Although wet (oil injected) and dry (oil free) screwcompressors are widely used in many applications,limited information is available regarding thepulsation, vibration, and noise problems associatedwith these types of compressors. This tutorialdiscusses such problems and includes case historieswhere field testing was performed, and providesdesign recommendations.Both wet and dry screw compressors normallygenerate pulsation at the pocket passing frequency(PPF) and its multiples. The pulsation amplitudes areaffected by many variables such as mole weight ofthe gas, operating pressures, speed, screw profile, andshape of the discharge port. The pulsationamplitudes are further amplified by acoustical naturalfrequencies of the compressor/silencer/piping system.Dry screw compressors are generally supplied withsuction and discharge silencers which are designed toattenuate the pulsation generated by the compressors.Most of the silencers are reactive type (Helmholtzfilters), absorptive type, or a combination of thesetwo. Although the silencers are designed to attenuatepulsation at certain frequencies, they can also amplifypulsation when the excitation frequencies arecoincident with the acoustical natural frequencies ofthe silencer itself. Pulsation in the silencers can alsoincrease the vibration levels of the compressor rotorsand can cause electrical problems to be fed into thelocal bus.Therefore, the silencers should be carefully designedto attenuate the pulsation levels over a widefrequency band.Wet screw compressors typically do not havesilencers, since oil separators are used. The oilseparators are primarily designed to remove the oilfrom the gas, but the separators can also attenuateand/or amplify the pulsation generated by thecompressor. The shell wall mechanical naturalfrequencies can also be excited by pulsation atmultiples of the PPF, which can result in excessivenoise levels and fatigue failures of the shell wall,attached small bore piping, instrumentation, and theoil injection lines.
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