Safety valves, relief valves, and pressure safety valves (PSVs) are all spring-operated pressure relief devices — but they serve different media and open in different ways. Using the wrong term (or the wrong valve) in a process specification can lead to unsafe overpressure conditions, regulatory non-compliance, or equipment damage.
This guide clarifies the terminology, design differences, applicable standards, and when to use each type.
1. Safety Valve
A safety valve is a spring-loaded pressure relief device that opens rapidly and fully (pop action) when system pressure reaches the set pressure. It is designed for compressible media — gases and steam.
- Pop-action opening — disc lifts to full open in a snap when set pressure is reached
- Required for steam boilers under IBR and ASME Boiler and Pressure Vessel Code (BPVC) Section I
- Once opened, reseats when pressure drops below the blowdown pressure (typically 7–10% below set pressure)
- Standard per API 526 (flanged design) and ASME PTC 25
2. Relief Valve
A relief valve opens proportionally to the overpressure — the disc lifts gradually as pressure rises above the set point. It is designed for incompressible media — liquids.
- Modulating action — disc lift is proportional to degree of overpressure
- Used in liquid-filled systems: pump discharge protection, heat exchanger thermal expansion
- Flow capacity increases as overpressure increases
- Standard per API 520 (sizing) and API 526 (design)
3. Pressure Safety Valve (PSV)
A pressure safety valve (PSV) is a combined design that can handle both gas/vapour and liquid service. It combines pop-action for gas and proportional opening for liquid — making it the most versatile and commonly specified pressure relief device in process plants.
- Used in two-phase (gas + liquid) systems and vessels that may contain either phase
- The most common type specified in oil & gas, refining, and petrochemical plant P&IDs
- Sized and selected per API 520 Part I (sizing) and manufactured per API 526
4. Comparison Table
| Parameter | Safety Valve | Relief Valve | PSV |
|---|---|---|---|
| Media | Gas / Steam only | Liquid only | Gas, Liquid, Two-phase |
| Opening action | Pop (full lift snap) | Proportional | Pop + Proportional |
| Standard | API 526, ASME Sec I | API 526 | API 520, API 526 |
| Typical use | Boilers, steam drums | Pump protection, liquid headers | Vessels, columns, heat exchangers |
| IBR applicability | Yes (boiler/steam) | No | Depends on service |
5. Key Sizing Terms
- Set Pressure: Pressure at which the valve begins to open. Must not exceed MAWP of the protected equipment
- Overpressure: Pressure above set pressure at which the valve reaches full rated capacity. API 520 allows 10% overpressure for process vessels
- Blowdown: Difference between set pressure and reseating pressure — typically 7–10%
- Back Pressure: Pressure at the valve outlet. Conventional spring-loaded PSVs are limited to ~10% back pressure; balanced bellows PSVs handle higher back pressure
6. Novel Valves Safety Valve Range
Novel Valves manufactures spring-loaded safety valves and pressure safety valves for steam, gas, and two-phase service — sizes DN 25 to DN 200, set pressures from 1.5 bar to 250 bar. IBR approval available for boiler and steam applications.
Need safety valves or PSVs for your plant or pressure vessel? Our team can assist with valve sizing per API 520 and provide IBR-approved safety valves for steam and boiler applications.
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