Flowserve Gaseous Noise Control - PRO-QUIP

flowserve.comThe Bernoulliprinciple:When the fluidpressure in thevalve drops, thefluid velocityrises.1. Introduction to Noise1.1 Pressure Profiles Through Control ValvesAs a fluid travels through a conventional single-seatedglobe-style control valve, a vena contracta (point of narrowestflow restriction) develops directly downstream ofthe narrowest throttling point. At the point of vena contractathe fluid reaches a minimum pressure and maximumvelocity which rapidly recovers to a lower pressure thanthe inlet pressure (see Figure 1.1: Pressure Drop througha Control Valve). Due to the Bernoulli principle, when thefluid pressure in the valve drops, the fluid velocity rises(see Figures 1.2: Velocity through a Control Valve). Asthe velocity of the fluid increases, the noise generated byturbulence in the fluid also increases. A significant rise invelocity can produce noise beyond safe limits.1.2 Gaseous Severe ServiceHigh pressure drops in gas services will generate turbulencein the fluid flow downstream of the pressure drop.As a direct result of the turbulence, noise is radiated tothe surrounding area by the downstream piping system(Figure 1.3: Noise Radiating from Downstream Piping).In situations where equipment damage or personal injurycould be caused by a noise source, attenuation is mandatory.Tests have demonstrated that control valve noise increasesproportional to the velocity cubed (SPL ~ V3). Moderatelyhigher velocities can produce significantly loudernoise. Substantial noise can be generated even when velocitiesare significantly less than sonic.Mechanical vibration accompanies high acoustic noiselevels. Acoustic noise and mechanical vibration levels aregreatly compounded (up to 50 times) when the frequencyof the excitation matches acoustic and/or mechanicalnatural frequencies of the system.Noise suppression solutions should always be consideredin high pressure drop, high flow rate or resonant noiseapplications.Control Valve noise is measured in decibels (dBa). Decibelsuse a logarithmic scale, doubling the energy level ofsound pressure wave will cause the noise level to increaseby about 6 dBa. The dBa scale is structured to the humanear, the lowest noise a human can hear is around 0 dBaand a rivet gun is around 100 dBa. Ear protection shouldbe considered when noise levels are above 85 dBa.Uncontrolled noise is a significant problem that can leadto serious health problems, vibration and in the mostextreme cases can cause damage to equipment. Noisecalculations should be performed on all control valveflowing gasses to verify the suitability of the equipment1.3 Factors Impacting NoiseFluid VelocityThe most frequently used solution to high levels of controlvalve noise is to reduce the pressure from the valve inlet tooutlet gradually, eliminating pressure and velocity spikesthroughout the trim (Figure 1.4: Pressure through a Multi-Stage Valve). By lowering the pressure gradually, lowervelocities are created. The lower velocities generate lowernoise levels. Successfully applying this technique requirescontrolling the gas velocity through the valve trim and atall points from the inlet to the outlet of the valve.Occasionally fluid velocities at the valve outlet or even inthe downstream pipe can cause excessive noise. In thesecases, simple trim solutions will not provide the needednoise attenuation.