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Surge ontrol inPumping StationsVal-Matic alve and Manufacturing Corporation

This primer presents basic surge control principles and thefunctions of various valves associated with pumping stations

Water pipelines and dis

tribution systems aresubjected to surges

almost daily, which over timecan cause damage to equipmentand the pipeline itsel£ urges arecaused by sudden changes in fluidvelocity and can be as minor asa few PSI to five times the staticpressure. he causes and effectsof these surges in pumping systems will be discussed, alongwith equipment that is designedto pr,event and dissipate surges.

Reference will be made to typicalinstallations and examples so thatan understanding of he applicableconstraints can be gained.

D ISTR IBUTION

SYSTEM \

Figure 1 illustrates a typical water pumping/distributionsystem where two parallel pumps -draw water from a wet well, thenpump the water through checkand butterfly valves into a pumpheader and distribution system.A surge tank and relief valve areshown as possible equipment onthe pump header to relieve andprevent surges. Each of these willbe discussed in greater detail.

Causes nd EffectsSurges are caused by suddenchanges in flow velocity thatresult from common causes suchas rapid valve closure, pump startsand StopS and improper filling

38 MARCH 2007

RESERVOIR

CHECK VALVE

WET WELL

BUTTERFLY VALVE

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SURGE TANK

PUMP HEADER

RELIEF VALVE

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practices. Pipelines often see their first surge during filling,when the air being expelled from a pipeline rapidly escapesthrough a manual vent or a throttled valve followed by thewater.

Being many times denser than air, water follows the air tothe outlet at a high velocity, but its velocity is restricted by theoudet, thereby causing a surge. t is imperative that the fillingflow rate be carefully controlled and the air vented throughproperly sized automatic air valves. Similarly, line valves mustbe closed and opened slowly to prevent rapid changes in flowrate.

The operation of pumps and sudden stoppage of pumpsdue to power failures probably have the most frequent impacton the system and the greatest potential to cause significantsurges. If the pumping system is not controlled or protected,contamination and damage to equipment and the pipelineitself can be serious.

The effects of surges can be as minor as loosening of pipejoints to as severe as damage to pumps, valves, and concretestructures. Damaged pipe joints and vacuum conditions cancause contamination to the system from groundwater andbackflow situations. Uncontrolled surges can be catastrophic aswell. Line breaks can cause flooding and line shi fting can causedamage to supports and even concrete piers and vaults. Lossescan be in the millions of dollars, so it is essential t hat surges beunderstood and controlled with the proper equipment.

Surge ackgroundSome of the basic equations of surge theory will be presented,so an understanding of surge control equ ipme nt can be gained.First, the surge pressure (H) resulting from an instantaneousflow stoppage is directly proportional to the change in velocityand can be calculated as follows:

H = a v g

where:

H = surge pressure, ft water columna = speed of pressure wave, ft sv = change in flow velocity, ftlsg = gravity, 32.2 ftls 2

The speed of the pressure wave a) varies with the fluid,pipe size, and pipe material. For a medium sized steel line, ithas a value of about 3500-ft/s. For PVC pipes, the speed willbe far less. For a 12-in steel line with water Bowing at 6-ft/sthe magnitude of a surge from an instantaneous Bow stoppageIS:

H = (3500 ft/s)(6 ft/s) I 32 ftls2)H = 656 ft water column

This surge pressure of 656-ft (285-psi) is in addition tothe static line pressure; therefore, the resultant pressure willlikely exceed the pressure rating of the system. Further, thishigh pressure will be maintained for several seconds as thewave reflects from one end of the piping system to the otherend, causing over pressurization of pipe seals and fittings. Thenafter a reflection, the pressure wave may cause a negative pressure and vacuum pockets for several seconds, allowing contaminated ground water to be drawn into the system throughseals or connections.

Even higher velocities than the pumping velocity areattainable in long piping systems. If the pumps are suddenlystopped due to a power failure, the kinetic energy of the watercombined with the low inertia of the pump may cause a separation in the water column at the pump or at a highpoint inthe pipeline. When the columns of water return via the statichead of the line, the reverse velocity can exceed the normal

velocity. The resultant surge pressure can be even higher thanthe 656-ft calculated above.

Transient analysis computer programs are normallyemployed to predict column separation and the actual returnvelocities and surges. Transient programs can also model metllods employed to control column separation, such as the use ofa surge tank, vacuum breaker, or air valve. These solutions willbe discussed in greater detail.

