CONTROLLING CAVITATION: - PRO-QUIP

CONTROLLING CAVITATION:HOW A DEEPERUNDERSTANDINGIMPROVES THE SOLUTIONavitation is a common problemC in most industrial processes.Without proper control, it can resultin equipment damage, which can beboth expensive and hazardous.In every valve application where asolution for control of cavitation issought, multiple factors must be considered.For example, the presence ofsolids in a fluid stream, high temperaturesor exotic fluids change whichcavitation control solution is bestsuited to an application. Understandingthe mechanisms that cause cavitationand what mechanisms controlit results in successful implementationof a solution. This article discussesthe causes of cavitation, howit can be predicted and the mechanismsused to control or eliminate it.VELOCITY ANDPRESSURE PROFILEAs a liquid travels through a simplecontrol valve, a “vena contracta” (apoint at which flow restriction is narrowest)develops directly downstreamof the throttling point. Theflow area at this point is smaller thanthe rest of the flow path. As the flowarea constricts, the fluid’s velocityrises. After the fluid passes the venacontracta, the velocity drops again.(See Figure 1, “Velocity through acontrol valve,” which demonstratesthe velocity profile through a conventionalsingle-seated, globe-style controlvalve with equal flow areasupstream and downstream.)This velocity increase at the venacontracta is caused by a transfer ofpressure energy to velocity energy inthe flow, which results in lower staticpressures. As the flow leaves thishigh-velocity area, the velocity energyis converted back into pressure,BECAUSE OFTHEWIDEVARIETY OFPROCESSES,FLUIDS AND SERVICECONDITIONS INVOLVED,NO ONEMECHANISM IS BEST FOR ADDRESSINGTHE PROBLEMS CAVITATION CAN CAUSE.UNDERSTANDING HOWTO PREDICTCAVITATION,ITSVARIOUS REGIMES ANDTHE MECHANISMS OF CONTROL LEADSTOAN OPTIMIZED SOLUTION FORCONTROLLING OR ELIMINATINGCAVITATION. BY JEFF PARISHand static pressure partially recovers. (See Figure 2, “Pressure through a controlvalve,” which shows a pressure profile through a conventional single-seated globe-stylecontrol valve.)Each time this conversion from pressure to velocity and back again occurs, totalenergy suffers a loss because of conversion inefficiencies. The initial pressure dropmay cause a large velocity increase at the vena contracta, but frictional losses andturbulence cause the pressure to not fully recover, even though the velocity returnsFigure 1.Velocity through a control valveAS SEEN IN THE SUMMER 2009 ISSUE OF...Figure 2. Pressure through a control valve© 2009 Valve Manufacturers Association. Reprinted with permission.S u m m e r 2 0 0 9 | 1

C O N T R O L L I N G C A V I T A T I O Nto its initial value. This complies withBernoulli’s energy equation and alsosatisfies the continuity equation forconservation of mass. These two conceptsare key in understanding controlvalve cavitation.CAVITATION PROFILEIn many control valve applications, thepressure at the vena contracta will dropto the vapor pressure of the liquid orbelow that pressure. When this occurs,small bubbles of vapor will form. As thelocal pressure rises above the vaporpressure again, these bubbles collapseor implode and the vapor condensesback into liquid. As the bubblesimplode, cavitation erosion damageoccurs. This implosion is very energetic,forming shock waves or jets of fluid thatcan tear small pits into the metal if thebubble implosion occurs next to valveparts or pipe walls. (See Figure 3,“Pressure profile for cavitation,” whichillustrates the profile of cavitation).Figure 3. Pressure profile for cavitationIn applications where downstreampressure does not recover above thevapor pressure, vapor and