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Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications
Self-acting Pressure Controls and
Applications
Why reduce steam pressure?The main reason for reducing steam pressure is rather fundamental. Every item of steam using
equipment has a maximum allowable working pressure (MAWP). If this is lower than the steamsupply pressure, a pressure reducing valve must be employed to limit the supply pressure to theMAWP. In the event that the pressure reducing valve should fail, a safety valve must also beincorporated into the system.
This is not, however, the only occasion when a pressure reducing valve can be used to advantage.
Most steam boilers are designed to work at relatively high pressures and should not be run at lower pressures, since wet steam is likely to be produced. For this reason, it is usually moreeconomic in the long term to produce and distribute steam at a higher pressure, and reducepressure upstream of any items of plant designed to operate at a lower pressure.
This type of arrangement has the added advantage that relatively smaller distribution mains can
be used due to the relatively small volume occupied by steam at high pressure.Since the temperature of saturated steam is closely related to its pressure, control of pressurecan be a simple but effective method of providing accurate temperature control. This fact isused to good effect on applications such as sterilisers and contact dryers where the control of surface temperature is difficult to achieve using temperature sensors.
Plant operating at low steam pressure:
o Can tend to reduce the amount of steam produced by the boiler due to the higher enthalpyof evaporation in lower pressure steam.
o Will reduce the loss of flash steam produced from open vents on condensate collecting tanks.
Most pressure reducing valves currently available can be divided into the following two main groups:o Direct acting valves.
o Pilot-operated valves.
Direct acting valves
Smaller capacity direct acting pressure reducing valves (Figure 7.3.1)
Method of operationOn start-up and with the adjustment spring relaxed, upstream pressure, aided by a return spring,holds the valve head against the seat in the closed position. Rotating the handwheel in a clockwise
direction causes a downward movement, which compresses the control spring and extends thebellows to set the downstream pressure.
This downward movement is transmitted via a pushrod, which causes the main valve to open.Steam then passes through the open valve into the downstream pipework and surrounds the bellows.
As downstream pressure increases, it acts through the bellows to counteract the adjustment spring force, and closes the main valve when the set pressure is reached. The valve plug modulatesin an attempt to achieve constant pressure.
In order to close the valve, there must be a build-up of pressure around the bellows. This requiresan increase in downstream pressure above the set pressure in proportion to the steam flow.
The downstream pressure will increase as the load falls and will be highest when the valve is
closed. This change in pressure relative to a change in load means that the downstream pressurewill only equal the set pressure at one load. The actual downstream pressure compared to theset point is the proportional offset; it will increase relative to the load, and this is sometimesreferred to as droop.
Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications
The total pressure available to close the valve consists of the downstream pressure acting on theunderside of the bellows plus the inlet pressure acting on the underside of the main valve itself and the small force produced by the return spring. The control spring force must therefore belarger than the reduced pressure and inlet pressure and return spring for the downstream pressureto be set.
Any variation in the inlet pressure will alter the force it produces on the main valve and so affect
the downstream pressure.This type of pressure reducing valve has two main drawbacks in that:
1. It suffers from proportional offset as the steam flow changes
2. It has relatively low capacity.
It is nevertheless perfectly adequate for a substantial range of simple applications where accuratecontrol is not essential and where steam flow is fairly small and reasonably constant.
Fig. 7.3.1 Small capacity direct acting pressure reducing valve
Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications
Larger capacity direct acting pressure reducing valves (Figure 7.3.2)Larger capacity direct acting pressure reducing valves are also available for use on larger capacityplant, or on steam distribution mains. They differ slightly to the smaller capacity valves in that the actuator force is provided by pressure acting against a flexible diaphragm inside the actuatorrather than a bellows.
As these are not pilot-operated, they will incur a change in downstream pressure as the steam
flow changes, and this should be taken into careful consideration when selecting and sizing thevalve.
This type of valve is installed with the actuator below the pipe when used with steam, and has awater seal pot to stop high steam temperatures from reaching and damaging the actuatorsflexible diaphragm, which is commonly made out of neoprene. A typical installation for thereduction of steam mains pressure is shown in Figure 7.3.3.
