Valtek Severe Service Equipment

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Valtek Severe Service EquipmentIntroductionPressure/Velocity Profiles in Globe ValvesAs a fluid travels through a conventional single-seatedglobe-style control valve, a vena contracta (point of narrowestflow restriction) develops directly downstreamof the throttling point. At this location of maximumvelocity, the fluid reaches a minimum pressure and thenrapidy recovers to a level less than the inlet pressure(Figure 1). Profiles representative of the pressure andvelocity in a gaseous application are shown in Figure 2.P 1P 2P(ValvePressureDrop)(Velocity atVena Contracta)P VC(Outlet Pressure)(Pressure atVena Contracta)Pressure Profile Single SeatFigure 1: Pressure Profile(single-seated globe-style valve)V 1V VCV 2P 1P 2Gaseous NoiseP VCFigure 2: Pressure and Velocity Profiles(single-seated globe-style valve)LIQUID SEVERE SERVICESWhen control valves fail in liquid service, cavitation isoften the root cause. Cavitation is a two-stage process,illustrated in Figure 3: First, as fluid velocity increasesthrough the restriction at the vena contracta, the pressureof the flowing liquid drops below the vapor pressureof the liquid, causing vapor bubbles to form. Second, asthe flow enters the enlarged flow area downstream of thevena contracta the fluid pressure increases. The result-ant pressure recovery raises the fluid pressure backabove the vapor pressure, causing the vapor bubbles tocollapse – or implode.Energy released by collapsing vapor bubbles can resultin extensive noise and vibration, often causing severedamage to unprotected metal surfaces (Figure 4).P 1P 2Cavitation in LiquidsP VCP V (VaporPressure)Figure 3: Pressure Profile for CavitationFigure 4: Damage to Trim Parts Causedby Cavitation2Flowserve Corporation, Valtek Control Products, Tel. USA 801 489 8611
Valtek Severe Service EquipmentIntroductionIncipient (or beginning) cavitation is heard in a pipingsystem as intermittent popping or cracking, but as thepressure drop increases the noise becomes a steadyhiss or rattle. Fully developed cavitation results in chokingof the liquid flow such that further increases inpressure drop cause no corresponding increase in theflow.The ideal solution to cavitation is to reduce the pressurefrom inlet to outlet gradually, avoiding the effects of alarge pressure drop at the vena contracta (Figure 5).P1P 2V 1V 2P V (VaporPressure)Figure 5: Pressure Profile for GradualPressure ReductionCavitation can be avoided entirely by not permitting thepressure to fall below the vapor pressure, thereby eliminatingany bubble formation and subsequent collapse.A related liquid service problem is flashing, which occurswhen pressure at the vena contracta falls below thevapor pressure followed by a pressure recovery thatremains below the vapor pressure (Figure 6). Thiscauses vapor bubbles to form in the liquid service,which continue downstream as a liquid-gas mixture.The increased volume of this mixture increases theoverall velocity, which leads to excessive noise anderosion. Eliminating flashing completely is generally notpractical and would involve increasing the valve downstreampressure or lowering the vapor pressure of theprocess. More practical approaches include using hardenedmaterials for affected surfaces, or relocating thevalve so that it discharges into a larger vessel and awayfrom critical surfaces.Sigma: The Cavitation IndexVarious cavitation indices have been used to correlateperformance data to improve designs of hydraulic processequipment. A cavitation index, called Sigma (σ),has been developed and applied to quantify cavitation incontrol valves:σ = (P - P ) 1 v(P 1- P 2)Where:P 1= Upstream pressure (psia), measured two pipediameters upstream from the valveP 2= Downstream pressure (psia), measured six pipediameters downstream from the valveP V= Vapor pressure of the liquid at flowingtemperatureSigma is seen as the ratio of the potential for resistingcavity formation to the potential for causing cavity formation.When Sigma is 1.0 or less, flashing is occurring.Through laboratory and field testing results, acceptableoperating Sigmas for eliminating cavitation (and itsassociated choking, noise, and damage) have beenestablished. For example:Tests indicate that a full-area, single-seated globe valveat 100 percent open with 80° F (vapor pressure of 0.5psia), 200 psia water and with flow-over-the-plug chokesat a downstream pressure of 56 psia. The chokedcavitation index is then:P 1P 2P VCP V(VaporPressure)Figure 6: Pressure Profile for Flashings choked= (200 - 0.5) = 1.39(200 - 56)These tests also indicate that cavitation damage (s damage)for this particular style of valve in continuous operationbegins at about s damage= 1.73. The point at whichincipient cavitation (s incipient) occurs can also be deducedfrom tests; it is found at a somewhat higher value thans damage(i.e. lower pressure drop).Flowserve Corporation, Valtek Control Products, Tel. USA 801 489 86113
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