Thermal Binding is generally caused when wedge gate style valves are closed hot and then are allowed to cool. The different expansion and contraction rates between the disc, seat, and valve body can lead to a stuck, closed position.
Since wedge style gate valves are torque seated, this leaves them more susceptible to thermal binding. By contrast, parallel disc gate valves are position seated. This design allows the discs to easily compress on and off the seating surface in spite of any thermal distortions in the body or seat area.
Pressure locking occurs when pressure in the bonnet cavity increases when the valve is in the closed position, or when line pressure decreases without an equal decrease in the bonnet area. Temperatures in the valve bonnet area can increase in response to heat up during plant operation, ambient air temperature rise due to leaking components, or pipe breaks.
Bonnet pressure can also decrease as a result of leakage past the seating surfaces or stem packing.
Thermal binding and pressure locking are common conditions in high temperature and/or high pressure operating conditions.
Pressure Locking & Thermal Binding Solutions
Disc Hole - Upstream modification to the gate valve wedge or disc that allows internal bonnet relief.
Equalizing Valve or Bypass - Installing a bypass pipe and valve on the inlet and outlet sides of the body of a Flex Wedge Gate Valve will allow warm up of both sides of the wedge to prevent any different thermal expansion and contraction rates.
Relief Device - Install an optional safety valve to prevent bonnet over pressurization
Drain - Installing a manual drain valve to exhaust to the atmosphere
For a complete listing of bypass solutions contact Pacific Customer Service at 877-841-9330.