Just like any other pressure valve, Pilot Operated Pressure Relief Valves are used for emergency relief during overpressure events. These valves have more options to control the pressure as compare to other valves. As they are pilot valves, the pilot is designed to open gradually, so that the system fluid is lost during each relief event. The Pilot Operated Pressure Relief Valves opens completely over a narrow pressure range. It allows the circuit to operate over a wider pressure range without any loss of fluid over the relief valve.
Like other pressure relief valves (PRV), pilot operated relief valves (PORV) are used for emergency relief during overpressure events (e.g. a tank gets too hot and the expanding fluid increases the pressure to dangerous levels). The distinction between PORV and conventional PRV is that pilot valves use system pressure to seal the valve. APRV typically uses a spring to hold the disc or piston on seat. The essential parts of a PORV are a pilot valve (or control pilot), a main valve, a pilot tube, the dome, a disc or piston, and a seat. The volume above the piston is called the dome.
The pressure from the pilot tube to the dome is routed through the actual control pilot valve. There are many designs but the control pilot is essentially a conventional PRV with the special job of controlling pressure to the main valve dome. When the pilot valve reaches set pressure it opens and releases the pressure from the dome. The piston is then free to open and the main valve exhausts the system fluid. The control pilot opens either to the main valve exhaust pipe or to atmosphere.
Snap Acting: At set pressure the valve snaps to full lift, it can be quite violent on large pipes with significant pressure. The pressure has to drop below the set pressure in order for the piston to reseat.
Modulating: The pilot is designed to open gradually, so that less of the system fluid is lost during each relief event. The piston lifts in proportion to the overpressure. Blowdown is typically short.
The PRECON Pilot Operated Relief Valve is used to replace weight-loaded or spring loaded valves in many applications to increase efficiency and reduce evaporation losses. Several advantages are obtained over the traditional type. For example, the process pressures may be closer to the set pressure than would be considered prudent and safe with the traditional valve. Additionally, greater conservation is obtained due to minimum product loss which in turn provides increased profits.