The pump manufacturer will state the NPSHr, Net Positive Suction Head required, to prevent cavitation or vaporization of the liquid in the pump. The NPSHa, Net Positive Suction Head available, must be greater than that required by the pump.
For a basic understanding of Net Positive Suction Head, you must think in terms of absolute pressure in feet. To determine the NPSHa, the Net Positive Suction Head available at the pump inlet, you first determine the pressure above the liquid. As atmospheric pressure varies with altitude (and weather), the site elevation must be known. For this discussion, we will be at sea level and pumping water.
At zero feet elevation, sea level, the atmospheric pressure is 33.9’ absolute (or 14.7psia). When you are pumping from a tank that is open to atmosphere, you automatically have 33.9’ absolute pressure helping you get liquid to the pump suction.
We will assume our tank has 10 feet of cold water level above the pump. We have a total of 43.9’ of pressure available above the pump (33.9’ atmos. pressure + 10’ liquid level).
That 33.9’ of atmospheric pressure is reduced first by the vapor pressure of the liquid (vapor pressure is the tendency for the liquid to become a vapor).
Using water as a common reference. At 33°F., the vapor pressure of water is basically zero. As the water temperature increases, so does its vapor pressure. At 212°F, the vapor pressure equals 33.9’, the point at which water turns to a vapor.
If we heat our water tank example to 212°F, the atmospheric pressure of 33.9’ minus the vapor pressure of 33.9’ leaves us with only the 10’ of liquid level to supply the pump.
The last part of the equation is deducting the friction loss of the suction piping. This calculation will vary from application to application.
Net Positive Suction Head Available = pressure above the liquid + liquid level above the pump (negative if level is below the pump) – the vapor pressure of liquid at pumping temperature – the friction loss of the suction piping.
Although NPSH calculations are not exact, they but must be considered for proper pump selection.