Cavitation in Pipes

[ldavison10]

Hi,
I am trying to produce a spreadsheet which calculates the point at which pipe cavitates based on the velocity in a siphonic system. As part of this i have produced the attached excel spreadsheet. A couple of things i have learned since i produced it is

1) the vapour pressure should be in m, and
2) this should be in the range of 0.12 - 0.56.

I need to update the sheet to reflect this and correct any errors i may have made (ie not KPa), can someone help complete this. The formula is from the british standard for siphonic systems.
If someone knows another way of writing the formula then im all ears.

Thanks!

Lee

 

[BigInch]

Pumps cavitate, pipe with a fluid reaching vapor pressure (or bubble point) will tend to contain some amount of vaporized fluid.

Your spreadsheet does not say if h_min is gage pressures or absolute. I assume it is gage, since adding Ho to each side of the equation would indicate that h_min is a gage pressure. It is much easier to understand the fluid state if you use absolute pressures.


I don't have the standard you mention, so I can't be sure but, I believe that you have used an equation for calculating net positive suction head available (a typical pump calculation), not an equation for calculating when fluid in a pipe vaporizes, which is the limit at which a siphon will fail to function.

I think a siphon will stop functioning when the absolute pressure at the high point is equal to vapor pressure of the fluid at that point only, H_min = H_vp and the velocity term should not appear.

I would say that the equation should be based on the following (neglecting capiliary pressures),

H_atmos + H_di - H_s >= H_vp

H_atmos = Atmospheric pressure at fluid surface
H_di = depth to siphon inlet
H_s = height from siphon inlet to fluid level in siphon

If the invert elevation of the pipe at the siphon high point is > H_s it won't flow. Vapor will be created in the goose neck and the fluid will not flow over the neck.

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