Understanding the differences between Pressure Safety Valves – PSV and Pressure Relief Valves – PSV is essential for selecting the appropriate device for a specific application.
Pressure relieving devices include mechanisms such as Pressure Safety Valves (PSV) and Pressure Relief Valves (PRV), although there are other types of pressure relieving devices as well, such as Rupture Disk Devices and Pin-Actuated Devices. These devices can come in many different sizes and shapes and allow pressurised fluids or gasses to escape through a secondary passage out of the system so that pressure cannot build up beyond safe operating limits.
Pressure Safety Valve – PSV
A Pressure Safety Valve (PSV) is a type of valve used to quickly release fluids from equipment in order to avoid over pressurisation and potential process safety incidents. PSV is activated automatically when pressure exceeds prescribed pressure limits in order to return equipment pressure to a safe operating level.
Pressure relief valve – PRV
A Pressure Relief Valve (PRV) is a type of valve used to release stored gas in various equipment in order to maintain an optimal pressure level. PRVs open gradually as pressure builds up in order to release the necessary amount of pressure. While the term PRV is sometimes used interchangeably with PSV, there is a difference between the two. A PRV opens gradually in relation to the pressure, while a PSV is opened suddenly once the pressure hits a certain level in order to avoid over pressurisation and a potential process safety incident.
Difference PSV and PRV
| Feature | Pressure Safety Valve | Pressure Relief Valve | Remarks |
|---|---|---|---|
| Protection | Pressure safety valve is the ultimate safety device to protect people, property and processes from major damage. It is safety device capable of operating at all times and is the last resort to prevent catastrophic failure in systems under overpressure conditions. | Pressure relief valve is designed to protect pressure-sensitive systems and equipment from the damaging effects of overpressure conditions. | |
| Function | It’s kind of a fail-safe device that actuates automatically in order to prevent the accumulation of pressure in a pressurised system beyond a preset limit. | The pressure system is set with a predetermined design limit called a setpoint, above which the valve begins to open to prevent overpressure conditions. | |
| Operation | The device is so designed so that the safety valve trips automatically when the given pressure is attained. It simply allows the excess pressure to escape in order to prevent any damage to the pressure equipment. Even a slight increment in pressure lifts the safety valve and it closes as soon as the pressure is reduced to the prescribed limit. | Open directly proportional to the increase in the pressure. This means the opening of the valve is rather gradual than sudden, allowing it to open only at a preset pressure level and release fluids until the pressure drops to the desired set pressure. | |
| Set point / set pressure | Designed to open at a certain pressure, generally referred to as non-adjustable”set pressure“. The set pressure is below the maximum allowable pressure PS or MAWP before the overpressure condition occurs. Overpressure is the pressure increase above the set pressure necessary for the safety valve to achieve full lift and capacity. The overpressure is usually expressed as a percentage of the set pressure. Codes and standards provide limits for the maximum overpressure. A typical value is 10%, ranging between 3% and 21% depending on the code, application and service characteristics; (see also ASME BPVC Section VIII Div. 1 UG-155) | Designed to open at a certain pressure level which is generally called as a “set point”. Is adjusted to the lowest maximum pressure rating meaning it is set below the maximum system pressure allowed before the overpressure condition occurs. The valve begins to open when the pressure reaches 10 % above the set point. | The set pressure is measured in [bar] or [psi] and must not exceed the maximum allowable working pressure (MAWP); see also EN 764-7, |
| Certifications and Codes | PSV are subject to stringent certification and code requirements, with precise standards that must be met to ensure their reliability in critical safety applications. Codes and Standards: – API STD 520 – ASME BPVC Section VIII Div. 1 UG-150 to UG-156 – ASME BPVC Section XIII – EN ISO 4126-1: Safety valve Certifications: ASME BPVC Section XIII (UV-Designator) ASME BPVC Section I (V-Designator) PED Category IV (CE-marked) | PRV often adhere to general industry standards and regulations. | |
| Application | Pressure Safety Valves are commonly used in gas or steam systems, where it is crucial to rapidly release pressure. | Pressure Relief Valves are more commonly used in liquid systems, where a gradual release of pressure is more appropriate. |
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ASME BPVC Section XIII: Overpressure protection of pressurised equipment
