SpiraxSarco B7 Self Acting Temperature Controls

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The Steam and Condensate Loop 7.3.1

Control Hardware: Self-acting ActuationBlock 7 Module 7.3Self-acting Pressure Controls and Applications

Module 7.3 Self-acting Pressure Controls

and Applications

The Steam and Condensate Loop7.3.2


Self-acting Pressure Controls andApplications

Why reduce steam pressure?The main reason for reducing steam pressure is rather fundamental. Every item of steam usingequipment 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 atlower 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 canbe 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 ofsurface temperature is difficult to achieve using temperature sensors.

Plant operating at low steam pressure:

� Can tend to reduce the amount of steam produced by the boiler due to the higher enthalpyof evaporation in lower pressure steam.

� 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:

� Direct acting valves.

� 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 clockwisedirection 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 adjustmentspring 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 isclosed. 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’.



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 itselfand 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 affectthe 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

Adjustment handwheel

Adjustment spring(control spring)

Bellows

Return spring

Flow

Valve and seat



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 thatthe 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 steamflow 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 actuator’sflexible 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

Flow

Separator

Stop valve

Strainer

Pressurereducing

valve

Safety valve

Stop valve

WS4water seal pot

1 m minimum

Steam

Condensate



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 proportionalto 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 pilotvalve 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 diaphragmchamber 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.



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 ofvariations 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 actingvalves. 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.

Over sizing should be avoided with all types of control valve and this is equally true of reducingvalves. 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 reducingvalves 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 theonly 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 ifthey are sized to match the correctly sized, larger diameters of the upstream and downstreampipework.

High pressuresteam flow

Condensate

Separator

Strainer

Isolatingvalve

Pressure reducingvalve

Safetyvalve

Isolatingvalve

Lowpressure



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



Summary of pressure reducing valvesA valve that senses and controls the downstream pressure is often referred to as a ‘let-down’valve 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 valveDirect acting Pilot-operated

Bellows operated Diaphragm operatedSmall capacity Very large capacity Large capacity

Compact Relatively large Compact for capacityLow cost Robust Extremely accurate

Steady load Steady load Varying loadsCoarse control Coarse control Fine control

Pressure maintaining valves

Some applications require that upstream pressure is sensed and controlled and this type of valveis 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 allowingthe boiler to maintain steam flow to the ‘essential’ plant at the proper pressure.

Fig. 7.3.7 Alternative positions for PMVs

Boiler

Separator PMV 1

PMV 2

Non essential line

Essential line

Drain pocketand trap set



It should be recognised that a PMV will not always cure the problems caused by insufficientboiler 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 ofsteam 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 operatejust 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 setpoint, 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 dependupon the circumstances involved, and will have a cost implication. The subject of accumulatorsis discussed in more detail in Module 3.22 ‘Steam accumulators’.

Boiler

PMV PRV 2

Lowpressure(LP) steam5 bar g

High pressure(HP) steam10 bar g

PRV 1

Accumulator



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 surplussingvalve 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 andany other source feeding into the flash vessel.

Fig. 7.3.9 Typical surplussing valve on a flash vessel application

Steammake-up PRV Surplussing valve

Excess steamto atmosphere

LP steamto plant

LP condensate

HP condensate

Flashvessel

Safetyvalve



Questions

1. In a self-acting pressure control system, which of the following is proportional to thecontrol valve opening?

a| The deviation of the downstream pressure from the set point �

b| The difference between upstream and downstream pressure �

c| The difference between upstream pressure and the set point �

d| The spring force �

2. What is ‘proportional offset’?

a| The rise in downstream pressure as flow increases through the control valve �

b| The fall in downstream pressure as flow decreases through the control valve �

c| The difference between the set point and actual downstream pressure �

d| The rise in upstream pressure when the control valve shuts �

3. Name an advantage that a pilot-operated pressure reducing valve has over a directacting pressure reducing valve?

a| It is usually smaller for the same capacity �

b| It has a much lower proportional offset �

c| It is more accurate over large changes in load �

d| All of the above �

4. What is the basic difference between a PRV and a PMV?

a| A PRV reduces pressure and a PMV increases pressure �

b| As downstream pressure drops, a PRV will close and a PMV will open �

c| As the sensed pressure drops, a PRV will open and a PMV will close �

d| As upstream pressure drops, a PRV will close and a PMV will open �

5. What can a PMV be used for?

a| To reduce non-essential loads, maintaining steam distribution pressure �

b| To maintain boiler pressure under overload conditions �

c| To exhaust surplus steam from a flash steam system �

d| All of the above �

6. Which of the following can a PMV not be used as?

a| A safety valve �

b| A pressure maintaining valve �

c| A pressure surplussing valve �

d| A pressure dump valve �

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

SpiraxSarco B7 Self Acting Temperature Controls

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