Birkett Technical Catalogue - Safety Systems UK Ltd

STAGES OF OPERATION - TYPES 4 AND 8
Type 4 & 8 Pilots are fully modulating. They are
intended for use with gas, liquid and mixed phase
fluids. Both types are non flowing designs.
STAGE 1
System pressure below set pressure
The feeding seat remains open with the drain seat
closed. This maintains equal pressures in the
dome and the system. The main valve is closed,
held tightly against the nozzle.
Pilot Valve Drain
STAGE 2
System pressure approaches set pressure
When the system pressure approaches the set
pressure the feeding seat closes. The drain seat
remains closed and the main valve is closed.
The pressure in the dome is now controlled by
the inlet system pressure acting against the pilot
diaphragm/piston and the dome pressure acting on
the feed back piston. The combination of these
two forces controls the opening of the drain seat,
thereby controlling the dome pressure and hence
the main valve lift.
When the drain seat opens it discharges a small
volume of fluid from the dome. At no time is the
drain seat continually flowing. It drains in short
bursts.
P
STAGE 3
System pressure reaches set pressure
As the system pressure gradually rises to the set
pressure, the dome pressure gradually falls
approximately 30%. Due to the differential size,
top and bottom of the piston, the main valve
opening and closing forces are now in equilibrium.
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