Thus far, the changes in velocity have been described assudden. ow sudden must changes in velocity be to cause

surges? If the velocity change is made within the time period,the pressure wave will travel the length of the' pipeline andreturn, the change in velocity can be considered instantaneous,

and the equation for surge pressure S) given earlier applies.This time period, often called the critical period can be calculated by the equation:

t = 2 L a

where:

t = critical period, secL = length of the pipe, fta = speed of the pressure wave, ftls

For the earlier example of he 12- in line, the critical periodwould be as follows for a 4-mi long steel pipeline:

t = 2 (21,120 ft) I (3500 ft/sect = 12 sec

To cause surges, a pump does not need to stop quicldynor does the valve need to close instantaneously (or even suddenly). A normal flow stoppage of 5 or 10 seconds may causethe maximum surge in long pumping systems. t follows that

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surge control strategies sho uld beemployed on all longpipelines.

umpsReferring again toFigure 1 a key tocontrolling surges inpumping systems isto control the rate ofincrease and decreaseof the Row velocity into the system.Pumps should besized for the expectedRow requirements.Multiple pumps canbe used to matchvaryIng demandsfor water. Oversizedpumps can createhavoc In certainpumpi ng systems.

Special pump

WELL PUMP

motor control systems are available to slowly rampup and ramp down the pumps by controlling theelectrical drive of the pump. These systems control supply and can prevent surges during normalpum p operation. However after a power f a i l u r ~ _the motor controls become inoperative and the -

pump will trip instantly and cause a sudden stoppage afRow.

Some pump station designs employ multiplepumps so that when one of the pumps is startedor stopped the stopped pump has a minor impacton the overall pipeline velocity. However these stations are likewise faced with the-severe impact ofa power failure. Almost all pumping systems needadditional surge equipment to prevent surges aftera power failure.

Vertical umps and Well Serviceir Valves

Vertical pumps as shown in Figure 2 lift waterfrom a tank or wet well into a pipeline. When thepump is off the suction water level is below thepump discharge pipe. The pump column refillswith air after each pump stoppage.

Air valves play an important role in automatically venting the pump column air and controlling surges in pump columns. If the vertical turbine pump is started without an air valve the airin the pump column would be pressurized and

40MARCH 2007

\ WELL SERVICE AIR VALVE

DUAL PORTTHROTTLINGDEVICE - - - - - I

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DISCHARGE PIPE

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OPTIONALDIFFUSER

SCREEN

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forced through the check valve intothe pipeline causing air related prob-lems. Air valves for pump dischargeservice called well service ir valves,are similar to air/vacuum valves butare equipped with either a throt

tling device or anti-slam device andare designed to exhaust air on pumpstart-up and admit air upon pumpshutdown

s shown in Figure 3 the wellservice air valve is a normally-openfloat-operated valve which relievesthe air in the pump column rapidly.When water enters the valve thefloat automatically rises and closes toprevent discharge of the water.

Throttling devices are providedon the outlet of 3-in and smallervalves to control the rate of airrelease especially wi th slow openingpump control valves. The throttlingdevice is adjusted with the externalscrew to slow the rise of the water intlle pump column. However afterpump shutdown a second port onthe top of the throttling device provides full flow into the pump columnto relieve the vacuum. The du l portth1 ottling device is importan t becauseit proyides full vacuum flow and pre

vents contaminated water from beingdrawn i ~ t othe pipeline which canhappen if the device has a commonexhaust and vacuum connection.

When a power operated pumpcontrol valve is used with a verticalpump an ir 1 elease valve equipped

ORIFICE

with a v cuum breaker can be used as shown in Figure 4. nthis case the pump is started and the opening of the controlvalve delayed a few seconds so that the air release valve canexpel the air slowly through its small orifice.

Dur ing the process the pump column will become pressurized to the pump shutof f head and force the air out at high

pressure. The momentarily trapped air will act as a cushion tocontrol the rise of the water in the pump column. The valveorifice is sized to control the rise of the water to a safe velocitytypically 2-ft/s.

Check ValvesAnother key element in pumping system design is the properselection and operation of the pump discharge check valve.Every pump station designer has been faced with check valveslam which is caused by the sudden stoppage of reverse flow

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42MARCH 2007

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through a closing check valve.To prevent slam, the check valve must either close very

quickly or very slowly. Anything in the middle is no-man'sland and a cause for concern. But just as important, the valveshould protect the pumping system ~ dpiping from suddenchanges in velocity if it is within its functional capabilities. Thecheck valve should also be reliable and offer low headloss.

Two categories of check valves will be discussed in detail.The first, fast-closing check valves represent the general category of check valves that operate automatically in less than asecond and without the use of external power or signals fromthe pumping system. The other category is pump ontrol valveswhich operate very slowly (i.e. 60 to 300 seconds) to carefullycontrol the changes in pipeline fluid velocity.

