CONTROLLING CAVITATION: - PRO-QUIP

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.