Page 67 - Compression Machinery for Oil and Gas

58 SECTION II Types of Equipment

Choke
At high flow, the head and efficiency will drop steeply, until the compressor will
not produce any head at all. This operating scenario is called choke. However,
for practical applications, the compressor is usually considered to be in choke
when the head falls below a certain percentage of the head at the best efficiency
point. Some compressor manufacturers do not allow operation of their machines
in deep choke. In these cases, the compressor map has a distinct high flow limit
for each speed line.
The efficiency starts to drop off at higher flows, because a higher flow
causes higher internal velocities, and thus higher friction losses. The head
reduction is a result of both the increased losses and the basic kinematic rela-
tionships in a centrifugal compressor: even without any losses, a compressor
with backwards bent blades (as they are used in virtually every industrial cen-
trifugal compressor) will experience a reduction in head with increased flow
(Fig. 3.26). “Choke” and “Stonewall” are different terms for the same
phenomenon.

Compressor Control

The interaction between a compressor and a compression system, in conjunc-
tion with control mechanisms and the compressor characteristic determine the
operating point of the compressor in a given situation.
Control mechanisms can be:
l power input from driver,
l compressor speed,
l guide vane settings,
l compressor suction or discharge pressure set by a throttle,
l compressor throughput modified by recycling gas, and
l multiple units.
External process objectives can be minimum suction pressure, maximum dis-
charge pressure, or delivered flow.
Compressor operational boundaries include surge, minimum speed, maxi-
mum speed, and in some instances minimum pressure rise (choke). The oper-
ating envelope of a centrifugal compressor is limited by the maximum
allowable speed (or, for other control means, the maximum guide vane angle),
the minimum flow (surge flow), and the maximum flow (choke or stonewall),
and the minimum speed (Fig. 3.28). Another limiting factor may be the avail-
able driver power.
Only the minimum flow requires special attention, because it is defined by
an aerodynamic stability limit of the compressor. Crossing this limit to lower
flows will cause a flow reversal in the compressor, which can damage the com-
pressor. Modern control systems prevent this situation by automatically