Page 181 - Compression Machinery for Oil and Gas

170 SECTION II Types of Equipment

have the advantage that they can handle a wide range of gas density (gas MW),
and even varying gas density. The screw and vane compressors are limited in
pressure to about 30bar. Reciprocating compressors have high adiabatic effi-
ciency because the compression take place within the cylinder and due to the
valve arrangement the amount of compression automatically matches the over-
all compression. For some screw and vane compressors the amount of compres-
sion is fixed and so may be greater or less than actually required, this results in a
loss. Some screw types have discharge valves which can be adjusted to match
the actual compression required but the adjustment is usually manual.
Centrifugal compressors are very simple and highly reliable with minimal
maintenance requirements. The mechanical efficiency is very high, over
99%, but the internal losses due to gas recirculation between the blades and
stages are relatively high with result that the adiabatic efficiency is lower than
is typical for a reciprocating compressor. The power density is very high, that is,
the size and weight of a centrifugal compressor is only a fraction of the size of a
reciprocating compressor of the same power. Centrifugal compressors operate
on the principle of speed-induced gas inertia forces generating pressure (veloc-
ity head), as this is a function of both gas velocity and gas density changes in gas
density (gas MW) will have a major effect on compressor performance. Cen-
trifugal compressors have a difficult time in handling low MW gases as well
where their MW varies over time. It is for this reason that large hydrogen com-
pressors are usually reciprocating compressors.

Speed Considerations
Rotative Speed
The rotative speed for reciprocating compressors used in the oil and gas indus-
tries is usually in the range of 200–1800rpm. Below 600rpm is considered slow
speed and over 600rpm is considered high speed. Historically speeds lower than
100rpm have been used but in modern times (since 1980) speeds of 200–
300rpm are used for hyper compressors (100–350MPa discharge pressure),
300–600rpm for slow speed used mainly in process applications and high speed
in most other applications with discharge pressure below 40MPa. One determi-
nation is the speed of the driver, a natural gas compressor is often driven by a gas
engine and the speed of the engine will vary between 750 and 1800rpm depend-
ing on the power.

Average Linear Piston Speed

2 Stroke Rotating Speed
Linear Piston Speed ¼ FPM,
12
2 Stroke Rotating Speedm=s
or
1000 60