Page 71 - Compression Machinery for Oil and Gas

62 SECTION II Types of Equipment

Centrifugal compressors, when driven by two-shaft gas turbines, are usually
adapted to varying process conditions by means of speed control. This is a very
elegant way of controlling a system, because both the centrifugal compressor
and the power turbine of a two-shaft gas turbine can operate over a wide range
of speeds without any adverse effects. A typical configuration can operate down
to 50% of its maximum continuous speed, and in many cases even lower. Reac-
tion times are very fast, thus allowing a continuous load following using mod-
ern, Programmable Logic Control (PLC)-based controllers.
A simple case is flow control: the flow into the machine is sensed by a flow
metering element (such as a flow orifice, a Venturi nozzle, or an ultrasonic
device). A flow set point is selected by the operator. If the discharge pressure
increases due to process changes, the controller will increase the fuel flow into
the gas turbine. As a result, the power turbine will produce more power and
cause the power turbine, together with the driven compressor, to accelerate.
Thus, the compressor flow is kept constant. Both the power turbine speed
and the power increase in that situation.
If the discharge pressure is reduced, or the suction pressure is increased due
to process changes, the controller will reduce the fuel flow into the gas turbine.
As a result, the power turbine will produce less power and cause the power tur-
bine, together with the driven compressor, to decelerate. Consequently, the
compressor flow is kept constant.
Similar control mechanisms are available to keep the discharge pressure
constant, or to keep the suction pressure constant. Another possible control
mode is to run the unit at maximum available driver power (or any other con-
stant driver output). In this case, the operating points are all on a line of constant
power, but the speed will vary.
The control scheme works for one or more compressors, and can be set up
for machines operating in series, as well as, in parallel.
If speed control is not available, the compressor can be equipped with a
suction throttle, or with variable guide vanes. The latter, if available in front
of each impeller is rather effective, but the mechanical complexity proves usu-
ally to be prohibitive in higher-pressure applications. The former is a mechan-
ically simple means of control, but it has a detrimental effect on the overall
efficiency.

Multiple Units
If a compressor station operates multiple compressors, either identical units or
dissimilar units, turning these units on or off provides a powerful means of con-
trol. For particular applications that see large swings in load, this control method
allows to operate the drivers near their best efficiency conditions for most of
time. It also has a positive impact on maintenance costs, since the drivers are
only operated when needed. The method works well if the selected equipment
has a high starting reliability. It is also advantageous if the compressors can