Page 320 - Compression Machinery for Oil and Gas

Screw Compressors Chapter 6 305

4 rpm. Excessive pulsations and vibrations can result in small-bore piping
failures. The failures could include cracked lube oil lines (for oil-flooded screw
compressors) and instrumentation, which could cause a gas leak and potentially
dangerous situation. Therefore, it is important to consider the system pulsations,
particularly for higher power compressors. Typically, the relatively standard-
ized silencer designs developed by many compressor manufactures have proven
to significantly reduce the compressor-generated pulsations, which results in
acceptably low vibrations and noise. Because of the standardized and proven
silencer designs and piping acoustic wraps downstream of the compressors,
most screw compressor installations do not require a pulsation and/or vibration
analysis. In addition to the compressor vibration, pulsation, and noise abatement
implementations, the gearboxes are typically placed in a noise enclosure. With
these measures many compressor manufacturers can typically achieve a sound
pressure level of 85dB. If the speed, pressure ratio, gas molecular weight, dis-
charge pressure, and over/undercompression are within the manufacturer’s
range of experience, a detailed acoustic or pulsation study is typically not nec-
essary. The advice of the manufacturer should be sought.
There are some instances when screw compressors can experience pulsa-
tion problems. Due to the relatively high running speeds of screw compres-
sors, the pulsation excitation frequencies are typically much higher than
those associated with reciprocating compressors. Therefore, one-dimensional
(1D) and three-dimensional (3D) acoustic modes can be excited (resonate).
Resonance can occur if the compressor frequencies of excitation coincide with
the 1D and/or 3D acoustic natural frequencies associated with the pipe and/or
vessel geometry and gas properties. However, the shape and location of the
acoustic natural frequencies determines whether an acoustic resonance exists
or does not exist. Excitation of the acoustic modes (acoustic resonance) can
result in piping and/or vessel shell wall vibrations/buzzing, and those vibra-
tions can be amplified significantly if the frequency of an acoustic resonance
coincides with the mechanical natural frequencies of the piping and/or vessel
shell modes.
Pulsation characteristics at lower frequencies can be evaluated with a 1D
acoustic analysis. At higher frequencies, acoustic modes associated with the
silencer vessel, gas-oil separator/reservoir, screw compressor internal gas-oil
passage (IGP), and connecting piping can be excited. These acoustic cross
modes (3D modes) can excite shell vibration/mechanical modes involving both
circumferential and axial deformation. An acoustic 3D finite element analysis
(FEA) can be performed to enable prediction of the acoustic radial or other 3D
mode shapes and frequencies associated with the silencers, compressor IGP,
and connecting piping to avoid the risk of exciting 3D acoustic and mechanical
modes. The 3D acoustic FEA can be coupled with a mechanical FEA of the
silencer to predict the significant response frequencies for the silencers, which
could lead to high vibration if coincident or nearly coincident with the compres-
sor excitation orders.