venturi

Nozzle

Paddle Type Orifice Plate

Sizing an Orifice for a Differential

Pressure Flow Indicator

b is the ratio of the orifice diameter to the pipe diameter.

• 0.2 < b < 0.7

• Pressure drop at minimum flow should be greater than 0.5 psi.

• Pressure drop across the orifice should be less than 4% of the line pressure.

• Choose the maximum value of b that satisfies each of the above specifications.

Electromagnetic Flow meter

•This instrument operates

based on Faraday’s Law.

•It is ideal for liquids that

conduct electricity.

•The magnetic field is

developed by electric coils.

•The conductive liquid forms

an electric conductor as its

move through the magnetic

field established inside the

flowmeter .

•This conductor in the

magnetic filed will generate

an electric voltage that is

proportional to its average

velocity.

Example of a Magnetic Flow Meter

Vortex Flow meter

Also know as vortex shedding

flowmeters or oscillatory flowmeters.

It measures the vibrations of the

downstream vortexes caused by the

barrier placed in a moving stream.

The velocity of vortex flowing in the

stream is directly related to the

stream velocity.

A device that counts the vortices

passing per second will also measure

the flowrate.

Advantage: Low cost installation-do not required impulse tubing and

valve manifold.

Disadvantages: Vortexes are inhibited in viscous fluid at low flow rate.

At high fluid velocity the obstructions may introduce

excessive pressure drop-limited to higher flow rate.

Turbine Flowmeter • Turbine meters have a spinning rotor with propeller-like blades that

is mounted on bearings in a housing on the central longitunidal axis of the pipeline.

• Magnets are embedded in the rotor housing and a pickup coil, isolated from the fluid is placed outside the rotor blades.

• The rotor spins as water or other fluid passes over it.

• The rotating magnets induce a voltage pulse in the coil each time they pass the coils.

• Pulse frequency is proportional to the velocity

• Disadvantages: sensitive to viscosity changes, require maintenance

at their bearing.

Ultrasonic Flow meter •Ultrasonic flowmeters can be categorized into two types based

on the installation method: clamped-on and inline.

•The clamped-on type is located outside of the pipe and there

are no wetted parts. It can easily be installed on existing

piping systems without worrying about corrosion problems.

Clamped-on designs also increase the portablility of the

flowmeter.

•The inline type, on the other hand, requires fitting flanges or

wafers for installation. However, it usually offers better

accuracy and its calibration procedures are more

straightforward.

•Ultrasonic flowmeters measure the traveling times (transit time

models) or the frequency shifts (Doppler models) of ultrasonic

waves in a pre-configured acoustic field that the flow is

passing through to determine the flow velocity.

• Time Transit Model

- A pair of transducers is placed on the pipe wall, one on the upstream and the other on the downstream. The time for acoustic waves to travel from the upstream transducer to the downstream transducer is shorter than the time it requires for the same waves to travel from the downstream to the upstream. The larger the difference, the higher the flow velocity.

Ultrasonic Flow meter

• Doppler Model

- rely on the Doppler effect to relate the frequency

shifts of acoustic waves to the flow velocity. It

usually requires some particles in the flow to reflect

the signals.

Ultrasonic Flow meter

Pressure-Measuring Devices

Most liquid and all gaseous materials in the process

industries are contained within closed

vessels. For the safety of plant personnel and

protection of the vessel, pressure in the vessel is

controlled. Types of pressure sensors are:

• U-Tube Manometer

• Bourdon Tube Sensor

• Bellows-Type Sensor

• Strain Gauge Sensor

U-Manometer

Bourdon, Bellow and Diaphragm Sensor

• Bourdon: A bourbon tube is a curved, hollow

tube with the process pressure applied to the fluid in the tube.

• Bellow: A bellows is a closed vessel with sides

that can expand and contract

• Diaphragm: A diaphragm is typically constructed of two

flexible disks, and when a pressure is applied to one face of the

diaphragm, the position of the disk face changes due to

deformation

• In all, the displacement can be related to pressure.

