Check Valve Back to Basics Article

8/22/2019 Check Valve Back to Basics Article

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TOO OFTENTHIS VALVEIS UNJUSTLYBLAMED WHENPROBLEMSOCCUR,BUTTHE REAL

CULPRITIS THEAPPLICATION.

BY MIKE JOHNSON

28 | Valve M A G A Z I N E

C heck valves are probably the mostmisunderstood valves everinvented. If you mention check valves tomost plant personnel, the typicalresponse is they dont work. In fact,

those personnel may well have takenout the internals or repiped the systemto avoid check valves. In other words,these valves are the least popular valvein use today.

This article will explore the basicsof check valves, how they work, whattypes there are, how to select andinstall them, how to solve their prob-lems, and, finally, why they are notalways the cause of the problem.

Simply put, a check valve allows

flow in one direction and automati-cally prevents back flow (reverseflow) when fluid in the line reversesdirection. They are one of the fewself-automated valves that do notrequire assistance to open and close.While some can be fitted with exter-nally weighted and dampened devicesfor special circumstances, the major-ity do not have any outside assis-tance, such as that found with on/offor other valves. Unlike other valves,

they continue to work even if the plant facility loses air, electricity, or the human

being that might manually cycle them.

As with other types of valves, check valves are found in a full range of sizes,

materials, and end connections. The line sizes range from 1/8 inch or smaller to 50

inches and larger. They are made of bronze, cast iron, plastics, carbon steel, and

various grades of stainless steel, alloys such as Hastelloy, Inconel, Monel, and tita-nium. End connections include threaded, socket weld, butt weld, flanged, Victaulic,

wafer, and insert type.

Check valves are found everywhere, including the home. If you have a sump

pump in the basement, a check valve is probably in the discharge line of the pump.

Outside the home they are found in industries such as automotive, desalination, avi-

ation, commercial construction, water and waste, chemical, colleges and universi-

ties, food and beverage, geothermal, hospitals, mining, oil and gas, pharmaceutical,

power, pulp and paper, refining, sanitary, marine, steel, tire, and ultrapure water.

Like other valves, check valves are used with a variety of media: liquids, air,

other gases, steam, condensate, and in some cases, liquids with fines or slurries.

Applications include pump and compressor discharge, header lines, vacuum break-

ers, steam lines, condensate lines, chemical feed pumps, cooling towers, loadingracks, nitrogen purge lines, boilers, HVAC systems, utilities, pressure pumps, sump

pumps, wash-down stations, and injection lines.

How They OperateCheck valves are flow sensitive and rely on the line fluid to open and close. Theinternal disc allows flow to pass forward, which opens the valve. The disc beginsclosing the valve as forward flow decreases or is reversed, depending on the design.The function or purpose of a check valve is to prevent reverse flow. Construction isnormally simple with only a few components such as the body, seat, disc, and cover.Depending on design, there may be other items such as a stem, hinge pin, disc arm,spring, ball, elastomers, and bearings.

Back toBASICS:Back toBASICS:

THE MISUNDERSTOOD

CHECK VALVE

AS SEEN IN THE SUMMER 2006 ISSUE OF...

2006 Valve Manufacturers Association. Reprinted with permission.


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B A C K T O B A S I C S : C H E C K V A L V E

Internal sealing of the check valvedisc and seat relies on reverse linepressure as opposed to the mechanicalforce used for on/off valves. Because ofthis, allowable seat leakage rates aregreater for check valves than with on/off

valves. MSS SP-61 Pressure Testing ofSteel Valves, published by the Manu-facturers Standardization Society, isone standard used by manufacturers toperform seat and shell closure tests forcheck valves (as well as other valves).Factors affecting check valve seat leak-age include reverse pressure, medium,and what the seat material is made of,such as metal or elastomer. Metalsealing surfaces generally will allowsome leakage while elastomers such asBuna-N and Viton provide bubble-tightshutoff (zero leakage). Because of this,elastomers should be considered forair/gas media and low-pressure sealing.Important considerations when usingelastomers for such valves are servicetemperature and compatibility of theelastomer with the medium.

So what is the ideal check valve?Regardless of type or style of valve, thelongest trouble-free service will comefrom valves sized for the application,not the line size, whereby the disc is sta-ble against the internal stop in the openposition or fully closed. When these con-ditions are met, no fluttering of the disc

will occur, resulting in premature fail-ure. Unfortunately, most check valvesare selected in the same way on/offvalves are selectedbased on line sizeand the desire for the largest Cv avail-able. This ignores the fact that, unlikeon/off valves, the flow conditions deter-mine the internal performance of thecheck valve since its disc is always inthe flow stream.

As mentioned earlier, unlike on/offvalves, check valve internals are flow

sensitive. If there is not enough flow,

disc movement occurs inside the valve,since the disc is always in the flowpath. This results in wear, potential forfailure, and a higher pressure dropthan calculated.

