chapter 85 controls and instrumentation

8/4/2019 Chapter 85 Controls and Instrumentation

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Controls and Instrumentation

Chapter 85

Introduction to Process measurement

Introduction

Process measuring devices are used either as stand-alone indicators of process conditions, or as

the sensing element components of control systems or alarm and logic systems.

Introduction

Depending on their specific application they may provide an output which is visual, mechanical,

pneumatic or electronic.

The most common process conditions which must be measured are pressure, level, flow,

temperature and composition.

Pressure Measuring Devices

Pressure is involved directly or indirectly in the measurement of other variables such as level,

flow, and density.

Since these applied pressures, located in various parts of a plant, can range from a few to many

thousand kilopascals, the same type of pressure sensing element cannot be used in all

applications.

Manometers

A manometer is one of the most accurate and useful sensors for measuring pressure in the lower

ranges. There are three general types that are used in industry.

1. U-Tube Manometer

This type can be a simple glass or other transparent material bent into the shape of a U as shown

in Fig. 1.

Figure 1. U-tube manometer.

The U-tube manometer contains a liquid, often referred to as the manometer fluid, which must not

mix or react chemically with the fluid whose pressure is being measured.

Typical manometer fluids are water, mercury and light oils. Mercury was a common manometer

fluid in the past, but has largely been replaced due to its environmental and health hazards.

With both sides of the manometer open to the atmosphere, the surface level on one side will be

the same as the level on the other side, or P1 = P2 as in Fig. 1 (a).

Suppose one end of the U-tube manometer is connected to an unknown pressure, P1 whose

value must be determined, while the other end is left exposed to the atmospheric pressure, P2. If

pressure P1 is greater than P2, the fluid on the right side of the manometer, Fig. 1 (b) will be


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displaced from its previous position and forced into the left side of the manometer until a balance

in pressure is created at points A and B, so:

If P2 is the atmospheric pressure, then the pressure represented by the liquid head, hpg, would be

equal to the amount that P1, the gage pressure, is above atmospheric pressure. P2 + hpg would

be equal to the absolute pressure applied on the right side of the manometer.

If the pressure P1 is below atmospheric, while P2 is still the atmospheric pressure, the manometer

fluid will be higher on the right side. The main purpose of a manometer is to measure the

difference between two pressures which is often called pressure differential measurement.

Manometers can be used to measure low gage pressures, or the amount of vacuum in a vessel.

A common application is the measurement of furnace pressure or draft in a boiler.

2. Inclined Manometer

The inclined manometer produces a longer scale of measurement than the U-tube or well-type for

the same differential in pressure between P1 and P2 so a more accurate reading can be obtained.

Figure 2. Inclined Manometer.

The pressure due to the vertical height of a manometer, h, is equivalent to the difference in

pressure between P1 and P2, where P2 often represents the atmospheric pressure.

Barometers

The earths atmosphere consists of a layer of gases, mainly nitrogen and oxygen, to an altitude of

about 75 km. This gas mixture in the atmosphere has a mass that creates a pressure of

approximately 101.3 kPa on the earths surface at sea level. This pressure varies from day to day

depending on the weather conditions and the elevation above sea level.

One form of atmospheric pressure measurement is by the use of a mercury barometer.

It consists of a glass tube of uniform bore, approximately 900 mm long, with one end sealed.

In Fig. 3 the tube is first completely filled with mercury, and is then inverted with the open end

submerged in an open dish of mercury.

The pressure exerted by the column of mercury, h, is balanced by the pressure of the

atmosphere.

Fig. 4 shows the actual components of a mercury barometer. As you can see, the mercury

barometer is essentially a manometer, with atmospheric pressure on one side and almost

vacuum, or zero pressure, on the other side.

Figure 4. Mercury Barometer.

Bourdon Tube Gage

One type of Bourdon pressure gage consists of an oval tube in the form of a C, having an arc of

about 270.


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The free end of the tube in Fig. 5 is sealed, while the other end contains the pressure inlet which

is connected to a socket.

When pressure is applied to the inside of the tube, the tube becomes slightly more circular in

cross section as shown by the dotted lines in Section A-A.

As the tube becomes more circular in cross section, it straightens out slightly, causing the free

end to move.

This linear motion of the free end is transmitted through a link to a geared sector and pinion that

causes rotation of the pointer. If the applied pressure decreases, the tube will act like a spring

and return to its original shape.