Displacement is convert to electrical signal ~ required

secondary element.. • Used elastic material

Bourdon Tube Sensor

Bourdon Tube Sensor

Bellows-Type Sensor

Diaphragms Sensor

Strain Gauge Sensor

• The electrical resistance of a metal wire depends on

the strain applied to the wire. Deflection of the

diaphragm due to the applied pressure causes strain

in the wire, and the electrical resistance can be

measured and related to pressure.

• Fluid – Force – Diaphragm Displacement (detected

by strain gauge sensor) –

and the resistance change (detected by Wheatstone

bridge).

Strain Gauge Sensor

Strain Gauge Sensor

Temperature control is important for separation and reaction

processes, and temperature must be maintained within limits to

ensure safe and reliable operation of process equipment.

Temperature can be measured by many methods; several of the

more common are described in this subsection. You should

understand the strengths and limitations of each sensor, so that you

can select the best sensor for each application.

In nearly all cases, the temperature sensor is protected from the

process materials to prevent interference with proper sensing and to

eliminate damage to the sensor. Thus, some physically strong,

chemically resistant barrier exists between the process and sensor;

often, this barrier is termed a sheath or thermowell, especially for

thermocouple sensors.

Temperature Sensing Systems

There are several methods used to measure temperature. The followings are just

few of these methods, which may be employed.

The capillary tube (fluid thermometer) A capillary tube is a very thin tube. This type of thermometer has a bulb filled

with mercury. When the temperature rises, the mercury in the bulb expands.

This expansion pushes the mercury higher in the capillary tube.

Temperature Sensing Systems

Thermocouple

• Consist of two dissimilar metal and connected ~ voltage generated

• Hot junction ~ measure temperature

• Cold junction ~ reference (known temperature)

• E1 = voltage generated by T1 (hot junction)

• E2 = voltage generated by T2 (cold junction)

• Et = E1 – E2

E1 E2

Hot

junctio

n

Cold

junctio

n

• When the junctions of two dissimilar metals are

at different temperatures, an electromotive force

(emf) is developed

• The emf is calculated using the following

equation:

emf (volts) = (Th – Tc)oC x b

b = constant (V/K) , T (K)

• Different combinations of metals result in

different voltage- temperature characteristics. In

industry, thermocouples are usually classified by

a one-letter type designation that describes their

response.

Standard thermocouples

_____________________________________________________

Type Materials Normal Range

J Iron-constantan -190oC to 760oC

T Copper-constantan -200 oC to 371 oC

K Chromel-alumel -190 oC to 1260 oC

E Chromel-constantan -100 oC to 1260 oC

S 90% platinum +

10% rhodium- platinum 0 oC to 1482 oC

R 87% platinum +

13% rhodium- platinum 0 oC to 1482 oC

•Thermocouple are often insulated electrically with ceramic

material (high temperature) and sheathed in stainless steel

•Used thermowell for effectively seal off the process fluid or gas.

•Advisable to use thermowell to prevent heat loss and personnel

injury

Temperature sensor

without thermowell

Temperature sensor

with thermowell

RTD (Resistance Temperature Detector )

• The electrical resistance of many metals changes

with temperature, metals for which resistance

increases with temperature are used in RTDs

• Temperature is determined from the change in the

electrical resistance of the metal wire.

• Linear relationship using equation RT= Ro(1+aT)

RT = the resistance at temperature, T

R0 = the resistance at base temperature of 0 °C

T = the temperature of the sensor (to be determined

from RT)

a = the temperature coefficient of the metal.

R100 = Resistance at 100oC (steam point)

R0 = Resistance at 0oC (ice point)

• Limitation: 0 - 100 oC

R100/ R0 - 1

100 a =

• RTD sensitivity, a can be noted from typical

value of metal used,

Platinum = 0.004 / oC

Nickel =0.005 / oC

• The effective range of RTDs principally

depend on the type of wire used

Platinum RTD = -100 to 650 oC

Nickel RTD = -180 to 300 oC

Thermistor:

This sensor is similar to an RTD, but applies metals for which the resistance

decreases with increasing temperature. The relationship is often very nonlinear,

but thermistors can provide very accurate temperature measurements for small

spans and low temperatures.