Fast Closing Check ValvesFast-closing check valves are simple, automatic, and cost effective, but are often plagued with the problem of check valveslam and a resultant system pressure surge. f the decelerationof the forward flow can be estimated, such as with a transientanalysis of the pumping system, the slamming potential ofvarious check valves can be predicted. Then, several non-slamvalve options will present themselves, and the performancefeatures and costs can be used to select the best check valve forthe application.

The most ubiquitous type of check valve is the traditional

swing check valve. Swing check valves are defined in AWWAC508 for waterworks service and are designed to rapidly closeto prevent backspinning of the pump during flow reversal.

Traditional swing check valves have 90-deg seats with longstrokes and are subject to slamming. These valves are thereforeoutfitted with a wide array of accessories, which are beyondthe scope of the AWWA C508 Standard. Probably the mostcommon accessory is a lever and weight. While it is normallyassumed that the weight makes the valve close faster, it actually reduces slamming by limiting the stroke of the disc, but

in return, causes a significant increase in headloss. The valve

closure is also slowed by the inertia of the weight itself and thefriction of the stem packing.

In more severe applications, an ail- cushion is sometimesused to slow down the impact of the valve closure. Everyonehas seen how effective an air cushion works on a slammingstorm door. But the conditions in a pipeline are significantlydifferent.

When a door slams, its momentum is smoothly absorbedby the air cylinder because as the door slows, the forces fromthe closing spring and outside wind become less and less.Conversely, when a check valve in a pipeline closes, the reverseflow is quickening at a tremendous rate, so every fraction of asecond that the valve closure is delayed, the forces on the discwill increase by an order of magnitude.

While it may be true that an air cushion prevents theweight from slamming the disc into the seat of a valve in aproduct display booth, in actual practice, the air CLlsh{onmerely holds the disc open long enough for the reverse flowto intensify and slam the disc even harder into the seat. Sinceair cushions are based on the use of air (which is compressible), they provide no positive restraint of the closing disc andcannot counteract the enormous forces being exerted by thereverse flow. In sum, the best setting of an air cushion is typically where the discharge needle valve is fully open and the airis expelled at the highest rate.

A far more effective accessory for controlling swing checkvalve motion is an oil cushion also referred to as an oil dashpot.Because oil is incompressible, the oil cushion will withstandthe high forces exerted on the disc by the reverse flow andproperly control the last 10 percent of valve closure. The pumpmust be capable of some significant backflow, though, becausethe oil dashpot will allow the check valve to pass a portion ofthe flow back through the pump.

Since the reverse flow forces on the valve disc are extremelyhigh, the oil pressure often exceeds 2000-psig, causing valves

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Pump ontrol ValvesEven though a fast-closing check valve may prevenr slam, itmay not fully protect pumping systems with long critical periods from velocity changes during pump startup and shutdown.For pumping systems where the critical period is long, a pumpconrrol valve is often used.

A pump control valve is wired to the pump circuit andprovides adjustable opening and closing times in excess of the system critical time period. Pump control valves arehydraulically operated so the motion ofthe closure member of the valve (i.e. abutterfly valve disc is unaffected by theflow or pressure in the line. Also, mostpumps in service today have low rotating inertia and come to a stop in lessthan 5 seconds.

The pump control valve can closerapidly during power outages or pumptrips to protect the pump. However,when rapid closure is required, additional surge equipment will be needed,as explained in the following section.First, though, the selection criteria pertaining to pump control valves will bepresented.

The list of possible pump conrrolvalves is long because many valves canbe equipped with the automatic controls necessary for pumping systems.Valves typically considered are butterfly,

plug, ball,' and globe-pattern controlvalves. Probably the most common criterion used to select a valve is initial cost,but for pumping systems, the selectionprocess should be carefully undertakenwith consideration given to:• valve and installation costs• pumping costs• seat integrity• reliability• flow characteristics

The installed costs for the varioustypes of pump control valves can varywidely. For example, a 12-in butterflyor plug valve with a hydraulic poweredactuator and conrrols can cost $5,000,while a ball valve or globe-pattern control valve can be 2 times to 4 times thatamounr. In addition to the purchaseCOSt the cost for making the flange connections, control wiring to the pumpmotor conrrols, and providing concrete

pedestals for the heavier ball and globe-pattern control valvesshould also be added.

