Page 52 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

• Larger oil and gas
installations use
centrifugal
compressors with 3-
10 radial wheels,
6000–20000 rpm
(highest for small
size), up to 80 MW
load at discharge
pressure of up to 50
bars and inlet
volumes of up to
3
500,000 m /hour.
Pressure differential
up to 10.
Photo: Dresser Rand
Most compressors will not cover the full pressure range efficiently. The
lowest pressure is atmospheric, for gas to pipeline, some 3 to 5 MPa (30-50
bar) pressure is used, while reservoir reinjection of gas will typically require
20 MPa (200 bar) and upwards since there is no liquid in the tubing and the
full reservoir pressure must be overcome. Therefore compression is divided
into several stages to improve maintenance and availability.
Also due to single unit power limitations, compression is often divided in
several parallel trains. This is not the case in this example since gas is not
exported, and reinjection can be interrupted during maintenance periods.
Compressors are driven by gas turbines or electrical motors (for lower power
also reciprocating engines, steam turbines are sometimes used if thermal
energy is available). Often several stages in the same train are driven by the
same motor or turbine.
The main operating parameters for a compressor are the flow and pressure
differentials. The product defines the total loading, so there is a ceiling set by
the maximum design power. Furthermore, there is a maximum differential
pressure (Max P d) and choke flow (Max Q), the maximum flow that can be
achieved. At lower flow, there is a minimum pressure differential and flow
before the compressor will "surge" if there is not enough gas to operate. If
variations in flow are expected or difference between common shaft
compressors will occur, the situation will be handled with recirculation. A
high flow, high pressure differential surge control valve will open to let gas
from the discharge side back into the suction side. Since this gas is heated it
will also pass through the heat exchanger and scrubber not to become
overheated by circulation.

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