The parameter Run Time defines how long it will take to run the slide from 0-100% at con-
stant up signal or from 100-0% at constant down signal. Usually slide speed is always adjus-
ted on the oil needle valves positioned on the capacity regulating block so that the slide speed
is approximately 60 seconds when the oil is warm from operation. This adjustment is usually
carried out by the service engineer who starts up the compressor unit.
If the slide speed for some reason cannot be adjusted for 60 seconds, the Run Time parame-
ter must be changed so that run time corresponds to the actual operating conditions. If this is
not done, it will appear as if the PID regulator is out of control and it may cause hunting.
Example 1
A screw compressor with a combination of slide regulation and frequency regulation has a
slide speed at 60 seconds from 0-100% slide movement. Depending on min. and max. fre-
quency this is shown as e.g. 0-42% capacity on the Unisab III display. When the slide has
reached 100%, the frequency converter will start to increase the frequency to the max. fre-
quency set. This will take 60 seconds and be shown as 42-100% in the Unisab III display.
Thus the Run time parameter must be set for 60+60=120 seconds for correct regulation.
However, the Run time parameter only corresponds to the slide regulation and should there-
fore be set to 60 seconds.
For screw compressors connected to frequency converters see also the Unisab III sequence
manual.
Example 2
PID = Yes
Values for outer loop:
Nz = 0.0
P band = 10.0
T. int. = 60.0
T. diff. = 0.0
If the compressor is running very unsteadily during regulation or two compressors cannot find
one another in parallel operation, raise the P band to e.g. 50°C.
If the running of the compressor has steadied, but the regulator seems slow to reach its set
point, reduce the P band a little at a time until the running of the compressor becomes un-
steady again. Now raise the P band to the last applicable value. If the suction pressure does
still not reach its set point (depending on the setting of Nz) a reduction in the integral time
may be necessary to fine-tune the regulator.
If the regulator for a long period of time seems to hunt, increase the T. int. to e.g. 100-200
sec. If the hunting stops, reduce the T. int. until the system starts hunting again. Now in-
crease the T. int. to the last applicable value.
Usually it will never be necessary to change the T. diff. to a value different than 0 sec. How-
ever, in special cases where the compressor is part of a process plant, e.g. in petrochemical
plants, it may be necessary to use another value than 0 sec. This will only be necessary if the
process requires a very fast regulation upwards or downwards to avoid that the suction pres-
sure removes itself too far from the set point.
The following diagrams show how the PID regulator reacts on a specific change in the input
signal. The diagrams are divided into P, I and D parts.
Compressor control and surveillance
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Engineering manual - Unisab III 1.10
001930 en 2014.09
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