Page 189 - Compression Machinery for Oil and Gas

178 SECTION II Types of Equipment

dynamic torques at the gear mesh comply with the API 618 max limit of 25%
P-P of the transmitted torque. This is necessary to avoid gear mesh chatter that
will destroy the gear box if too severe.

Couplings
Couplings are used to connect the driver to the compressor. The simplest form is
when the crankshaft is direct bolted to the motor shaft, that is, without a cou-
pling. This is the normal arrangement for bolting a slow-speed compressor to a
single-bearing synchronous motor. Alignment is critical and is accomplished by
aligning the outboard bearing so that the crank web deflections at the drive end
throw are close to zero (typical limit is no greater than 0.025mm runout using a
dial indicator in the crank web). This is highly reliable, torsionally and laterally
stiff and will accept a high torque pulsation including a torque reversal.
All other coupling types will accept some degree of misalignment, lateral,
and angular, they are selected basis the torsional analysis based on the torques
applied, stiffness, and damping requirements. Flexible disc couplings are eco-
nomical and torsional stiff, they will accept lateral and angular misalignment,
they will accept large torsional pulsations but only limited torque reversal. A
flywheel is often used on the crankshaft to absorb torsional pulsations to avoid
overloading and to limit torque reversals on the coupling.
Elastomeric couplings come in two basic varieties, rubber in compression
and rubber in shear, the elastomer has a limited life and needs to be replaced
at the original equipment manufacturer (OEM)’s recommended replacement
interval. Also properties are temperature dependent and so the torsional analysis
should consider maximum and minimum temperature properties they are usu-
ally limited in power to approximately 3MW due to the heat dissipation limits
of the elastomer. The rubber in compression is moderately stiff and has only
limited misalignment capability. It consists of matching steel or nodular iron
halves with male teeth on one side with female teeth on the other, between
the teeth are synthetic or elastomer blocks. The blocks provide the torsional
stiffness along with torsional damping. They are used when the torsional analyst
determines a moderate torsional stiffness and some level of damping is required.
Rubber in shear couplings is highly torsionally flexible, but the lateral misalign-
ment is limited. They also provide good damping. They are typically used when
it is desired to place the first torsional natural frequency below the running
speed, this would normally be on variable speed applications.
Steel couplings with coil springs in compression are used when a torsionally
soft coupling is required usually to bring the first TNF (torsional natural fre-
quency) below 1 . Stiffness quoted is quite accurate and is generally constant
with load and temperature. Damping is possible using damper friction blocks
(coulomb damping), however the damping capacity is quite limited. These
are used in high kW applications, and below 3MW elastomeric couplings
are usually more economical.