Page 465 - Compression Machinery for Oil and Gas

Compressor System Design and Analysis Chapter 11 445

reference, Ker Wilson “Practical Solution of Torsional Vibration Problems”
[20, 21]). In general a torsional analysis is necessary for reciprocating, screw,
and centrifugal compressors and should include: preparing an appropriate mass
elastic model; accounting for all gear speed ratio effects (if applicable) on stiff-
ness, inertia, torque, and stress; determining the system critical speeds; evalu-
ating the mode shapes for potential excitation coupling mechanisms; preparing
interference diagrams (for each shaft speed in the train) to determine operating
regimes where critical speeds and excitation energy are likely to coincide; and
evaluating forced response dynamic stress and torque levels for the entire train
during anticipated steady-state operating conditions.
The torsional analysis should consider all potential excitation sources,
including: lobe and/or pocket passing frequencies from the screw compressors,
engine drive order multiples, electric motor slip frequencies, variable frequency
drive integer or noninteger excitation components, gear mesh frequencies, and
similar sources. In addition, a transient torsional analysis is recommended when
synchronous electric motor or variable frequency drives are involved, in order
to determine if the machinery can tolerate the stress and torque levels developed
during transients such as start-up or motor short-circuit events.

Mechanical: Skid and Piping Analyses
As previously discussed, reciprocating compressors (or any positive displace-
ment compressor) will generate significant mechanical excitation and pulsation
energy at discrete frequencies that will be transmitted throughout the machin-
ery, vessels, and piping system. This excitation can occur at many different fre-
quencies and directions simultaneously. When this excitation energy is
coincident in frequency with one of many MNFs that are inherent in any com-
plex structure, excessive vibration can easily occur. To reduce the risk of vibra-
tion, the mechanical characteristics of a new compression system should be
analyzed at the design stage, or when troubleshooting existing problems.
A typical system can often be divided into several key areas such as the com-
pressor manifold system (including the compressor, the pulsation bottles, and
the directly attached piping), the compressor skid and the piping system. For
simple piping systems, clamp spacing guidelines can be applied to achieve cer-
tain minimum MNFs in the piping, depending on the operating speed of the
compressor. API 618 recommends all mechanical responses of the piping
and vessels be above 2.4 the highest compressor operating speed (rpm/60).
While there is no similar standard for mechanical responses in piping associated
with centrifugal compressors, clamp spacing guidelines can also be used to
ensure the MNFs of the piping are not easily excited by flow turbulence (see
the FIT section). More advanced finite element-based modeling techniques
can also be applied for mechanical piping analyses.
For the mechanical analysis of a reciprocating compressor manifold or skid,
the system is typically modeled in detail using finite element modeling