SpiraxSarco-B8-Control Applications

8/14/2019 SpiraxSarco-B8-Control Applications

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The Steam and Condensate Loop 8.1.1

Block 8 Control Applications Pressure Control Applications Module 8.1

Module 8.1

Pressure Control Applications


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The Steam and Condensate Loop8.1.2


Pressure Control Applications

There are many reasons for reducing steam pressure:

o Steam boilers are usually designed to work at high pressures in order to reduce their physicalsize. Operating them at lower pressures can result in reduced output and carryover of boilerwater. It is, therefore, usual to generate steam at higher pressure.

o Steam at high pressure has a relatively higher density, which means that a pipe of a given sizecan carry a greater mass of steam at high pressure, than at low pressure. It is usually preferableto distribute steam at high pressure as this allows smaller pipes to be used throughout most ofthe distribution system.

o Lower condensing pressures at the point of use tend to save energy. Reduced pressure willlower the temperature of the downstream pipework and reduce standing losses, and alsoreduce the amount of flash steam generated when condensate from drain traps is discharginginto vented condensate collecting tanks.

It is worth noting that if condensate is continuously dumped to waste, perhaps because of the

risk of contamination, less energy will be lost if the condensing pressure is lower.o Because steam pressure and temperature are related, control of pressure can be used to control

temperature in some processes. This fact is recognised in the control of sterilisers and autoclaves,and is also used to control surface temperatures on contact dryers, such as those found inpapermaking and corrugator machines. Pressure control is also the basis of temperature controlin heat exchangers.

o For the same heating duty, a heat exchanger designed to operate on low-pressure steam willbe larger than one designed to be used on high-pressure steam. The low-pressure heat exchangermight be less expensive because of a lower design specification.

o The construction of plant means that each item has a maximum allowable working pressure

(MAWP). If this is lower than the maximum possible steam supply pressure, the pressure mustbe reduced so that the safe working pressure of the downstream system is not exceeded.

o Many plants use steam at different pressures. A stage system where high-pressure condensatefrom one process is flashed to steam for use in another part of the process is usually employedto save energy. It may be necessary to maintain continuity of supply in the low pressure systemat times when not enough flash steam is being generated. A pressure reducing valve is ideallysuited for this purpose.


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Fig. 8.1.1 General arrangement of a direct operating, self-acting pressure reducing station

Direct operating, self-acting pressure reducing valve

bellows type

DescriptionWith this self-acting type of pressure controller, the downstream (control) pressure is balanced(via a bellows) against a spring force.

Advantages:1. Inexpensive.2. Small.3. Easy to install.4. Very robust, giving long life with minimum maintenance.5. Tolerant of imperfect steam conditions.6. Self-acting principle means that no external power is required.

Disadvantages:1. Proportional only control.2. Proportional band is 30% to 40% of the upstream pressure.3. Wide proportional band means that maximum flow is only achieved when the downstream

pressure has dropped considerably. This means that the reduced pressure will vary dependingon flowrate.

4. Limited in size.5. Limited flowrate.6. Variation in upstream pressure will result in variation in downstream pressure.

Applications:Non-critical, moderate load applications with constant running flowrates, for example:1. Small jacketed pans.2. Tracer lines.3. Ironers.

4. Small tanks.5. Acid baths.6. Small storage calorifiers.7. Unit heaters.8. Small heater batteries.9. OEM equipment.

Points to note:1. Different versions for steam, compressed air, and water.2. Soft seat versions may be available for use on gases.3. A wide range of body materials means that particular standards, applications and preferences

can be satisfied.

4. A wide proportional band means care is needed if the safety valve needs to be set close to theworking pressure.

High pressuresteam in

Separator

Safety valvePressurereducing

valve

Low pressuresteam out

Condensate


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Fig. 8.1.2 General arrangement of a direct operating, self-acting pressure reducing station

Direct operating, self-acting pressure reducing valve

diaphragm type

Description:With this self-acting type of pressure controller, the downstream (control) pressure is balanced(via a diaphragm) against a spring force.

Advantages:1. Very robust.2. Tolerant to wet and dirty steam.3. Available in large sizes, so high flowrates are possible.4. Easy to set and adjust.5. Simple design means easy maintenance.6. Self-acting principle means that no external power is required.7. Able to handle pressure drops of 50:1 in small sizes, and 10:1 in large sizes.

Disadvantages:1. Large proportional band means that close control of downstream pressure is improbable with

large changes in load.2. Relatively high purchase cost, but lifetime cost is low.3. Bulky.

Applications:1. Distribution mains.2. Boiler houses.

Points to note:1. Because the diaphragm is subject to fairly low temperature limitations, a water seal is required

on steam applications. This adds to the cost slightly.2. Because of the large proportional band, this type of valve is better suited to reducing steam

pressure to plant areas rather than individual plant items.3. A bellows sealed stem ensures zero maintenance and zero emissions.4. Although wide proportional band provides stability, care is needed if a safety valve needs to

be set close to the apparatus working pressure.5. Suitable for liquid applications.6. More expensive than a pilot operated valve, but less expensive than a pneumatic control

system.


Safety valve

Pressurereducing

valve


Separator

Condensate


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Fig. 8.1.3 General arrangement of a pilot operated, self-acting pressure reducing station

Pilot operated, self-acting pressure reducing valve

DescriptionThese have a more complex self-acting design, and operate by sensing the downstream pressurevia a pilot valve, which in turn operates the main valve.

The effect is a very narrow proportional band, typically less than 200 kPa.

This, together with low hysterisis, results in very tight and repeatable control of pressure, evenwith widely varying flowrates.

Advantages:1. Accurate and consistent pressure control, even at high and variable flowrates.2. A variety of pilot valves may be used on one main valve. Pilot valve options include electrical

override, multi-pilot for a choice of control pressures, a surplussing option and remote control,as well as different temperature/ pressure control combinations.

3. Self-acting principle means that no external power is required.4. Tolerant of varying upstream pressure.

Disadvantages:1. More expensive than bellows operated direct acting controls.2. Small clearances mean that steam must be clean and dry to ensure longevity, but this can be

achieved by fitting a strainer and separator before the pressure reducing valve.

Applications:1. A system which requires accurate and consistent pressure control, and installations which

have variable and medium flowrates. For example: autoclaves, highly rated plant such asheat exchangers and calorifiers.

2. A system where installation space is limited.

Points to note:1. Installation must include a strainer and separator.2. Size for size, pilot operated valves are more expensive than bellows type self-acting controls,

but cheaper than diaphragm type self-acting controls.3. Size for size, they have higher capacity than bellows type self-acting controls, but less than

diaphragm type self-acting controls.4. Can be installed before temperature control valves to maintain a constant upstream pressure,

and hence stabilise control.5. Not suitable for liquid applications.6. Do not use if the plant is subject to vibration, or other equipment is causing pulses in flow.


Safety valvePressurereducing

valve


Separator

Condensate


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Fig. 8.1.4 General arrangement of a pneumatic pressure reducing station

Pressure reduction pneumatic

Description:These control systems may include:

o P + I + D functions to improve accuracy under varying load conditions.

o Set point(s), which may be remotely adjusted.

Advantages:1. Very accurate and flexible.2. No limit on valve size within the limits of the valve range.3. Acceptable 50:1 flow rangeability (typically for a globe control valve).4. Suitable for hazardous environments.5. No electrical supply required.6. Fast operation means they respond well to rapid changes in demand.7. Very powerful actuation being able to cope with high differential pressures across the valve.

Disadvantages:1. More expensive than self-acting controls.2. More complex than self-acting controls.3. Not directly programmable.

Applications:A system which requires accurate and consistent pressure control, and installations which havevariable and high flowrates and/or variable or high upstream pressure. For example: autoclaves,highly rated plant such as large heat exchangers and calorifiers.

