Pressure Control Valve Application

7/31/2019 Pressure Control Valve Application

1/20

PRESSURE CONTROL VALVE APPLICATION

There are many reasons for reducing steam pressure:

Steam boilers are usually designed to work at high pressures in order to reduce their

physical size. Operating them at lower pressures can result in reduced output and 'carryover' ofboiler water. It is, therefore, usual to generate steam at higher pressure.

Steam at high pressure has a relatively higher density, which means that a pipe of a givensize can carry a greater mass of steam at high pressure, than at low pressure. It is usuallypreferable to distribute steam at high pressure as this allows smaller pipes to be used throughoutmost of the distribution system.

Lower condensing pressures at the point of use tend to save energy. Reduced pressurewill lower the temperature of the downstream pipework and reduce standing losses, and alsoreduce the amount of flash steam generated when condensate from drain traps is discharging intovented condensate collecting tanks.

It is worth noting that if condensate is continuously dumped to waste, perhaps because of the riskof contamination, less energy will be lost if the condensing pressure is lower.

Because steam pressure and temperature are related, control of pressure can be used tocontrol temperature in some processes. This fact is recognised in the control of sterilisers andautoclaves, and is also used to control surface temperatures on contact dryers, such as thosefound in papermaking and corrugator machines. Pressure control is also the basis of temperaturecontrol in heat exchangers.

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

The construction of plant means that each item has a maximum allowable workingpressure (MAWP). If this is lower than the maximum possible steam supply pressure, the pressuremust be reduced so that the safe working pressure of the downstream system is not exceeded.

Many plants use steam at different pressures. A 'stage' system where high-pressurecondensate from one process is flashed to steam for use in another part of the process is usuallyemployed to save energy. It may be necessary to maintain continuity of supply in the low pressuresystem at times when not enough flash steam is being generated. A pressure reducing valve isideally suited for this purpose.

Top

Direct operating, self-acting pressure reducing valve - bellows type

DescriptionWith this self-acting type of pressure controller, the downstream (control) pressure is balanced (via abellows) 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.
http://www.spiraxsarco.com/resources/steam-engineering-tutorials/control-applications/pressure-control-applications.asp#top%23tophttp://www.spiraxsarco.com/resources/steam-engineering-tutorials/control-applications/pressure-control-applications.asp#top%23tophttp://www.spiraxsarco.com/resources/steam-engineering-tutorials/control-applications/pressure-control-applications.asp#top%23tophttp://www.spiraxsarco.com/resources/steam-engineering-tutorials/control-applications/pressure-control-applications.asp#top%23top


2/20

3. Wide proportional band means that maximum flow is only achieved when the downstreampressure has dropped considerably. This means that the reduced pressure will vary depending onflowrate.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 andpreferences can be satisfied.4. A wide proportional band means care is needed if the safety valve needs to be set closeto the working pressure.

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

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 adiaphragm) 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.


3/20


4/20

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 includeelectrical override, multi-pilot for a choice of control pressures, a surplussing option and remotecontrol, 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 canbe 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 whichhave variable and medium flowrates. For example: autoclaves, highly rated plant such as heatexchangers 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-actingcontrols, but cheaper than diaphragm type self-acting controls.3. Size for size, they have higher capacity than bellows type self-acting controls, but lessthan diaphragm type self-acting controls.4. Can be installed before temperature control valves to maintain a constant upstreampressure, 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.

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


5/20

Pressure reduction - pneumatic

Description:These control systems may include:

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

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 thevalve.

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 have variable andhigh flowrates and/or variable or high upstream pressure. For example: autoclaves, highly rated plant suchas 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 arerequired for calibration and commissioning.3. The control is 'stand-alone', and cannot communicate with PLCs (Programmable LogicControllers).4. The failure mode can be important. For example, a spring-to-close on air failure is normalon steam systems.


6/20

Fig. 8.1.4 General arrangement of a pneumatic pressure reducing stationTop

Pressure reduction - electropneumatic

DescriptionThese control systems may include:


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

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 thevalve.

