Page 182 - Compression Machinery for Oil and Gas

Reciprocating Compressors Chapter 5 171

Piston speed¼ Feet per minute (fpm) m/s
Stroke¼ Inches mm
Rotating speed¼ Revolutions per minute (rpm) rpm

For example, for a rotating speed of 400rpm, a stroke of 304.8mm (12in.),
the average piston speed is
2 12 400 2 304:8 400
Piston Speed ¼ Piston Speed ¼
12 1000 60
Piston Speed ¼ 800fpm Piston Speed ¼ 4:06m=s

At piston speeds greater than about 6.1m/s then the inertia forces in the
reciprocating parts become so high that they approach the rod load limits of
the machine, this provides an upper limit on piston speed.
Fixed vs. Variable Speed
It is generally lower in cost and simpler to provide efficient capacity control by
varying the volumetric efficiency (VE) using devices such as variable timing of
the inlet valves, inlet valve unloaders to inactivate a cylinder end or by using
clearance pockets to vary the cylinder clearance (and thereby vary VE). The
highest efficiency capacity control is by using clearance pockets followed by
compressor speed variations. Speed variation is inherent (within limits) using
an engine driver, but also by using a variable frequency drive (VFD) using
an electric motor drive. Variable speed may involve additional driver cost
(for motor applications) plus technical complications in the compressor valves,
inlet and discharge bottle and piping pulsation and also in the torsional system
(these issue will be discussed in later sections).

Driver Options
Reciprocating compressors used in the oil and gas industry today are driven by
electric motors or natural gas fueled engines. Electric motors are either induc-
tion or synchronous. Induction motors are typically chosen for speeds of
400rpm or higher, synchronous motors are often preferred for speeds less than
400rpm and power greater than 2MW. Motors are generally fixed speed but can
incorporate a VFD to provide variable speed.
Reciprocating compressors impose a cyclic torque and lateral vibration
transmitted through the crankshaft to the motor shaft and lateral vibration trans-
mitted to the motor through the foundation. This means that driving a recipro-
cating compressor needs to be considered severe duty and appropriate
considerations are required.

Effect of Torque Pulsations
At each revolution the reciprocating components are accelerated and deceler-
ated as the pistons travels out to the outer end and back to the inboard end