Points to note:1. A clean, dry air supply is required.2. A skilled workforce is required to install the equipment, and instrument personnel are required

for calibration and commissioning.3. The control is stand-alone, and cannot communicate with PLCs (Programmable Logic

Controllers).4. The failure mode can be important. For example, a spring-to-close on air failure is normal on

steam systems.

Safetyvalve

Pneumatic controller


Pneumaticpressurereducing

valve


Separator

Condensate


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Fig. 8.1.5 General arrangement of an electropneumatic pressure reducing station

Pressure reduction electropneumatic

DescriptionThese control systems may include:


o Set point(s) which may be remotely adjusted, with the possibility of ramps between set points.

Advantages:1. Very accurate and flexible.2. Remote adjustment and read-out.3. No limit on valve size within the limits of the valve range.4. Acceptable 50:1 flow rangeability (typically for a globe control valve).5. Fast operation rapid response to changes in demand.6. Very powerful actuation being able to cope with high differential pressures across the valve.

Disadvantages:1. More expensive than self-acting or pneumatic controls.2. More complex than self-acting or pneumatic controls.3. Electrical control signal required. Costly for hazardous areas.

Applications:A system which requires accurate and consistent pressure control, and installations which havevariable and high flowrates and/or variable or high upstream pressure, including autoclaves, highlyrated plant such as large heat exchangers and calorifiers, and main plant pressure reducing stations.

Points to note:1. A clean, dry air supply is required.2. A skilled workforce is required to install the equipment, and instrument personnel are required

for calibration and commissioning.3. Can be part of a sophisticated control system involving PLCs, chart recorders and SCADA

systems.4. Always consider the failure mode, for example, spring-to-close on air failure is normal on

steam systems.

Safetyvalve

Separator

Electroniccontroller

Pressuretransmitter




valve

Condensate


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Fig. 8.1.6 General arrangement of an electric pressure reducing station

Pressure reduction electric

Description:These control systems may include:



Advantages:1. Both controller and valve actuator can communicate with a PLC.2. No compressed air supply is required.

Disadvantages:1. If a spring return actuator is required, the available shut-off pressure may be limited.2. Relatively slow actuator speed, so only suitable for applications where the load changes slowly.

Applications:1. Slow opening / warm-up systems with a ramp and dwell controller.2. Pressure control of large autoclaves.

3. Pressure reduction supplying large steam distribution systems.

Points to note:1. Safety: If electrical power is lost the valve position cannot change unless a spring return

actuator is used.2. Spring return actuators are expensive and bulky, with limited shut-off capability.


Safetyvalve

Separator

Pressuretransmitter


Electronicpressurereducing

valve


Condensate


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Fig. 8.1.7 Parallel pressure reducing station

Pressure reduction (other possibilities)

Parallel pressure reducing stations

Description:Pressure reducing stations may be configured as shown below for one of two reasons:

1. The valves are serving a critical application for which downtime is unacceptable

The equipment is operated on a one in operation, one on stand-by basis to cover for breakdownand maintenance situations

2. The turndown ratio between the maximum and minimum flowrates is very highThe equipment is operated on a pressure sequence principle with one valve set at the idealdownstream pressure, and the other at a slightly lower pressure.

When demand is at a maximum, both valves operate; when flow is reduced, the valve set at thelower pressure shuts off first, leaving the second valve to control.

Point to note:The valves selected for this type of application will require narrow proportional bands (such aspilot operated pressure reducing valves or electro-pneumatic control systems) to avoid thedownstream pressure dropping too much at high flow rates.


Safetyvalve

Pressurereducing

valve


Separator

Pressurereducing

valve

Safetyvalve

Condensate


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Fig. 8.1.8 Typical series pressure reducing station

Pressure reduction (other possibilities)

Series pressure reducing stations

A pressure reducing station may be configured in this manner if the ratio between theupstream and downstream pressure is very high, and the control systems selected have a lowturndown ability. 10:1 is recommended as a practical maximum pressure ratio for this type ofreducing valve.

Consider the need to drop pressure from 25 bar g to 1 bar g. The primary reducing valve mightreduce pressure from 25 bar g to 5 bar g, which constitutes a pressure ratio of 5:1. The secondaryreducing valve would drop pressure from 5 bar g to 1 bar g, also 5:1. Both valves in series providea pressure ratio of 25:1.

It is important to check the allowable pressure turndown ratio on the selected reducing valve, thismay be 10:1 on a self-acting valve, but can be much higher on electrically or pneumaticallyoperated valves. Be aware that high pressure drops might have a tendency to create high noiselevels. Refer to Module 6.4 for further details.

The trapping point between the two reducing valves (Figure 8.1.8) is to stop a build up ofcondensate under no-load conditions. If this were not fitted, radiation losses would causecondensate to fill the connecting pipe, which would cause waterhammer the next time theload increased.


Safety

valve


Separator

Pilot operatedreducing valves

Condensate Condensate

Trappingpoint

Pilot operatedreducing valves


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Fig. 8.1.9 Simple steam atomising desuperheater station

Desuperheaters

Desuperheating is the process by which superheated steam is either restored to itssaturated state, or its superheated temperature is reduced. Further coverage of desuperheaters isgiven in Block 15.

The system in Figure 8.1.9 illustrates an arrangement of a pressure reducing station with a

direct contact type pipeline desuperheater.In its basic form, good quality water (typically condensate) is directed into the superheated steamflow, removing heat from the steam, causing a drop in the steam temperature.

It is impractical to reduce the steam temperature to its saturated value, as the control system isunable to differentiate between saturated steam and wet steam at the same temperature.

Because of this, the temperature is always controlled at a value higher than the relevant saturationtemperature, usually at 5C to 10C above saturation.

For most applications, the basic system as shown in Figure 8.1.9 will work well. As the downstreampressure is maintained at a constant value by the pressure control loop, the set value on thetemperature controller does not need to vary; it simply needs to be set at a temperature slightly

above the corresponding saturation temperature.

However, sometimes a more complex control system is required, and is shown in Figure 8.1.10.Should there be a transient change in the superheated steam supply pressure, or a change in thewater supply temperature, the required water/steam flow ratio will also need to change.

A change in the water/steam flow ratio will also be required if the downstream pressure changes,as is sometimes the case with certain industrial processes.

Good quality water in

Pressurecontroller

Temperature

control valve

Superheatedsteam in

Pressurecontrolvalve

Desuperheaterunit

PT100temperature

sensor Pressure

transmitter

Temperaturecontroller

Steam out


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Fig. 8.1.10 Steam atomising desuperheater station with downstream pressure / temperature compensation

The system shown in Figure 8.1.10 works by having the pressure controller set at the requireddownstream pressure and operating the steam pressure control valve accordingly.

The 4-20 mA signal from the pressure transmitter is relayed to the pressure controller and the

saturation temperature computer, from which the computer continuously calculates the saturationtemperature for the downstream pressure, and transmits a 4-20 mA output signal to the temperaturecontroller in relation to this temperature.

The temperature controller is configured to accept the 4-20 mA signal from the computer todetermine its set point at 5C to 10C above saturation. In this way, if the downstream pressurevaries due to any of the reasons mentioned above, the temperature set point will also automaticallyvary. This will maintain the correct water/steam ratio under all load or downstream pressureconditions.

Good quality water in

Superheatedsteam in

Pressurecontroller

Saturationtemperature

computer

Temperaturecontrolvalve

Pressurecontrolvalve

Desuperheaterunit

PT100temperature

sensor Pressuretransmitter

Steam out

Temperaturecontroller


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Controlling pressure to control temperature

DescriptionThese are applications which utilise the predictable relationship between saturated steam pressureand its temperature.

Advantages:

1. The pressure sensor may be located in the steam space, or close to the control valve ratherthan in the process medium itself. This is an advantage where it is difficult to measure theprocess temperature.

