Difference between PSV and PRV
'Pressure Safety Valve' and 'Pressure Relief Valve' are commonly used terms to identify pressure relief devices on a vessel. Although terms Pressure Safety valves and Pressure Relief valve are used interchangeably, these two are different pressure relieving devices.
These devices are used in refineries and chemical plants to prevent pressure vessels and other equipment from overpressurization by releasing excess pressure. Properly functioning of pressure safety valves are most important to prevent equipment from damage and for safety of plant personnel.
1. Pressure Safety Valve
A safety valve is used to protect the system against overpressure. Overpressure occurs when the pressure exceeds the Maximum Allowable Working Pressure (MWAP) or the pressure for which the system is designed. Safety valves can open very quickly compared to relief valves. A safety valve opens from a set pressure; the valve first opens a little, after which it opens fully so that the unwanted pressure is removed from the system as quickly as possible.
PSV are used to prevent pressure increases, leading to malfunctions, fire hazards, or explosions. Safety valves only have mechanical parts, so they are used when electronic or pneumatic safety devices fail. A safety valve is fully actuated by the system's media, keeping it working in a power failure.
There are different safety valve types:
- Spring-loaded (Conventional type)
The opening and closing of a safety valve are determined by the balance between the spring force and the input force.
- Balanced Bellows
The bellows ensure that the back pressure does not influence the operation of the valve.
- Pilot operated
With Pilot operated valve, the response pressure can be closer to the working pressure without leaks or unwanted openings.
2. Pressure Relief Valve
Pressure relief valves are essential components in keeping hydraulic and pneumatic systems below a set pressure.
PRVs are used to either reduce the downstream pressure to a constant level whenever it exceeds a threshold or maintain sustained pressures down- or upstream from the valve.
Difference
- When the set pressure of the PSV is reached, the valve opens almost fully whereas PRV opening is proportional to an increase in the system pressure.
- PSV are stamped and certified to ASME/API standards while PRV are non ASME/API certified.
- Relief valve is used to limit pressure in the system while safety valve is designed to release excess pressure from the system.
- Safety valves are used in most of the vapor and steam systems whereas relief valves are used in liquid systems.
Video courtesy of Jakarta Engineering.
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