Trunnion Ball Valve - Fire Safe Seat Design

[Gray76]

Hi,

Many trunnion ball valve manufacturers show a fire safe graphite seal located in a closed groove on the trunnion seat carrier.
As this is a closed groove, the graphite will have to be split/scarfed in order to fit. Maybe die formed ring or braided packing section.
Does anyone know how this design will seal against pressure if the O-Ring/Lipseal burns out during a fire test?
I'm puzzled as to how the split seal will hold pressure - granted, there is a generous leakage rate during a fire test.

Thanks in advance.

 

[gerhardl]

Hello Gray76

Your question started me thinking.

Please take into consideration the difference between a firesafe designed valve and a firesafe tested valve. As you know, a (real) firesafe valve is firesafe only after it has passed a test of going trough a real fire, according to one of the approved firesafe tests regulations.

This twists the question to how the sealing will behave and seal according to the details of the approved leakage test after the fire. There is also the question if all such designs as described will require a split O-ring. Without knowledge details, I wold imagine this problem is technically overcome, one way or another.

 

[Gray76]

Hi Gerhardl,

I was referring specifically to fire tested valves - API 607 etc.
Not sure about split O-Rings, never seen those before on a trunnion seat.
I'm still none the wiser as to how a split graphite seal will hold pressure sufficiently to pass fire test.
I supplose it is possible (on larger valves or thick walled forged valves) that the soft seat seals do not actually burn out within the test period.

 

[bcd]

When assembled into the valve, the OD of the graphite seal is compressed. When the OD is compressed, the split ends are pushed together and provide a good enough seal to minimize the leak during and after the fire test.

 

[Gray76]

Thanks bcd,

So the amount of compression will be key.
I would also imagine that the seal density and scarf profile will also be critical?