Pump Related Terms

Vapor pressure, cavitation, and NPSH are subjects widely discussed by engineers, pumps users, and pumping equipment suppliers, but understood by too few. To grasp these subjects, a basic explanation is required.




VAPOR PRESSURE


Vapor pressure is the pressure absolute at which a liquid, at a given temperature, starts to boil or flash to a gas. Absolute pressure (psia) equals the gauge pressure (psig) plus atmospheric pressure.


Water and water containing dissolved air will boil at different temperatures. This is because one is a liquid and the other is a solution. A solution is a liquid with dissolved air or other gases. Solutions have a higher vapor pressure than their parent liquid and boil at a lower temperature. While vapor pressure curves are readily available for liquids, they are not for solutions. Obtaining the correct vapor pressure for a solution often requires actual laboratory testing.


CAVITATION


When a liquid boils in the suction line or suction nozzle of a pump, it is said to be “flashing” or “cavitating”
(forming cavities of gas in the liquid). This occurs when the pressure acting on the liquid is below the vapor pressure of the liquid. The damage occurs when these cavities or bubbles pass to a higher pressure region of the pump, usually just past the vane tips at the impeller “eye,” and then collapse or “implode.” Cavitation can create havoc with pumps and pumping systems in the form of vibration and noise. Bearing failure, shaft breakage, pitting on the impeller, and mechanical seal leakage are some of the problems caused by cavitation.


NPSH


Net Positive Suction Head is the difference between suction pressure and vapor pressure. In pump design
and application jargon, NPSHA is the net positive suction head available to the pump, and NPSHR is the net positive suction head required by the pump. The NPSHA must be equal to or greater than the NPSHR for a pump to run properly. One way to determine the NPSHA is to measure the suction pressure at the suction
nozzle, then apply the following formula:

NPSHA = PB – VP ± Gr + hv

where PB = barometric pressure

in feet absolute,

VP = vapor pressure of the liquid at maximum pumping temperature in feet absolute,

Gr = gauge reading at the pump suction, in feet absolute (plus if the reading is above barometric

pressure, minus if the reading is below the barometric pressure), and

hv = velocity head in the suction pipe in feet absolute.

NPSHR can only be determined during pump testing. To determine it, the test engineer must reduce the NPSHA to the pump at a given capacity until the pump cavitates. At this point the

vibration levels on the pump and system rise, and it sounds like gravel is being pumped. More

than one engineer has run for the emergency shut-down switch the first time he heard cavitation on

the test floor. It’s during these tests that one gains a real appreciation for the damage that will occur if a pump is allowed to cavitate for a prolonged period.

By Rajendra