Definition of NPSH
The margin of pressure over vapor pressure, at the pump suction nozzle, is net positive suction head (NPSH). NPSH is the difference between suction pressure (stagnation) and vapor pressure. In equation form:
Since vapor pressure is always expressed on the absolute scale, suction pressure must also be in absolute terms. In U.S. customary units, both pressures must be in pounds per square inch absolute (psia). Gauge pressure is converted to absolute pressure by adding atmospheric pressure.
The above equation provides an answer in units of pressure (psi). This can be converted to units of head (feet) by the following equation:
Importance of NPSH
NPSH is a subject of extreme importance in all pumping systems. It has been estimated that 80 percent of all pump problems are due to inadequate suction conditions, and most suction problems are related to NPSH. (Either the system does not provide as much as anticipated, or the pump requires more than anticipated.) It is therefore probable that most pump problems are NPSH problems.
Units of NPSH
For centrifugal pumps, NPSH values are expressed in units of specific energy (equivalent column height) such as feet or meters. For displacement pumps (rotary and reciprocating), NPSH values are normally expressed in pressure units such as pounds per square inch (psi), kilopascals or bars.
NPSH values are neither gauge pressures nor absolute pressures. The “g” in psig means that the pressure is measured above atmospheric pressure. The “a” in psia means that the pressure is measured above absolute zero, a perfect vacuum. NPSH is a measurement of pressure above vapor pressure, so the units of NPSH (in the U.S.) are just psi or feet.
NPSH Available: A System Characteristic
NPSHa stands for NPSH available from the system. It can be calculated by measuring suction pressure at the pump suction nozzle, correcting to datum, adding atmospheric pressure, adding velocity head and subtracting vapor pressure. In equation form:
If desired, all units can be converted to head (feet) prior to plugging into the equation.
If the system has not been built, it is necessary to calculate the NPSHa by starting with the pressure in the suction tank. Add atmospheric pressure, add (or subtract) the liquid level above (below) datum, subtract all losses from the tank to the pump and subtract vapor pressure.