Pressure sensing connection
Fig. 7.3.2 Large capacity direct acting pressure reducing valve
Pressure reducing valve
Adjustment nut
Actuator
Spring
Fig. 7.3.3 Typical steam pressure reducing station for a large capacity direct acting pressure reducing valve
Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications
Fig. 7.3.4 Pilot-operated pressure reducing valve
A pilot-operated pressure reducing valve works by balancing the downstream pressure via apressure sensing pipe against a pressure adjustment control spring. This moves a pilot valve tomodulate a control pressure. The control pressure transmitted via the pilot valve is proportional
to the pilot valve opening, and is directed, via the control pipe to the underside of the mainvalve diaphragm. The diaphragm moves the pushrod and the main valve in proportion to themovement of the pilot valve. Although the downstream pressure and pilot valve position areproportional (as in the direct acting valve), the mechanical advantage given by the ratio of theareas of the main diaphragm to the pilot diaphragm offers accuracy with small proportionaloffset.
Under stable load conditions, the pressure under the pilot diaphragm balances the force set onthe adjustment spring. This settles the pilot valve, allowing a constant pressure under the maindiaphragm. This ensures that the main valve is also settled, giving a stable downstream pressure.
When downstream pressure rises, the pressure under the pilot diaphragm is greater than theforce created by the adjustment spring and the pilot diaphragm moves up. This closes the pilot
valve and interrupts the transmission of steam pressure to the underside of the main diaphragm.The top of the main diaphragm is subjected to downstream pressure at all times and, as there isnow more pressure above the main diaphragm than below, the main diaphragm moves downpushing the steam underneath into the downstream pipework via the control pipe and surpluspressure orifice. The pressure either side of the main diaphragm is balanced, and a small excessforce created by the main valve return spring closes the main valve.
Any variations in load or pressure will immediately be sensed on the pilot diaphragm, which willact to adjust the position of the main valve accordingly, ensuring a constant downstream pressure.
The pilot-operated design offers a number of advantages over the direct acting valve. Only avery small amount of steam has to flow through the pilot valve to pressurise the main diaphragm
chamber and fully open the main valve. Thus only very small changes in control pressure arenecessary to produce large changes in flow. The fall in downstream pressure relative to changesin steam flow is therefore small, typically less than three hundredths of a bar (3 kPa; 0.5 psi) fromfully open to fully closed.
Adjustment spring
Pilot diaphragmPressure sensing pipe
Pilot valve
Main valve return spring
Main valve and pushrodSurplus pressure orifice
Main diaphragmPilot pressure directed to
underside of diaphragmby control pipe
High pressure
Low pressure
Control pressure
Pilot-operated valvesWhere accurate control of pressure or a large flow capacity is required, a pilot-operated pressurereducing valve can be used. Such a valve is shown schematically in Figure 7.3.4. A pilot-operatedpressure reducing valve will usually be smaller than a direct acting valve of the same capacity.
Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications
Although any rise in upstream pressure will apply an increased closing force on the main valve,the same rise in pressure will act on the underside of the main diaphragm and will balance theeffect. The result is a valve which gives close control of downstream pressure regardless of variations on the upstream side.
In some types of pilot-operated valve, a piston replaces the main diaphragm. This can beadvantageous in bigger valves, which would require very large size main diaphragms. However,
problems with the piston sticking in its cylinder are common, particularly in smaller valves.It is important for a strainer and separator to be installed immediately prior to any pilot-operatedcontrol valve, as clean dry steam will prolong its service life.
Selection and installation of pressure reducing valves
The first essential is to select the best type of valve for a given application.
Small loads where accurate control is not vital should be met by using simple direct acting valves. In all other cases, the pilot-operated valve is the best choice, particularly if there areperiods of no demand when the downstream pressure must not be allowed to rise.