2. This arrangement can be used to control a number of different elements from a single point.

Disadvantage:1. Control is open loop, in that the sensor is not measuring the actual product temperature.

Applications:1. Autoclaves and sterilisers2. Presses and calenders3. Constant pressure plant, for example, jacketed pans, unit heaters, and steam-jacketed pipes.

Point to note:Good air venting is essential (refer to Module 11.12 for further details)

Fig. 8.1.11 Pressure control of an autoclave

High pressuresupply

Separator

Pilot

operatedpressure reducingvalve

Low pressure to autoclave

Autoclave

Condensate

Safety valve

Fig. 8.1.12 Pressure control on a jacketed pipe application

High pressuresupply

Pilot operated pressurereducing valve

Condensate

Jacketed pipe

Condensate

Condensate

Condensate

Jacketed pipe

Automatic air vent

Automatic air vent


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Fig. 8.1.13 Pressure control on a multi platen press

Fig. 8.1.14 Pressure / temperature control on a jacketed pan

Fig. 8.1.15 Constant pressure steam supply to a control valve supplying a plate heat exchanger

High pressuresupply

Pilot operated pressure

reducing valve with on-offfunction

Condensate

Safety valve

Low pressureto press

Multi-platenpress

Jacketed pan

Safetyvalve

Condensate

Direct actingpressure reducing

valve

High pressuresteam supply

Automaticair vent

High pressure supply

Pilotoperatedpressurereducing

valve

Electropneumaticcontrol system

Condensate

Flow

Return


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Fig. 8.1.16 Differential pressure control

Differential pressure control

DescriptionIn these applications the control valve will open and close to maintain a set differential pressurebetween two points.

Advantages:

1. A constant differential steam pressure is maintained in the system.2. The differential pressure ensures that condensate is actively purged from the heat exchange

system. This is particularly important where accumulated condensate could act as a heatbarrier, and create a temperature gradient across the heat transfer surface.

This temperature gradient could, in turn, result in a distorted or poorly heated product.3. Different operating temperatures can be achieved.

Disadvantage:A complex system is required if efficiency is to be maintained. This might involve flashvessels and/or thermo-compressors, as well as downstream applications which use thelower pressure pass-out steam.

Application:Blow-through drying rolls in a paper mill.

Point to note:A special controller or differential pressure transmitter is required to accept two inputs; onefrom the primary steam supply and the other from the flash vessel. In this way, the pressuredifferential between the flash vessel and the primary steam supply is maintained under allload conditions.


High pressure condensate discharging into a flash vessel

Differentialpressurecontroller


valve

Flash vessel

Condensate

Condensate


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Surplussing control

DescriptionThe objective is to maintain the pressure upstream of the control valve. Surplussing valves arediscussed in further detail in Module 7.3, Self-acting pressure controls and applications.

Applications:

1. Boilers on plants where the load can change by a large proportion over a very short period.The sudden reduction in boiler pressure may result in increased turbulence and rapid flashingof the boiler water, and large quantities of water being carried over into the pipework system.

2. Accumulators where surplus boiler output is used to heat a mass of water under pressure.This stored energy is then released when the boiler has insufficient capacity.

Points to note:1. Minimum pressure drop is usually required over the fully open control valve; this may mean

a line size valve is needed.2. Not all self-acting controls are suitable for this application and it is important to consult the

manufacturer before use.

Fig. 8.1.18 Steam accumulator

Fig. 8.1.17 Surplussing control on a steam boiler

Surplussing valve

Surplussingvalve

Steam from boiler


valve

Steam to plant

Accumulator

Condensate

Dry steam at all times

Drain (normally closed)

Overflow


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Cascade control Limiting pressure and temperature

with one valve

DescriptionWhere it is necessary to control two variables with one valve it is necessary to employ twoseparate controllers and sensors. It is always the case that the control valve accepts its controlsignal from the slave controller.

The slave controller is configured to accept two input signals, and its set point will change (withindefined limits) depending on the electrical output signal from the master controller.

This form of control is very important where the pressure to the apparatus must be limited,despite the heat demand.

Application:The steam heated plate heat exchanger shown in Figure 8.1.19 is heating water circulating in asecondary system. The heat exchanger has a maximum working pressure, consequently this islimited to that value in the slave controller.

In order to control the secondary water temperature, a master controller and temperaturetransmitter monitors the heat exchanger outflow temperature and sends a 4-20 mA signal to theslave controller, which is used to vary the slave set point, between pre-determined limits.

Points to note:1. An adequate pressure margin must exist between the set pressure of the safety valve and the

pressure limitation imposed by the controller.2. The safety valve must not be used as a device to limit pressure in the heat exchanger; it must

only be used as a safety device.

Fig. 8.1.19 Cascaded controllers on the steam supply to a heat exchanger

Steam in

4-20 mA

Slavecontroller 4-20 mA

Pneumaticpressurecontrolvalve

Pump trap

Pressuresensor

Temperaturesensor

Mastercontroller

Condensate

Safetyvalve

Flow

Return


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Typical settingsThe output from the master controller is direct acting, that is, when the upstream pressure is at orabove its proportional band, the masters output signal is maximum at 20 mA; when at thebottom of, or below the proportional band, the control signal is minimum at 4 mA.

When the control signal is 20 mA, the slave set point is the required downstream pressure; whenthe signal is 4 mA, the slave set point is at a pre-determined minimum.

Consider the normal upstream pressure to be 10 bar g, and the maximum allowabledownstream pressure to be 5 bar g. The minimum allowable upstream pressure is 8.5 bar g,which means that if this pressure is reached the valve is fully shut. The minimum reducedpressure is set at 4.6 bar g.

These conditions are recorded in Table 8.1.1

Table 8.1.1

P1 P1 and Master Master output signal Master output signal Slave set point

bar g output signal mA and slave set point bar g

10.0 20 5.0

9.5 20 5.09.0 12 4.8

8.5 4 4.6

8.0 4 4.6

Fig. 8.1.20 General schematic arrangement of a reducing/surplussing valve

Cascade control Combined pressure reduction and

surplussing with one valve

DescriptionThe objective is to reduce steam pressure but not at the expense of overloading the availablesupply capacity.

Application:The upstream pipework is a high-pressure distribution pipe possibly from a distribution manifoldor steam boiler supplying plant of a non-essential nature (Figure 8.1.20). Should the demand behigher than the supply capacity, the valve closes and throttles the steam flow, maintaining thepressure in the upstream pipework.

The master controller is set at the normal expected supply pressure. If the master detects a dropin upstream pressure below its set value (due to an increase in demand) it reduces the set pointin the slave controller, in proportion to pre-determined limits.

The slave closes the valve until the steam demand falls to allow the upstream pressure tore-establish to the required value. When this is achieved, the set point of the slave controller is

set at its original value.

Steam flow

Highpressure

Mastercontroller

4-20 mASlavecontroller

4-20 mA

LowpressureReducing / surplussing valve

Output signal

Upstream pressure

Output signal

Slave set point


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Fig. 8.1.21 Schematic diagram showing a pasteuriser control using the cascade principle

Cascade control Limiting and controlling temperature

with one valve

DescriptionThe main objective is to limit and regulate the temperature to a particular process, wheresteam is the available heat source but it cannot be used directly to heat the final product foroperational reasons.

Application:A typical application is a dairy cream pasteuriser requiring a pasteurisation temperature of 50C.Because of the low control temperature, if steam were applied directly to the pasteurisation heatexchanger, it is possible that the relatively large amount of heat in the steam would make controldifficult, causing the system temperatures to oscillate, overheating and spoiling the cream.

To overcome this problem, the system in Figure 8.1.21 shows two heat exchangers. The pasteuriseris heated by hot water supplied from the primary steam heated heat exchanger.

