PSV Pressure Safety Valve Selection Guide

PSV Selection for Beginner,
Pressure Safety Valve or PSV, is one of safety devices in oil-and gas production facility, which ensure that pipes, valves, fittings, and pressure vessels can never be subjected to pressure higher than their design pressure. Therefore, the selection of PSV to be installed must be conducted in a careful and proper manner. These are the questions worth to be asked when you are going to specify details of PSV. What type of PSV we will have for our process requirements? Is there any easier way for PSV sizing or PSV calculation rather than calculate it manually? What kind of material shall be chosen for our process requirements? Now we are going to discuss the first question, What type of PSV we will have for our process requirements?In fact, there are three types of PSV you can select to suit the process requirement. They are, Conventional, Bellows and Pilot types. Conventional type of PSV,Conventional PSV is the simplest one . This type of PSV is used whenever the existence of back pressure is relatively small and less than 10% of set pressure, or nearly zero. Due to its low immunity to back pressure, the conventional type outlet is vented into atmospheric, and mostly, the fluid to be vented is non-hazardous fluid like water, air and steam. Bellows type of PSV,PSV with bellows or balanced-bellows type is used when there is backpressure and the backpressure does not exceed 50% of the set pressure. This type of PSV is almost the same with the conventional ones, but there is additional bellows in it. The bellows itself has a function to reduce the effect of the backpressure force over the disk. The bellows contained the upper side of the disc and the rod which connected to the spring from pressure force of process fluid or pressurized system – in which connected through PSV outlet – and the inside chamber of the bellow will be vented to the atmospheric, which obviously has constant pressure. Commonly, this type of PSV does not have a wide range of PSV, hence, it is not so flexible in alteration of set pressure. Pilot type of PSVA pilot-operated pressure safety valve consists of the main valve, which normally encloses a floating unbalanced piston assembly, and an external pilot. The piston is designed to have a larger area on the top than on 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 the operating pressure increases, the net seating force increases and tends to make the valve tighter. This feature allows most pilot-operated valves to be used where the maximum expected operating pressure is higher than 90% of Maximum Allowable Working Pressure. PSV Sizing guide