Page 317 - Compression Machinery for Oil and Gas

302 SECTION II Types of Equipment

Screw compressors can operate with a large variation of molecular weights
as indicated in Figs. 6.31 and 6.33. The main concern with high mole weight gas
is the tendency of the gas to remain entrained in the oil instead of degassing.
Generally, gases with hydrocarbons C6 and heavier should be approached with
caution. The oil supplier must confirm the dilution levels and the performance
of the oil at the given operating points.

Application Examples for Screw Compressors

The examples in Tables 6.6–6.9 are chosen to demonstrate the range and ver-
satility of dry screw compressors.

Rotordynamics
Lateral Rotordynamics

A detailed evaluation of the lateral rotordynamics of screw compressors are typ-
ically not necessary, since compressor manufacturers typically design the com-
pressors per API Standard 619 [1] to avoid lateral rotordynamic issues. In all
screw compressors designed per API Standard 619 specifications, the rotor runs
subcritical (below the first lateral mode), which avoids synchronous energy (1
running speed) from passing through a critical speed and typically avoids the
need for a lateral critical analysis.
If it is determined that a screw compressor could potentially be experiencing
vibration associated with a lateral rotordynamics issue, the issue should be
investigated in a manner generally consistent with other modern rotating equip-
ment. Several notable resources, (such as API619, ISO 10440-1:2007, and
API684, among others) provide specific guidance on carrying out such an anal-
ysis for machinery of this type. As noted above, the rotor always runs subcritical
(below the first lateral mode), which is generally advantageous from an unbal-
ance standpoint as synchronous energy (1 running speed) is not required to
pass through a critical speed. Generally speaking, stiffness and damping coef-
ficients 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 speed 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.
In addition, the lateral rotordynamic characteristics of screw compressors
are subject to some specific considerations. The lateral critical speeds of many
of these compressors are relatively high compared to other rotating equipment,
due to the comparatively short and stiff rotors involved. In all screw compressor
designs, the rotor runs subcritical (below the first lateral mode), which is