Page 463 - Compression Machinery for Oil and Gas

Compressor System Design and Analysis Chapter 11 443

(in the limit case of a very fast change in compressor power) to a change in flow
without a change in head. If the power setting is maintained, the compressor
operating point would then start to approach the steady-state line again, albeit
at a higher speed, pressure ratio, flow, and power. As shown in Fig. 11.9, the
steady-state compressor map is still valid, however, the compressor operating
path is not known and must be mapped out through a dynamic simulation of the
system.

Interaction Between the System and the Compressor
For any situation, the process determines the suction and the discharge pressure
the compressor ‘sees.’ Based on some control setting (available power, speed,
guide vane setting) the compressor will react to the situation by providing a cer-
tain amount of flow to the system. Thus, the flow into the system is a result of
the compressor characteristic (its map) and some external control setting.
Different controls elicit different scenarios in these control situations: if the
compressor is controlled by the level of power that’s supplied, then the speed at
which the compressor runs is an outcome of the interaction between compressor
and process. If the compressor speed is controlled, the required power is an out-
come. This also applies to transient situations such as line pack. From a process
standpoint, typically flow or pressure is controlled. Note however that pressure
changes relatively slowly due to the usually large pipe volumes, while flow can
change quickly. Thus, the path for the starting condition to the end condition is
different depending on the control method used (see Figs. 11.10 and 11.11, Kurz
et al. [19]).

Rotordynamic Analyses

There are two types of rotordynamic analysis commonly performed on com-
pressor drive trains: lateral and torsional. Torsional analysis evaluates the twist-
ing interaction between rotors and couplings. A lateral analysis evaluates the
rotor vibration in a compressor or train of equipment.
The lateral rotordynamics of centrifugal and screw compressors should be
investigated in a manner generally consistent with the appropriate standard
(API 610, 617, 541, 619, among others). Generally speaking, stiffness and
damping coefficients should be developed for the bearings based on anticipated
loads and geometry. The flexibility of any underlying structural supports should
also be considered and accounted for. An undamped critical speeds analysis
should be performed, along with a damped unbalance response analysis for
unbalance distributions that excite the relevant lateral modes. The stability of
the system should be investigated using a damped Eigenvalue analysis to ensure
that whirl-related problems and instabilities are avoided.
Several notable resources are available that outline the requirements for a
complete torsional analysis (e.g., see API 618, 684 as well as a more historical