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

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  • Thread starter Thread starter KristianCE
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SUMMARY

This discussion focuses on accurately determining the pressure at vena contracta to prevent cavitation in water flow through a specific orifice plate design. The user seeks a method to calculate vena contracta pressure, which can be approximated by subtracting the differential pressure (dp) across the orifice plate from the inlet pressure. For improved accuracy, the use of vena contracta taps is recommended, as the location of the vena contracta is influenced by the beta ratio and pipe diameter. ASME's report on fluid meters provides valuable insights into the vena contracta's location relative to the upstream face based on beta ratios.

PREREQUISITES
  • Understanding of cavitation and its implications in fluid dynamics
  • Familiarity with orifice plate design and flow coefficients
  • Knowledge of differential pressure measurement techniques
  • Awareness of ASME standards related to fluid meters
NEXT STEPS
  • Research the use of vena contracta taps for accurate pressure measurement
  • Study ASME's report on fluid meters for insights on vena contracta location
  • Explore the relationship between beta ratio and vena contracta positioning
  • Learn about advanced methods for calculating pressure drops in orifice plates
USEFUL FOR

Fluid dynamics engineers, mechanical engineers, and anyone involved in the design and analysis of flow systems using orifice plates.

KristianCE
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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
 
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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
 

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