Valve Cavity Relief and Body Drain Design

Why Cavity Relief Matters

Trapped fluid in a valve body cavity can create hazardous pressure buildup during thermal expansion or fire events. Proper cavity relief design prevents seal blowout and structural damage.

Design Approaches

• Self-relieving seats that vent trapped pressure to the higher-pressure side

• Body relief valves set slightly above system pressure

• Drain/bleed connections per API 6D or customer specifications

• Upstream cavity relief for DBB valves handling thermal expansion

API 6D Cavity Pressure Requirements

API 6D requires that trunnion-mounted ball valves relieve cavity pressure automatically when it exceeds the higher of the two end pressures. The relief path must be proved by testing at the factory.

Body Drain Connections

• 1/4-inch or 1/2-inch NPT drain plugs at body low points

• Radiographic testing of drain port welds in critical service

• Fire-safe plugs in hydrocarbon service

• Isolation valves on drain connections for process service

Installation Considerations

Valve orientation affects drain effectiveness. Body drains should point downward for gravity drainage. Drain lines must be routed to safe discharge locations away from ignition sources in flammable service.