Valve Cavity Fill and Drain Connections for Process Safety

Trapped cavities in double-seated valves can accumulate pressure during thermal expansion or fluid entrapment. Cavity fill and drain connections are required for safe operation and maintenance.

Trapped Cavity Pressure Hazard

When a double block valve isolates a cavity between two seats, thermal expansion of trapped liquid or gas can build pressure far above the valve pressure rating. This can rupture the body or blow out seals.

• Thermal expansion: Liquid volume increases about 0.1% per °C; creates large pressure rise in sealed cavities

• Gas expansion: Trapped gas pressure rises proportionally with temperature

• Cryogenic flash: LNG or liquid nitrogen vaporizes if not vented, exceeding valve rating

• Relief requirement: Any isolated cavity containing liquid must have pressure relief or venting

Cavity Relief Valve Installation

The preferred solution for trapped cavity hazard is a small pressure relief valve connected to the valve body cavity. It relieves excess pressure automatically and reseats when pressure drops.

• Set pressure: Typically 1.1x maximum cavity pressure or body MAWP

• Body connection: Threaded connection on valve body at cavity location

• Discharge: Vent to safe location or connected to flare/vent header

• Size: Small flow capacity sufficient; thermal expansion rate is low

Double Block and Bleed (DBB) Valve Connections

DBB valves include a bleed valve between the two block valves. The bleed verifies both seats are holding and vents the cavity pressure before any downstream work begins.

• Bleed valve: 1/2 or 3/4 inch valve on body cavity; normally closed

• Bleed before breaking flange: Open bleed to confirm both block valves are holding

• Cavity monitoring: Pressure gauge on bleed connection detects seat leakage

• Drain location: Cavity drain at lowest point; vent at highest point for complete draining

Specification of Cavity Connections

Include cavity connection requirements in valve datasheets. Specify size, rating, material, and end connection type for all cavity fill, drain, and vent connections.

• Standard connections: 1/2 inch NPT or flanged cavity connections per ASME B16.34

• Material: Match body material or use corrosion-resistant insert

• Plugging: Unused connections must be plugged with pressure-rated plugs

• Marking: Tag each connection with purpose (cavity vent, cavity drain, etc.)