PILOT OPERATED PRESSURE RELIEF DEVICES

                Pilot operated pressure safety valves (POPSV) consists of main valve which normally encloses a floating unbalanced piston assembly and an external pilot. 

                        Figure: Parts of POPSV

 Operating mechanism of Pilot operated PSVs is described in following steps

Step 1: Piston is designed to have a larger area on the top than the bottom. Up to the set pressure, the top and bottom areas are exposed to the same inlet operating pressure. Because of the larger area on the top of the piston, the net force holds the piston tightly against the main valve nozzle.

As operating force increases ---- ->Net seating force increases ------>which makes valve tighter 

 The net seating force of pilot operated PSV increases with increase in operating pressure unlike the conventional PSV as shown in the below figures. 

Step 2: At set pressure, the pilot vents the pressure from the top of the piston, the resulting net force is now upward causing the piston to lift, and process flow is established through the main valve.

Step 3: After over pressure incident, the pilot will close the vent from the top of the piston, thereby re establishing the pressure and the net force will cause the piston to re seat.

The lift of the main valve piston or diaphragm is not affected by built up back pressure.

 

                 A back flow preventer (NRV) is required when the possibility exists of developing a pressure on the discharge side of the PSV that exceeds the inlet pressure of PSV.

The pilot that operates the main valve can be either a pop action or modulating action pilot.

Pop action pilot -------> Causes the main valve to lift fully at set pressure without over pressure. (Similar to gas operated Pop action conventional PSV)

Modulating pilot -------> opens the main valve only enough to satisfy the required relieving capacity. (Similar to liquid operated pressure relieving device)

 

 Also the pilots can be classified as Flowing and Non flowing

Flowing pilot allows process fluid to continuously flow through the pilot when the main valve is open and where the non flowing type does not.

Applications

·         POPSVs are used when operating pressure is above 90% of set pressure.

·         Used during high back pressures (> 50%) as valve lift is not affected by back pressure.

·         Used in critical applications where passing of PSV is not allowed as closing force of valve increases with increase in operating pressure.