Boiling (cavitation) starts starts when the water pressure at the
impeller is reduced to the vapor pressure of water at the water
temperature. You see this addressed in the pump literature as "NPSH"
The term net positive suction pressure (NPSH) is defined as the
difference between the suction pressure and the vapor pressure of water
and must be a positive number to avoid cavitation. At 68 degrees F water
temperature, the vapor pressure is 0.35 psi. So, as long as the inlet
pressure is greater than 0.35 psi absolute or -14.3 gauge, no cavitation.
In practical terms, so long as the suction pressure is above -12 to -13
psi, you should be fine. When a typical centrifugal fire pump starts to
cavitate, you'll know it. (sounds like the pump is full of rocks), and
you're not going to damage the pump unless you let it cavitate for an
extended period of time. The problem, of course, is that the pump
performance deteriorates significantly.
Joe
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