liquid remainmixed in the downstream flow, whichincreases the volume and flow velocity.This is known as flashing and requiresspecial handling beyond the solutionsdiscussed in this article.CAVITATION EFFECTS,DAMAGE,AND SOUNDCavitation damage can compromise theintegrity of both piping and controlvalves, sometimes resulting in catastrophicfailure. Erosion or pitting damagecan cause valves to leak by erodingseat surfaces or by weakening pressurevesselwalls.Cavitation damage forms a roughsurface of small pits that are easy to seeor can be seen with slight magnification(See Figure 4, “Cavitation-damagedparts”). However, certain types of corrosioncan mimic the effects of cavitation.In these cases, cavitation can beidentified by the location of the damage,which, unlike crevice and pitting corrosion,rarely occurs in narrow gapsand/or static liquid regions. Cavitationdamage is almost always located downstreamof the control valve seating areasor other high-velocity flow regions.Figure 4. Cavitation-damaged partsOccasionally, cavitation bubbles are carriedfar downstream by high-velocityflow before the pressure recovers sufficientlyto collapse the bubbles, whichcan cause damage to downstream pipingand fittings.When cavitation bubbles implode,they make a distinctive sound. Lowlevel,or incipient, cavitation can beheard in a piping system as intermittentpopping or crackling. As the pressuredrop increases and cavitationbecomes more severe, the noisebecomes a steady hiss or rattle thatgradually gets louder. Fully developedor choked cavitation is often describedas a sound similar to gravel or smallrocks flowing through the pipe.Figure 5.Multi-stage pressure reduction profileCAVITATION CONTROLThe ideal solution for handling heavycavitation conditions is to reduce thepressure from inlet to outlet gradually,thus avoiding large pressure excursionsto the vapor pressure. Cavitation can beavoided entirely by not allowing thepressure to fall to the vapor pressure,which eliminates vapor formation andsubsequent bubble collapse (See Figure5, “Multi-stage pressure reduction profile,”which illustrates cavitation elimination).At the other end of the spectrum,adifferentsolution for controlling(not eliminating) lower levels of cavitationinvolves isolating the bubbles fromthe metal surfaces and safely dissipatingthe energy of the implosion into the surroundingliquid. Mechanisms for accomplishingthese solutions are discussedbelow in the “Anti-Cavitation and ControlMechanisms” section.CAVITATIONMEASUREMENTCavitation in fluid flows can be measuredusing the vibration or noise ofimploding bubbles or by examiningdamaged parts. Using vibration measurementshas advantages, but thismethod may not always be practical inthe field. However, under laboratoryconditions, vibration measurements canprovide a quick way to identify andmeasure cavitation severity. Fortunately,there are also methods to predict andeliminate cavitation before a valve isexposed to damaging conditions.SIGMA:THE CAVITATIONINDEXVarious cavitation indices have beenused to correlate performance data toimprove designs of hydraulic processequipment. A cavitation index, calledSigma (σ), has been developed andapplied to quantify cavitation in controlvalves. The inverse of Sigma (1/σor X F is used by the International ElectrotechnicalCommission (IEC) in IEC60534-8-4 to correlate hydrodynamicnoise with cavitation. Sigma has beenadopted by the International Society ofAutomation (ISA) in RP75.23.01 asan industry-recommended practice toevaluate cavitation in control valveapplications. Sigma represents theratio of the potential for resisting cavi-2 | Valve M A G A Z I N E© 2009 Valve Manufacturers Association. Reprinted with permission.