Of course, the installed OSt of the v lve is important andrepresents an imporranr investment. But equally imporranr isthe pumping OSt associated with the head loss through thevalve. The electrical current draw of the pump is a function

of the system head loss and flow rare. The additional elec-

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characteristic for long pipelines is equal percentage as providedby butterfly and ball valves.

total pipe line headloss in controlling the flow rate. Initial fieldsettings will normally be three 0 five times greater than thecritical period 0 minimize the surge.ll of the selection criteria discussed, inc luding cost, head

loss, reliability, and flow characteristics, should be consideredtogether when selecting a valve. No single valve type will excelin all categories. The benefits of the expected performancemust be weighed against the costS and impact on the system

One additional function of the pump control valveshould be considered: prevent the pump from backspinningafter power failure or overload trip. Since pumps today are nolonger equipped with flywheels, as with old diesel units, rhey

surge potential.

Pump Control ValveOperationUtilizing a butterfly valve, let us consider the operation of a typical pumpcontrol valve. A butterfly valve is operated by rotating its shaft 90-deg andis normally equipped with a hydrauliccylinder actuator. The cylinder can bepowered with pressurized water fromthe line or from an independent oilpower system.

We learned earlier that negativesurge conditions can occur for severalseconds, so a backup water or oil systemis appropriate. Figure 11 illustrates atypical installation. Hydraulic controlselectrically wired into the pump circuitare mounted on the valve. Four-way andtwo-way solenoid valves (SV) direct theoperating medium to the cylinder POrtSto cycle the valve. The speed of opening and closing is controlled by independently adjustable flow cont rol valves

Fey). Flow control valves are specialneedle valves with a built-in reversecheck valve to allow free flow into thecylinder but controlled flow out of thecylinder.

When the pump is started andpressure builds, a pressure switch (PS)located on the pump header signals thebutterfly valve to open. During shut

. down, the valve is signaled to close whilethe pump continues to run. When the

valve nears the closed position, a limitswitch LS) located on the valve willstop the pump.

The safe operating time for thepump control valve is usually muchgreater than the critical period. A longoperating time is needed on pipelineapplications because the valve's effectiveclosing time is a fraction of its total closing time due to the fact that the valveheadloss musr be combined wirh the

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have a low rotating inertia and come to restin a just a few seconds. Therefore, after apower outage or pump trip, the pump control valve must close more rapidly to prevent backspinning.

The valve hydraulic controls are

equipped with a bypass line equipped witha 2-way solenoid valve (SV) to send thecontrolled cylinder flow around the normalflow control valve and through a large flowcontrol valve (FeV) , thereby closing thepump control valve automatical ly in 5 t 10seconds after power failure. T his is essentialto prevent excess pump backspin and toprevent depletion of the hydro-pneumaticsurge tank water back through the pump ifone is utilized.

- - - - - - T ~ - - - - - - - - - - - - - , ....... As an alternative to the special bypasscircuit, a fast-closing check valve is sometimes installed upstream of the pump control valve to back-up the control valve. Thefast-closing check valve not only preventsreverse flow through the pump, but alsoprovides redunda nt protection of the pumpshould the pump control valve fail to close

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due to loss of pressure orequipment malfunction.

The rapid closure ofeither the pump controlvalve or a fast-closing check

valve in a long piping systemposes a dilemma. It was previously explained that thecontrol valve should close inthree to five times the criticalperiod. On the other hand,the valve must close in fiveseconds t protect the pumpafter a power failure. Hence,on these systems, excessivesurges will be caused onpower outages so additionalsurge protection s usuallyneeded.

Surge Reliefquipment

PIPELINECONNECTION

MAIN VALVE

STRAINERNEEDLE VALVE

REDUCINGCONTROL PILOT

RELIEFCONTROL PILOT

ISOLATIONVALVES

Since it is impractical to usepipe materials, which canhandle high surge pressures

INLET

or slow the operating flowvelocity to a crawl, surgerelief equipment is neededt anticipate and dissipatesurges from sudden veloc

ity chaIlges after power outages. Surge relief equipment: rillalso provide protection against malfunctioning valves,Improper filling, or other system problems.

Standpipes and Surge TanksMany. types of surge relief equipment are used to safeguardpumpmg systems. For low-pressure systems, a standpipe open~ oatmosphere will relieve pressure almost instantly by exhaustIng w a ~ e rFor s y s t e ~ swith higher pressure, the height of astandpIpe would be Impractical, so a bladder-type accumulator or surge tank with pressurized air over water can be usedto absorb shocks and prevent column separations see Figure12).