However, even with this arrangement, if only the master controller operated the valve, a time lagwould be introduced into the system, and poor control might again be the result.

Two controllers are therefore used, working in cascade, each receiving a 4-20 mA signal fromtheir respective temperature transmitters.

The slave controller is used to control the final temperature of the product within clearly definedlimits (perhaps between 49C and 51C). These values are altered by the master controller relativeto the product temperature such that, if the product temperature increases, the slave set pointreduces in proportion.

Master

4-20 mA

Steam flow

Temperature sensor

Water Cream flow

Steam/ water heat exchanger

Condensate

Pasteuriser

Cream return

Temperature sensor

Slave


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Questions

1. What is MAWP?

a| Maximum attenuated working pressure

b| Minimum allowable working pressure

c| Maximum allowable with pressure

d| Maximum allowable working pressure

2. One large and one small steam-heated heat exchanger have exactly the sameheating duty. Which will operate at the lower pressure?

a| The smaller one

b| The larger one

c| They will both operate at the same pressure

d| There is not enough information to answer the question

3. Name one disadvantage of a direct acting pressure reducing valve

a| It only has proportional control

b| It has proportional and integral control but no derivative control

c| It operates in an on /off fashion

d| An external power source is required for it to operate

4. What type of pressure reducing station is required when the pressure ratio

is greater than 10:1a| A parallel station

b| A pilot operated station

c| A series station

d| A surplussing station

5. Why is cascade control used?

a| To control the flow of water over a weir

b| When more than one input is necessary to secure good control

c| When more than one valve is required to secure control

d| When two pressures are being sampled

6. Why is it sometimes necessary to reduce pressure?

a| To increase the pipe size

b| Because the apparatus pressure is lower than the supply pressure

c| Because the boiler pressure is too high

d| To increase the steam flowrate

1:d,2:b,3:a,4:c,5:b,6:b Answers


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Block 8 Control Applications Temperature Control for Steam Applications Module 8.2

Module 8.2

Temperature Control forSteam Applications


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Temperature Control for Steam Applications Module 8.2Block 8 Control Applications

Temperature control for steam applications

There are a number of reasons for using automatic temperature controls for steam applications:

1. For some processes, it is necessary to control the product temperature to within fairly closelimits to avoid the product or material being processed being spoilt.

2. Steam flashing from boiling tanks is a nuisance that not only produces unpleasant environmentalconditions, but can also damage the fabric of the building. Automatic temperature controlscan keep hot tanks just below boiling temperature.

3. Economy.

4. Quality and consistency of production.

5. Saving in manpower.

6. Comfort control, for space heating.

7. Safety.

8. To optimise rates of production in industrial processes.

The temperature control system employed should be matched to the system, and capableof responding to the changes in heat load. For example:

o On a low thermal mass system experiencing fast load changes, the control system needs to beable to react quickly.

o On massive systems, such as oil storage tanks, which experience slow changes in temperature,the control may only have to respond slowly.

o The temperature control system selected may need to be capable of coping with the start-upload without being too big, to provide accurate control under running conditions.


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Direct operating, self-acting temperature control

DescriptionThe direct operating, self-acting type of temperature control uses the expansion of liquid in asensor and capillary to change the valve position.

Advantages:

1. Inexpensive.2. Small.3. Easy to install and commission.4. One trade installation.5. Very robust and extremely reliable.6. Tolerant of imperfect steam conditions and of being oversized.7. Self-acting principle means that no external power is required.8. Simple to size and select.9. Many options are available, such as different capillary lengths and temperature ranges.

Disadvantages:1. The control is stand-alone, and cannot communicate with a remote controller or

PLC (Programmable Logic Controller), although a high temperature cut-out may signalclosure via a switch.

2. Limited sizes.3. Limited pressure ratings.4. Limited turndown.5. Sensors tend to be much larger than the pneumatic and electronic equivalents and also much

slower acting.

Applications:Applications would include those with low and constant running flowrates:1. Small jacketed pans.2. Tracer lines.3. Ironers.

4. Small tanks.5. Acid baths.6. Small storage calorifiers.7. Small heater batteries.8. Unit heaters.

Point to note:The proportional band is influenced by the size of the valve.

Fig. 8.2.1 General arrangement of a direct operating, self-acting temperature control system

on a DHWS (Domestic Hot Water Services) storage calorifier

Steam

supply

Highlimitvalve

Controlvalve

Spring loadedcut-out unit

Fail-safe control system

Condensate

Flow

CalorifierReturn

Cold watermake-up

Separator

Condensate

Vacuumbreaker


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Pilot operated, self-acting temperature control

DescriptionThe pilot operated self-acting type of temperature controller uses the expansion of liquid ina sensor and capillary to operate a pilot valve, which in turn changes the main valve position.

Advantages:

1. Easy to install and commission.2. One trade installation.3. Very robust.4. Self-acting principle means that no external power is required.5. Simple to size and select.6. Remote adjustment (option).7. Can be switched on and off (option).8. Dual set point (option).

Disadvantages:1. The control is stand-alone, and cannot communicate with a PLC.2. Small clearances within the valve body mean that steam should be clean and dry to ensure

longevity, but this can easily be achieved by fitting a separator and strainer before the valve.3. Proportional only control, however, the proportional offset is much smaller than for direct

operating, self-acting controls.

Applications:1. Jacketed pans.2. Tracer lines.3. Tanks.4. Acid baths.5. Hot water storage calorifiers.6. Heater batteries.

7. Unit heaters.Points to note:1. The temperature ranges of controllers tend to be narrower than direct operating, self-acting

controls.2. Installation must include a strainer and separator.

Fig. 8.2.2 General arrangement of a pilot operated, self-acting temperature control injecting steam into a tank

Steam in

Pilot operatedtemperaturecontrol valve

Sensor

TankInjector

Condensate

SeparatorVacuum breaker


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Pneumatic temperature control




Advantages:1. Very accurate and flexible.2. No limit on valve size within the limits of the valve range.3. Excellent turndown ratio.4. Suitable for hazardous environments.5. No electrical supply required.6. Fast operation means they respond well to rapid changes in demand.7. Very powerful, and can cope with high differential pressures.

Disadvantages:1. More expensive than direct operating controls.2. More complex than direct operating controls.

Applications:1. Which need accurate and consistent temperature control.2. With variable and high flowrates, and/or variable upstream pressure.3. Which require intrinsic safety.

Points to note:1. A clean, dry air supply is required2. A valve positioner is generally required except for the smallest and simplest of applications.

Air is continually vented from the positioner and controller, and there is a need to ensure thatthis quiescent air flow is acceptable to the surroundings.

3. A skilled workforce is required to install the equipment, and instrument personnel for calibrationand commissioning.

4. The control is stand-alone, and cannot directly communicate with a PLC.5. The failure mode must always be considered. For example, spring-to-close on air failure is

normal on steam heating systems, spring-to-open is normal on cooling systems.

Fig. 8.2.3 General arrangement of a pneumatic temperature control system on a heating calorifier

Condensate

Condensate

Heating calorifier

Steam in

Hot water out

Cold water in

Pneumatictemperaturecontrol valve

Pneumaticcontroller

Separator Vacuumbreaker

Temperature sensor


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Electropneumatic temperature control



o Set point(s) may be remotely adjusted, with the possibility of ramps between set points.

Advantages:1. Very accurate and flexible.2. Remote adjustment and read-out.3. No limit on valve size within the limits of the valve range.4. Excellent turndown ratio.5. Fast operation means they respond well to rapid changes in demand.6. Very powerful, and can cope with high differential pressures.

Disadvantages:1. More expensive than self-acting or pneumatic controls.2. More complex than self-acting or pneumatic controls.3. Electrical supply required.

Applications:1. Which need accurate and consistent temperature control.2. With variable and high flowrates, and/or variable upstream pressure.

