Figure 25: Suction check valve by-pass
Check valve CV-4 is installed on all units with suction pressure below atmospheric or installed with
economiser option utilised on multiple compressor plants.
On high-stage systems, the check valve, CV-4, should be installed with a 3.1 bar [45 psi] spring to
avoid the possibility of back-feeding to a shutdown compressor from a common economiser vessel.
On booster systems, the check valve, CV-4, should be installed with a 1.7 bar [25 psi] spring to avoid
the possibility of air ingress into the system if the system suction pressure is below atmospheric.
NV-2 must be closed if the oil pump should run for long periods of time when the compressor is
stopped. This is important to prevent oil being pumped up the suction line.
Balance piston pressure regulator
All SGC compressors (SAB 193, 233, 283, and 355) have a balance piston on the male rotor inlet end.
Oil is fed to the balance piston to counteract a portion of the force of gas pressure.
For the SAB 193, 233, and 283, the balance piston oil feed is internal using the common oil
connection through the SB-3 port (standard configuration). A separate configuration is available
for high pressure operation, where an active regulation of the oil pressure to SB-2 is needed by a
Balance Piston Regulator (BPR). On the SAB 355 compressors the oil is fed separately through the
SB-2 port.
55
SAB 283-355 A-Frame OHU 6042 and OHU 7650 (including ATEX) Screw Compressor Unit
