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

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In summary, the conversation discusses the topic of cavitation and the difficulties in calculating the pressure at vena contracta. The speaker is seeking a more accurate solution and suggests using vena contracta taps or referring to ASME's report on fluid meters for guidance. There is no direct calculation for determining the pressure at vena contracta and it is dependent on various factors such as beta ratio and pipe diameter.
  • #1
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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  • #2
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
 
  • #3
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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.
 

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

1. What is the Vena Contracta?

The Vena Contracta is the narrowest point in a fluid stream where the velocity of the fluid is the highest, usually occurring just downstream of a constriction or restriction.

2. How is the Vena Contracta related to cavitation?

The Vena Contracta is closely related to cavitation because it is at this point that the pressure of the fluid drops to its lowest value, which can lead to the formation of vapor bubbles and subsequent cavitation.

3. What is cavitation and why is it important to study?

Cavitation is the formation and sudden collapse of vapor bubbles in a fluid due to low pressure. It is important to study because it can cause damage to machinery and structures, and can also have negative effects on fluid flow and efficiency.

4. How can the Vena Contracta be measured?

The Vena Contracta can be measured using various techniques such as pressure sensors, flow visualization methods, and ultrasonic probes. These methods can provide information on the size, location, and intensity of the Vena Contracta.

5. What are some factors that can affect the Vena Contracta and cavitation?

The Vena Contracta and cavitation can be affected by factors such as the shape and size of the constriction, the viscosity and density of the fluid, and the velocity of the flow. Other factors like temperature, surface roughness, and dissolved gases can also play a role.

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