On the issue of calculated specific pressure for fixed ball valves
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For a single piston, why is the condition where both the valve chamber and the inlet are at pressure P1 not taken into account when calculating the actual working specific pressure? The relevant materials all assume zero pressure in the cavity when calculating the working specific pressure! Consider the effect of the pressure in the middle cavity only when designing for pressure relief in that cavity! The same applies to dual pistons as well; assuming that when the valve is closed, the upstream and downstream pipes as well as the valve’s internal chamber are all at pressure P1, then Tz should equal PI/4*P1*(Dm squared – D2 squared) + F_spring. The specific pressure calculated in this way must be less than the required specific pressure for the valve to function properly! I hope an expert can help resolve my doubts; thank you in advance! I believe the formula for T1 in the image is incorrect; it should be PI/4*P1*((D1 squared – d squared)). Moreover, the reaction force T4=PI/4*P1*((Dm squared – d squared)), which arises due to the force acting away from the spherical surface, is not taken into account. When there is pressure inside the cavity, part of the force acting on T1 is balanced out. If, during design, D2 equals DM, then the remaining force on T1 is equal to T4, and this force as well is canceled out! So TZ = PI/4 * P1 * (Dm squared – D2 squared) + F_spring! That’s my understanding!
When calculating the design of a single-piston middle chamber, it should not be considered as having pressure; rather, it should be regarded as the outlet pressure. I think there’s a problem with the dual-piston company as well, but yours seems incorrect too. The calculation should be related to DM; how can it be related to d?
Ball valves require a relatively low pressure difference, which is different from conventional valves.
The design of valves takes into account not only the pressure conditions before and after the pipeline. For reasons related to performance, safety, and redundancy, valves are generally designed with a sealing pressure ratio based on full pressure differences. Otherwise, in situations such as pipeline maintenance when it is necessary to release the full pressure difference, some valves may experience seal failure.
What I mean is that he ignores a force related to d; if the pressure in the cavity is taken into account, the force exerted on the sphere by the dual-piston medium is only PI/4*P1*(Dm squared – D2 squared). When this force is converted into a sealing pressure ratio, it is far less than the sealing pressure ratio required for a ball valve!
But there are certainly situations where the ball valve is closed with pressure on both the upstream and downstream sides; in such cases, could there be a problem with the sealing?
The width of the sealing surface is not large, so no matter what approach is taken, the sealing pressure will be sufficient.