Pressure Safety Valve – PSV & Pressure Relief Valve – PRV is the terms used interchangeably and may entirely depend on particular project standards in order to identify all pressure relief devices either as safety valves or relief valves. These terms are used in the process industry because every pressurized system needs some specialized devices in order to protect life, property, and environment.
First of all, one has to keep in mind that Relief valves & safety valves are the two principle safety devices that are mainly designed to prevent overpressure conditions in process industries. Although these two devices are used almost for the same purpose, the significant difference between these two devices lies in the way they operate.
Primarily, a relief valve device is immune to the back pressure effects of a system and is subject to periodic strip down whereas the safety valve allows the excess pressure to escape so that it can prevent any damage to the vessel. For PRV, the opening is proportional to the increase in the vessel pressure and for PSV the opening is sudden. Since you are wondering about the difference between PRV and PSV, here’re a few major differences between Pressure and Safety Relief Valves. So let’s dive in;
Pressure relief valves are primarily used in the hydraulic systems in order to limit the pressure in the system to a specific preset level. When the pressure reaches the limit of safety design, the relief valve responds simply by releasing the excess flow from an auxiliary passage from the system back to the tank, thereby preventing equipment failure.
So what’s the major purpose of a safety valve? It is meant to protect property, life, and the environment against failure in the control system pressure. Precisely, a safety valve will open whenever the pressure exceeds the designed set pressure limit.
Operation of Relief Valve vs. Safety Valve
When it comes to a safety relief valve, the opening is directly proportional to the increase in vessel pressure. It clearly means that the valve opening is rather gradual than sudden. Perhaps that’s the reason why it allows it to open only at a current pressure level while releasing fluids until the pressure drop to the desired pressure.
On the other hand, a safety valve opens immediately when the system pressure reaches the set pressure level to system failure. Inevitably, this safety device can operate at all times and is the last option to prevent catastrophic failure in systems in case the pressure exceeds its limit.
If we talk about a pressure relief valve, it is mainly designed to open at a certain pressure level that’s known as “setpoint”. A setpoint must not be confused with the set pressure. In fact, a setpoint of a relief valve is also adjusted to the lowest pressure rating. It clearly means that its set below the maximum system pressure allowed before even the overpressure condition occurs.
When the pressure reaches above the setpoint, the valve starts opening. The unit in which the setpoint is measured in Pounds per Square Inch – PSIG for short. It must not exceed the maximum allowable working pressure. In safety valves, normally the setpoint is set at 3% above the working pressure level whereas it is set at 10% in relief valves.