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
NPSH = NPSH available from the system, at the pump inlet, with the pump running
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.
Suction Pressure: The First Half of the NPSH Equation
Suction pressure must be determined at the pump suction nozzle when the pump is running. If suction pressure is measured with a gauge, the atmospheric pressure (at the pump location) must be added to the gauge reading to convert the reading to absolute pressure. The elevation of the gauge must also be added (if the gauge is above datum) or subtracted (if the gauge is below datum). Although often negligible, the velocity head in the pipe at the gauge connection should be added to obtain total (stagnation) pressure. For a reciprocating pump (and some rotaries), the acceleration head must be subtracted. (More on acceleration head later.)
Vapor Pressure: The Second Half of the NPSH Equation
Vapor pressure is more difficult to determine than suction pressure. It is a measure of the "desire" of a liquid to boil to a gas. Some liquids, such as butane and ammonia, have high vapor pressures. They must be kept under pressure, or they will boil (flash). An open container of pure ammonia would quickly boil away, filling the area with noxious ammonia gas.
Cool water has a low vapor pressure. An open container of cool water, on the earth's surface, would not boil, but would evaporate slowly over a period of days. The desire of cool water to boil is therefore low. If we were to place that same open container of cool water on the surface of the moon, it would boil away, similar to the ammonia. Why? The atmospheric pressure on the moon is zero, a perfect vacuum. The vapor pressure of pure, air‑free water at 80-deg F is about 1/2-psia. This means that if the pressure on the water is reduced below 1/2- psia, the water will boil.
A Function of Temperature
Vapor pressure is a function only of temperature. As the temperature of the liquid increases, its vapor pressure increases until the critical temperature is reached. At the critical temperature, vapor pressure vanishes. Above the critical temperature, there is no distinction between a liquid and a gas. It is all fluid.
Boiling Reestablishes Equilibrium Conditions
Any liquid at its vapor pressure is on the verge of boiling (flashing). In such a condition it is said to be in equilibrium, at its bubble point or saturated. If the pressure is reduced slightly, it will start to boil. If the temperature is held constant (which requires heat input) and the pressure held constant (below the vapor pressure), the liquid will continue to boil until it has all flashed to vapor.