How Can I Accurately Determine Pressure at Vena Contracta to Prevent Cavitation?

[KristianCE]

Hello
I am trying to perform a calulcation to see if I may get cavitation problems for water flow and a specific orifice plate design. As I understand it cavitation occurs when the pressure at vena contracta is low enough to make the water boil.

My problem is that I do not know the pressure at vena contracta. I have a calulation including the inlet pressure, the remaining pressure loss, the dp across the orifice plate and the orifice plate discharge coeficient.

I assume that I can use the inlet pressure and subtract dp to get a value close to the vena contracta pressure. If possible I would like to find a better and more accurate solution and I would be very grateful for any ideas or pointers to sources of information

 

[FredGarvin]

There is no direct calculation that I know of. That is why we use the flow coefficient so as to take into these kinds of effects. If you really need a more accurate pressure drop, I would recommend looking into using vena contracta taps on your flow station. The tough part is that the location of the vena contracta is very dependent on beta ratio and pipe diameter. For smaller pipes, the VC can be very close, if not inside the flanges.

ASME's report on fluid meters has a plot that gives a rough outline of the location of the VC at # of D downstream from the upstream face as a function of beta ratio. This is totally dependent on numerous observations. There is no calculation related. The stated tolerance is ±.2D at β=.2 to ±.5D at β=.75