Points to note:1. A clean, dry air supply is required.2. A skilled workforce is required to install the equipment, electrical personnel are required for

power supplies, and instrument personnel to calibrate and commission.3. Can be part of a sophisticated control system involving PLCs, chart recorders and SCADA

systems.4. The failure mode must always be considered. For example, spring-to-close on air failure is

normal on steam heating systems, spring-to-open is normal on cooling systems.5. Probably the most common control system - it has the sophistication of electronics with the

pace / power of pneumatics.

Fig. 8.2.4 General arrangement of an electropneumatic temperature control system on a heating calorifier

Condensate

Condensate

Heating calorifier

Steam inHot water out

Cold water in

Pneumatictemperaturecontrol valve


Separator

Vacuumbreaker

Temperature sensor


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Temperature control (other possibilities) -

Parallel temperature control station

DescriptionAn arrangement, as shown in Figure 8.2.6, can be used where the ratio between maximum andminimum flowrates (the flowrate turndown) is greater than the maximum allowable for theindividual temperature control valve.

For example, if a specific application has to be brought up to operating temperature very quickly,but the running load is small, and plant conditions dictate that self-acting controls must be used.

To satisfy the application:1. A valve and controller, which could satisfy the running load, would be selected first, and set to

the required temperature.2. A second valve and controller, capable of supplying the additional load for warm-up would

be selected, and set to a couple of degrees lower than the running load valve. This valve islikely to be larger than the running load valve.

With this configuration:1. When the process is cold, both control valves are open, allowing sufficient steam to pass to

raise the product temperature within the required time period.2. As the process approaches the required temperature, the warm-up valve will modulate to

closed, leaving the running load valve to modulate and maintain the temperature.

Fig. 8.2.6 General arrangement of a parallel temperature control station

Warm-up load valve leg

Running load valve leg

Condensate

Steam in

Separator

To temperaturesensor and controller

To temperaturesensor and controller


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High temperature fail safe control

DescriptionThere are many applications where a totally independent high limit cut-out device is eitherdesirable, or even a legal requirement.

Options:

1. A self-acting control, where the expansion of the fluid releases a compressed spring in acut-out unit, and snaps the isolating valve shut if the preset high limit temperature is exceeded.

This particular type of self-acting control has additional advantages:

a. It can incorporate a microswitch for remote indication of operation.

b. It is best if it has to be reset manually, requiring personnel to visit the application andascertain what caused the problem.

2. Spring-to-close electrical actuator where an overtemperature signal will interrupt theelectrical supply and the valve will close. This may be accompanied by an alarm.

3. Spring-to-close pneumatic actuators where an overtemperature signal will cause theoperating air to be released from the actuator. This may be accompanied by an alarm.

Application:Domestic hot water services (DHWS) supplying general purpose hot water to users such ashospitals, prisons and schools.

Points to note:1. There may be a legal requirement for the high temperature cut-out to be totally independent.

This will mean that the high temperature cut-out device must operate on a separate valve.2. Generally, the high temperature cut-out valve will be pipeline size, since a low pressure drop

is required across the valve when it is open.

Fig. 8.2.7 General arrangement of a high temperature cut-out on a DHWS storage calorifier

Steamsupply

High limitvalve

Controlvalve

Spring loadedcut-out unit

Fail-safe control system

Condensate

CalorifierReturn

Cold watermake-up

Separator

Condensate

Flow


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Questions

1. Name one disadvantage of direct operating temperature control

a| It is relatively inexpensive

b| The sensors tend to be large compared to EL (electronic) and PN (pneumatic) sensors

c| Systems are difficult to size and select

d| Systems are difficult to install and commission

2. A temperature control application in a hazardous area, and which haslow thermal mass, is subject to fast load changes and periods ofinoperation. Which would be the best control solution from the following?

a| A direct operating temperature control system

b| A pilot operated self-acting temperature control system

c| A pneumatic temperature control system

d| An electric temperature control system

3. In Figure 8.2.6, the warm-up valve is shown in the upper leg of theparallel supply system. Is this logical?

a| Yes, otherwise condensate would tend to collect in the warm-up leg during low loads,when the warm-up valve would be shut

b| Yes, it makes maintenance easier

c| No, either leg is acceptable

d| Yes, the warm-up valve needs more installation space

4. Is the fail-safe self-acting high limit temperature cut-out only suitable forDHWS storage calorifiers?

a| Yes

b| It is suitable for any application requiring high limit temperature control

5. In Figure 8.2.5, a shell and tube heating calorifier uses electrical control.Is this really suitable for this type of application?

a| No, it was the only example drawing available

b| No, the valve would not react quickly enough

c| No, an electropneumatic system should always be chosen for this type of application,especially when steam is the energy provider

d| Yes, because changes in load will occur slowly

1:b,2:c,3:a,4:b,5:dAnswers


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Block 8 Control Applications Level and Flow Control Applications Module 8.3

Module 8.3

Level and FlowControl Applications


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Level and Flow Control Applications Module 8.3Block 8 Control Applications

Level Control Applications

The control of liquid levels, for example in a process tank, is an important function. An examplewould be a hot water tank where water is removed, perhaps for washing down, and the levelneeds to be restored ready for the next wash cycle.

Control of water level and alarms for steam boilers is specifically excluded from this Module, andthe reader is referred to Block 3 (The Boiler House), which deals with the subject in depth.

Many different types of level control systems are used in industry, covering a wide range ofprocesses. Some processes will be concerned with media other than liquids, such as dry powdersand chemical feedstock. The range of media is so wide that no single instrument is suitable for allapplications.

Many systems are available to serve this wide range of applications. The following list is notexhaustive but, in most cases, the final control signal will be used to operate pumps or valvesappropriate to the application:

o Float operated types a float rises and falls according to the change in liquid level and operates

switches at predetermined points in the range.o Solid probe types these measure conductivity or capacitance and are discussed in more

detail in the following pages.

o Steel rope capacitance types a flexible steel rope is suspended in the liquid, and the changein capacitance is measured relative to the change in water level.

o Ultrasonic types a high frequency acoustic pulse is directed down from a transducer to thesurface of the medium being measured and, by knowing the temperature and speed of soundin air, the time it takes for the pulse to rebound to the sensor is used to determine the level.

o Microwave radar types similar in principle to the ultrasonic type but using high frequency

electromagnetic energy instead of acoustic energy.o Hydrostatic types a pressure transmitter is used to measure the pressure difference between

the confined hydrostatic pressure of the liquid head above the sensor and the outsideatmospheric pressure. Changes in pressure are converted into a 4-20 mA output signal relativeto the head difference.

o Differential pressure types similar to hydrostatic but used where the application beingmeasured is subjected to dynamic pressure in addition to static pressure. They are capable ofmeasuring small changes in pressure in relation to the output signal range. Typical applicationsmight be to measure the level of water in a boiler steam drum, or the level of condensate ina reboiler condensate pocket.

o Magnetic types a float or cone is able to rise and fall along a stainless steel probe held in thetank fluid being measured. The float can interact magnetically with switches on the outside ofthe tank which send back information to the controller.

o Torsion types a moving float spindle produces a change in torsion, measured by a torsiontransducer.

It is important that the level control system is correct for the application, and that expert adviceis sought from the manufacturer before selection.

It is not within the scope of this Module to discuss the pros and cons and potential applicationsof all the above control types, as the types of level control systems usually employed in the steam

and condensate loop and its associated applications are float and solid probe types. The operationof float types is fairly self-explanatory, but conductivity and capacitance probes may requiresome explanation. Because of this, this section will mainly focus on conductivity and capacitanceprobe-type level controls.


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Methods of achieving level control

Fig. 8.3.1 A four tip level probe

Fig. 8.3.2 A capacitance level probe

There are three main methods of achieving level control:

o Non-adjustable on/off level control.

o Adjustable on /off level control.

o

Modulating level control.Non-adjustable on/off level control (Figure 8.3.1)The final control element may be a pump which is switchedon/off or a valve which is opened/closed.

