DE-3000 IOI 7-17
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47
ased turn valve, or slide valve, the 4-20mA. CAPACITY output can be connected
to an appropriate I/P transducer which then connects via tubing to the actuator
in order to move the valve
The second set of capacity control outputs is brought out of the controller in
a digital format (discrete transistors). The digital or discrete outputs consist of
two transistors which switch on to ground and are designed to drive relay coils or
pilot duty solenoid valves. One switch is labeled DIGITAL OUT 1 and energizes
to LOAD the compressor the other is labeled DIGITAL OUT 2 and energizes to
UNLOAD the compressor. When used with the hydraulically positioned slide
valve on a screw compressor application, these outputs are typically connected
to a three-way solenoid valve. On reciprocating compressors, the digital outputs
can be used to actuate a motor controlled bypass valve via relay contacts which
energize the motor to move in the open or close directions.
25.5 The PRIMARY control input should be selected on the basis of the prevailing operat-
ing conditions at the compressor site as well as considerations of loading fluctua-
tions, 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 vari-
ous reclamation or vapor recovery programs. This is an inverse acting relation-
ship — increasing the throughput of the compressor causes the suction pressure
to decrease.
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 compres-
sor site, but by how much has been consumed by the destination site. This is a
direct acting relationship — increasing the throughput of the compressor causes
the suction pressure to increase.
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.
25.6 When programming the DE-3000 system, the basic relationship of the Primary
Control Inputs (A1, A2, S1), Primary Control Outputs (AO1 and AO2), and Out-
put Actuators needs to be defined.
The relationship between a Primary Control Input and Primary Control Output is
defined as either direct or inverse acting. direct acting means that to increase
the value of the Primary Control Input, the throughput of the compressor is in-
creased. 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 pa-
rameter that is inverse acting. In order to increase suction pressure the com-
pressor throughput must be reduced.
25.7 The secondary control setpoint options have been modified to add more flex-
ibility as detailed below.
•
INHIBIT AN OUTPUT INCREASE
The output of one or more of the control loops can be limited in the increasing
direction only, while allowing the assigned out-put to freely decrease.
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