suction performance of centrifugal pumps

[25362]

Can anybody submit a reasonable explanation why suction performance of a centrifugal pump handling water is slightly improved by rising water temperatures, and at what temperature levels is this effect felt ?

 

[scalleke]

Have you experienced this? First time I hear of it.

Best Regards
Scalleke

 

[25362]

To Scalleke, I haven't personally witnessed that effect, however books on pumps keep repeating that strange statement.

At higher temperatures, the NPSH_a clearly drops as the water vapor pressure increases, however the NPSH_r also seems to come down. I pressume it should be a result of the vapour bubbles' (steam) properties.

Taking extremes for an easier understanding, at 70^oF saturated steam has a specific volume of 869 ft³/lb while at 200^oF, just 33.7 ft³/lb. Therefore you need to evaporate 26 more water at 200^oF than at 70^oF to get the same volume of vapour. However, since the latent heat at 200^oF is 93% of that at 70^oF, the amount of heat needed to evaporate the same volume of steam would be 26*0.93=24 times larger at 200^oF than at 70^oF.
Since the time allowed for bubble formation in the low pressure zones inside the pump is quite short, a smaller volume of vapour would be generated at 200^oF than at 70^oF, meaning less cavitation intensity.

Although cavitation is not eliminated, I feel this is the explanation, could you confirm ?

A pump handling water under conditions of incipient cavitation, would seem be somehow "protected" by heating the water a few degrees, wouldn't it ?

If the above assumption is right, why hasn't this effect received more attention? Thanks.

 

[Scipio]

Actually it is a proven effect, though I don't know the mechanics behind it myself, but there are published curves for NPSHR reductions for hot water & hydrocarbon condensates based on tests comparing pump cavitation in cold water and again at various other liquids. The Hydraulic Institute and Karassik's "Pump Handbook" have these curves in them. The Pump Handbook does refer to theories by Stepanoff and others explaining this, maybe you can find something in one of these references;

Stepanoff, A.J.: Pumps and Blowers: Two Phase Flow, Wiley, New York, 1967.

Stahl, H.A., and A.J. Stepanoff: "Thermodynamic Aspects of Cavitation in Centrifugal Pumps," Trans. ASME 78:1691 (1956)

Salemann, V.: "Cavitation and NPSHR Requirements of Various Liquids," Trans ASME, J. Basic Eng., ser. D, 81.167 (1959)

Stepanoff, A.J.: "Cavitation Properties of Liquids," Trans. ASME, J. Eng. Power, ser. A, 86:195 (1964)

I've never seen the NPSHR reduction pumping hot water, I have seen the effects of hydrocarbon reduction, I've got pumps running happily away in hydrocarbon condensate service with about half the manufacturer's stated NPSHR.

I haven't read any of those references, and I hadn't thought about steam properties since it also effects condensate - I'd been thinking about the fact that hot water or hydrocarbons have lower densities. Because of the lower densities, the shockwaves of implosion would travel at a lower velocity, possibly reducing the damage inflicted on metal components. Just a guess though.

 

[25362]

To Scipio, thanks a lot for your comments and advice.
Following your thoughts, I went looking for the speed of sound in liquids. The speed of sound in liquids is given by the square root of the ratio: modulus of elasticity divided by the density.
The slower-moving shockwaves could indeed be one reason, as you propose, for the weaker effects of cavitation at higher temperatures for most liquids, including hydrocarbons.
Water, as usual, is an exception, i.e., sound velocity increases a bit with temperature, and this happens because the modulus of elasticity rises with temperature. Thus, perhaps the explanation based on thermodynamics suits better the cavitation effects at higher temperatures for water.
As a first thought it would be risky to use heat to reduce cavitation because pump materials may suffer a drop in strength at higher temperatures.

Any comments ? Thanks again.