A Bourdon tube can also be shaped in the form of a spiral or helix as illustrated in Figs. 6 and 7

respectively.

These types are used to develop sufficient power and rotation to position a pen directly on a chart

without the use of gears. A greater degree of rotation is achieved as more windings are added.

Figure 6. Spiral Bourdon Spring.

The Bourdon tube gage is generally a stand-alone device, more versatile and rugged than

manometers and probably the most commonly used instrument in the process industry.

Diaphragm Elements

Bourdon tube gages are not well suited to low pressure ranges because of the stiffness of their

metal parts.

A diaphragm element gage, using more flexible material, can be used in low pressure

applications.

Diaphragms are made of materials such as leather, cotton lined rubber, impregnated silk, copper,

and stainless steel, depending upon the pressure applied and the temperature of the fluid.


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The diaphragm is a flexible disc, either flat or with concentric corrugations to increase motion and

sensitivity.

Nonmetallic or limp diaphragms are used for pressure from 0 to 0.5 kPa while the metallic type

are suitable for pressures of 0 to 2500 kPa.

Figure 8. Diaphragm-type pressure indicator.

Bellows Elements

The bellows element is most often used for pressure measurement between a pressure

equivalent to 250 mm water head to approximately 350 kPa.

It consists essentially of a corrugated metal tube that will expand in the direction of its length

when a pressure is applied.

Special calibrating springs are also used (Fig. 9) to provide greater accuracy.

Figure 9. Low range element.

The expansion and (or contraction)of a bellows can be used as a transducer, that is, to convert

different types of signals.

Bellows are commonly used in pneumatic devices to position nozzle and flapper clearances, and

in electronic devices to position rheostats and solenoid cores.

Thus, unlike the Bourdon tube elements, they are often incorporated in control loops rather than

as stand-alone devices.

Fig. 9 illustrates a bellows used as a low-pressure gage.

Pressure exerted on the outside of the bellows tends to compress it, stretching the calibration

spring and moving the pen linkage on a chart an amount that is proportional to the applied

pressure.

When the maximum design pressure is reached, the bellows comes to rest against the over-

range tube which protects the linkage and recording parts from damage.

Strain Gages

A strain gage is an extremely sensitive transducer that can be used to measure pressure or force

on a column, shaft, etc. This pressure or force is converted into a proportional electrical signal.

The most common type of strain gage is the resistance type similar to the one in Fig. 10 (a)

It consists of a fine wire grid, the size of a small postage stamp, which is cemented to a paper or

plastic base.

This base is then bonded firmly to the column or shaft on which the force is measured.

When a tensile force is applied, the wire grid will increase in length while its cross-sectional area

will decrease.

These two physical changes will cause an increase in electrical resistance of the conductor.


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If this conductor is part of a bridge circuit, the voltage imbalance across the bridge will be

proportional to the pressure or force.

Since the output of a strain gage is electrical these devices are well suited to being sensing

elements for control loops using electronic signals.

Figure 10. Strain Gages.

Fig. 10 (b) shows a strain gage that can be used to measure the pressure in a cylinder.

A change in pressure, acting on the flush diaphragm, will vary the compressive force on the tube

on which the strain gage is bonded, causing a proportional change in electrical resistance of the

strain gage.

The imbalance of voltage across the bridge is amplified to provide sufficient power to operate a

controller and/or a recorder.

Pressure Sensing Element Protection Devices

Special devices are used to protect pressure sensing elements against such conditions as hightemperature, cycling pressures, vibrations, and corrosive or congealing fluids.

A low cost device, shown in Fig. 11 (a), consisting of a U-shaped tubing filled with a suitable seal

fluid, will prevent the process fluid from entering the pressure sensing pipe and the instrument.

The seal fluid must have a greater density than the line fluid for this system to be effective.

Figure 11. Pressure sensor protectors.

Another method of preventing the above problems is through the use of a diaphragm box as

shown in Fig. 11 (b).

The space above the diaphragm containing the capillary and the sensing element is filled with

seal fluid so the line fluid does not enter the pressure gage.

Figure 11. Pressure sensor protectors.

In many different processes, equipment such as reciprocating and metering pumps may produce

vibrations or pulsations in pressure.

These vibrations are amplified considerably by the time they reach the pressure gages. This

effect can be lessened by a pulsation dampener illustrated in Fig. 12.

Figure 12. Line pulsation dampener.

Fig. 12 (a) consists of a rubber bulb and felt plug that are connected to a pressure gage and

completely filled with glycerine.

