Page 321 - Compression Machinery for Oil and Gas

306 SECTION II Types of Equipment

In general, dry screw compressors are more prone to pulsation and vibration
issues; however, wet (oil-flooded) screw compressors can sometimes have pul-
sation and/or vibration issues also. Oil-flooded screw compressors normally do
not have suction or discharge silencers, so that there is less pulsation attenuation
between the compressor and connecting piping. However, suction silencers
are typically less critical, and the oil and “foam” (oil-gas mixture) that is created
by adding the oil to the flow path tends to add significant damping to the
compressor-generated pulsations.
For higher power screw compressors (>370kW), a Helmholtz resonator-
type acoustic filter/silencer (reactive-type pulsation dampener) is typically
designed to reduce upstream and/or downstream pulsation amplitudes to accep-
table levels. These types of silencers are volume-choke-volume arrangements
that are designed such that the acoustic responses associated with the silencer
design are below the screw compressor lobe passing frequency (4 running
speed) and results in a flow with significantly reduced pulsation amplitudes.
This type of design can also result in reduced noise being emitted from the sys-
tem. However, a reactive silencer that is improperly designed for a given appli-
cation can result in high shell vibrations and noise amplification. Acoustic filter
vessels are most effective at attenuating pulsations at the lower end of the fre-
quency spectrum and can provide some attenuation at the middle and higher
frequencies.
Passive- or absorptive-type vessels and piping applications can also success-
fully attenuate pulsations. Absorptive-type vessels are typically good at atten-
uating the higher frequency content of the pulsations, but these types of vessels
are less effective at attenuating the lower and middle pulsation frequencies.
Some examples of absorptive vessels are vessels that are lined with or have
internals that include sand contained in a relatively thin wall of steel or steel
wool restrained with perforated metal sheets and/or steel mesh. Materials
restraining the steel mesh can sometimes experience fatigue failure, which
could then result in a release of some or all of the absorptive materials. Those
materials will then travel downstream of the vessel or fall into the compressor,
in the case of top-connected silencers.
To attenuate pulsation amplitudes at both lower and higher frequencies,
reactive and absorptive designs are sometimes implemented. Absorptive mate-
rials can be installed inside volume-choke-volume-type vessels to significantly
reduce pulsation amplitudes across the frequency spectrum.
Compressor design for the given application can play a significant role in
determining whether the resulting pulsation amplitudes are excessive or not.
If the V i does not result in a compression ratio that corresponds with the system
application, undercompression or overcompression will occur. Undercompres-
sion or overcompression by an excessive amount will result in significantly
higher discharge system pulsation amplitudes; however, undercompression is
less likely to cause higher pulsation amplitudes.