Two main types of on/off level control systems are usuallyencountered; float operated types and types usingconductivity probes. Float type level controls either relyupon the direct movement of a control valve, or uponelectrical switches being operated by a float moving on thesurface of the liquid. Conductivity probes (see Figure 8.3.1)

may have several probe tips; the control points beinglocated where the separate tips have been cut todifferent lengths.

Adjustable on/off level control (Figure 8.3.2)Again, the final control element may be a pump which isswitched on /off or a valve which is opened/closed.

One method used to adjust the control points is that of acapacitance probe (see Figure 8.3.2). The probe willmonitor the level, with control points adjusted by thecontroller. Capacitance probes are not cut to length to

achieve the required level and, of course, the whole probelength must be sufficient for the complete control range.

Modulating level control (Figure 8.3.2)The final control element may be a valve that is adjusted toa point between fully open and fully closed, as a functionof the level being monitored. Modulating level controlcannot be achieved using a conductivity probe. Capacitanceprobes are ideal for this purpose (see Figure 8.3.2).

In systems of this type, the pump can run continuously,and the valve will permit appropriate quantities of liquid

to pass. Alternatively, the final control element may be avariable speed drive on a pump. The speed of the drivemay be adjusted over a selected range.

Alarms are often required to warn of either:

o A high alarm where there is a danger of the tank overflowing and hot liquid being spilled,with the attendant danger to personnel.

o A low alarm where there is a danger of the tank water level becoming too low, with thepotential to damage a pump drawing from the tank, or running out of liquid for the process.

Installation of floats and probes in turbulent conditionsIn some tanks and vessels, turbulent conditions may exist, which can result in erratic andunrepresentative signals. If such conditions are likely to (or already) exist, it is recommendedthat floats or probes be installed within protection tubes. These have a dampening effect onthe water level being sensed. The rest of this Module concerns itself with probes rather thanfloats for level control applications.

Cableentry

Insulationsleeving

Probe tips

Amplifierconnection

Mainbody

Insulatedprobe


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Non-adjustable on/off level control

DescriptionNon-adjustable on/off level control uses a conductivity probe connected to an electronic controller.The probe typically has three or four tips, each of which is cut to length during installation toachieve the required switching or alarm level (see Figure 8.3.3).

o

When the tip of the probe is immersed in liquid it uses the relatively high conductivity of thewater to complete an electrical circuit via the tank metalwork and the controller.

o When the water level drops below the tip, the circuit resistance increases considerably, indicatingto the controller that the tip is not immersed in the liquid.

o In the case of a simple pumping in system with on/off level control:- The valve is opened when the tank water level falls below the end of a tip.- The valve is closed when the water level rises to contact another tip.- Other tips may be used to activate low or high alarms.

Advantage:A simple but accurate and relatively inexpensive method of level control.

Applications:The system can be used for liquids with conductivities of 1 S/cm or more, and is suitablefor condensate tanks, feedwater tanks and process vats or vessels. Where the conductivity fallsbelow this level it is recommended that capacitance based level controls are used.

Point to note:If the tank is constructed from a non-conductive material, the electrical circuit may be achievedvia another probe tip.

Fig. 8.3.3 General arrangement of a non-adjustable on/off level control system for a tank

Conductivity probe controller

Rotarypneumatic

valveSolenoid

valve

Four elementconductivityprobe

The 4thconductivity

probe is usedas an earth

Valveclosed

600 mm

Valveopen

750 mm

Lowalarm

850 mm

Watersupply

Water outflow

Tank


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Adjustable on/off level control

Description:An adjustable on/off level control system consists of a controller and a capacitance probe (seeFigure 8.3.4), and provides:

o Valve open/closed control plus one alarm point.

o Alternatively two alarms - high and low.

The levels at which the valve operates can be adjusted through the controller functions.

Advantage:Adjustable on/off level control allows the level settings to be altered without shutting down theprocess.

Disadvantage:More expensive than non-adjustable on/off control.

Application:

Can be used for most liquids, including those with low conductivities.Point to note:Can be used in situations where the liquid surface is turbulent, and the in-built electronics can beadjusted to prevent rapid on/off cycling of the pump (or valve).

On-offcontrolvalve

Watersupply

Water outflow

Controller

Fig. 8.3.4 General arrangement of an adjustable on/off level control system for a tank

Capacitance probe

Tank


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Modulating level control

DescriptionA modulating level control system consists of a capacitance probe and appropriate controller,which provides a modulating output signal, typically 4-20 mA. Refer to Figure 8.3.5. This outputsignal may be used to affect a variety of devices including:

o

Modulating a control valve.o Operating a variable speed pump drive.

Advantages:1. Because the probe and controller only provide a signal to which other devices respond, rather

than providing the power to operate a device, there is no limit on the size of the application.2. Steady control of level within the tank.

Disadvantages:1. More expensive than a conductivity probe system.2. More complex than a conductivity probe system.3. Supply system must be permanently charged.4. Less suitable for stand-by operation.5. Possibly greater electricity consumption.

Point to note:To protect the supply pump from overheating when pumping against a closed modulating valve,a re-circulation or spill back line is provided to ensure a minimum flowrate through the pump(neither shown in Figure 8.3.5).

Modulatingcontrol

valve

Watersupply

Water outflow

Controller

Fig. 8.3.5 General arrangement of a modulating control system maintaining the level in a tank

Capacitance probe

Tank

Air supply


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Steam flow control applications

The control of steam flow is less common than pressure and temperature control, but it is used inapplications where the control of pressure or temperature is not possible or not appropriate toachieving the process objectives. The following sections give more information on measuringand controlling the flow of steam.

Flow control system

Typical applications:1. Feed-forward systems on boiler plant, where the rate of steam flow from the boiler will

influence other control points, for example: feedwater make-up rate, and burner firing rate.2. Re-hydration processes, where a measured quantity of steam (water) is injected into a product,

which has been dried for transportation or storage. Examples of this can be found in thetobacco, coffee and animal feedstuff industries.

3. Batch processes, where it is known from experience that a measured quantity of steam willproduce the desired result on the product.

The selection and application of components used to control flowrate require careful thought.

Pneumaticcontrol valve

Air supplyto valve

Flowmeter

Differentialpressure

transmitter

The flowmeter (pipeline transducer)The flowmeter is a pipeline transducer, which converts flow into a measurable signal. Themost commonly used pipeline transducer is likely to relate flow to differential pressure. Thispressure signal is received by another transducer (typically a standard DP (differential pressure)

transmitter) converting differential pressure into an electrical signal. Some pipeline transducersare capable of converting flowrate directly to an electrical signal without the need for aDP transmitter.

Figure 8.3.6 shows a variable area flowmeter and standard DP transmitter relating differentialpressure measured across the flowmeter into a 4 - 20 mA electrical signal. The standardDP transmitter is calibrated to operate at a certain upstream pressure; if this pressure changes,the output signal will not represent the flow accurately. One way to overcome this problemis to provide a pressure (or temperature) signal if the medium is saturated steam, or a pressureand temperature signal if the fluid is superheated steam, as explained in the next Section.Another way is to use a mass flow DP transmitter, which automatically compensates forpressure changes.

Fig. 8.3.6 General arrangement of a flow control system

Condensate

Steamsupply

Separator

Controller

Measuredsteam flow

AC Vac


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The possible need for a computer

If steam is the fluid in the pipeline, then other temperature and/or pressure sensors may benecessary to provide signals to compensate for variations in the supply pressure, as shown inFigure 8.3.7.

