Page 230 - Compression Machinery for Oil and Gas

Reciprocating Compressors Chapter 5 219

FIG. 5.39 Reciprocating compressor with small diameter high pressure cylinders. (Courtesy of Ariel
Corporation.)

suction pressure creating a bypass. When the piston is on the compression and
discharge portion of the cycle the gas simply goes back to suction and is not
compressed thus bypassing and the head end is deactivated.
The head-end bypass is attractive to use when the suction pressure is too
high for a finger-type suction valve unloader (about 7MPa and higher) or when
the compressor valves are too small to accommodate a hole through the center
for a plug-type suction valve unloader. This means the head-end bypass can be
an attractive option for use on small diameter higher pressure cylinders
(Fig. 5.39).

Pulsation Control

Reciprocating compressors receive and deliver gas in discrete slugs (or pulses)
as the pistons moves back and forth. For a double-acting compressor there will
be two pulses of gas for each revolution. As these pulses of gas enter or exit the
compressor cylinder they will give rise to pressure pulsation that moves in both
directions at the speed of sound down the pipe. These pulses can be quite large
and so attenuation devices generally called “pulsation bottles” or “dampers” are
mounted on the inlet and discharge of the cylinder. Usually there will be one,
two or three or even more cylinders per stage and so all the cylinders operating
in parallel on each bank will have a common pulsation bottle. So for a two-
throw single-stage compressor there are one cylinder on each bank therefore
there will be two inlet and two discharge pulsation bottles. For a six-throw
two-stage machine the three first-stage cylinders would normally be on one