valve seats
valve seats
(OP)
Does anyone know what exactly is meant by 'double piston seats'?
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RE: valve seats
For a trunnion ball valve, with the ball closed, the force that presses the seat sealing surface against the ball is generated by the annular area of the seat ring (the distance outside the bore). With the ball closed, you should be able to open the body vents because the pressure is sealed off.
In the event of leakage past the upstream seat, you will get pressure in the body cavity. For double piston seats, the sealing surface diameter is larger than the O-ring diameter, so pressure in the body cavity goes around and pushes the downstream seat back against the ball and you have a secondary seal. Single piston seats have the O-ring larger than the sealing surface and thus cavity pressure will push the seat off the ball and you will get leakage downstream of the ball.
Double piston seats sound like the way to go, right? Well, they require body pressure vents/equalizers to release pressure inside the body whereas single piston seats vent excess body pressure back into the flow when the valve is open. Single piston seats also allow "double block & bleed" for checking the seals and maintenance. Also single piston seats generate much larger sealing force for a given pipe size and valve geometry, so require less material (money) to make.
There may be some other things that I've never had explained to me.
RE: valve seats
You can, of course, mix the seats: have the upstream seat a 'standard' and the downstream seat as DPE. This means that once the cavity pressure exceeds the UPSTREAM pressure PLUS the spring force of the seat spring the cavity pressure would relieve UPSTREAM. It does mean that the valve will only work as intended in one direction and you need to be confident that the upstream pressure will never be full class rated if a secondary pressure relief device is not used - I am not sure what standards, if any, address this installation.
RE: valve seats
Just want to add that in both configuration, single and double piston, the valve (trunnion) is always double block and bleed.
API 6D rules the construction of these valves.
Ciao
RE: valve seats