Cavity relief for Trunnion Ball Valve

[1psb]

My custome has required a Trunnion mounted ball with cavity relief by seat leakage to inside of valve.
Normaly the Trunnion type are self relief sealing but in this case i dont' understood what's mean about cavity relief by seat TO INSIDE OF VALVE !!

 

[zdas04]

Seats occasionally leak by. If you overpressure a trunnion boy cavity it will self relieve through the other seat. A PSV threaded into the body bleed port can prevent the self relieving if you just can't allow any flow all the way across the valve.

Set point is a big issue. It only takes a couple of psi of over pressure to lift the seat from the back. If you set the PSV at a couple of psi then it will lift every time you shut the valve.

Tough problem. You can solve it with a dP control valve that samples upstream and downstream and sends the lower pressure to the static side of a dP PSV which is set to lift when the cavitiy is 2-3 psid higher than the low pressure side of the Trunion. It sounds complicated, but it is pretty easy.

David

 

[dcasto]

forget a PSV. Install a 1/2" tubing check valve with a 10 psi spring. The valve is yubed from the body bleed tap to a point on the upstream or downstream side of the vale (the low p[ressure sink of cooling side/underground run).