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 have variable andhigh flowrates and/or variable or high upstream pressure, including autoclaves, highly rated plant such aslarge heat exchangers and calorifiers, and main plant pressure reducing stations.

Points to note:


7/20

1. A clean, dry air supply is required.2. A skilled workforce is required to install the equipment, and instrument personnel arerequired for calibration and commissioning.3. Can be part of a sophisticated control system involving PLCs, chart recorders and SCADAsystems.4. Always consider the failure mode, for example, spring-to-close on air failure is normal onsteam systems.

Fig. 8.1.5 General arrangement of an electropneumatic pressure reducing stationTop

Pressure reduction - electric

Description:These control systems may include:


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

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 changesslowly.

Applications:


8/20

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

Fig. 8.1.6 General arrangement of an electric pressure reducing stationTop

Pressure reduction (other possibilities) - Parallel pressure reducingstations

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 unacceptableThe equipment is operated on a 'one in operation, one on stand-by' basis to cover for breakdown

and 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 the lowerpressure shuts off first, leaving the second valve to control.

Point to note:


9/20

The valves selected for this type of application will require narrow proportional bands (such as pilot operatedpressure reducing valves or electro-pneumatic control systems) to avoid the downstream pressure droppingtoo much at high flow rates.

Fig. 8.1.7 Parallel pressure reducing stationTop

Pressure reduction (other possibilities) - Series pressure reducingstations

A pressure reducing station may be configured in this manner if the ratio between the upstream anddownstream pressure is very high, and the control systems selected have a low turndown ability. 10:1 isrecommended as a practical maximum pressure ratio for this type of reducing valve.

Consider the need to drop pressure from 25 bar g to 1 bar g. The primary reducing valve might reducepressure from 25 bar g to 5 bar g, which constitutes a pressure ratio of 5:1. The secondary reducing valvewould drop pressure from 5 bar g to 1 bar g, also 5:1. Both valves in series provide a pressure ratio of 25:1.

It is important to check the allowable pressure turndown ratio on the selected reducing valve, this may be10:1 on a self-acting valve, but can be much higher on electrically or pneumatically operated valves. Be

aware that high pressure drops might have a tendency to create high noise levels. Refer to Tutorial 6.4 forfurther details.


10/20

Fig. 8.1.8 Typical series pressure reducing stationThe trapping point between the two reducing valves (Figure 8.1.8) is to stop a build up of condensate underno-load conditions. If this were not fitted, radiation losses would cause condensate to fill the connecting pipe,which would cause waterhammer the next time the load increased.

Top

DesuperheatersDesuperheating is the process by which superheated steam is either restored to its saturated state, or itssuperheated temperature is reduced. Further coverage of desuperheaters is given in Block 15.

The system in Figure 8.1.9 illustrates an arrangement of a pressure reducing station with a direct contacttype pipeline desuperheater.

In its basic form, good quality water (typically condensate) is directed into the superheated steam flow,removing heat from the steam, causing a drop in the steam temperature.

Fig. 8.1.9 Simple steam atomising desuperheater stationIt is impractical to reduce the steam temperature to its saturated value, as the control system is unable todifferentiate 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 saturation


11/20

temperature, 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 the temperaturecontroller does not need to vary; it simply needs to be set at a temperature slightly above the correspondingsaturation temperature.

However, sometimes a more complex control system is required, and is shown in Figure 8.1.10. Shouldthere be a transient change in the superheated steam supply pressure, or a change in the water supplytemperature, 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 issometimes the case with certain industrial processes.

Fig. 8.1.10 Steam atomising desuperheater station with downstream pressure/temperaturecompensation

The system shown in Figure 8.1.10 works by having the pressure controller set at the required downstreampressure 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 saturationtemperature computer, from which the computer continuously calculates the saturation temperature for thedownstream pressure, and transmits a 4-20 mA output signal to the temperature controller in relation to thistemperature.

