I was doing some research on the gate valve bonnet vent system, and I came across the following from the US Department of Energy standards (DOE-HDBK-1018/2-93).

"Some parallel disk gate valves used in high pressure systems are made with an integral Figure 8   Parallel Disk Gate Valve bonnet vent and bypass line.   A three-way valve is used to position the line to bypass in order  to  equalize  pressure  across  the  disks  prior  to  opening.   When  the  gate  valve  is closed,  the three-way  valve  is  positioned  to  vent the  bonnet to  one  side or  the  other. This  prevents  moisture  from  accumulating  in  the  bonnet.     The  three-way  valve  is positioned to the high pressure  side of the gate valve when closed to ensure that flow does not bypass the isolation valve.   The high pressure acts against spring compression and  forces  one  gate  off  of  its  seat."

I am having difficulty visualizing this, so if anyone has pictorial illustration of the above paragraph, and can share that, please let me know.

When a parallel-disc gate valve seats, there is a wedge between the discs that drives them outwards to snug against the seats.  It is theoretically possible to trap a volume of liquid between the discs, inside the internal volume of the valve.  If something (usually temperature change)  causes that volume to expand, pressures adequate to rupture the bonnet can occur.  So, you need to have the bonnet vented. Usually the bonnet vent is directed to the upstream side.

With the valve closed, the differential pressure from the upstream of the valve to the downstream assists the downstream disc in loading against the seat.  At high differentials, the loading can be quite high and it can make it nearly impossible to start moving the disc, or galling of the sealing surfaces can occur due to the high loading. (10" valve with 1000 psi differential: 79000 lbf loading due to differential. this corresponds to 2500 lbf per linear inch of seat. Bypassing the valve to equalize the pressure relieves the process seat loading so the disc can move.

With ball valves or plug valves it is a little simpler.  Just drill a hole in the upstream side of the ball/plug to prevent trapped-liquid expansion damage. You could do the same thing with a parallel-seat double disc gate valve, it would just make it directional instead of being able to seal in either direction.  

Large ball valves are frequently trunnion-style, so the seat loading vs pressure can be controlled to manage the seat loading and the resultant torque.  Trunnion-style balls could vent to the downstream since the upstream seat does the sealing- or the upstream seat loading can be controlled to relieve cavity pressure.

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