PSV Calculations

Introduction

Pressure Safety Valve (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 (PSV calculation) rather than calculate it manually?

What kind of material shall be chosen for our process requirements?

Prior to the PSV selection, it would be better if we know how the PSV works which will lead us in understanding of critical parts of PSV.Then, the PSV selection process can be done with awareness of some strong points.

Pressure Safety Valve by definition

Cited from API 520 part 1

(Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries;Sizing and Selection)

about Safety Valve definition: “A safety valve is a spring loaded pressure relief valve actuated by the static pressure upstream of the

valve and characterized by rapid opening or pop action. A safety

valve is normally used with compressible fluids.” Figure 1 shows Conventional PSV, which is purposed for description only.

Figure 1. Conventional Pressure Safety Valve How does it work?

                                         Fig.2 Sketch of Pressure Relief Valve

Figure 2 is a simple sketch of pressure relief valve which shows the disc held in the closed position by the spring. When system pressure reaches the desired opening pressure, the pressure force of  the process fluid pass through the inlet and then it is acting over Area A1 equals the force of the spring, and the disc will lift and allow fluid to flow out through the outlet. When pressure in the system returns to a safe level, the valve will return to the closed position.

Certain area of the disc and nozzle will allow certain amount of the gas/liquid volume. The area of the nozzle (So called as “Orifice”) needs to be calculated in order to have proper amount flow of the process fluid. This certain area has been standardized in API 526 (Flange Steel Pressure Relief Valves) and designated into certain alphabetic as shown on Table 1.

Since PSV will most likely to be in closed position, it is a good idea to choose some kind of “seal” between disc and the nozzle to keep the process fluid from leaking to the outlet of the PSV. Conventional, Bellows or Pilot type? Back-pressure considerations Types of PSV are created due to existence of backpressure. The effect of  back-pressure can be depicted by Figure 3 which incorporate forces from spring (Fs), fluid from the pressurized system (Pv,An) and backpressure (PbAn).The Pv is the pressure due to the changes over the pressurized system, and the Pb is the pressure which exist in the outlet of the PSV, we recognise this as a back pressure.As you may see, that the spring denotes with the Fs-is having main contribution to the balance, and have a positive direction along the PB.The over pressure in the pressurised system will increase the magnitude of the Pv ,and eventually it will affect the balance of the pressure force,and hence the sum of PB AN and the Fs will be less than PvAN.The spring, which holds the disc and isolates the pressurized system into the outlet of the PSV, is moving upward and the disc will not contain the pressurized system any more.