The temperature controller is configured to accept the 4-20 mA signal from the computer to determine its setpoint at 5C to 10C above saturation. In this way, if the downstream pressure varies due to any of thereasons mentioned above, the temperature set point will also automatically vary. This will maintain thecorrect water/steam ratio under all load or downstream pressure conditions.

Top

Controlling pressure to control temperature


12/20

DescriptionThese are applications which utilise the predictable relationship between saturated steam pressure and itstemperature.

Advantages:

1. The pressure sensor may be located in the steam space, or close to the control valve

rather than 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 singlepoint.

Disadvantage:

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

Applications:

1. Autoclaves and sterilisers.2. Presses and calenders.3. Constant pressure plant, for example, jacketed pans, unit heaters, and steam-jacketedpipes.

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

Fig. 8.1.11 Pressure control of an autoclave


13/20

Fig. 8.1.12 Pressure control on a jacketed pipe application

Fig. 8.1.13 Pressure control on a multi platen press


14/20

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 exchangerTop

Differential pressure control

DescriptionIn these applications the control valve will open and close to maintain a set differential pressure between twopoints.

Advantages:

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


15/20

Disadvantage:A complex system is required if efficiency is to be maintained. This might involve flash vessels and/orthermo-compressors, as well as downstream applications which use the lower 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; one from the primarysteam supply and the other from the flash vessel. In this way, the pressure differential between the flashvessel and the primary steam supply is maintained under all load conditions.

Fig. 8.1.16 Differential pressure controlTop

Surplussing control

DescriptionThe objective is to maintain the pressure upstream of the control valve. Surplussing valves are discussed infurther detail in Tutorial 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 shortperiod. The sudden reduction in boiler pressure may result in increased turbulence and rapidflashing of the boiler water, and large quantities of water being carried over into the pipeworksystem.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.


16/20

Points to note:

1. Minimum pressure drop is usually required over the fully open control valve; this maymean a 'line size' valve is needed.2. Not all self-acting controls are suitable for this application and it is important to consult themanufacturer before use.

Fig. 8.1.17 Surplussing control on a steam boiler


17/20


18/20

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

Cascade control - Combined pressure reduction and surplussing with onevalve

DescriptionThe objective is to reduce steam pressure but not at the expense of overloading the available supplycapacity.

Application:The upstream pipework is a high-pressure distribution pipe possibly from a distribution manifold or steamboiler supplying plant of a non-essential nature (Figure 8.1.20). Should the demand be higher than thesupply capacity, the valve closes and throttles the steam flow, maintaining the pressure in the upstreampipework.

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

The slave closes the valve until the steam demand falls to allow the upstream pressure to re-establish to therequired value. When this is achieved, the set point of the slave controller is set at its original value.


19/20

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

Typical settingsThe output from the master controller is direct acting, that is, when the upstream pressure is at or above itsproportional band, the master's output signal is maximum at 20 mA; when at the bottom of, or below theproportional 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; when the signalis 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 allowable downstream pressureto be 5 bar g. The minimum allowable upstream pressure is 8.5 bar g, which means that if this pressure isreached the valve is fully shut. The minimum reduced pressure is set at 4.6 bar g.

These conditions are recorded in Table 8.1.1

Table 8.1.1Top

Cascade control - Limiting and controlling temperature with one valve

DescriptionThe main objective is to limit and regulate the temperature to a particular process, where steam is theavailable heat source but it cannot be used directly to heat the final product for operational reasons.

Application:

A typical application is a dairy cream pasteuriser requiring a pasteurisation temperature of 50C. Because ofthe low control temperature, if steam were applied directly to the pasteurisation heat exchanger, it ispossible that the relatively large amount of heat in the steam would make control difficult, causing thesystem temperatures to oscillate, overheating and spoiling the cream.

To overcome this problem, the system in Figure 8.1.21 shows two heat exchangers. The pasteuriser isheated 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 lag would beintroduced into the system, and poor control might again be the result.


20/20

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

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

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

Pressure Control Valve Application

Documents

Transcript of Pressure Control Valve Application