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 Teflon Operating pressure is 60psig Differential pressure when closed condition is 70psid First, focus on contact stress. In a perfect world, if the contact stress equals the pressure trying to seal, then you get a seal. The world is not perfect, not perfectly round, surfaces are not perfectly smooth, so you need a little more contact stress than the pressure you are trying to seal. Since ptfe is resilient, 25 to 50% more contact stress will suffice in most cases. Now you know how much contact stress you need. Now figure out the contact area, which will allow you to calculate the required force. The leakage rate you mention is a value selected to account for variations in the contact surfaces. It is not be part of your analysis on determining how much force you need to get a seal. Without in depth knowledge of the range of ball  valve roundness, surface finish variability on the ball, and ball seat and dimensional tolerances, you will never be able to get a consistent leak rate of 0.5 cc/min for a given force. Way to many variables that are outside your control. You need to shoot for tight shut-off to ensure you are always under the allowable. Ultimately, you will have to test the final result to validate your calculations.

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