Severe Service Equipment - PRO-QUIP

Turbulence ControlMegaStream trim effectively lowers the sound pressure levels associated with any turbulence generatedwithin the valve. This turbulence is often carried into the downstream piping, where the localized pressurechanges (resulting from the turbulence) vibrate the relatively thin pipe wall that radiates noise to thesurroundings. With the MegaStream attenuator, each successive stage has additional holes or orifices,which increases the flow area and handles the increased gas volume resulting from the pressure drop.As the flow leaves the final stage of the MegaStream attenuator, the turbulence is limited by controllingthe physical size of the individual fluid streams. (In other words, the smaller fluid stream size exiting thefinal stage of the MegaStream trim limits the amount of turbulence energy present.) Further, the smallerturbulent eddies are more easily dissipated. The stages also effectively limit much of the sound vibrationgenerated at the throttling area. This attenuating effect is made possible by the acoustic impedancecharacteristics of the material and design, which provide resistance to further transmission of incidentsound energy. This acoustic impedance is especially effective in controlling noise when the plug isthrottling close to the seat.VelocitiesOne of the fundamental design considerations with Valtekcontrol valves with MegaStream trim is the establishment ofacceptable velocities at every point as the flow passes throughthe valve. For gaseous control valve applications, it isdesirable to keep the maximum outlet gas velocity below 0.33Mach although higher velocities can be accommodated.Velocities must be assessed for the most difficult flowingconditions at the following critical points (as shown in Figure14.8.3):1. The inlet passageway to the valve2. The internal flow area of the MegaStream attenuator at allplug positions3. The gallery flow area formed between the outside diameterof the attenuator and the inside diameter of the valve body4. The valve outlet passage flow area. For proper noisecontrol, the downstream piping must be equal to or largerthan the valve outlet sizeFigure 14.8.3: MegaStreamVelocity CheckpointsFlow CharacteristicsAs with the standard Mark One trim design, the flow characteristic for MegaStream trim is determined bythe contour of the plug head. Three characteristics are available: equal percentage, linear and quickopen.SIZING TIPS:• UNITS: Double-check to ensure that each of your process conditions is using the correct units.• HIGH MACH LEVELS: at the valve exit can generate high sound pressure levels eclipsing the noisegenerated by the valve itself. For this reason, it is recommended to maintain exit velocities less than0.30 mach.• MULTI-STAGE: trim calculations under the current IEC 1 standard focus on the last stage of the trim topredict the noise generation for the entire system, using a factor to estimate the effects of all the otherstages. This can result in inaccurate noise prediction for certain trim. For this reason, specialconsideration should be given to trim with variable pressure drops across the different stages. Amodification to the IEC standard is anticipated, enabling it to better assess trims with this attribute.Performance!® version 7.3.0 and later, provides an additional modified IEC calculation for valves withthis characteristic.• VALVE STYLE: selection plays an important role in noise control. For example, 50% of the noisegenerated in butterfly valves is transmitted downstream, but only 25% of the noise created by globevalves is transmitted downstream. This information can be found on a table in Section 15.1 CEI/IEC 60534-8-3:2000 Second Edition, Global Engineering Documents, July 2000Sizing & Selection: Severe Service Equipment Selection, 14-MegaStreamSS014, Rev 09/2002 Flowserve Corporation, Flow Control Division, Tel. USA 801.489.8611