Pump Cavitation and Water Temperature

<...max fluid temp allowable before cavitation?...>

One looks it up in Steam Tables, in the columns for saturated water and steam.

• "Thermodynamic Properties of Fluids" by Mayhew & Rogers
• Kempe's Engineer's Yearbook, any edition.
• Perry, "The Chemical Engineer's Handbook", any edition.
• etc.

Is there a way to calculate it? If I don't know the temp at which cavitation occurs, how could I know the density or vapour press?

One looks it up in Steam Tables. <sigh>

Hmmm...actually hard to convince a lecturer without any workings <sigh>...

Maximum 80C after which pump cavitation takes place with condensate flasing into steam and imploding onto impellers. This is why boiler feed tanks are installed at a height of minimum 4.5 metres for 100c temperature since boilers are rated capacity at 100c feed water. This is your homework so you do your own .

"This is your homework so you do your own ."

Yep, I know it's mine but I'm having trouble inderstanding that's why I ask. Though I thought I would get a little assistance or guidance from the "gurus" but obviously I was wrong with only attitude from some "better than thou" wankers...

Thanks to those that posted without prejudice.

Wait a minute! Did anyone ever ask the guy "What Fluid?"

he mentioned water.

Oops, now I see it.

The vapour pressure vs temp chart is no use on its own, you also need to consider suction conditions. You will need to use the VP chart to calculate the NPSHa at different temperatures. The temperature which gives an NPSHa equal to the NPSHr of the pump at duty point is the temperature at which cavitation should start to occur (I say 'should' because the pump manufacturer may incorporate a safety margin in their curve).

This might help: http://www.mcnallyinstitute.com/11-html/11-12.html

Add a suitable safety margin to the NPSHr (typically 1m) to give you NPSHa which the system must provide. The formula for NPSHa includes a vapour pressure term. Calculate the maximum vapour pressure allowable to give the NPSHa worked out above. Then look up in a table water temperature corresponding to that vapour pressure.

If you don't have a table to hand, saturated vapour pressure as function of temperature (°C) is given to a good approximation by P_v = A*e^(B*T/(T+C)) where A = 6.105 mbar, B = 17.074, C = 234.011. You can solve that to give T in terms of P_v (left as an exercise!). Need to convert mbar to m or ft water.

Cheers..........Codey

It seems the GA's question is quite specific -

How do I work out the max fluid temp allowable before cavitation?

Not:

What is the max suction temp at which a pump should be run under x conditions to avoid cavitation problems?

The answers to these are quite different, and maybe the question should be re-phrased accordingly.

If you are actually trying to establish the point at which cavitation commences, then you want to be eliminating safety margins, not adding them. Furthermore, even NPSHr vs. NPSHa needs to be discounted, as it is usually defined (in terms of pump performance) as the point at which pump head is reduced by 3%, so how exactly do you determine the point of initiation of cavitation?.

If you are just trying to ensure your pump runs OK at given suction/temp conditions, then the formulae for establishing NPSHa are very simple(see above), and you just ensure they exceed NPSHr by a comfortable margin, depending on how optimistic or pessimistic you believe the pump suppliers NPSHr figures to be (this is the difficult part of the equation). If you have control over suction conditions and temperature, just leave plenty of margin. If NPSHr vs. NPSHa looks too close for comfort, ask the pump supplier for actual NPSHr test results.

NPSH and cavitation are not easily understood, even by the cognoscenti, and no one should be mocked for missing the point. It's a bit like a conundrum that, once you know the answer, it seems simple and obvious.

Having re-read the last paragraph of wot I wrote, I should apologize if it seems I am infering you are amongst the mockers. This is not what I intended, and your comments are equally true and valid. This was just a general statement.

OK Holzfeller, no problem. You're right, he did say (or implied) temperature at which cavitation actually occurs, rather than a design figure.

Defining NPSHr as point of 3% flow reduction is somewhat arbitrary, but it's the onset of cavitation that causes the flow reduction (isn't it?), and NPSHr is the only relevant data he has. I can't think of anything published by pump suppliers that gives a better idea. Also your post #9 says just that. I hadn't noticed it when I posted #11 and repeated some of what you'd said.

Cheers.........Codey

Hello Guest,

Cavitation can occur under several means first is steam (100*C) being created in water pipe line delivering. Also in cold water cavitaion can occur by vane motion beating the air out the water. Also it can come from pinhole air leak in delivery line, always hard to spot as now water exits line at point air being drawn in.