Research shows that morethan half of unscheduled shut-downs of reciprocating com-pressors are the result ofproblems associated withvalves and unloading systems.As a result, these componentsreceive a great deal of atten-tion. Without a doubt, prop-erly designed and maintainedvalves are critical to achievingmaximum reliability. However,there are many misconcep-tions regarding exactly howvalve performance can bemaximized and precisely whatchanging or redesigningvalves can accomplish. Thefollowing discussion addressesthe most common of thesemisconceptions and suggestshow operators can avoid thepitfalls associated with them.

If a Little Oil is Good, Moreis That Much Better

Compressor valve designhas changed over the past 30years. Metallic-plated valvesare rarely used in mainstreamapplications, having been re-placed with new, engineeredthermoplastics. Field experi-ence has shown that one ofthe major advantages of plas-tic elements is their perform-ance in mini and nonlubri-cated applications.

Metallic-element valves re-quire design adjustments inorder to reduce sliding in-duced wear in nonlube envi-ronments. Teflon and othernonmetallic materials wereregularly inserted as guides inthe guard to avoid prematurefailure due to wear. In lubri-cated cylinders, high-viscosityoils combined with generousfeed rates were the primaryprotection from metal-to-metalwear in these older-style designs.

Modern, nonmetallic valvedesigns should operate equallywell in lube and nonlube en-vironments. Although a thincoating of lube oil can helpprotect parts from corrosiveelements in the gas and allowfor a broader selection of ma-terials, a little oil goes a longway.

Excessive lube oil or ex-

tremely high-viscosity lube oilcan greatly reduce compressorvalve reliability by creating anadhesive effect called “stick-tion,” which is readily appar-ent when taking apart a valvefor service. Usually, the ele-ments are so thoroughly stuckto the seat that they cannoteasily be pulled off, but mustbe pushed off with a slidingmotion. During operation, theviscous adhesion of sticktioncan cause the element to ad-

here to the seat (thus delayingplate opening), cause the ele-ment to adhere to the guard(thus delaying plate closing)and in excessive amounts, actas a liquid (which it is). Theharmful effects of sticktion be-come more significant as luberates and oil viscosity increase.

If valve opening is delayed,the valves will open later inthe stroke, usually closer tothe midstroke (maximum ve-locity) of the piston. This can

greatly increase the elementopening speed. If valve clos-ing is delayed, the elementsmay close after top dead cen-ter (TDC), resulting in slam-ming and the impact failurespreviously discussed. Becausethe contact area between theelement and guard is usuallymuch greater than that be-tween the seat and element,the respective adhesive forcewill be much greater as wellon valve closing. This force issimply one more obstacle thatthe springs must overcome inorder to close the elementsagainst the seat before TDC.Because of this, valves withnonmetallic plates will gener-ally run better with a lowerquantity of light oil.

PEEK is PEEKAs the use of plastic for

valve elements has increased,PolyEtherEtherKeytone (PEEK)has emerged as one of the in-dustry standards. Numerousother materials are marketedby various companies and, formany natural gas compres-sors, nylon and other mid-temperature materials offer ac-ceptable service. For theultimate performance in proc-ess applications, however,PEEK still dominates. Un-fortunately, not all PEEK partsare created equal. In additionto having multiple base resins(which are all the same color)and fillers (which usually donot change the color of thepart), PEEK can also be a dif-ficult material to process.

There are two primary injection-molded PEEK resinsavailable: the high-density450G and the lower-density or“easy flow” 150G. On paper(i.e. tensile, yield and flexural

EXPOSING FIVE COMMONMYTHS ABOUT

COMPRESSOR VALVESA Valve Manufacturer Addresses

Commonly Held MisbeliefsBy John Metcalf

MAY 2004 COMPRESSORTechTwo

■ Modern, nonmetallic valve designs, such as Cook Manley’s patentedMOPPET valve, run better with a low quantity of light oil.

