Johnson Controls SAB 355 - Compressor Protecting System, Pos. 220 and 221

Engineering manual - SAB 355 Screw compressor unit (including ATEX)
004100 en 2020.10
59/156
Technical description
Booster applications
For booster applications, the valve is equipped with a light spring, which produces 0.5 bar [7 psi]
oil pressure above suction pressure before it fully powers open. An oil pump is required to ensure
compressor lubrication.
The unit is also equipped with a suction check valve by-pass. The oil separator will slowly bleed
down to approximate system suction pressure when the unit is stopped. This allows the compres-
sor drive motor to have an easier start, and the discharge check valve will seat more tightly. See
section 4.1.17 Suction check valve by-pass for operation.
Note: For alarm descriptions and shutdown or cut-out parameters, see separate control system
manual.
Fig. 34: Cold-start valve
4.1.26 Compressor protecting system, pos. 220 and 221
Fig. 35: System with closed valve
In order to protect the compressor against an inadmissible high pressure (if the electric system
has not already stopped the compressor), the compressor is fitted with a protecting system with
one or two main valves, pos. 220 and corresponding pilot valve(s), pos. 221, see Fig. 35.
O
pen
Closed
P3 - To suction
P2 - To
condenser
P1 - From
oil separator
P1
P2
P3
Pilot valve BSV8
(Pos. 221)
Main valve POV
(P
os. 220)
From evaporator
To oil separator
c
ondenser
Condensing
pr essure
Evaporating
pr essure