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22.5 The PRIMARY control input should be selected on the basis of the prevailing operating
conditions at the compressor site as well as considerations of loading fluctuations ,etc. Some
basic approaches to compressor load control are listed below :
Suction pressure control - By holding suction pressure at a nearly constant value, a large
number of limited flow rate wells can be kept productive with minimal upset conditions. Usually
this approach is characterized as a relatively limited supply, or a low flow rate supply of gas, at
a given site. This approach may also be required as part of various reclamation or vapor
recovery programs.
Discharge pressure control - By holding discharge pressure at a constant value, a trunk line
feeding a larger compressor, or pipeline system, permits a supply of gas to be delivered at a
rate approximately equal to the rate at which it is to be consumed. The amount of gas being
compressed is not necessarily limited by its availability at the compressor site, but by how much
has been consumed by the destination site.
Engine Manifold pressure control - By adjusting the compressor throughput on the basis of
engine manifold pressure, compressed gas is being produced at a rate that is determined by
the horsepower available at the site. This approach would be used where there is plenty of gas
available at the wellhead and all of it that is produced can be sold or consumed. In this
situation, the only limitation on compressor loading is how much work the engine can do without
subjecting it or the compressor to an overload. In the case of electric motor driven
compressors, a motor current sensor or kW sensor works in the same manner as the engine
manifold pressure sensor on a gas engine.
22.6 When programming the DE-2500 system the basic relationship of the PRIMARY CONTROL
INPUT, PRIMARY CONTROL OUTPUT and OUTPUT ACTUATOR need to be defined.
The relationship between the PRIMARY CONTROL INPUT and PRIMARY CONTROL OUTPUT
is defined as to be either DIRECT or INVERSE acting. DIRECT acting means that to increase
the value of the PRIMARY CONTROL INPUT the throughput of the compressor is increased.
INVERSE acting means that to increase the value of the PRIMARY CONTROL INPUT the
compressor load must be decreased. In the examples of common control approaches given;
discharge pressure and engine manifold pressure or motor amps are direct acting. Suction
pressure is an example of a control parameter that is inverse acting. In order to increase
suction pressure the compressor throughput must be reduced.
In addition to defining the input/output relationship, the relationship of the control output value
to the actuator must also be defined as either direct or inverse acting. As in the case of the
control input/output relationship, the direct acting output mechanism is one where an increase
in current from the controller causes an increase in load on the compressor. An inverse
operating actuator is one where the current is decreased to increase the load on the
compressor.
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