Engineering manual - SAB 193-233-283 S A-frame (including ATEX)
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008831 en 2020.10
Technical description
4.26 Compressor protecting system, pos. 220 and 221
Fig. 33: 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. 33.
Functional description
The pilot valve, pos. 221, is actuated by the compressor discharge pressure P1 and is kept closed
by a built-in spring whose closing force is adjusted to the pressure shown on the valve plate. The
reference pressure of the pilot valve is the atmospheric pressure Po, and the valve is dimensioned
in such a way that the pressure P2" does not affect the opening pressure of the valve.
If the pressure P1 rises above the set opening value on the valve plate, the pilot valve, pos. 221,
will modulate open and raise the pressure P2" below the piston, pos. 5, in the main valve.
In the chamber above the piston, pos. 5, there is the same pressure P1 as under the valve cone,
pos. 2, since the piston rod, pos. 3, is pierced. When the main valve is closed, the pressure P2" in
the chamber, pos. 6, below the piston will be the same as the compressor discharge pressure P2
since the pressure P2" is equalised through the nozzle, pos. 7.
If the pilot valve, pos. 221, opens due to too a high pressure P1, gas will flow from the pilot valve
to the main valve through the P2" line. If there is more gas that can flow through the nozzle, pos.
7, the pressure will rise in the chamber, pos. 6, and modulate open the main valve.
Thus the way is open for a gas flow through the main valve from the compressor discharge side to
the suction side.
When the pressure P1 falls below the set pressure of the pilot valve again, the pilot valve will
close. The pressure P2" in the chamber, pos. 6, is equalised to the P2 pressure through the nozzle,
pos. 7, and the main valve closes in less than 50 seconds. The activation of the compressor pro-
tecting system will usually not result in valve leaks or in the need for regulation.
Pilot valve BSV8
(Pos. 221)
Main valve POV
(P
os. 220)
From evaporator
To oil separator
c
ondenser
Condensing
pr essure
Evaporating
pr essure
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