Oversizing should be avoided with all types of control valve and this is equally true of reducing valves. A valve plug working close to its seat when passing wet steam can suffer wiredrawing andpremature erosion. In addition, any small movement of the oversized valve plug will produce arelatively large change in the flow through the valve, making it more difficult for the valve tocontrol accurately.
A smaller, correctly sized reducing valve will be less prone to wear and will provide more accuratecontrol. Where it is necessary to make big reductions in pressure or to cope with wide fluctuationsin load, it may be preferable to use two or more valves in series or in parallel.
Although reliability and accuracy depend on correct selection and sizing, pressure reducing valves also depend on correct installation. Figure 7.3.5 illustrates an ideal arrangement for theinstallation of a pilot-operated pressure reducing valve.
Fig. 7.3.5 Typical steam pressure reducing valve station
Many reducing valve problems are caused by the presence of moisture or dirt. A steam separatorand strainer with fine mesh screen, if fitted before the valve, will help to prevent such problems.The strainer is fitted on its side to prevent the body filling with water and to ensure that the fullarea of the screen is effective. Large isolation valves will also benefit from being installed ontheir side for the same reason.
All upstream and downstream pipework and fittings must be adequately sized to ensure that the
only appreciable pressure drop occurs across the reducing valve itself. If the isolating valves arethe same size as the reducing valve connections, they will incur a larger pressure drop than if they are sized to match the correctly sized, larger diameters of the upstream and downstreampipework.
Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications
If the downstream pipework or any connected plant is incapable of withstanding the maximumpossible upstream pressure, then a safety valve or relief valve must be fitted on the downstreamside. This valve should be set at, or below, the maximum allowable working pressure of theequipment, but with a sufficient margin above its normal operating pressure. It must be capableof handling the full volume of steam that could pass through the fully open reducing valve, at themaximum possible upstream pressure.
Pilot operation also allows the reducing valve to be relatively compact compared to other valvesof similar capacity and accuracy, and allows a variety of control options, such as on-off operation,dual pressure control, pressure and temperature control, pressure reducing and surplussing control,and remote manual adjustment. These variations can be seen in Figure 7.3.6.
Direct acting and pilot-operated control valves can be used to control either upstream ordownstream pressures. Pressure maintaining valves (and surplussing valves) sense upstreampressure, while pressure reducing valves sense downstream pressure.
Fig. 7.3.6 Four complementary versions of pilot-operated pressure reducing valve
With temperature control With dual pressure control
With on-off controlBasic pilot-operated pressure reducing valve
A solenoid valve which interruptsthe signal to the main diaphragm
Switchable pilot valvesto change the control pressure
Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications
Summary of pressure reducing valvesA valve that senses and controls the downstream pressure is often referred to as a let-downvalve or pressure reducing valve (PRV). Such valves can be used to maintain constant steampressure onto a control valve, a steam flowmeter, or directly onto a process.
Pressure reducing valves are selected on capacity and type of application.
Table 7.3.1 Typical characteristics for different types of pressure reducing valve
Direct actingPilot-operated
Bellows operated Diaphragm operated
Small capacity Very large capacity Large capacity
Compact Relatively large Compact for capacity
Low cost Robust Extremely accurate
Steady load Steady load Varying loads
Coarse control Coarse control Fine control
Pressure maintaining valves
Some applications require that upstream pressure is sensed and controlled and this type of valve
is often referred to as a Pressure Maintaining Valve or PMV. Pressure maintaining valves arealso known as surplussing valves or spill valves in certain applications.
An example of a PMV application would be where steam generation plant is undersized, andyet steam flow is critical to the process. If steam demand is greater than the boiler output, orsuddenly rises when the boiler burner is off, the boiler pressure will drop; progressively wettersteam will be supplied to the plant and the boiler operation may be jeopardised. If the boilercan operate at its design pressure, optimum steam quality will be maintained.
This can be achieved by fitting PMVs on each non-critical application (perhaps heating plant ordomestic hot water plant), thereby introducing a controlled diversity to the plant. These willthen progressively shut down if upstream pressure falls, giving priority to essential services.