Pneumaticcontrol valve

Separator

Steamsupply

Condensate

Flowcontroller

Air supplyto valve

Flowmeter

Flowcomputer

Differentialpressuretransmitter


Multiple inputs will mean that an additional flow computer (or PLC) containing a set of electronicsteam tables must process the signals from each of these flow, pressure and temperature sensors

to allow accurate measurement of saturated or superheated steam.If a flow computer is not readily available to compensate for changes in upstream pressure, itmay be possible to provide a constant pressure; perhaps by using an upstream control valve, togive stable and accurate pressure control (not shown in Figure 8.3.7).

The purpose of this pressure control valve is to provide a stable (rather than reduced) pressure,but it will inherently introduce a pressure drop to the supply pipe.

A separator placed before any steam flowmetering station to protect the flowmeter from wetsteam will also protect the pressure control valve from wiredrawing.

Using a mass flow DP transmitter

By using a mass flow DP transmitter instead of a standard DP transmitter, the need for a computerto provide accurate measurement is not required, as shown in Figure 8.3.8.

This is because the mass flow transmitter carries its own set of steam tables and can compensatefor any changes in saturated steam supply pressure.

However, a computer can still be used, if other important flowmetering information is required,such as, the times of maximum or minimum load, or is there is a need to integrate flow over acertain time period.

A controller is still required if flowrate is to be controlled, whichever system is used.

Measuredsteamflow

AC Vac

Pressuretransmitter


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The controllerEven if the output signal from the DP transmitter or computer is of a type that the control valveactuator can accept, a controller will still be required (as for any other type of control system) forthe following reasons:

1. The output signal from certain flowmeters/computers has a long time repeat interval(approximately 3 seconds), which will give enough information for a chart recorder to operatesuccessfully, but may not offer enough response for a control valve. This means that if thecontroller or PLC to which the transmitter signal is being supplied operates at higher speeds,then the process can become unstable.

2. PID functions are not available without a controller.

3. Selecting a set point would not be possible without a controller.

4. The signal needs calibrating to the valve travel - the effects of using either a greatly oversizedor undersized valve without calibration, can easily cause problems.

Summary

It is usually better to install the flowmetering device upstream of the flow control valve. Thehigher pressure will minimise its size and allow it to be more cost effective. It is also likely that theflowmeter will be subjected to a more constant steam pressure (and density) and will be lessaffected by turbulence from the downstream flow control valve.

In some cases, the application may be required to control at a constant flowrate. This means thatfeatures, such as high turndown ratios, are not important, and orifice plate flowmeters areappropriate.

If the flowrate is to be varied by large amounts, however, then turndown becomes an issue thatmust be considered.

The subject of Flowmetering is discussed in greater depth in Block 4.


Pneumatic control valve

Separator

Steamflow

Condensate

Flowcontroller

Air supplyto valve

Flowmeter

Mass flowdifferential

pressuretransmitter

AC Vac


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Questions

1. Condensate has a conductivity of 0.1 s/cm. Name the best choice of solid probeto give on/off level control for this application.

a| A single tip conductivity probe

b| Two single tip conductivity probes

c| A four tip conductivity probe

d| A capacitance probe

2. Name an advantage of modulating control over on/off control.

a| It tends to control at a steady level

b| It allows the level settings to be altered without removing the probe

c| It allows the alarm settings to be altered without removing the probe

d| All of the above

3. Why is a separator recommended before a flow control station?

a| It protects the pipeline transducer from the effects of a wet steam

b| It protects the pressure control valve from wiredrawing

c| It ensures that only dry steam is being measured

d| All of the above

4. Why is a flow computer recommended when controlling steam flow?

a| The system wont work without it

b| It compensates for changes in supply pressure to give accuracy

c| It contains a set of electronic steam tables

d| All of the above

5. What does a pipeline transducer actually do?

a| It always converts flow into a measurable signal

b| It always converts flow into an electrical signal

c| It always converts flow into a pressure signal

d| It converts differential pressure into a flow signal

6. What does a DP transmitter actually do?

a| It converts differential pressure into an electrical signal

b| It converts an electrical signal into differential pressure

c| It converts upstream pressure into an electrical signal

d| It converts differential pressure into a flow signal

1:d,2:d,3:d,4:b,5:a,6:a Answers


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Block 8 Control Applications Module 8.4Control Installations

Module 8.4

Control Installations


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Module 8.4Control InstallationsBlock 8 Control Applications

Control Installations

The service life and accuracy of a control system is influenced not just by the component parts,but also by the installation.

Temperature sensors

Sensor locationThe position of the sensor is important, and it must be located where it can sense a representativepressure, temperature or level.

The length of the sensor must also be considered. If the sensor to be used is large or long,provision has to be made for this in the pipework into which it is installed.

Sensors for self-acting control systems can come in many different shapes and sizes. Generally,the sensors for electronic and pneumatic control systems are smaller than those for self-actingcontrols.

The next requirement is to position the sensor in a location where it is not susceptible to damage,

and perhaps to fit it in a pocket if necessary.The pocket must be long enough to enable the whole sensor to be immersed in the liquid. If, in Figure8.4.1, the stub connector were longer, the sensor might not be properly immersed in the fluid.

Short stub connector

Self-acting sensorSensor element is immersedwell in the fluid flow

Fig. 8.4.1 A good installation with the sensor properly immersed in the fluid

Sensor protectionIf the sensor is to be installed in a tank, it may be better to locate it close to one of the corners,

where the greatest wall strength might be expected, with less chance of flexing.

With some fluids it is necessary to protect the sensor to prevent it from being corroded or dissolved.Pockets are usually available in various materials, including:

o Stainless steel.

o Mild steel.

o Copper and brass, which are suitable for the less severe applications.

o Heat resistant glass, which offers good general protection against corrosive products like acidsand alkalis, but these can be fragile.

Self-acting control capillary tubes can usually be supplied covered with a PVC coating, which isuseful in corrosive environments.

Where it is possible to fit the sensor through the side of the tank, the provision of a pocket alsoallows the sensor to be removed without draining the contents.


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A pocket will tend to increase the time lag before the control can respond to changes in solutiontemperature, and it is important to make arrangements to keep this to a minimum. There will, forinstance, be an air space between the sensor and the inside of the pocket, and air is an insulator.To overcome this, a heat conducting paste can be used to fill the space.

Controllers

The controller:

o Should be installed where it can be accessed and read by the authorised operator.

o Should be installed where it is safe from accidental damage inflicted by passing personnelor vehicles.

o Must be appropriate to the environment in terms of enclosure rating, hazardous gases and/orliquids.

o Must comply with standards relating to radio frequency interference.

Valves and actuators

The preferred actuator position will depend upon the type of control system used. Forself-acting control valves, it is generally preferable if the actuator is fitted underneath the valve.Conversely, it is usually better to fit an electrical or pneumatic actuator above the valve, otherwiseany leakage from the stem may result in process fluid, which may be hot or corrosive, spillingonto the actuator.

Horizontal fitting is not recommended as over a period of time:

o Uneven stem wear may occur.

o The valve plug may not present itself squarely to the valve seat.

The material construction of electric actuators must be appropriate to the environment in terms

of the enclosure rating against excess moisture, and hazardous gases and liquids.The valve and actuator will be heavier than an equivalent length of pipe, and will need adequatesupport.

It is important, before and after installation, to check that the valve is installed with its flowarrow in the correct direction.

Enough space must be left around the valve and actuator for maintenance, and to lift the actuatoroff the valve.

Radio frequency interference (RFI)Radio frequency interference is electrical noise that can cause corruption of control signals

and affect the operation of electronic controllers.

There are two forms of RFI:

o Continuous

o Impulse (transient).

Radio transmitters, computers, induction heaters, and other such equipment emit continuoushigh frequency radio interference.

Impulse interference is generated from electrical arcing, which can occur on the opening ofswitch contacts especially those responsible for switching inductive components, such as motorsor transformers.

