A relief valve is a type of pressure control valve designed to limit pressure in a system by allowing excess fluid (liquid or gas) to escape when the pressure exceeds a preset limit — thereby protecting the system from overpressure damage.
How a Relief Valve Works
- It stays closed during normal operation.
- When system pressure exceeds the setpoint, the valve gradually opens to release fluid.
- Once pressure drops back to a safe level, it closes again to maintain system integrity.
Unlike safety valves (which snap fully open), relief valves typically open proportionally to the pressure increase.
Types of Relief Valves
- Spring-Loaded Relief Valve
- Uses a spring to maintain set pressure.
- Common in hydraulic and liquid systems.
- Pilot-Operated Relief Valve
- Uses a pilot valve to control the main valve.
- Suitable for high-capacity systems requiring precise control.
- Pressure and Temperature Relief Valve (PTRV)
- Combines pressure and temperature sensing.
- Used in water heaters and thermal systems.
Applications
- Hydraulic systems
- Pumps (to prevent dead-heading or overloading)
- Compressors
- Pipelines carrying liquids
- Pressure vessels for slow or controlled relief
Relief Valve vs Safety Valve
| Feature | Relief Valve | Safety Valve |
|---|
| Activation | Gradual opening based on pressure rise | Instant, full lift at set pressure |
| Typical use | Liquids (hydraulics, water, oil) | Gases and vapors (steam, air, gas) |
| Design purpose | Process control | Emergency protection |
| Set pressure tolerance | Narrow | Wider range (to handle spikes) |
Benefits
- Prevents system damage from overpressure
- Maintains safe operating conditions
- Extends equipment life
- Essential for compliance with safety standards (e.g., ASME, API, ISO)