Pressure Relief Valve Terms You Should Know
Understanding key terminology is essential for grasping the factors that influence pressure relief valves and their impact on system operations. Here are some of the most important terms to know in the pressure relief valve industry:
Pressure Relief Valve (PRV)
A pressure relief valve (PRV) is designed to open when vessel pressure exceeds a predetermined limit and close once pressure normalizes. PRVs include various types such as safety valves, safety relief valves, and relief valves.
Safety Relief Valve
A safety relief valve serves a dual function, acting as both a safety valve and a relief valve. A safety valve quickly and fully opens—often "popping" open—when static upstream pressure exceeds its limit, making it suitable for use with compressible fluids. A relief valve, on the other hand, opens in proportion to the pressure increase and is commonly used with incompressible fluids.
Operating Pressure
The normal pressure of a vessel during operation. This pressure is typically maintained at least 10% below the Maximum Allowable Working Pressure (MAWP) to ensure safe operation.
Set Pressure
Also known as nameplate set pressure, this is the inlet pressure at which the pressure relief valve must open, as required by regulatory codes.
Maximum Allowable Working Pressure (MAWP)
The highest pressure a vessel can safely handle at a specific temperature. MAWP is a key factor in determining the set pressure of pressure relief valves.
Opening Pressure
The pressure level at which the relief valve begins to open.
Design Pressure
The maximum pressure the valve is engineered to withstand while maintaining proper functionality.
Overpressure
Pressure that exceeds the valve’s set pressure, potentially leading to equipment damage or safety concerns.
Back Pressure
The pressure present at the outlet of a pressure relief valve, which can be either constant or variable. Back pressure may result from post-opening flow buildup or from pre-existing conditions in the discharge header before the valve activates.
Blowdown
The difference between the set pressure and the reseating pressure of a relief valve. Proper blowdown settings ensure that the valve doesn’t reseat too soon or too late after activation.
Reseating Pressure
The pressure at which a relief valve closes after releasing excess pressure. This is an important factor in preventing unnecessary valve cycling.
Chattering
Rapid opening and closing of a pressure relief valve, which can lead to valve wear, performance degradation, and system instability.
Set Pressure Tolerance
The allowable deviation in the set pressure at which a valve opens, defined by industry standards to ensure safety and compliance.
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