Thermisters are made from oxides of metals such as copper, nickel, cobalt and

lithium which are blended to produce the required resistance-temperature

characteristics. Most Thermistors have negative non-linear temperature

coefficients. Thermistors are produced in many shapes and sizes such as beads,

discs and probes and may be coated in a glass or steel sheath for added

protection and strength.

Filled systems:

A fluid expands with increasing temperature and exerts a varying

pressure on the containing vessel. When the vessel is similar to a

bourbon tube, the varying pressure causes a deformation that

changes the position detected to determine the temperature.

The tube in a filled-system temperature indicator can be filled with a

liquid and vapour. When the temperature rises, more liquid is changed

to vapour. The increase vapour pressure straightens the spiral

bourdon tube pressure instrument. The figure below show how the

movement of the bourdon tube, caused by a pressure change, is read

as a temperature change

Sensor Type Limits of

Application (°C)

Accuracy1,2 Dynamics:

t (s)

Advantages Disadvantages

Thermocouple

type E:

chromel-constantan -100 to 1000

±1.5 or 0.5%

for 0 to 900 °C

see note 3

-good reproducibility

-wide range -minimum span of 40

°C

-temperature vs. emf

not exactly linear

-drift over time

-low emf corrupted by

noise

type J:

iron-constantan 0 to 750 ±2.2 or 0.75%

type K:

chromel-nickel 0 to 1250 ±2.2 or 0.75%

type T:

copper-constantan -160 to 400

±1.0 or 1.5%

for -160 to 0 °C

RTD -200 to 650 0.15 + 0.2|T| see note 3

-good accuracy

-small span possible

-linearity

-self-heating

-less physically rugged

-self-heating error

Thermister -40 to 150 ± 0.10 °C see note 3 -good accuracy

-little drift

-highly nonlinear

-only small span

-less physically rugged

-drift

Bimetallic - ± 2% - -low cost

-physically rugged -local display

Filled system -200 to 800 ± 1% 1 to 10 -simple and low cost

-no hazards

-not high temperatures

-sensitive to external

pressure

Table: Summary of temperature sensors

• Level = a measurement of the height of the free surface of the liquid from a fixed datum or reference.

• Level accuracy:- – Smoothen the process operation.

– Comply the custom and taxing regulation.

• Level can represent the amount of asset and related to money matter

• Tank contains liquid and sometime the solid.

• Sometimes the liquid solidifies, very corrosive, and vaporizes – create difficulties.

LEVEL MEASUREMENT

The difference in pressures between to points in a vessel depends

on the fluids between these two points. If the difference in

densities between the fluids is significant, which is certainly true

for a vapor and liquid and can be true for two different liquids, the

difference in pressure can be used to determine the interface level

between the fluids. Usually, a seal liquid is used in the two

connecting pipes (legs) to prevent plugging at the sensing points.

Differential Pressure Level Measurement

Differential Pressure Level Measurement

DPT

Vapor

Diaphragm

Lower Tap

Upper Tap

Liquid

Buoyancy Sensor

• Buoyancy = displacement

• By Archimedes principle, a body immersed

in a liquid is buoyed by a force equal to the

weight of the liquid displaced by the

body. Thus, a body that is more dense than

the liquid can be placed in the vessel, and

the amount of liquid displaced by the body,

measured by the weight of the body when in

the liquid, can be used to determine the

level .

Buoyancy Sensor

Buoyancy Sensor

Float Sensor

The float of material

that is lighter than

the fluid follows the

movement of the

liquid level.

The position of the

float, perhaps

attached to a rod,

can be determined to

measure the level.

Capacitance Sensor

A capacitance probe can

be immersed in the

liquid of the tank, and

the capacitance between

the probe and the vessel

wall depends on the

level.

