Johnson Controls SABROE Unisab III Control User Manual

Plug valve limiters (SAB 157HR)
The high pressure (HPSH) screw compressor type SAB 157HR is equipped with a relief valve
for unloading. This relief or plug valve is used for start relief and torque reduction. The follow-
ing limiter functions will unload the plug valve.
Low lube pressure (SAB 157HR)
To secure the bearing (Lube) oil pressure at start-up Unisab III will start with unloaded plug
valve. Under operation the plug valve will unload if the lube oil pressure is lower than a pre-
defined limit.
If P
oil,abs
< P
suct,abs
* 1.25 + 1.0: unload plug valve
If P
oil,abs
> P
suct,abs
* 1.30 + 1.0: allow loading plug valve
When the plug valve is unloaded, a Lube pressure unload alarm is given.
Insufficient main oil pressure (SAB 157HR)
To secure main oil pressure at start-up and during operation the plug valve will unload if the
absolute oil pressure is too low compared to the absolute suction pressure.
If P
oil,abs
< P
suct,abs
* 1.50 + 1.0: unload plug valve
If P
oil,abs
> P
suct,abs
* 1.50 + 1.1: allow loading plug valve
When the plug valve is unloaded a Low main oil pressure alarm is given with a 180 second
delay.
See also Fig. 9: Insufficient main oil pressure during low differential pressure, SAB 157HR in
subsection 2.2.45. Oil pressure diagrams.
High suction pressure (SAB 157HR)
If the suction pressure is higher than the high alarm limit, unload the plug valve. When the
suction pressure is lower than the high alarm limit – 1.0 bar, allow loading the plug valve.
When the plug valve is unloaded a High suction pressure alarm is given.
High internal pressure (SAB 157HR)
To secure the internal threat pressure at start-up and during operation, the plug valve will un-
load if the internal 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 157HR)
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 157HR)
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.
Compressor control and surveillance
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Engineering manual - Unisab III 1.10
001930 en 2014.09

Johnson Controls SABROE Unisab III Control Specifications

General

Brand	Johnson Controls
Model	SABROE Unisab III Control
Category	Control Systems
Language	English