Page 264 - Compression Machinery for Oil and Gas

Reciprocating Compressors Chapter 5 249

indicate that some test locations are very sensitive to the modes of interest,
while other potential test locations are not likely to be sufficiently sensitive.
Another common issue involves recording start-up or shutdown data with
sufficient fidelity to adequately capture the transient events. Historically, it
has been useful to record the signal with digital tape recorders (usually sensitive
to 20kHz or so), or some other means, which allows for quickly playing the
signal back to the computerized DAS (Data Acquisition System) at several sam-
ple rates. This allows for optimized data reduction, since it can be difficult at
times to estimate the optimal sample rate in advance of observing the events.
Such a procedure avoids a potential need to start and or stop the machine several
times while the DAS is optimized. Another consideration for potentially
increasing the shutdown time of a train (and thus increasing the amount of cap-
tured data for a given sample rate) is to reduce the load as much as possible on
the driven equipment during the testing.

Reciprocating Compressor Lateral Rotordynamics
Lateral rotordynamic studies describe the vibration behavior of a rotating shaft
due to the various radial forces acting on the rotor. The lateral rotordynamics of
reciprocating compressors are often not considered a priority, as compared to
other machinery, but a few special considerations are noteworthy and discussed
further. This text is not intended to be a tutorial on subject of lateral rotordy-
namics or vibration, but rather give insight to the unique considerations for
reciprocating compressors. Furthermore, it is noted that lateral rotordynamics
must be carefully considered for many other types of rotating machinery,
including driving equipment, connecting equipment such as couplings and gear-
boxes, and other gas machinery, such as centrifugal compressors. A more in-
depth discussion of lateral rotordynamics for centrifugal compressors is pro-
vided in Chapter 3.
This section will cover four main topics, including a discussion on lateral
natural frequencies of a crankshaft internal to a compressor, lateral modes
involving overhung mass, lateral implications of rigid couplings between
drivers and crankshafts, and lateral-torsional coupling.
In general, the lateral natural frequencies (or critical speeds) of a crankshaft
within a reciprocating compressor (lateral mode shapes that involve amplitude
internal to the compressor) are typically not a concern for two main reasons:
(1) the crankshaft natural frequencies are high in frequency and therefore well
above potential excitations, and (2) the damping from the crankshaft bearings
helps dampen the rotor response. As described earlier in this chapter, the crank-
shaft includes multiple journal locations for the bearings, which are spaced axi-
ally in between or on either side of the crank throws. Fluid-film stiffness and
damping forces exist at all journal-bearing locations. The relatively short axial
spacing between multiple-bearing constraints results in crankshaft natural fre-
quencies (internal to the compressor) that are relatively high in frequency, and