By measuring the

capacitance of the

liquid, the level of the

tank can be determined

Capacitance Sensor

• The term analyzer refers to any sensor that measures a physical property of the process material. This property could relate to purity (e.g., mole % of various components), a basic physical property (e.g., density or viscosity), or an indication of product quality demanded by the customers in the final use of the material (e.g., gasoline octane or fuel heating value).

• Used to measure the physical properties

• Standalone in laboratory or installed near to equipment.

ANALYTICAL MEASUREMENT

pH Meter

Discuss and answer all the questions

• What is the purpose of pH meter??

• How it’s measured??

• Explain standard hydrogen electrode.

• List and describe types of electrodes.

Conductivity Meter

Discuss and answer all the questions

• What is the purpose of conductivity meter ??

• Explain polarization effect.

• How to measure conductivity?

Called Actuator System

• Control Valve

– Valve body

– Valve actuator

• I/P converter

• Instrument air system

Pneumatic Control Valve

Cross-section of a Globe Valve

Typical Globe Control Valve

Air to Close Valve, Fail Open

Pneumatic Control Valve

Pneumatic Control Valve

Air to Open Valve, Fail Close

Types of Globe Valves

• Quick Opening- used for safety by-pass

applications where quick opening is desired

• Equal Percentage- used for about 90% of

control valve applications since it results

in the most linear installed characteristics

• Linear- used when a relatively constant

pressure drop is maintained across the valve

Inherent Valve Characteristics

0

0.5

1

0 20 40 60 80 100

Stem Position (% Open)

f(x)

=%

QO

Linear

Control Valve Design Procedure

• Choose a control valve so that the average

flow rate results when the valve is 2/3 open.

• After the valve has been sized, check to

ensure that the maximum and minimum

flow rates will be accurately metered.

Additional Information Required to

Size a Control Valve

• CV versus % open for different valve

sizes.

• Available pressure drop across the valve

versus flow rate for each valve.

• Note that the effect of flow on the upstream

and downstream pressure must be known.

Valve Sizing Example

• Size a control valve for max 150 GPM of

water and min of 50 GPM.

• Therefore, choose the valve size so that

valve is approximately 67% open at 100

GPM.

Determine CV at 100 GPM

• Use the valve flow equation (Equation 2.1) to

calculate Cv

• For DP, use pressure drop versus stem position

(e.g., Table 2.2)

/)(

PK

FxC m

vD

Cv versus % Valve Travel for

Different Sized Valves

• Body % Valve Opening

• Size in 50 60 70

• 1 3.63 5.28 7.59

• 1.5 4.30 6.46 9.84

• Cv 2 11.1 20.7 32.8

• 3 21.7 36.0 60.4

• 4 31.2 52.6 96.7

Check Max and Min Flows

• Ensure that the flow rate will be accurately

controlled at the maximum and minimum

flow rates.

• At minimum flow rate valve should be at

least 10-15% open.

• At maximum flow rate the valve should be

at most 85-90% open.

Valve Deadband

• It is the maximum change in instrument air

pressure to a valve that does not cause a change in

the flow rate through the valve.

• Deadband determines the degree of precision

that a control valve or flow controller can provide.

• Deadband is primarily affected by the friction

between the valve stem and the packing.

Valve Actuator Selection

• Choose an air-to-open for applications for

which it is desired to have the valve fail

closed.

• Choose an air-to-close for applications for

which it is desired to have the valve fail

open.

Optional Equipment

• Valve positioner- a controller that adjusts the instrument air in order to maintain the stem position at the specified position.

• Greatly reduces the deadband of the valve.

• Positioners are almost always used on valves serviced by a DCS.

• Booster relay- provides high capacity air flow to the actuator of a valve.

• Can significantly increase the speed of large valves.

Control Relevant Aspects of

Actuator Systems

• The key factors are the deadband of the actuator

and the dynamic response as indicated by the

time constant of the valve.

• Control valve by itself- deadband 10-25% and a

time constant of 3-15 seconds.

• Control valve with a valve positioner or in a flow

control loop- deadband 0.1-0.5% and a time

constant of 0.5-2 seconds.

See ex 9.1 for A to O and A to C