Flowserve Gaseous Noise Control - PRO-QUIP

Gallery Flow AreaInlet PassagewayOutlet PassageInternal Trim Flow AreaFigure 1.5: Critical Velocity ZonesA common method to raise the peak frequency is to makesmaller outlet holes in the noise control device. Cutting ahole diameter in half can lower the overall noise level byup to five decibels.WaveCracker TechnologyWaveCracker is a patented technology that reduces noiseas flow passes through passages with irregularly shapedcross sections. Tests have shown this technology effectivelycan reduce noise by more than ten decibels.WaveCracker works by forming an irregular cross sectionshape (see Figure 1.6: WaveCracker Passages). Soundwaves reflecting off the walls of the passage have irregularpatterns. These irregular patterns cause the soundpressure wave to lose intensity as it moves down thepassage.Mass FlowLower pressure drops across control valves will producesignificant noise if the mass flow rate is high. This happensmost often in large valves when many small noisegeneration sites combine to produce a loud cumulativenoise level. Combating this problem is more difficultsince taking the pressure drop in small stages may notbe effective.Acoustical AttenuationAcoustical attenuation can provide a barrier which blocksnoise. This can be done in many ways, from insulatingthe pipe to increasing the distance to the noise source.Careful engineering of a noise solution includes evaluatingany existing or potential attenuation.Harmonic VibrationHarmonic vibration occurs when the natural frequency ofthe pipe and valve approach a common frequency. Thiscauses a noise with a single frequency and a characteristicsound that is often called screech. Since screech occurswhen the frequency of the valve and pipe match, itis not easily predicted. However, simple changes in thevalve geometry can usually solve the problem.