The Definition of Overpressure Protection

Definition of Overpressure Protection

Overpressure protection is the combination of devices, controls and design features that keep pressure vessels, boilers, piping, tanks and process equipment from exceeding their maximum allowable working pressure (MAWP). Typical devices include pressure relief valves, safety valves, rupture discs and vacuum breakers, backed by alarms and automatic shutdowns. Without working overpressure protection, equipment can rupture violently, releasing energy, hot fluids or hazardous chemicals.

Key Facts About Overpressure Protection

• Relief devices are generally set at or below the equipment's MAWP, as required by the ASME Boiler and Pressure Vessel Code.
• API 520 and API 521 provide widely used guidance for sizing relief devices and designing relief systems.
• OSHA's Process Safety Management standard (1910.119) includes relief systems in its mechanical integrity and process safety information requirements.
• Relief device outlets must discharge to a safe location, away from people and ignition sources.
• Blocking, gagging or removing a relief device without authorization is one of the most dangerous practices in a plant.

Example of Overpressure Protection

An air compressor's receiver tank is rated for 200 psi. Its relief valve is set to open at 200 psi, and a pressure switch shuts off the compressor at 175 psi. When the switch fails during a night shift, the relief valve lifts and vents air safely, preventing a tank rupture.

How to Maintain Overpressure Protection

1. Keep an inventory of relief devices with set pressures and capacities.
2. Inspect, test and recertify relief valves on a set schedule.
3. Control any isolation valves under relief devices with car seals or locks.
4. Review relief device sizing through management of change when processes change.
5. Investigate every relief event to find out why pressure rose.

Related Terms

• Boiler Safety
• Explosion Hazard
• Preventive Maintenance
• Gas Cylinder Safety

Frequently Asked Questions: Overpressure Protection

What is the difference between a relief valve and a rupture disc?

A relief valve opens to release pressure and recloses when pressure drops. A rupture disc is a one-time device that bursts at a set pressure and must be replaced afterward.

How often should relief valves be tested?

Intervals depend on service conditions, codes, jurisdiction and inspection history, and are often set between one and five years, with more frequent checks in harsh service.

Why is gagging a relief valve dangerous?

Gagging stops the valve from opening, removing the last line of defense against overpressure and allowing the equipment to rupture if pressure rises.