Manian,<br>
All manufactors publish the cracking pressure for their style of valve. Velocity is not the a factor, although you do need flow to create pressure. <br>
Because all valves are made different, you should contact the manufactor to answer your question.
To calculate you would need to measure or assume some geometry conditions. <br>
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I list assumptions I would use for general conditions below. I believe most swing check valves seat at 30 degrees or less so use this. You will need to assume some flapper geometry so use the internal diameter of the pipe size and figure the check pivot point is at the top of the circle.You will need to assume some bearing friction, I would assume zero.<br>
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You can then calculate based upon static pressures. I think you will find that you need a few inches of w.c. to open a swing check. <br>
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Regarding flow if the flow is small enough you will find there is almost zero pressure required. This is because there is little seating pressure for a swing check. A swing check only develops seating pressure from the hydraulic pressure of a return flow.
Below a general formula that can be used to determine the minimum velocity necessary to hold the check valve in awide open stable position :<br>
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v = 45 x ( V )^0.5<br>
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v : velocity in (m\s)<br>
V : specific volume of the fluid in (m^3 / kg)<br>
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I hope this helpful<br>
<br>
Khaled Muhsen<br>
