I hope I understand your request. You want to calculate via an equation how much an installed valve would leak under a specific condition?
I would think that how much a valve is leaking depends on the particular valve, such as the current condition of the seats. Also, you did not specify type of valve. I would think that different types of valves would leak differently as well. Hence, I don't think it is possible to calculate it.
The only way I can think of to determine how much your particular valve is leading is to meaure it.
Of course, once you do, and put it back in service, it will continue to deteriorate, and the leakage rates will again change with it.
>>No, I need to calcutate how much I can allow it to leak.<<
That depends on what your process will do with the dribble of shutoff leakage. If it will cause something to overtemp or overflow. or otherwise exceed limits, then that's too much. Steam can leak quite a lot because it condenses and the steam traps take out the condensate.
It seems that your valve is on air service. If it is a vent line or goes to a flare stack, the the leakage can be quite high. If it goes to a dead-end process, you don't want any leakage.
API 598 and the other standards provide criteria for the allowable leakage rate for new valves in various applications. Your question may be about the leakage rate of the valves available. However, I sense the need to understand the reason for the leakage specifications so that you can make a judgement about your application.
If I operate a plant with 1000 psi pure oxygen on one side of a valve and high temperature hydrocarbon on the other side of the valve, my requirement is zero leakage. What is your application?
I have a manometer that is measuring differential pressure over a filter. The valve for which I need to calculate the maximal acceptable leakage rate is placed between plus side and minus side of the manometer. What I need is to calculate is the leakage that won’t affect the measurements of the manometer more then 10% .
