NPSHR reduction with rising temperatures
NPSHR reduction with rising temperatures
(OP)
The relevant chart appears on:
http://www.mcnallyinstitute.com/Charts/NPSH_reduct...
Karassik et al., in their Pump Handbook (McGraw-Hill) show this chart (constructed from test data) saying that it could be used subject to some listed limitations.
I'd appreciate to read your opinions about the pertinent reason(s) for those NPSHR reductions (meant for pure deaerated liquids).





RE: NPSHR reduction with rising temperatures
Independent events are seldomly independent.
RE: NPSHR reduction with rising temperatures
True and thanks. I'm after the underlying reason (s). I assume it has to do with the vapor specific volumes (L/kg) decreasing as temperatures rise.
For example with butane, when comparing its (saturation) properties at 30 and 50oC:
Vapor, L/kg; latent heat, kcal/kg; sound speed in liquid, m/s; pressure, atm; are respectively:
30oC => 140.1, 85.2, 874, 2.8
50oC => 81.7, 79.8, 767, 4.9
We see a reduction of more than 40% in specific volume, and only about 6% in latent heat. A small reduction of sound speed in the liquid of some 12% may also help in reducing the impact of shock waves when bubbles implode.
I'd much appreciate reading yours and other experts' opinions on the subject.
RE: NPSHR reduction with rising temperatures
BTW, this strange effect doesn't mean that by heating the fluid stream we avoid cavitation, since the NPSHA drops (vapor pressures rise) by rising temperatures, probably even more than the allowable decrease in NPSHR.
RE: NPSHR reduction with rising temperatures
Independent events are seldomly independent.
RE: NPSHR reduction with rising temperatures
"…If the suction system may be susceptible to transient changes in absolute pressure or temperature, a suitable margin of safety in NPSH should be provided. This is particularly important with hot water and may exceed the reduction that would otherwise apply with steady-state conditions."
An exercise with water between 30 and 90°C shows, respectively:
Vapor pressure, atm: 0.0419 & 0.0693;
Saturated vapor, m3/kg: 32.9 & 2.36;
Latent heat, kcal/kg: 600.7 & 545.5;
Sound speed, m/s: 1509 & 1556.
Note that sound speed rises with temperature.
For 90°C the allowable reduction would be about 0.7 ft. According to the graph, for 150oC, it would be about 4 ft.
RE: NPSHR reduction with rising temperatures
Water is indeed a strange liquid. This shows a slight reduction of sonic velocity between 150 and 212F. http://www.engineeringtoolbox.com/sound-speed-wate...
Independent events are seldomly independent.
RE: NPSHR reduction with rising temperatures
You are absolutely right, indeed, water properties are weird.