Pressure locking and thermal binding of gate valves on steam and hot water
Steam and hot-water systems have a distinctive failure: the valve is sound, yet even its actuator cannot open it. The cause is pressure trapped in the body cavity or the body contracting as it cools. Both effects are well known and are prevented at the selection stage.
TAZ Engineering Team · · 6 min
Pressure locking
A closed wedge gate valve has a cavity between the two seating faces: the inside of the body and bonnet. If liquid is trapped there and the valve then heats up, the cavity pressure rises sharply because water is nearly incompressible. That pressure forces both wedge faces against the seats, and the actuator may not develop enough thrust to open the valve. In extreme cases the bonnet or gasket is damaged.
Thermal binding
If a valve is closed hot, the body contracts faster than the heavy wedge as it cools and clamps it between the seats. Solid-wedge valves closed with high torque are most prone to this.
Prevention
Use flexible-wedge gate valves for steam and hot water. Where pressure locking is credible, provide a hole in the wedge on the high-pressure side or a cavity vent to the line; the valve becomes unidirectional and carries a flow arrow. External bonnet relief devices are also used. Avoid closing hot valves with excessive torque, and check actuator torque margins on critical automated valves with these effects in mind.
What to state in the RFQ
Describe the duty: warm-up and cool-down cycles, whether the valve may be closed hot, and which side sees higher pressure when closed. TAZ will select the wedge design and, where needed, provide a relief hole with direction marking.
Checklist
- medium and temperature
- heat-up and cool-down cycles
- closing hot
- high-pressure side
- wedge type
- cavity relief
- actuator torque margin