Ball Valve Internal Cavity Relief: Why It Matters

Introduction

Ball valves trap fluid in the cavity between the ball and body seats when the valve is closed. If that trapped fluid is a cryogen or a liquid that vaporizes at elevated temperature, the cavity pressure can rise to dangerous levels, potentially causing valve body rupture or seal failure.

Thermal Expansion Problem

LNG, liquid propane, or liquid ammonia expands dramatically when warmedA 1-liter cavity of LNG at minus 162 degrees C becomes about 600 liters of gas at ambientCavity pressure can exceed 700 bar without reliefThis exceeds body design pressure, causing catastrophic failureParticularly hazardous for valves that may be closed during plant startup

Relief Mechanisms

The most common solution is a small hole drilled through the ball into one flow port (cavity vent hole or body vent). This connects the cavity to the upstream or downstream piping, relieving overpressure to the pipeline system. The vent also makes the valve double-block-and-bleed (DBB) capable.

Pressure Relief Valve Option

Body-mounted PRV set at 110% of valve MAWP provides controlled reliefMore complex but preferred when cavity venting to pipe is unacceptableRequired for isolated cavity sections with no vent path to pipingPRV must be compatible with process fluid and temperatureDocument PRV set pressure and replace at scheduled maintenance intervals

Standards and Specifications

API 6D requires cavity relief for pipeline ball valves in liquid service. ASME B31.3 requires thermal relief for any closed system where liquid can be trapped. Project specifications should explicitly address cavity relief requirements, vent orientation, and plugging/blinding requirements for gas service.

Vent Hole Sizing

Vent holes are typically 1.5 mm to 6 mm diameter depending on valve size and service. Too small and the hole may plug with debris; too large and it compromises the valve as a double-isolation device. Most manufacturers publish standard vent hole sizes for each valve size and pressure class.