John Metcalf is general man-ager of Cook Manley inHouston, Texas, U.S.A.

The Five Myths1. If a little oil is good, more is that much better.2. PEEK is PEEK.3. Changing finger unloaders to plug unloaders will

reduce efficiency.4. My valves failed — I must have a valve design

problem.5. Valve changes can increase compressor capacity.

■ The harmful effects of “sticktion”become apparent when a valve istaken apart for service.

strength, Izod impact etc.),these materials appear similar.In a compressor valve ele-ment, however, there is a dra-matic difference in perform-ance. Regardless of the fillersused (most high-end valvesuppliers have proprietaryblends made specifically forthem), the 150G resin, while itis easier to mold and morecost-effective for the manufac-turer, will always exhibit sig-nificantly lower impact resis-tance than the 450G version.Unfortunately, it is impossibleto look at a finished part andtell the difference between thetwo resins.

The other area of concern isthe use of re-ground materialin valve parts. PEEK is expen-sive — currently approaching$30 per pound even in largequantities. As a result, there isa temptation to re-use materialleft over from the originalmolding and machining proc-ess in order to maximize prof-its. The use of reground mater-ial cannot be visually detected,but does change the mechani-cal properties of the material.Each time the PEEK material ismelted, molded and ma-chined, the resin degradesslightly and the fillers used arechopped into smaller andsmaller pieces. The end resultis a valve element that looksnormal, but will likely fail pre-maturely. Although many sup-pliers will not divulge their se-cret PEEK recipes, all shouldbe willing to certify that a fin-ished part is made from virginblends of 450-grade PEEK.

Changing Finger Unloadersto Plug Unloaders WillReduce Efficiency

The unloaded efficiency ofan unloader depends on itscompletely unloading the endin question and the brakehorsepower (bhp) of the un-loaded cylinder compared toits bhp in the loaded state. Ingeneral, determining the un-loaded bhp for a given cylin-der end is the same as for aloaded cylinder. The higherthe flow area of the unloader,the lower the bhp will be inthe unloaded state (just asmore compressor valve areareduces the horsepower re-quirements of a cylinder inthe loaded state).

When finger unloaders areapplied, it is almost alwaysnecessary to use one on eachof the inlet valves in a givenend. This provides the gaswith the full lift effective flowarea (EFA) entering the cylin-der and some value less thanthat in the opposite direction.Since valves are designed forflow entering the seat andexiting the guard, the flowcoefficient for reverse flow issomewhat lower. Althoughthis reverse-flow EFA variesfrom valve design to valvedesign, it is generally ac-cepted to be no more than80% of the forward flow EFA.This will certainly have an ef-fect on the unloaded bhp,but the real determining fac-tor for unloaded efficiencywhen using finger unloadersis the cylinder swept volumevs. valve area ratio, plus

some other extraneous factors.Because the cylinder is us-

ing the valve porting as theunloader area, there can neverbe more unloader EFA thanwhat is available through thevalves when a finger unloaderis used. This level of unload-ing area will almost always besufficient to unload the end,but will result in varying un-loaded bhp. As applicationwindows and cylinder designsshift, so will the efficiency ofany unloader. With a fingerunloader, however, there is little flexibility to adjust un-loader area. There is typicallyonly one plug unloader ap-plied per cylinder end (al-though this can vary), regard-less of whether it is used inconjunction with an activevalve or not. Despite this fact,plug unloaders usually alsooffer more unloader EFA thanfingers and lower unloaderhorsepower.

The loaded cylinder horse-power will always be affectedby the use of unloaders, re-gardless of which style isused. The extent of the effectthat a specific unloader designhas on loaded cylinder horse-power often dictates whethera finger or plug unloader isused for a given cylinder de-sign. Plugs are often not effec-tive in single-valve-per-cornercylinder or cylinders with verysmall diameter valves. Regard-less of the size and number ofvalves in the cylinder, activevalve area is always sacrificedto provide a hole for a plugunloader — this is well

known and documented.There is a common miscon-ception, however, that no ac-tive valve area is sacrificedwhen using finger designs.Although the lift area (thearea between the elementsand seat at full lift) of thevalve is not affected, the EFAof the valve can be signifi-cantly lower when a fingerunloader is applied.