When the pressure from the process fluid increases, the rubber bulb is compressed, causing the

glycerine to be forced through the plug which acts as a restriction to the flow.

The degree of damping depends on the compression of the felt plug which is adjusted before the

bulb is filled with glycerine.


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Another practical means of restricting pulsations in the line leading to the pressure gage can be

achieved by means of a needle valve, Fig. 12 (b), which is placed between the pressure source

and the gage.

Level Measurement

Level measurements include boiler water level, fuel tank level, water storage and heater levels.

In many applications, gage glasses are used to indicate the level of liquids while other devices

are used to record and control.

Level Measurement

Float-Type Level Measurement

One method of measuring and indicating level in an open tank is shown in Fig. 13 (a) where a

float is attached to a weight by means of cables and pulleys. The float is positioned within the

tank while the weight hangs outside adjacent to a scale which is marked in units of level.

Fig. 13 (b) shows a float-type level control system where a float positions the inlet valve through alinkage arrangement. By moving the pivot to the left, a greater change in level is required to

produce full valve travel.

When it is not practical to have the float in a vessel or tank, a float cage or chamber (Fig. 14) is

mounted on the outside, with the bottom part connected to the liquid space and the top to the

vapor space in the vessel.

Figure 14. Float Cage Unit.

If the level in the vessel increases, the float rises to open the control valve further so more liquid

will flow from the vessel.

Note that the cage will be under the same pressure as that in the vessel.

Similar float arrangements can be connected to a boiler drum where the float may operate a

switch to start and stop a feedwater pump and to operate a low water-level fuel cutoff in case the

water becomes dangerously low in the boiler.

Diaphragm-Type Level Indicator

Fig. 15 shows a level indicator that can be placed in a remote location when the boiler is high and

the gage on the drum is not visible from the operating floor.

This indicator has a large diaphragm with the upper side connected to the steam space and the

lower side to the water space of the boiler.

A condenser at the boiler drum maintains a fixed head of water or pressure on the upper side of

the diaphragm, while the lower side is subjected to a varying head or pressure dependent upon

the water level in the boiler.

The difference in pressure due to the liquid head between the two sides of the diaphragm is

balanced by a spring so the diaphragm moves in accordance with the water level. As the water

rises in the drum, the pressure under the diaphragm increases causing it to rise and move the


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indicator upwards. Full boiler pressure is exerted on both sides of the diaphragm, so the boiler

pressure has no effect on its movement.

Colored screens illuminate the inside of the indicator; blue represents the water in the lower

portion of the gage and red the steam in the upper part. If the water level rises in the boiler drum,

the diaphragm will move the shutter and pointer upward so more blue color is visible in the gage

glass.

Differential Pressure Manometers

The manometer, discussed in the section on pressure measurement, can also be applied to liquid

level measurement.

The difference between the one shown previously in Fig. 1 and the illustration in Fig. 16 (a) is that

the movement of the manometer fluid is sensed and transmitted by means of a float mechanism.

The body of the one in Fig. 16 (a) is made of steel.

Float motion is transmitted to an indicator by a lever connected to a spindle in a pressure sealed

bearing.

The manometer is connected to the pressurized vessel so that vapor pressure is applied

simultaneously to both the float chamber and the range tube, and the manometer reading is

dependent only on the pressure exerted by the height of liquid in the vessel. In other words the

vessel pressure is cancelled out and only head pressure is measured.

Remember a manometer is only able to measure small pressures.

The external leg, frequently called the reference or constant head, is usually filled with seal fluid

whose relative density is greater than that of the liquid in the vessel.

Figure 16. Differential pressure sensors.

In Fig. 16 (b) the differential pressure manometer has been replaced by a differential pressure

transmitter that uses a diaphragm as a sensing element, similar to the type used for the remote

level indicator in Fig. 15. Fig. 16 (b) shows a differential pressure transmitter being applied to the

same type of level measurement as in Fig. 16 (a).

An increase in level will move the diaphragm slightly to the right causing a proportional increase

in the output of the transmitter.

If a diaphragm-actuated differential pressure transmitter is used to measure level in an open tank,

the right side of the diaphragm is open to the atmosphere.

Flow Measurement

Flow is probably one of the most widely measured process variables.

Many methods are used to measure the rate of flow of steam, feedwater, fuel, and air, but only

the most common sensing elements will be discussed at an introductory level.