Whenever a metal part rubs againstanother metal part, wear is a result,which leads to eventual failure of thecomponent. A component failure canresult in the valve not performing itsfunction, which in the case of a checkvalve, is to prevent reverse flow. In

extreme cases, failure could result inthe component or components escapinginto the line, causing failure or non-performance of other valves or equip-ment in the line.

Typically, pressure drop is calculatedbased on the check valve being 100%open as with on/off valves. However, if

the flow is not sufficient to achieve fullopening, but instead, the check valve isonly partially open, the pressure dropwill be greater than calculated since theflow passage is restricted by the discbeing in the flow path. In this situation,a large rated Cv actually becomes detri-mental to the check valve (unlike withon/off valves), resulting in fluttering ofthe disc and eventual failure. Such isnot the case with some other valves.With a gate valve, for example, if the

valve is fully open, the wedge is out of

the flow path and the flow through thevalve does not affect the performance ofthe wedge whether that flow is low,medium, or high.

Various types of check valves areavailable. Some of the more populartypes are included below. All these canbe used for clean media. As with othertypes of valves, specialty check valvescan be found for special applications.While no one type of valve is good forall applications, each has its advan-tages. Taking time to contact the manu-facturer to assist in selection can helpyou find the best fit, especially if you areincurring problems with whatever typeof check valve is presently installed.

Ball ChecksBall check valves use a ball inside thebody to control the movement of flow.The ball is free to rotate, resulting ineven wear and a wiping action betweenthe ball and seat. This feature makes ballchecks useful for viscous media. Ballchecks are typically found in smallersizes of 2 inches and less. Some designsinclude a spring to assist in closing andfor use in 90-degree styles installed in

vertical lines. Depending on the bodydesign, pressure drops with ball types canbe higher than with other types of checkvalves. Ball checks are available in vari-ous end connections including threadedand socket weld. Some body designs per-mit in-line repair/inspection.

Dual PlateDual plate, or double-door, check valvesconsist of two plates hinged in the cen-ter and spring loaded to assist in clos-

ing. They can be used for most media

The ball check valve is similar to the lift check, but uses a ball instead of a guided disc.

Lift Ball

Regardless of type or style of valve, thelongest trouble-free service will come fromvalves sized for the application, not the line

size, whereby the disc is stable against theinternal stop in the open position or fullyclosed.



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and possess good flow capacity. Sizesrange from 2 inches to 60 inches andlarger. Because of their design, dualplate check valves are lightweight andhave compact face-to-face dimensions.As with butterfly valves, the compact

design of dual plate types can requireless support and shrink the needed sizefor a mechanical room. Dual platevalves are available in various bodystyles that include wafer, lug, andflanged; some meet the popular API594 face-to-face dimensions. Dual platevalves require removal from the line toperform inspection or repair.

Spring Assisted In-Line/Nozzle CheckIn-line check valves are also known asnozzle checks or silent check valves.They are designed to prevent water ham-mer as well as reverse flow. The designuses a spring-assisted disc in line withthe flow and has a short travel distance,resulting in a fast closing valve. As for-ward velocity begins to slow, the springassist starts to close the disc. By the timethe forward velocity reaches zero, thevalve disc is closed against the seatbefore reverse flow can occur, preventingpressure surges in the line and thus pre-

venting water hammer. Most in-linedesigns can be installed in any position,including flow down if the proper springis installed. As with other check valves,spring-assisted valves can be used formost media. Line sizes range from 1/4inch and smaller to 30 inches and larger.They are available in various stylesincluding globe, wafer, insert, andthreaded/socket weld. Some designs

meet API 594 and ANSI B16.10 face-to-face dimensions. End connectionsinclude threaded, socket weld, flanged,wafer, and butt weld. In-line check valveshave to be removed from the line to per-form internal inspections or repair.

Piston CheckPiston, or lift, check valves are avail-able as inclined (Y pattern) or conven-tional (90 degree) body designs. Ineither case, a body-guided disc moveswithin the body bore. The body guideensures alignment of the seat and discwhen the valve closes. Piston checkvalves are available from 1/4 inch to24 inches and larger. Smaller valves,1/4 inch to 2 inches, are normally pro-vided with a spring to assist in closingand to ensure the disc slides back tothe seat when installed in vertical

lines. The body design selected willdetermine the pressure drop; inclineddesigns will provide the best flow per-formance. Piston check valves areavailable with threaded or socket weldends for 2 inches and smaller. Larger

valves are available with flanged orbutt weld ends. Special end connec-tions are available, but you should con-sult the valve manufacturer. Pistoncheck valves can normally be inspectedand repaired in line. Exercise cautionwhen these valves are used for dirtymedia because that media could causethe disc to stick inside the body bore.