The EFA of the valve is theproper tool for the evaluationof compressor valve pressuredrop and cylinder horsepower.EFA takes into account notonly the lift area, but the seatand guard area and the flowcharacteristics through each.Because the finger assemblyblocks some portion of thetop of the seat and the fingersextend down into the seatholes (greatly reducing the ef-fective seat port area), the useof a finger unloader will, in-disputably, reduce valve EFA.In laboratory tests, it has beenshown that the EFA of a valvecan be reduced up to 30%when used with a finger as-sembly. This is an area ofanalysis that is often over-looked when predictingloaded cylinder horsepower.

My Valves Failed –I Must Have a Valve Design Problem

Just because an operatorhas broken valve parts on hisdesk does not necessarilymean he has a valve problem.The performance of thevalves is influenced by thecylinder operating environ-

MAY 2004 COMPRESSORTechTwo

■ PEEK parts that appear identical to the unaided eye may actuallyhave different base resins and fillers. Therefore, their performancecharacteristics may vary significantly.

■ Cook Manley recommends that buyers should always specify partsmade from 100% virgin raw material (PEEK pellets, left), with no re-ground material (right).

ment more than any other sin-gle component.

Many incompressible sub-stances can be entrained ingas, and all of them are a po-tential threat to compressorvalve reliability. Lubricationoil type and quantity (as pre-viously discussed) and otherliquids (water, light ends, etc.)combine with dirt and debristo form a barrier to valve runtime. In addition, gas dynam-ics resulting from pressurepulsations and cylinder pas-sage, head and cage designcan dramatically alter elementmotion. Aside from all this,valves receive literally hun-dreds of thousands of impactsdaily in often high-temperatureand corrosive environments.As one engineer put it, “It’snot amazing that valves workas well as they do, it’s amaz-ing that they work at all.”When something goes wrongin the cylinder or the process,the valves are usually the firstto feel the effect. The result istypically to focus on thevalves instead of the rootcause of the problem.

Naturally, many of theseoutside influences cannot bealtered or avoided. Some aresimply the nature of the proc-ess, application or compressortype. If valve life is to be max-imized, however, it is impor-tant to concentrate on thosethat can be controlled, or atleast influenced. Using properknock-out systems and sepa-rators can quickly turn a diffi-cult environment into afriendly one. Filters should beused whenever feasible andregularly monitored andcleaned to avoid clogging andeventual collapse. Proper se-lection of lubrication oil forspecific applications, as wellas careful inspections to deter-mine the proper feed rates,

can do wonders for valve reli-ability. In short, the cleanerand dryer the gas stream, themore likely that the valveswill operate as intended.

Even if a difficult operatingatmosphere cannot be avoid-ed, there are usually steps thatcan be taken by the valvemanufacturer to improve reli-ability. Computerized dynam-ics can often be thrown outthe window as outside influ-ences on the valves negatenormal operation.

However, knowledgeablevalve designers can call onpast experiences in similar ap-plications to make alterationsto lift, springing, materials,coatings and other specifica-tions to make the valve asdurable as possible. For thisto happen, two conditions arenecessary. First, the operatorand the vendor must haveopen and honest communica-tion about the actual processconditions. Second, the ven-dor must have experiencedand qualified technical staff.

Valve Changes canIncrease CompressorCapacity

This is perhaps the mostcommon myth of all. Numer-ous companies have ap-proached suppliers regardingvalve upgrades for the pur-pose of increasing unit capac-ity. Unfortunately, unless thevalves removed are leakingprofusely or are otherwise in-sufficiently designed for theapplication from a flow areaor springing standpoint, thereis little chance that onlychanging the valves can sig-nificantly alter cylinder flow.

