Engineering manual - SAB 355 Screw compressor unit (including ATEX)
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004100 en 2020.10
Technical description
Fig. 29: P&I diagram, liquid injection with dual port
4.1.17 Suction check valve by-pass
The unit is equipped with a low-pressure-drop suction check valve bolted directly to the compres-
sor housing. Units with an 8" stop valve or larger will be piped as shown in Fig. 30. During normal
operation, valve NV-1 is closed. This is a pump-out connection to allow refrigerant removal to the
system suction prior to evacuation for servicing. In most systems, valve NV-2 must be open at all
times. It should normally be cracked open to allow the oil separator to slowly bleed down to ap-
proximately system suction pressure when the unit is stopped (having this valve cracked open al-
lows the compressor drive motor to have an easier start and the discharge check valve will seat
more tightly). If the drive coupling backspins, start closing the valve until the backspin stops. If
the separator oil level foams excessively on shutdown, NV-2 should be closed slightly. If the sepa-
rator takes more than 20-30 minutes to equalise to suction pressure after shutdown, NV-2 can be
opened slightly. See Fig. 30.
Fig. 30: 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.
Compressor
Low Vi
Compressor
H
igh Vi
To separator
Tubing line
Motorised
e
xpansion
valve
Solenoid
v
alve
Strainer
Sight glass
Liquid
line
Liquid refrigerant
fr
om receiver
Compressor
suc
tion pressure
5
170
165
171
F
rom
separator
cold-start
valve
For 8” and larger suction valve
customer supplied line
Compressor end view
Suc
tion gas
to the compr.
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