Practical use of Cavitation Number 5

[SiggiN]

Hi!

I'm trying to compare a number of fluids wrt. cavitation through a connector

This is not my area of expertis, so I was hoping someone could explain to me if the "cavitation number" is a good comparison between fluids?

Ca = (p-p_v)/(0.5*rho*v^2)

Is there a target value for Ca?

Is Ca conservative for estimating cavitation?

Thank you!

Regards
Sigurd

 

[LittleInch]

I looked this up as I'd never seen it before but no one seems to give any numbers for when is ok and when it isn't.

Maybe >1?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

I don't see Ca as a means of making meaningful fluid comparisons. The only fluid properties that enters the equation are Vapor Pressure and Density, so other than a ratio comparing volatility to density, there doesn't appear to be anything else that you can derive from the Ca number in regards to comparing fluids to one another.

It may serve to compare the ability to pump fluids at low pressures, but I don't see the need for that either. In the real world, you seldom if ever get any choice between fluids that need to be pumped. You either have to do gasoline today, or diesel today, or visa versa, and usually you have to be able to pump either one.

But Ca describes the ratio of Net Positive Suction Head, Suction Pressure - Vapor Pressure, to velocity head, rho V^2/2/g which could be useful knowledge when designing a pump system.

Suction Pressure - Vapor Pressure can be thought of as your operating suction pressure's margin of safety against cavitating conditions. The "Cavitation Number" compares that margin of safety to Velocity Head, 1/2 rho V^2/2/g

When calculating Net Positive Suction Head Available, NPSHA, for a pump system, usually velocity head is ignored in order to arrive at a conservative calculation for NPSHA, but velocity head can be included in the NPSHA calculation, if the system designer feels it is warranted.

So in the usual case where velocity head is ignored in the NPSHA calculation, the Ca number would then compare the margin of suction pressure's margin of safety against cavitation to the velocity head that really is there, but was not considered. A Ca of 1 would indicate that the cavitation margin is comprised of 50% pressure head and 50% velocity head, which is kind of like a safety factor of 2 against cavitation, since you have a pressure margin against cavitation equal to the velocity head's margin of safety against cavitation. A Ca value close to 0 would indicate that suction pressure is nearly at vapor pressure and the only thing preventing immenent cavitation is the velocity head.

For your connection problem, I think I'd tend just to look at absolute connection pressure ratioed to the fluid's vapor pressure as an indication of its flashing characteristics. 1 = Flash

Maybe somebody else knows other instances of where Ca could be useful, (evaporation rate of light or heavy coatings???) , but I don't.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[Latexman]

Same here. First time I've seen this. I've never seen it used in practice.

You can look here

https://www.engineeringtoolbox.com/cavitation-number-d_584.html

but it has no numbers, so . . .

Closest thing I have to it is an old magazine article on Eliminating Cavitation from Pressure-Reducing Orifices. It recommends using multiple orifices to stay away from the vapor pressure. It does NOT use this definition of a Cavitation Number.

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[1503-44]

I couldn't find anything at all about Ca in any of my textbooks. I don't see much of a use for it. Maybe it is an extremely important number for designing ink jet printers using some kind of highy volitile fluid ... or something?

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[Latexman]

I see the utility of the numerator, (p-p_v), but I don't see what the denominator, (0.5*rho*v^2), the kinetic energy term, brings to the table.


Good Luck,
Latexman
Pats' Pub's Proprietor

 

[bimr]

It appears that there is no standard that exists for predicting cavitation damage.

You can probably use the same method that valve manufacturers use. One is a mathematical calculation; the other is empirical based on the noise volume of the fluid disturbance.

Link

 

[1503-44]

Thanks bimr, nice read. The most I've ever seen about cavitation in any one place.
It gave me some more ideas about Ca. I haven't looked into these yet, but Ca might be useful to compare,
1. Different fluids when considering cavitation downstream of a valve; cavitation from high velocity inside the valve followed by pressure recovery in a lower velocity downstream environment.
2 Different fluids propensities for pump cavitation, comparing NPSHa for specific fluids, other than water; with NPSHr from the pump manufacturer, which is always given relative to cool water.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"