the operating sigma is:(500 - 7.5)σ operating = __________ = 2.59(500 - 310)Figure 6.Typical valve recovery coefficients.Values will vary with specific makes, models andpercentage open.* Values apply to water at nominal test pressures. Scale factors are not included here.** Choking may not occur, or valve may be designed to operate choked.Note: These values are supplied by the manufacturer of the valve types used for this illustration.ty formation to the potential for causingcavity formation. This cavitationindex is defined as follows:(P 1 - P V )σ = ____________(P 1 - P 2 )Where all pressures are in consistentabsolute pressure units, and:P 1 = Upstream absolute pressure,measured two pipe diametersupstream from the valve,P 2 = Downstream absolute pressure,measured six pipe diametersdownstream from the valve,P V = Vapor pressure of the liquidat flowing temperature.Through laboratory and field testing,manufacturers can establish recommendedoperating Sigma limits for variousvalve designs. In actual applications,additional sizing and scaling factors needto be considered. For example:A manufacturer determines bytesting that water flowing overthe-plugthrough a fully open, single-seatedthree-inch globe valveat 100.5 psia and 80° F (vaporpressure = 0.5 psia), starts tocause cavitation damage at adownstream pressure of 61 psia.The sigma damage (σ damage ) orsigma incipient damage index(σ id ) for this case is:(100.5 - 0.5)σ id = __________ = 2.53(100.5 - 61)The Manufacturer’s RecommendedSigma (σ mr ) for this particularstyle of valve in continuousoperation at these reference conditionsis generally set equal to avalue slightly higher than σ id .The pressure drop at whichcavitation first occurs, sigmaincipient (σ incipient ), is muchlower than for incipient damage.σ incipient can be determined fromvibration tests described in ISA-RP75.23.01 and the value of σ incipientwill be significantly greaterthan σ mr or σ id . At the other endof the spectrum is choked flow,which is caused by an increasedvolume of vapor due to cavitation.Choked flow is determinedwhere the flow rate reaches andremains at a maximum valueregardless of further increases inpressure drop from a constantupstream pressure. This sametest valve will choke at a sigmavalue (σ choked ) somewhat smallerthan σ mr .If this same type of valve in a 10-inch size operates wide open at anupstream pressure (P 1 ) of 500 psia anda downstream pressure (P 2 ) of 310psia, and with a water temperature of180° F (vapor pressure = 7.5 psia),At this point, the value for the σ operatingappears to indicate that this valvewill operate without problems since theσ operating is greater than σ id . However,size and pressure scaling effects mustbe considered.To apply the lab results to an actualapplication, scale effects for the actualvalve size and pressure conditions needto be applied to σ mr . Methods of scalingthe sigma index for such variables havebeen established in ISA-RP75.23.01.The following equation for the scaledvalve coefficient, σ v , is used to apply thescaling effects for size (SSE) and pressure(PSE) to a reference or recommendedcoefficient represented by σ R .σ v = [σ R (SSE) -1] * PSE +1Where:σ R = The reference Sigma,or in this case σ mrSSE = Size Scale Effect(determined by themanufacturer)PSE = Pressure Scale Effect(determined by themanufacturer)Guidelines for determining andapplying SSE and PSE scale effects canbe found in ISA-RP75.23.01. Usingdata from the manufacturer, SSE is calculatedto be 1.29 and the PSE is calculatedto be 1.19 for this example. Substitutingthese numbers into theequation above we have:σ v = [2.59 (1.29) -1] *1.19 +1 = 3.69This means that the σ v value of 3.69is the true coefficient of σ mr to whichthe σ operating should be compared.Because the σ operating value is less thanσ v , the valve is likely to be damaged atthese conditions unless cavitation controltrim or harder materials are used.The type of valve used can make adifference in the level of resistance to© 2009 Valve Manufacturers Association. Reprinted with permission.S u m m e r 2 0 0 9 | 3