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Head Meters

Head type or differential pressure flow meters include a number of sensing devices for fluid flow

measurement such as orifice plates, venturi tubes, and nozzles

Each of these devices will cause a restriction in the flow stream which will result in a pressure

drop, which has a certain relationship with the velocity, and with the flow.

If the net cross-sectional area of the fluid stream is reduced, the velocity of flow increases and

hence results in an increase in kinetic energy.

Since energy cannot be created or destroyed, the increase in kinetic energy results in a reduction

in pressure downstream from the restriction.

1. Orifice Plate

An orifice plate is the most common form of head meter that is used in flow measurement.

It consists of a flat metal plate with an opening of a fixed area, Fig. 17. The concentric type shown

in (a) is the most common, but the eccentric and segmental in (b) and (c), respectively, are alsoused in special applications. The outside of the plate is designed to fit inside the bolt circle on

standard flanges.

Figure 17. Orifice plates.

Fig. 18 illustrates the pressure drop or differential pressure across an orifice plate.

Note that the flow pattern shows an effective decrease in cross section beyond the plate, with the

maximum velocity and maximum change in static pressure occurring at the narrowest point of

flow called the vena contracta. After this point in the flow stream only some of the pressure drop

is recovered, as turbulence and friction have created considerable permanent pressure loss.

The pressure differential across an orifice is measured by two pressure connections, one before

the plate and another downstream from the plate. Fig. 19 (a) and (b) shows two types of

connections. In (a) the pressure connections or taps are located directly on the flanges, while in

(b) the taps are located on the pipe at a specific distance from the orifice plate.

Figure 19. Flange and pipe taps.

The orifice plate is easy to install and replace. It is low in cost, and different sizes may be easily

substituted if the flow range is varied, but it is the least accurate and creates the highest

permanent pressure loss.

2. Venturi Tube

This flow sensing element, installed between flanges, Fig. 20 converges to a minimum cross

section, called the throat, and then diverges to the original pipe size.

High- and low-pressure connections are installed at specified locations as indicated.

Figure 20. Venturi tube.


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The venturi tube produces less permanent pressure loss than an orifice plate. On the other hand,

it has the disadvantages of higher cost and bulkiness.

3. Flow Nozzle

The flow nozzle in Fig. 21 is an adaptation of the venturi tube.

It consists essentially of a venturi tube without the diverging section, but its pressure recovery is

not as efficient.

Flow nozzles are used principally for measurement of high velocity flows. The high-pressure

connection is located one internal pipe diameter before the inlet face of the nozzle, while the low-

pressure tap is usually one-half the pipe diameter downstream.

Figure 21. Flow nozzle.

Variable Area Meter

The variable area meter, or rotameter, Fig. 22 (a), consists of a tapered tube in which a float or

rotor can move freely up or down, while the fluid being measured flows upward in the tube.

As the flow increases, the float assumes a higher position and the area around the float is

increased, thus the differential pressure across the float remains constant when the flow

stabilizes.

The flow rate can be read from a graduated scale located on the tube or adjacent to it.

Figure 22. Rotameter.

When comparing the head or differential pressure meter with a variable area meter, one finds that

the area is fixed and the differential pressure varies in a head meter, while in a variable area

meter or rotameter, the differential pressure is constant and the area varies with a change in flow

rate.

This rotameter is used mainly as an indicating device. It is fairly low in cost, simple in

construction, and quite accurate.

Temperature Measurement

Glass Stem Thermometers

A glass stem thermometer, shown in Fig. 23, consists of a glass bulb, stem, and an indicating

scale.

The bulb is completely filled with liquid, while the stem has a uniform bore called the capillary in

which the liquid expands when its temperature increases.

Usually the space above the liquid is evacuated, but in some thermometers this space may be

filled with an inert gas such as nitrogen to increase the temperature range of the thermometer.

The scale may be marked on the stem or it may be mounted beside the thermometer, Fig. 23.


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Figure 23. Glass stem thermometers.

Mercury is the most common thermometer liquid because its coefficient of expansion is nearly

linear for temperatures between -39C and 450C. If the space above the mercury is pressurized

with nitrogen or argon, the upper temperature limit may be increased to about 550C. At

temperatures lower than -39C, the mercury will freeze.

Alcohol and other hydrocarbons can be used at lower temperatures but at higher temperatures

the liquid will evaporate.

Bimetal Thermometers

The bimetal thermometer consists of two thin strips of metal, with different coefficients of

expansion, laminated together without the use of any fi ller metal.

