Engineering manual - UniSAB III 1.10.8
001930 en 2021.06
99/346
Compressor control and surveillance
When the plug valve is unloaded a High suction pressure alarm is given.
High internal pressure (SAB 157 HR)
To secure the internal threat pressure at start-up and during operation, the plug valve will unload if the in-
ternal pressure is high.
If P
suct,abs
* Vic^kappa > 80: unload the plug valve
If P
suct,abs
* Vic^kappa < 75: allow loading plug valve
When the plug valve is unloaded, a High internal pressure alarm is given.
High motor current (torque) (SAB 157 HR)
For torque reduction in case of current overload, unload the plug valve but continue to allow increasing
motor speed.
If I
current
> SP1/SP2 for motor current: unload plug valve
If I
current
< SP1/SP2 – NZ: allow loading plug valve
When the plug valve is unloaded, UniSAB III will show Running Overload in compressor state.
Low Vi ratio (SAB 157 HR)
If the compressor is not running at a Vi ratio corresponding to the block fixed Vi ratio, it is possible to use
the plug valve to compensate for the lower Vi ratio.
The limiter operates in two situations, under 95% of maximum speed where the compressor can run
more efficiently in the unloaded plug valve mode and at 100% of maximum speed to protect the com-
pressor. Speed between 95% and 100% is the speed hysteresis.
If speed < 95%:
If Vi < 0.80 * fixed Vi ratio: unload plug valve
If Vi > 0.85 * fixed Vi ratio: allow loading plug valve
This part of the Vi limiter can be enabled or disabled in the menu Compressor block.
See subsection 6.4.3 Compressor block 5: VI comp. plug valve.
If speed = 100%:
If Vi < 0.65 * fixed Vi ratio: unload plug valve
If Vi > 0.70 * fixed Vi ratio: allow loading plug valve
When the plug valve is unloaded a Low Vi alarm is given. This part of the Vi limiter cannot be disabled.
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