Page 319 - Compression Machinery for Oil and Gas

304 SECTION II Types of Equipment

generally advantageous from an unbalance standpoint as synchronous energy
(1 running speed) is not required to pass through a critical speed. However,
the machines can still respond to typical mechanical excitations (such as mis-
alignment, oil-film instabilities, etc.) and are often subjected to significant
energy at the lobe and/or pocket passing frequencies. Care must be taken to
ensure that sufficient separation margins are maintained between the predicted
lateral modes and prevalent excitation energy over the entire anticipated oper-
ating speed range. Another somewhat unique aspect of these designs is that the
radial loads developed between screw compressor lobes can generate signifi-
cant forces, which typically are greater than the rotor weight. As such, these
loads should be included, in addition to the gravitational loads, when calculating
bearing coefficients.

Torsional Rotordynamics
From a torsional behavior standpoint, trains containing screw compressors are
similar in many respects to other forms of rotating equipment. Several notable
resources are available that outline the requirements for a complete torsional
analysis (e.g., see W. Ker Wilson “Practical Solution of Torsional Vibration
Problems” [2], and API 684 [3], among others). However, in general a torsional
analysis for such a system should include: preparing an appropriate mass elastic
model; accounting for all gear speed ratio effects on stiffness, inertia, torque,
and stress; determining the system critical speeds; evaluating 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.

Pulsation and Vibration
Screw compressors are positive displacement compressors that inherently gen-
erate pulsations (pressure fluctuations). The male rotor typically consists of four
lobes; therefore, the main frequency component of the discharge flow is at 4
the compressor speed (4 rpm). Thus, for four lobe compressors, the dominant
pulsation excitation frequency is at the primary lobe pass frequency (PPF) of