When one end is fixed, as illustrated in Fig. 24 (a), the other end will deflect in nearly direct

proportion to the change in temperature. Brass and invar, an iron-nickel alloy, are the metals

often used because the linear coefficient of brass is over twenty times higher than that of invar.

Figure 24. Bimetal thermometer.

To amplify motion, the bimetal strip may be wound into a helix or spiral. Fig. 24 (b) shows an

industrial bimetal thermometer that uses a helical bimetal element whose motion is transmitted to

a pointer by a shaft.

Filled System Thermometers

The filled system thermometer, illustrated in Fig. 25, can be used to provide an indication of

temperature, or to produce a pneumatic signal proportional to the measured temperature so it can

be used for recording and controlling some distance away.

Figure 25. Filled system thermometer.

A basic system consists of a temperature sensitive bulb, capillary, a pressure sensing device

such as a bourdon tube, bellows, or diaphragm, and some indicating or transmitting device.

The system is completely filled with fluid (liquid or gas).

With an increase in measured temperature, the fluid in the bulb will expand and increase the

pressure in the bulb, capillary, and bourdon tube.

The pressure sensing element responds to the increase in pressure by moving the pointer up the

scale.

When used in a pneumatic transmitter, the movement of the pressure sensing device producesan output from the transmitter that is proportional to the temperature.


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Thermocouples

The thermocouple is probably one of the most widely used temperature sensing devices.

Basically, it consists of two wires, each made of a different metal or alloy.

These wires are connected at one end to form the measuring junction as shown in Fig. 26.

Figure 26. Basic thermocouple circuit.

The free ends of the two wires are connected, directly or by means of extension wires, to a

measuring instrument to form a loop in which current can flow. This connection is often called the

reference junction. When the measuring and reference junctions are at different temperatures a

voltage is produced by the thermocouple causing a current to flow. The magnitude of the dc

voltage is not quite proportional to the temperature difference between the two junctions.

basic thermocouple and measuring circuit is illustrated in Fig. 26 where the thermocouple circuit

is connected to a millivoltmeter whose scale is calibrated in degrees Celsius.

An increase in temperature will cause an increase in current flow through the moving coil, and acorresponding movement of the pointer on the temperature scale.

In actual practice, the measuring junction of a thermocouple is placed at the point of temperature

measurement, while the meter with the reference junction may be some distance away.

Various combinations of metals may be used depending on the temperature range required.

Some types of thermocouples and their approximate temperature ranges are shown in Table 1.

Table 1. Thermocouple Ranges

Composition Determination

A chromatograph, Fig. 27 (a), is used to analyse the components of a gaseous mixture or of a

liquid in vapor form.

It operates on the principle that if a gaseous mixture is forced through a certain material that

resists its flow, gases that have a lower density or boiling point will pass through more quickly

than the ones that are more dense.

A basic chromatograph consists of a separation column, packed with an absorbent material and

installed in an oven that is maintained at a constant temperature.

The column is connected to a regulated supply of inert carrier gas, such as helium or argon,

indicated in Fig. 27 (a).

The gas sample mixes with the carrier gas and flows through the column.

Each component of the gas mixture is identifiable by the time that elapses between the injection

of a sample into the column and the emergence of that component.

Quantitative measurement of each component depends on the difference in thermal conductivity

between the mixture of carrier gas in the reference leg and that of the carrier gas and a

component in the detector leg.


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The heat sensitive elements in the detector are often thermistors or semiconductors whose

electrical resistance decreases rapidly with an increase in temperature.

When a greater quantity of a specific gas passes through the detector, the wheatstone bridge, to

which these heat sensitive elements are connected, will become more unbalanced.

The resulting trace on a chart will appear like that shown in Fig. 27 (b). Peak heights and peakareas above the base line are used to calculate the quantity of a particular gas component in the

mixture.


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Figure 26. Basic thermocouple circuit.


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Figure 25. Filled system thermometer


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Figure 24. Bimetal thermometer.


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Figure 23. Glass stem thermometers.


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Figure 22. Rotameter.

Figure 21. Flow nozzle.


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Figure 19. Flange and pipe taps

Figure 20. Venturi tube


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Figure 17. Orifice plates.

Fig. 18 illustrates the pressure drop or differential pressure across an orifice plate


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Figure 16. Differential pressure senso


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Figure 14. Float Cage Unit.

chapter 85 controls and instrumentation

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