Safety Valves vs. Relief Valves: Key Differences
When it comes to overpressure protection, safety valves and relief valves serve similar purposes but operate differently. Here’s what sets them apart:
Opening & Closing Speed
- Safety valves open and close rapidly (“pop action”), making them ideal for gases and steam.
- Relief valves open and close gradually, allowing proportional pressure release, mainly used for liquids.
Overpressure & Blowdown
- Safety valves typically have an overpressure of 10%, 21% in fire case, and 3% in boilers (ASME Sec I).
- Blowdown (pressure drop before reseating) is around 10%.
- Relief valves usually handle lower overpressure and gradual reseating.
Applications
- Safety valves protect steam, gas, and compressed air systems.
- Relief valves are common in liquid systems and thermal relief applications. When exothermic reactions occur in a pipeline, part of the fluid may vaporize, increasing pressure. Relief valves prevent excessive pressure buildup in such cases.
Codes & Standards
- ASME (Sec I & VIII), API 520/526, EN 4126 apply to safety and relief valves.
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