Many believe that a valvewith a higher flow area will re-sult in higher cylinder flow.This is incorrect in almost allcases. Except in high-speed or

very low-pressure applications,the only aspect of valve designthat affects capacity is theclearance it adds to the cylin-der. This assumes that thevalve is otherwise operatingcorrectly. In low-pressure (usu-ally less than 100 psi or 6.9bar) and/or high-speed (<1000rpm) applications, it is possiblethat very restrictive valve flowarea can create such a throt-tling effect that capacity can bereduced. In very low-pressurecases, excessive springing canhave a similar effect.

In general, however, theonly compressor characteris-tics that can significantly affectcompressor flow are suctionpressure (higher PS results inhigher flow), cylinder dis-placement (larger swept vol-ume and higher speed resultin higher flow), compressionratio (lower compression ratioresults in higher flow), cylin-der fixed clearance (lowerfixed clearance results in

higher flow) and, to a lesserextent, suction temperatureand gas properties (lower TSand ZS result in higher flowdue to increased gas density).

Cylinder flow is basicallycontrolled by the inlet volu-metric efficiency, which issimply the percentage of thestroke in which the inletvalves are open. The fixedcylinder clearance has a directinfluence on this property.Since the valves contribute tothe overall cylinder clearance,changes in valve design canresult in changes in flowbased on the comparativeclearance properties of thetwo designs. The valves con-tribute whatever internalclearance is contained in theinlet guard and discharge seat(when the valve elements arein the closed position) to thefixed cylinder clearance. Thecylinder clearance is impor-tant because it determines therate at which the pressure in

MAY 2004 COMPRESSORTechTwo

■ Thermoplastics molding operations at Cook Manley’s facility inHouston, Texas, U.S.A.

■ Plug unloaders (cutaway in photo) usually offer more unloader EFAand lower unloader horsepower than finger unloaders.

the cylinder rises or falls whenthe valves are closed. This di-rectly influences the point inthe stroke at which the inletvalves open and start to allowgas into the cylinder.

Note that in this case, weare mostly concerned with theinlet volumetric efficiency(VE). The discharge VE willfollow accordingly, based onthe amount of gas brought induring the inlet event. Theamount of influence the clear-ance has on VE is also tied tothe compression ratio. Thehigher the compression ratio,the more added clearancewould reduce flow. Therefore,when different valves are in-stalled, they may or may notnoticeably affect the flowbased on a number of factors.

First, the new valves musthave significantly higher orlower internal clearance thanthe original design. Poppetvalves typically add signifi-cantly more clearance thanplate valves. Also, when double- and single-deckvalves are swapped, signifi-cant changes in clearance canbe expected. And if the newvalve design is thicker thanthe original, additional clear-ance can be expected.

Second, the compression ratio of the cylinder must behigh enough that the changein clearance will have a meas-urable effect. Gas transmis-

sion applications almostnever show changes in cylin-der flow with valve changes.The compression ratio (RCusually less than two) is solow that very large changes inclearance are required formeasurable capacity effects.Yet small, single-stage aircompressors are very suscep-tible to capacity changeswhen new valves are in-stalled. The compression ratio(RC usually four or greater) isso high that any change inclearance will have a notice-able effect on flow. This canalso be true in process appli-cations with compression ratios ≥ 3.0.

Assuming proper valve de-sign and minimal leakage, atleast one of these scenariosmust occur and, in mostcases, both must be true in order to see any appreciablechanges. Because of this, thescenarios in which the valveclearance contributes signifi-cantly are rare.

Compressor operation is acomplex and evolving task.However, a proper under-standing of these commonmisconceptions will helpthose responsible for com-pressors to improve produc-tivity, avoid chronic problemsand become better-informedbuyers of related productsand services. ■

■ The skill and experience of the design engineer at customizingvalves (such as this Manley radiused-disc valve) can greatly enhancedurability, especially in challenging process conditions.