What Is a Pressure Relief Valve?

A pressure relief valve is a safety device that automatically opens to discharge fluid when the system pressure exceeds a predetermined set point. It is the last line of defense against overpressure in piping systems, pressure vessels, boilers, and storage tanks.

Term	Meaning
PSV	Pressure Safety Valve; pops open at set pressure (gas/vapor)
PRV	Pressure Relief Valve; opens proportionally (liquid)
SRV	Safety Relief Valve; acts as PSV on gas or PRV on liquid
Set pressure	Pressure at which the valve begins to open
MAWP	Maximum Allowable Working Pressure of the protected equipment
Blowdown	Difference between set pressure and reseating pressure

Mandatory Application

Relief valves are mandatory on any pressure-containing equipment where overpressure is possible. ASME Boiler and Pressure Vessel Code (Section VIII) and API 521 require relief protection on pressure vessels, heat exchangers, fired heaters, and piping systems with blocked-in sections that could be exposed to thermal expansion or upstream pressure excursions.

PSV vs PRV vs SRV

Feature	PSV	PRV	SRV
Service	Gas, vapor, steam	Liquid	Gas or liquid
Opening action	Pop action (full lift at set pressure)	Proportional (gradual opening)	Pop on gas, proportional on liquid
Typical application	Steam boilers, gas systems, compressor discharge	Pipelines, pumps, liquid-filled vessels	Versatile, general process equipment
Standard	API 526, ASME Section VIII	API 526, ASME Section VIII	API 526, ASME Section VIII
Sizing	API 520 Part I (vapor)	API 520 Part I (liquid)	API 520 Part I

Key Standards

• API 520 Part I: sizing and selection of pressure-relieving devices
• API 520 Part II: installation guidelines
• API 521: pressure-relieving and depressuring systems (flare, vent)
• API 526: flanged steel pressure relief valve dimensions and orifice sizes
• API 527: seat tightness testing
• ASME Section VIII, Div. 1, UG-125 to UG-137: mandatory overpressure protection requirements

Set Pressure Guidelines

Per ASME Section VIII, the set pressure of a single relief valve must not exceed the MAWP of the vessel. For multiple relief valves, one must be set at or below MAWP; the others may be set up to 105% of MAWP.

Common Mistake

Confusing set pressure with operating pressure. The set pressure must be above the normal operating pressure (with sufficient margin to prevent spurious lifting) but at or below the MAWP. A typical margin is 10% above operating pressure.

Relief valve types (spring-loaded, pilot-operated, balanced bellows) are covered in detail in the full safety and pressure relief valve guide.

Read the full guide to valve types