The control engineer is often most concerned about impulse interference. The pulses are ofvery high intensity and very short duration, and can disturb genuine electrical control signals.


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Transmission of RFIRadio interference can travel via two modes:

o Conduction.

o Radiation.

Conducted interference is communicated to the controller via mains supply cables. Having an

interference suppressor in the supply as close to the controller as possible can reduce its effect.

Radiated interference is a greater problem because it is harder to counteract. This form ofinterference is like a broadcast transmission being picked up by aerials naturally formed by thesignal wiring, and then re-emitted within the controller box to more sensitive areas.

The electronic components within the controller can also receive transmissions directly,especially if the interference source is within 200 mm.

Effects of RFIController types respond to different forms of interference in different ways.

Analogue controllers will usually respond to continuous rather than transient interference butwill usually recover when the interference ceases. The symptoms of continuous interference arenot easily recognisable because they usually influence the measurement accuracy. It is oftendifficult to distinguish between the effects of interference and the normal operation of the device.

Transient interference is more likely to affect relay outputs, as its occurance is faster than thatwhich the analogue circuits can respond.

Microprocessor based controllers are more subject to corruption from transient impulseinterference but have a higher immunity to continuous interference.

The first indication that interference has occurred is often that the display has locked up,is scrambled or contains meaningless symbols in addition to the normal display.

More difficult symptoms to detect include measurement inaccuracies or incorrect actuatorposition, this may continue undetected until the system is clearly out of control.

Installation practice to limit RFIThe correct selection and installation of control signal wiring is vital to reduce susceptibilityto RFI. Twisted pairs of wires are less susceptible to interference than parallel run cables(Figure 8.4.2). Earthed screened cables are even less susceptible to interference than twistedpairs of wires, but this cannot always be relied on, especially near high current cables.

Signal wire(unprotected)

Fig. 8.4.2 Unprotected signal wire

Screened cable (Figures 8.4.3) should only be earthed at one end, see Figure 8.4.3 (A and B);earthing at both ends will lead to a deterioration in this situation.


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Signalwiring

Screen

Earthed

A - Screened and earthed wiring

Screen

Earthed

Twisted pairsignal wiring B - Twisted pair, screened and

earthed at one end

Other power cables

Instrument power wiring

Signal wiring

Conduit

C - Unprotected wiring in conduit

with other cables

Earthed

Fig. 8.4.3 Correct earthing of screened cable

Keeping wires separate from power wiring (Figure 8.4.4) can reduce pick-up via the signalwires. BS 6739: 1986 recommends that this separation should be at least 200 mm for instrumentpower wiring and 250 mm for other power cables.

Fig. 8.4.4 Cable separation

Other power cables

Instrument powerwiring

Signal wiring

200 mmminimum

250 mmminimum


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Electrical protection standardsElectrical equipment such as electronic controllers must be suitable for the environment inwhich they are to be used. Hazardous environments may be found in oil refineries, offshoreplatforms, hospitals, chemical plants, mines, pharmaceutical plants and many others. The degreeof protection will alter depending on the potential hazard, for example the risk of sparks or hotsurfaces igniting flammable gases and vapours which may be present.

It is equally important to safeguard equipment against moisture, dust, water ingress, and severechanges in temperature.

Standards and procedures exist to reduce the chance of equipment inducing faults, which mightotherwise start fires or initiate explosions in adjacent equipment.

Basic standards of protection have been devised to cater for specific environments.

IP ratingsThe IP, or international protection rating stated for an enclosure, is a means of grading theprotection level offered by the enclosure, by using two figures, as shown in Tables 8.4.1 and 8.4.2.

The first figure (see Table 8.4.1) refers to the protection offered against the intrusion of foreign

objects such as levers, screwdrivers or even a persons hand. The range consists of seven digitscommencing with 0, designating no protection offered from material objects or humanintervention; up to 6, offering meticulous protection against the entry of dust or extremely fineparticles.

Table 8.4.1 Degrees of protection offered by the 1st characteristic numeral

First

characteristicDegree of protection

numeral Short description Definition

0 Non-protected No special protection.

Protected against solid objects A large surface of the human body, like a hand, but

1 larger than 50 mm diameter. no protection against attempted deliberate access.

2Protected against solid objects Fingers, or similar objects, not exceeding 80 mm in length.larger than 12 mm diameter.

3Protected against solid objects Tools, wires etc of diameter greater than 2.5 mm.larger than 2.5 mm diameter.

4Protected against solid objects Tools, wires etc of diameter greater than 1.0 mm.larger than 1.0 mm diameter.

Ingress of dust not prevented, but does not enter in sufficient5 Dust protected. quantity to interfere with satisfactory operation of the equipment.

6 Dust-tight. No ingress of dust.


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The second figure (see Table 8.4.2) indicates the degree of protection against water intrusion.The range commences with 0 meaning no protection against water. The highest is 8, givingoptimum protection for equipment being continuously immersed in water.

Table 8.4.2 Degrees of protection offered by the 2nd characteristic numeral

First

characteristicDegree of protection

numeral Short description Definition

0 Non-protected. No special protection.

1 Protected against dripping water. Dripping water shall have no harmful effect.

2Protected against dripping water Dripping water shall have no harmful effect when tilted at any

when tilted up to 15. angle up to 15 from its normal position .

3Protected against Water falling as a spray at an angle up to 60 from the vertical

spraying water. shall have no harmful effect.

4Protected against Water splashed against the enclosure from any direction

splashing water. shall have no harmful effect.

5 Protected against water jets.Water projected by a nozzle against the enclosure shall

have no harmful effect.

6 Protected against heavy seas.Water from heavy seas or water projected in powerful jets

shall not enter the enclosure in harmful quantities.

Ingress of water in a harmful quantity shall not be possible

7Protected against the effects

when the enclosure is immersed in water under definedof immersion.

conditions of pressure and time.

8 Protected against submersion.The equipment is suitable for continuous submersion in water

under conditions which shall be specified by the manufacturer.

Example 8.4.1An electrical enclosure having the following IP34 rating can be defined as follows:

Code letters IP An enclosure which has been given an International Protection rating

1st characteristic numeral 3 Protects equipment inside the enclosure against ingress of solid foreign objects

having a diameter of 2.5 mm and greater.

2nd characteristic numeral 4 Protects equipment inside the enclosure against harmful effects due to water

splashed onto the enclosure from any direction.

It is not the intention of this Module to enter into detail regarding the subject of enclosure protection.The subject is discussed in much further depth in International Standards, BS EN 60529:1992being one of them. The reader is advised to refer to such standards if information is required forspecific purposes.

Explosion protected electrical equipment

It has been shown briefly how IP ratings cover two important areas of protection. There are,however, numerous other types of hazard to contend with. These may include corrosion, vibration,fire and explosion. The latter are likely to occur when electrical equipment produce sparks,operate at high temperatures, or arc; thus igniting chemicals, oils or gases.

In practice, it is difficult to determine whether or not an explosive atmosphere will be present ata specific place within a potentially hazardous area or plant. This problem has been resolved byassigning an area within the plant where flammable gases may be present to one of the followingthree hazardous zones:

o Zone 1 - An area where the explosive gas is continuously present or is present for longperiods of time.

o Zone 2 - An area where the explosive gas is likely to occur during normal operation.

o Zone 3 - An area where the explosive gas is not likely to occur during normal operation andif it does, will exist only for a short period of time.


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There have been many attempts to formulate internationally accepted standards of protection.The IEC (International Electrotechnical Commission) were the first to produce internationalstandards in this area, however, CENELEC (European, Electrical Standards Co-ordinationCommittee) currently unites all the major European manufacturing countries under one set ofstandards.

Measurement and control equipment is covered by an intrinsic safety protection method, which

is based upon the reduction of explosive risk by restricting the amount of e

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