Vena contracta and 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

 

[sir-pinski]

.

Hello, thank you for sharing your question about vena contracta and cavitation. Vena contracta is the point of minimum cross-sectional area in a fluid flow, typically occurring at the outlet of a constriction such as an orifice plate. This is important in flow calculations as it affects the velocity and pressure of the fluid.

Cavitation occurs when the pressure in a fluid drops below the vapor pressure, causing the formation of vapor bubbles. These bubbles can then collapse and cause damage to equipment or decrease flow efficiency. In your case, if the pressure at vena contracta is low enough, it can lead to cavitation.

To accurately determine the pressure at vena contracta, you can use Bernoulli's equation which relates the velocity, pressure, and elevation of a fluid at different points in a flow. This equation can be used to calculate the pressure at vena contracta by considering the pressure at the inlet, the pressure loss across the orifice, and the velocity of the fluid at that point.

In addition to Bernoulli's equation, there are also other methods and equations that can be used to calculate the pressure at vena contracta, such as the continuity equation and the energy equation. It is important to also consider the properties of the fluid, such as its density and viscosity, in these calculations.

I recommend consulting with a fluid dynamics expert or referring to reputable sources for more accurate solutions and information. I hope this helps and good luck with your calculations.