Operating Cycle of a pilot operated safety valve
LESER Pilot Operated Safety Valve (POSV) is controlled by process medium. To achieve this, the system pressure is applied to the pilot valve (= control component for the main valve) via the pressure pickup. The pilot valve then uses the dome above the main valve piston to control the opening and closing of the main valve.
There are two types of pilot-operated safety valves, showing their difference in the opening characteristics. While one of them opens and closes abruptly (LESER Series 810 – Pop Action), the other one is characterized by proportional opening (LESER Series 820 – Modulate Action).
During operation, the POSV goes through these basic operating states:
1. Below set pressure: normal operation
During normal operation, the system pressure is picked up at the main valve inlet and routed to the dome (see illustration).
Since the dome area is larger than the area of the main valve seat, the closing force is greater than the opening force. This keeps the main valve tightly closed.
2. At set pressure: actuating state
At set pressure, the pilot valve actuates. The medium is no longer routed to the dome (see illustration). This prevents a further rise in dome pressure.
Also, the dome is vented. As a result, the closing force ceases as a precondition for the system overpressure to push the main valve open.
3. Main valve opening
The main valve opens. Depending on the design of the pilot valve, this opening is either rapid and complete (Pop Action) or gradual and partial following system pressure (Modulate Action).
4. At closing pressure: refilling the dome
If system pressure drops to closing pressure, the pilot valve actuates and again routes the medium to the dome.
The pressure in the dome builds up and the main valve recloses either rapid and complete (Pop Action) or gradual and partial following system pressure (Modulate Action).