C O N T R O L L I N G C A V I T A T I O NSudden Expansion and Contraction –With every sudden change, eitherthrough an expansion or a contractionin volume, energy losses occur as totalenergy is converted back and forthbetween pressure and velocity. This isaccounted for in Bernoulli’s energyequation and also satisfies the continuityequation for conservation of mass.As a fluid passes from a large flowarea to a smaller area, the pressurewill drop and the velocity will increase.As the fluid exits a small area andenters a larger area, the pressure partiallyrecovers as the velocity decreases.Each time this occurs, energy is lostto friction, resulting in lower and lowerfinal pressures upon each subsequentexpansion.cavitation achievable for a givenprocess (see Figure 6, which listsexample sigma values of various valvetypes and trims). The σ mr for a givenvalve configuration is determined bythe manufacturer based on testing andexperience. Methods other than vibrationtesting may be necessary for somespecialty multistage valves to determineσ values for incipient cavitationor choked flow. Experience shows thata high degree of correlation betweenpredicted sigma values and actual performancevalues can be developed.ANTI-CAVITATION/CONTROL MECHANISMSProfiles were presented earlier (SeeFigures 1, 2 and 5) to illustrate theenergy transfer between velocity andpressure. A variety of mechanisms thattrigger this energy conversion can beemployed to control or eliminate cavitation.Many types of anti-cavitationtrim solutions will employ multiplemechanisms to achieve the most effectiveresults.Figure 7. Mechanism combination A(Illustrations are not to scale.)Figure 8. Mechanism combination B(Illustrations are not to scale.)Cavitation Bubble Isolation – The simplestmethod of controlling mild cavitationis by isolating the cavitationbubbles away from metal surfaces. Themost common method for doing this isto use cages with opposing drilledholes. This method can only control lowlevels of cavitation and does not eliminateit. The resulting collision of thestreams causes the cavitation bubblesto implode in the middle of the flowaway from the cage and control surfaces.The surrounding liquid absorbsmuch of the bubble collapse energy.Frictional Losses in Small Passages –When a fluid flows through many smallpassages, the boundary surface areathat a viscous fluid comes in contactwith increases friction near the boundarysurface, which can cause significantenergy to be lost, resulting inadditional pressure reduction.Turbulent Mixing – When two or morestreams converge from wide (oblique)angles, considerable energy is lost inthe mixing. The resulting turbulencefrom two or more colliding streamswill generate a pressure drop downstreamas the fluid velocity profile stabilizes.Directional Changes – Abrupt directionalchanges dissipate energy as turbulenceis generated at each turn.Each subsequent directional changeresults in a net loss of pressure. Theangle of the directional change has animpact on the pressure reduction.Impingement – When a fluid streamimpinges on a surface or another fluidstream, changes in velocity, direction,and turbulent mixing occur together,resulting in a non-recoverable energyloss. As the fluid velocity profile stabilizes,a reduction in pressure occurs.MECHANISMCOMBINATION EXAMPLESWhen combining all of the mechanismsmentioned (see Figure 7, “Mechanismcombination A,” which illustrates away to combine all mechanisms),atrim that is capable of effectively eliminatinghigh levels of cavitation isobserved. While this may look like aperfect solution, it has limitations. Ifthe passages are small, large debriscan clog them. This combination ofmechanisms is very compact, easy tomanufacture, and moderate in cost;however, it is best for clean service.Another solution (see Figure 8,4 | Valve M A G A Z I N E© 2009 Valve Manufacturers Association. Reprinted with permission.

“Mechanism combination B,” whichillustrates a simplified combination)uses fewer of the mechanisms, yeteffectively eliminates cavitation whilepermitting the passage of relativelylarge solids without clogging. Althoughnot as compact or as inexpensive tomanufacture as other combinations ofmechanisms, this configuration iseffective for controlling cavitation indirty service.CONCLUSIONBecause of the wide variety of processes,fluids and service conditions, no onemechanism or combination of mechanismsis best for all cavitating services.While the expected life of equipment insevere conditions may vary, properapplication of industry standards, engineeringfundamentals and experiencewill ensure cost-effective operationover the entire life cycle. By understandinghow to predict cavitation, itsvarious regimes and the mechanisms ofcontrol, an optimized solution can befound for controlling or eliminatingcavitation. VMJEFF PARISH is a senior product manager forFlowserve’s Flow Control Division(www.flowserve.com). Reach him atjparish@flowserve.com.© 2009 Valve Manufacturers Association. Reprinted with permission.S u m m e r 2 0 0 9 | 5