For typical pumping systems, however, these tanks tendto be large and expensive and must be supplied with a compressed air system. When used, an additional fast-closino- checkvalve is also needed to prevent surge tank water from e;capingback t ~ r o u g hthe pump. This is a common example of whenyou WIll see both a pump control valve and a fast-closino- checkvalve installed. b

Further, the surge tank creates extremely high deceleration rates i.e. 25 ft/s2 , so fast-closing check valves or checkvalves equipped with bottom-mounted oil dashpots should be

FLOW

•used to prevent slamming.

Surge Relief Valves

OUTLET

Surge relief valves are often a more practical means of relievingpressure. In these valves, a pressure surge lifts a disc, allowingthe valve to rapidly relieve water to the atmosphere or back tothe wet well.

Surge relief valves have the limitation that they may notopen rapidly enough t dissipate surges in cases where columnseparation can occur. For these cases where the transient computer model predicts steep or rapid pressure surges, surge relief

valves equipped with anticipator controls should be considered. A globe-pattern control valve equipped with suro-e reliefand anticipator controls is shown in Figure 13. A surg anticipator valve will open rapidly upon the sensing of a high or lowpressure event.

When a pump suddenly stops, the pressure in the headerwill drop below the static pressure and trigger the surge anticipator valve to open. The valve will then be partially or fullyopen when the return pressure surge occurs. Anticipator valvestypically open in less than five seconds, pass high low rates,and reclose slowly at the pump control valve closure rate 60

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ro 300 seconds). he sizing of surgerelief valves s critical and shouldbe overseen by transient analysisexperts.

Anti Slam Combination

Air ValvesAir valves help reduce surges in pipelines by preventing the formationof air pockets in pipelines duringnormal operation. Air pockets cantravel along a pipeline and causesudden changes in velocity andadversely affect equipment operation such as flow measuring devices.Air valves are also designed ro openand allow air to be admitted to thepipeline to prevent the formationof

a vacuum pocket associated withcolumn separation. Transient analysis computer programs are equippedto analyze the surge reduction fromusing various size air valves.

When column separation isexpected at the air valve location, theair valve should be equipped withan anti-slam device which controlsthe flow of water into the air valveto prevent damage to the valve floatsee Figure 14).

he anti-slam device allows

air to pass through it unrestrictedduring either the air discharge or

AIRNACUUMVALVE

ANTI-SLAMDEVICE - - - - -

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air re-entry cycle. When water(because of its greater density)enters the device, the disc willrapidly close and provide slowclosure of the air valve float.

he disc contains ports, whichallow water to flow through theanti-slam device when closed tofill the air valve at about 5 percent of the full fill rate, preventing the air valve from slammingclosed.

ASSEMBLY

FLOAT

Vacuum Breaker

ValvesAnother type of air valve usedat critical points in the pipelinewhere column separation mayoccur is a vacuum breaker (VB)see Figure 15).

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The VB has components very similar t the anti-slamdevice, except the VB disc is held closed by a spring whilethe anti-slam disc is held open. Hence, the vacuum breakercannot expel air; it only admits air t prevent the formation ofa vacuum pocket. This keeps the pipeline at a positive pressureand reduces the surge associated with a column separation. In

essence, a large cushion of air is admitted and trapped in thepipeline after a pump trip. The air is then slowly released over afew minutes through the adjoining air release valve, which has

References:

a small (i.e. lA-in) orifice. Again, transient analysis programsare designed t model this type of air valve solution as well.

P S

1. American Water Works Association, Steel water Pipe: a Guide for Design and Installation M I l Water Hammer and PressureSurge , 4th ed. 2004, pp. 51-56.

2. Bosserman, Bayard E. Control of Hydraulic Transients , Pumping Station Design, Butterworth-Heinemann, 2nd ed., 1998,Sanks, RobertL., ed., pp. 153-171.

3. Hutchinson, J.W ISA Handbooko Control Valves, 2nd ed., Instrument Society of America, 1976, pp. 165-179.

4. Kroon, Joseph R., et. al., Water Hammer: Causes and Effects , AWWAJoumal November, 1984, pp. 39-45.

5. Val-Matic Valve & Mfg. Corp, 1993 Check Valve Selection Criteria , waterworld Review, NovlDec 1993, pp. 32-35.6. Rahmeyer, William, 1998. Reverse Flow Testing of Eight-Inch Val-Matic Check Valves , Utah State University Lab Report No.

USU-609, Val-Matic Valve Test Report No. 117, Elmhurst, IL, [Confidential].

7. Tullis, J. Paul, Hydraulics o Pipelines, 1984 Draft Copy, Utah State University, pp. 249-322.

8. Val-Matic Valve & Mfg. Corp., Dynamic Characteristics of Check Valves , 2003.

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