Swing CheckSwing checks are a simple design

using a disc attached to an arm that is

hinged at the top of the valve (at the12 oclock position). Reverse flow andgravity assist the valve in closing.Swing checks can be used for mostmedia and generally provide good flowcapacity. They range in size from 1/2inch and smaller to 50 inches andlarger, and are available withthreaded, socket weld, flanged, or buttweld end connections. Swing checksare typically easy to inspect and main-tain. In most cases, repairs can be per-formed with the valve in the line.Because of their design, swing checksare not fast-closing valves due to thetravel distance from full open to close.Most swing check valves meet ANSI

B16.10 face-to-face dimensions, andwill permit pigging of the line.

SelectionAmong the many factors to considerwhen selecting a check valve are mate-rial compatibility with the medium;valve rating (ANSI); line size; applica-tion dataflow, design/operating condi-tions; installationhorizontal, verticalflow up or down; end connection; enve-lope dimensions, especially if replacing

an existing valve to avoid pipe modifi-cations; leakage requirements; andspecial requirements such as oxygencleaning, NACE, CE Mark, etc.

Problem SolvingWhen replacing a check valve it helpsto ask the following simple questions:

Why am I replacing this valve? What was the problem?Sometimes we get so busy or

absorbed in other things, we forget the

cause can help with the solution. Com-



A lift check valve has a guided disc that is

raised from the seat by upward flow pressure.

There are many different check valve designs,

with the oldest and most common being the

swing check.

Shown here is an example of an in-line check

valve.This valve is also known as a nozzle

check or silent check valve.



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mon check valve problems include noise(water hammer), vibration, reverseflow, sticking, leakage, missing inter-nals, component wear, or damage.However, it is worth mentioning thatnormally the real cause is the wrong

style check valve for the application. Insuch cases, the problem is the applica-tion, notthe check valve.

Two of the most common problemswith check valves are reverse flow andwater hammer. In both situations, afast-closing valve is desired. Reverseflow can be costly, especially if itoccurs at the discharge of a pump andthe pump spins backwards. The cost torepair or replace the pump, plus theplant downtime far exceeds the cost ofinstalling the right check valve in thefirst place. With water hammer, youneed a faster-closing check valve toprevent pressure surges and resultingshock waves that occur when the discslams into the seat, sending noise,vibration, and hammering sounds thatcan rupture pipelines and damageequipment and pipe supports.

If the internals are missing orexhibiting wear, two factors may beoccurring. First, if the check valveselected does not have enough flow

passing through to keep it against itsstop, a valve with a lower Cv is neededto prevent the moving/fluttering of theinternals. Second, if the check valve isused at the discharge of a reciprocatingair or gas compressor, a valve with a

damped design or dashpot to handlehigh-frequency cycling is needed.

Sticking can occur when scale or dirtis trapped between the disc and bodybore. Leakage can happen from damageto the seat or disc or simple trash in the

line. An elastomer is needed to providezero leakage.

InstallationWhen installing check valves, point theflow arrow in the direction of theflow to allow the valve to perform itsfunction. The flow arrow can be foundon the body or tag. Make sure the valvetype will work in the installed position.For example, not all check valves willwork in a vertical line with flow down,nor will conventional or 90-degree pis-ton check valves perform in a verticalline without a spring to push the discback into the flow path. The disc insome check valves extends into thepipeline when the valves are fully open.This could interfere with the perform-ance of another valve bolted directly tothe check valve. If possible, install thecheck valve a minimum of five pipediameters downstream of any fittingthat could cause turbulence. Notice, Isaid if possible. After all, how many

check valves have you seen bolted to thedischarge of a pump? Many! A goodsource of reference for installing checkand other styles of valves is MSS SP-92Valve Users Guide, published by theManufacturers Standardization Society.

How Are Check ValvesLike Doors?Lastly, I like to compare check valves todoorswhether that door is to youroffice or home. Typically, you open youroffice door at the start of the day and

close it at the end, which is similar towhat happens when a pump is cycled onand off. However, if someone stands atyour door and constantly cycles it openand closed, what could happen? In mostcases, the hinge pins would fail, sincethey are the weak link in the operation ofyour door.

Check valves face a similar situation.Pins, stems, springs, or other compo-nents that are constantly cycled can fail.That is why it is important to properlyselect check valves for their possibleapplications. A check valve with a highCv in a low flow application is doomedfrom the start. It is not the check valvesfault; it is the fault of the wrong selectionfor the application. The selected valvewould have worked fine in proper flowconditions. Unfortunately, the installedcheck valve is blamed for the failure,when in reality the culprit was the appli-cation. It is always best to review theapplication and service conditions withthe manufacturer before purchasing a

check valve to make sure the correctstyle is selected. VM

MIKE JOHNSON is sales and marketing manager,

DFT, Inc. (www.dft-valves.com), located in Exton,

PA. Reach him at 610.363.8903 or via email at

[email protected].




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