Valtek Severe Service Equipment

Valtek Severe Service EquipmentIntroductionIf this same valve operates wide open at an upstreampressure (P 1) of 500 psia and a downstream pressure(P 2) of 200 psia, and the water temperature increasedto 180° F (vapor pressure = 7.5 psia), the operatingSigma is:σ operating= (500 - 7.5) = 1.64(500 - 200)Because this Sigma value is greater than σ choked, thevalve is not choked at these conditions. However, theSigma is less than σ damage; therefore, the valve mayexperience cavitation damage unless special anticavitationtrim or harder materials are used.Some of the other factors that affect the intensity ofcavitation are the magnitude of the actual service pres-sure compared with test pressures, the flow path geometryand the fluid purity. By researching these factors,methods of scaling the index for such variables havebeen established. This geometry and pressure scalingis not accounted for in the use of liquid pressure recoveryfactor (F L) and liquid cavitation factor (F i) in previousvalve sizing equations.Globe valves experience minimal cavitation damagewhen operating at low pressure (Sigmas between 2and 1.7). Generally, in these cases, no cavitation controltrim is necessary. However, at between Sigma 1.7 and1.15, some cavitation control is usually required. Whenthe Sigma index for a valve is less than 1.15, thepotential for severe cavitation damage exists and astaged pressure drop severe service trim must beincluded in the valve’s sizing.Table 1: Valtek Trim Solutions to Cavitation Associated with Control ValvesGlobe-styleTrimTiger-ToothChannelStreamMicroCavCavControlDesignStackof expanding teeth discsCartridge or plug with cylindersusing intersecting channelsPlugwith expansion groovesDiametricallyopposed jetsTo eliminateformationApplicationor minimize cavitationTo eliminate or minimize cavitationformationTo eliminate or minimize cavitationformation in low flow servicesForSigmasapproaching1.002SeePages6 - 91.00210 - 141.00415Toeliminate or minimize cavitation damage1.1516 - 18GASEOUS SEVERE SERVICESIn gas services, noise is generated by high pressuredrops across the valve and by subsequent turbulencedownstream. As a direct result, noise is radiated to thesurrounding area by the downstream piping system(Figure 7). In situations where equipment damage orpersonal injury could be caused by a noise source, noiseattenuation is not only desired, it is mandatory.With gases, superimposing a velocity profile on thepressure profile (Figure 2) reveals the main componentin control valve noise generation. For single throttlingpoint control valves, because of the sharp pressurereduction at the vena contracta, the velocity is greatlyincreased. Tests have demonstrated that a control valve’ssound pressure level increases proportional to the velocitycubed (SPL ~ V 3 ). Noise is generated as velocitiesin the valve increase and substantial noise can begenerated even where velocities are significantly lessthan sonic.OSFigure 7: Downstream Turbulence Causedby Vena Contracta in a Valve4Flowserve Corporation, Valtek Control Products, Tel. USA 801 489 8611