Page 265 - Compression Machinery for Oil and Gas

250 SECTION II Types of Equipment

well above potential excitations. In summary, lateral natural frequencies (or
critical speeds) of a crankshaft that involve mode shapes with amplitudes within
a reciprocating compressor are not a typical concern.
Lateral natural frequencies for reciprocating compressor crankshafts that
involve an overhung mass, such a coupling hub or flywheel, can become impor-
tant. Specifically, this is referring to a lateral natural frequency that involves a
mode shape with large amplitude at the overhung mass. These particular modes
are controlled by the shaft stiffness of the overhung portion, and the mass of
the overhung component. These modes are potentially excited by imbalance
at 1 , misalignment at 1 and 2 , and also by the rod loads acting from
the connecting rod at the two throws adjacent to the drive end, primarily at
1 and 2 . With relatively high amplification and little damping (mostly from
the crankshaft-bearing oil films), operating on or very near an overhung lateral
natural frequency (primarily at 1 and 2 ) can cause significant lateral vibra-
tion and potentially failed parts. In most cases, a simple hand calculation con-
sidering the crankshaft held at the two drive-end bearings, shaft, and overhung
mass may be adequate to determine that the lateral natural frequency is separate
from 1 and 2 running speed. Such a check (or a complete lateral analysis)
should be performed for these types of designs during the design phase to deter-
mine the potential for any overhung modes to be excited in the running
speed range.
The lateral rotordynamics of a reciprocating compressor also become
important when the crankshaft is rigidly connected to the driving equipment.
When a rigid coupling is used, the lateral rotordynamic response of the crank-
shaft is coupled to the driver shaft. Therefore, the lateral rotordynamic model
and analysis should (at a minimum) include both the driver and reciprocating
compressor crankshaft through the first two throws and crank bearings at the
drive end. Traditional calculation of natural frequencies, mode shapes, and
imbalance response should be considered for the coupled driver and compressor
crankshaft model. It is noted that most oil and gas reciprocating compressors
today are driven through flexible couplings, where the lateral response of the
driver and compressor is de-coupled, or independent of one another.
Lateral and torsional coupling, from a vibration standpoint, only occurs
when a kinematic constraint relates lateral motion to torsional motion. This
can be seen in a geared system or gearbox where lateral motion of the bull
or pinion can influence or become affected by the torsional motion through
the connection at the gear mesh. Typically, lateral-torsional coupling is seen
when a torsional mode of a machinery train is excited, resulting in measureable
lateral vibration of the pinion. There is the possibility, although considered very
rare, of a coupled lateral-torsional mode in a geared system. Further information
on lateral-torsional coupling in rotordynamics is discussed in Childs [18].
In addition, lateral and torsional vibration coupling has been noted in recip-
rocating compressors. The literature shows that lateral and torsional vibration
coupling can be problematic in reciprocating compressors. Stephens et al.