The 3-line bypass and equalizer combination pictured above allows the end user to achieve a variety of results.
By keeping the upper valve closed and opening the two lower valves, the “bypass” part of the combination allows for “warming” of the downstream pipe prior to opening the large primary valve, thus reducing “cold shock", and it can be used to equalize pressure on either side of the gate, which lowers the torque required to open or close the valve.
All wedge gate valve designs have the ability to seal upstream and downstream at the same time. Media that is trapped in the center section of the valve can increase in pressure, if the media is the type that expands when heated. This additional pressure could cause the valve to lock up, and in the worst case it can cause pressure boundary failure.
This possible pressure boundary failure is recognized by both ASME B16.34 Paragraph 2.3.3 and ASME B31.1 paragraph 107.1c. They both require the purchaser to provide a means in system design, installations and/or operations to assure pressure in the valves remains within the pressure and temperature rating.
The “equalizer” part of the combo (the top valve) taps into the Bonnet Cavity area that goes around and above the disc. By opening this top valve and one of the lower valves (usually the upstream side), this will vent the cavity to the line and eliminate this overpressurization possibility.
T-equalizer/Bypass combination with a drain valve at the tee (now a cross)
Composed by: Dave Romphf, Product Engineering Specialist for QRC Valves