Should all supplies be considered essential, a variety of options are available, each of which hasa different cost implication.
The cheapest solution might be to fit a PMV in the boiler steam outlet, (see PMV 1 in Figure 7.3.7).This will maintain a minimum steam pressure in the boiler, regulate maximum flow from theboiler and, in so doing, retain good quality steam to the plant.
If it is possible to shut off non-essential equipment during times of peak loading, PMVs can beinstalled in distribution lines or branch lines supplying these areas of the plant. When the steamboiler becomes overloaded, the non-essential supplies are gradually shut down by PMV 2 allowing the boiler to maintain steam flow to the essential plant at the proper pressure.
Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications
It should be recognised that a PMV will not always cure the problems caused by insufficient boiler capacity. Sometimes, when there is little plant diversity, only one real alternative is available,which is to increase the generating capacity by adding another boiler.
However, there are occasions when the cheaper alternative of a steam accumulator is possible.This allows excess boiler energy to be stored during periods of low load. When the boiler isoverloaded, the accumulator augments the boiler output by allowing a controlled release of
steam to the plant (see Figure 7.3.8).In Figure 7.3.8, the boiler is designed to generate steam at 10 bar g, which is distributed at both10 bar g and 5 bar g to the rest of the plant.
PRV 1 is a pressure reducing valve, and is sized to pass the boiler capacity minus the highpressure steam load.
Fig. 7.3.8 Typical boiler and accumulator arrangement
For sizing purposes, the capacity of the pressure reducing valve PRV 2 should equal the maximumdischarge rate and time for which the accumulator has been designed to operate, whilst thedifferential pressure for design purposes should be the difference between the minimumoperating accumulator pressure and the LP (Low pressure) distribution pressure. In this example,PRV 2 would probably be set to open at about 4.8 bar g.
PMV is a pressure maintaining valve whose size is determined by the recharging time requiredby the accumulator and the available surplus boiler capacity during recharging. When recharging,the pressure drop across the PMV is likely to be relatively small, so the PMV is likely to be quitelarge, typically the same size as the line in which it is installed. The PMV is usually set to operate
just below the boiler maximum pressure setting .
When the total plant load is within the boiler capacity, PRV 2 is shut and the boiler supplies theLP steam load through PRV 1 which is set to control slightly higher than PRV2. Any excess steamavailable in the boiler will cause the boiler pressure to rise above the PMV set point, and thePMV will open to recharge the accumulator. Recharging will continue until the accumulatorpressure equals the boiler pressure, or until the plant load is such that the boiler pressure againdrops below the PMV set point.
Should the LP steam load continue to increase, causing the LP pressure to drop below PRV 2 set point, PRV 2 will open to provide steam from the accumulator, in turn supplementing the steamflowing through PRV 1.
There is more than one way in which to design an accumulator installation; each will depend
upon the circumstances involved, and will have a cost implication. The subject of accumulatorsis discussed in more detail in Module 3.22 Steam accumulators.
Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications
Pressure surplussing valves
The ability to sense upstream pressure may be used to release surplus pressure from a steamsystem in a controlled and safe manner. The surplussing valve is essentially the same as a PMV,opening when an increase in upstream pressure is sensed. The surplussing valve is sometimesreferred to as a dump valve when releasing steam to atmosphere.
A surplussing valve is often used to control the maximum pressure in a flash recovery system.Should the demand for flash steam be less than the available supply, the flash pressure will riseand the surplussing valve will open to release any excess steam to atmosphere. The surplussing valve will be set to operate at a pressure below the safety valve setting.
Important: Whilst this allows the controlled release of steam to atmosphere, it does not replacethe need for a safety valve, should the plant conditions require it.
In Figure 7.3.9 the PRV replenishes any shortfall of flash steam generated by the high pressure(HP) condensate, and the surplussing valve releases any excess flash steam to either a condenseror to atmosphere.
The safety valve is sized on the full capacity of the PRV plus the capacity of the steam traps and
any other source feeding into the flash vessel.
Fig. 7.3.9 Typical surplussing valve on a flash vessel application