Page 259 - Compression Machinery for Oil and Gas

244 SECTION II Types of Equipment

different coupling. Two types of couplings widely used in reciprocating systems
are of the flexible disc and elastomeric varieties. These couplings both have
advantages and disadvantages and can be exchanged to produce a significant
shift in torsional rotordynamics. Similarly, couplings within a series of a
particular type may be exchanged for a less dramatic shift in the critical speeds.
Flexible disc couplings are very commonly specified in reciprocating
applications. These couplings can provide a low cost alternative in many sit-
uations, generally require little maintenance, and are available for a wide vari-
ety of torque and power ranges. Elastomeric couplings can be useful when a
low torsional stiffness is required, for adding damping to a system, or allowing
operation over a wide operating speed range with no lock-out (exclusion)
speeds. However, elastomeric couplings generally require more maintenance
than comparable flexible disc designs and may not be available for some high
torque applications (see Ref. [5], section 3.1.2 for a more detailed discussion
of this subject).
In cases where the machinery must transition through or run on a critical
speed for some operating conditions, it may be useful to increase the damage
tolerance of the shafting. Some common systems that might require this
approach would be a synchronous motor driving a reciprocating compressor
during a start-up condition, or reciprocating compressors utilizing disc-pack
couplings over a wide speed range. One common method for increasing the
damage tolerance in new machinery designs is to increase the shaft diameter.
This change has the additional effect of increasing the torsional stiffness of
the shafting, which may be useful as well. Another common approach in
new machines is to specify a shaft material with an increased UTS. Many equip-
ment manufacturers offer an alternate shaft material for this purpose, which can
greatly increase the ability of the machines involved to tolerate dynamic torque.
Yet another approach is to decrease stress concentration in the shafting an asso-
ciated rotating elements by removing keyways, increasing fillet radii, etc.
In some cases involving reciprocating compressors, loading strategies may
also be modified to reduce shaft stress to acceptable levels. In general, asym-
metric loading strategies (some cylinders loaded, others unloaded) tend to pro-
duce the highest dynamic torques. A common misconception is that the most
loaded (highest power) case would result in the most cumulative damage. In
actuality, the total stress level experienced by a shaft is a combination of mean
stress (which generally does increase with power setting) and dynamic stress
(which can be significantly influenced by excitation of the torsional modes).
It is not unusual for the highest composite stress level to occur at a load step
that produces less power than the full-load case. As such, it may be possible
to utilize a more symmetric loading strategy to achieve a particular operating
condition, while limiting the shaft stress to a tolerable level.
VFDs may also introduce significant alternating torques in a system that can
result in excitation of a critical speed. The design and operation of these drives is
considered beyond the scope of this chapter. However, in some cases it may be