how much seating force required to control leakage for following requirements. Allowable leakage when closed condition is 0.5cc/min.The seating material used is TeflonOperating pressure is 60psigDifferential pressure when closed condition is 70psidEarlier i were trying with yeild stress value of Teflon for seating load(250psi)168lbf force,however one of articale from emerson defines 40lbf is enough to control 0.5cc/min at 50psig.More the force more the actuation torque required to actuate the ball.If you actually test comparable ball valves from 50 different vendors, you'll get 50 different results. Seat material, seat profile, seat preload, surface finish of the ball are >some< of the variables that affect this. USUALLY there is some preload built in to the assembly stackup so that the valve seals at zero differential when new. Virgin PTFE seats are bad to cold-flow so they may seal when new, but if you take the valve up to pressure and then relax the pressure, the valve may leak at low differntials.

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