What is Net Positive Suction Head?
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:
NPSH = Ps ‑ Pvap
Ps = Stagnation suction pressure, at the pump inlet, with the pump running
Pvap = Vapor pressure of the pumpage at inlet temperature
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 psia. Gauge pressure is converted to absolute pressure by adding atmospheric pressure. In equation form:
absolute pressure = gauge pressure + 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:
h = 2.31p/SG
h = Head, feet
p = Pressure, psi
SG = Specific gravity of the liquid
Problem No. 1: NPSH
Stagnation suction pressure is determined to be 1-psig at a sea level installation. The vapor pressure of the liquid is 8-psia. Calculate NPSH in PSI and feet for a 0.9 SG liquid
NPSH = Ps ‑ Pvap = 1 + 14.7 ‑ 8 = 7.7 PSI
NPSH = 2.31p/SG = (2.31) (7.7)/0.9 = 19.8 FEET
Velocity Head is Included
Note that suction pressure is stagnation pressure (total pressure); it includes velocity head. Adding velocity head puts all pumps on the same basis, otherwise a pump would require different amounts of NPSH when tested with different sizes of suction lines (assuming that suction pressure is measured in the suction line, which is normally the case).
It should also be noted that the velocity head is normally quite small, relative to the NPSH, so it can usually be ignored.
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.
What Symbol Should We Use?
Because units of pressure are typically used to express the value of NPSH for a displacement pump, some authors, and companies, use symbols such as NPIP (for net positive inlet pressure) and NIP (for net inlet pressure), because the units are pressure units, not head units. For simplicity, I'll stick with NPSH, regardless of the units used.