Engineering manual - SAB 193-233-283 S A-frame (including ATEX)
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008831 en 2022.02
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 pres-
sure P2" in the chamber, pos. 6, below the piston will be the same as the compressor dis-
charge 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 noz-
zle, 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 com-
pressor protecting 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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