Page 187 - Compression Machinery for Oil and Gas

176 SECTION II Types of Equipment

four-throw compressor in order to meet the current pulsation requirement. The
motor should be heavy duty (also called severe duty). For heavy duty applica-
tions the motor manufacturer will typically provide a large shaft, heavy duty
fan, more internal bracing of windings, etc. Keyways are best avoided, but
if necessary they should include a radius fillet to minimized torsional
stresses [3]. It is best if the motor shaft at the drive end from the core through
shaft end (including bearing) be no less than the crankshaft stub shaft [3].

Lateral Pulsation The compressor imposes a strong lateral pulsation on the
motor. This comes from the rod loads of the compressor. They are transmitted
to the motor in two ways: through the baseframe and foundation and through
the crankshaft to the motor shaft. At each revolution the two throws adjacent
to the drive apply the rod load to the crankshaft at the instantaneous angle of
the connecting rod. As the crankshaft rotates it roughly orbits and because the
two drive end throws are in phase but opposed, the crankshaft also angles. The
motion of the crankshaft is limited by the bearing clearance which will typically
not exceed 0.1% of the shaft diameter as a diametral clearance. The motion at the
crankshaft is primarily at 1 but there is some excitation at all higher harmonics.
For a two-bearing motor connected via a flexible coupling the crank lateral
motion will cause vibration of the flywheel (0.2mm p.p. for a 200-mm crankshaft
assuming nonresonant motion), but only a limited amount is transmitted through
the coupling to the motor. However, for rigidly connected motors, for example, a
single-bearing synchronous motor, the entire lateral motion of the crankshaft is
transmitted directly. The outboard bearing and motor shaft needs to be designed
for the crankshaft orbital motion while limiting the vibration at the bearing to
acceptable limits. Because of the compressor pulsating loads the vibration at
the drive motor will exceed what is normally considered acceptable. For example,
ISO 10816-3 list an acceptable vibration at the motor of 4.5mm/s rms, however
this standard specifically excludes motors driving reciprocating compressors.
A more realistic limit would be the acceptable value listed for the compressor
frame which is 8mm/s rms as a typical vibration limit for the motor frame and
the bearings.

Variable Frequency Drives
A VFD allows an otherwise fixed-speed electric motor to run with variable
speed. Variable speed allows almost infinite and very simple capacity control.
Refer to Chapter 7 for more detail about VFDs.

Engines
The majority of reciprocating compressors used in the upstream and midstream
industry segments today are driven by natural gas fueled engines. The gas being
compressed is also used to fuel the engine. The compressor is matched to the
engine power and speed connected by a flexible coupling, commonly referred