zdas, you need both a differential pressure gage and a pitot tube to measure the whole of what's going on in there. Cracking pressure is a misnomer for what should be termed "cracking head", which would actually be the sum of static head and velocity head, so the cracking pressure concept is only valid at zero flow, after which it becomes the sum of static differential head plus developing velocity head.
dP
total= P
c + SG * (Q/C
v)
2
BTW, Smith Valves shows C
v for swing check valves which are substantially higher than even their globe valves,
These swing-checks don't show an opening, or cracking pressure, probably because in most systems the downstream head would be many magnitudes greater than that anyway and it really wouldn't matter.
In a counterbalanced swing check, initially the differential pressure x the Sin(swing_angle)- the counterbalance weight x cos of the counterbalance angle would have to be greater than zero to open. As soon as flow began, the initial static differential pressure would decrease from the Bernoulli effect, as that static pressure converts to velocity head. Its not the static pressure vs the velocity head, or one or the other, its the sum of both that works to keep the swing plate open. Then the valve would remain open when flowing only when the sum of upstream pressure plus velocity head impacting the angled surface (from the vertical) of the swing-plate, V^2/2/g * Sin([α]) plus any frictional drag effects on the plate remains greater than the counterbalance force x the Cosine of its angle (measured from horizontal) - any downstream backpressure.
So, the "cracking pressure" is really the static head equivalent of the energy required to open the swing with no flow, which then becomes the sum, if you will, of all forces, pressure, drag and velocity impact forces acting on the plate, I think more appropriately should be called what, "the total differential head across the valve".
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