Net positive suction head (NPSH) is a crucial topic for the design, selection and operation of pump systems in chemical process industries (CPI) facilities. This one-page reference provides information about NPSH available (NPSHa) and NPSH required (NPSHr).
Hydrostatic pressure
Hydrostatic pressure (liquid head pressure) is the downward force exerted by a column of liquid due to the earth’s gravitational pull. In industrial applications, it is often expressed in units of distance (feet or meters), where the height of a vertical sample of liquid corresponds to a pressure at the bottom of its vessel or container, and depends on the density of liquid. For example, one vertical foot of pure water has a pressure head of 0.433 pounds per square inch (psi).
When discussing NPSH and pumping liquids, it is important to keep track of the pressure units and recognize the difference between psig (the gage pressure) and psia (psi absolute — gage pressure plus atmospheric pressure). In the example for pure water, psig at the bottom of a container is 0.433 psi and 0 at the water’s surface, but psia adds the standard atmospheric pressure (at sea level, this value is 14.7 psia, and varies with elevation and weather). In the example of pumping a liquid from an open tank, the atmospheric pressure can increase the total head of a liquid because it is pushing down on the water’s surface, helping to deliver fluid to the pump.
NPSH
The mechanical energy in a liquid system is expressed by the total head of that liquid. The total head combines static head (energy stored in a fluid due to its hydrostatic pressure), velocity head (kinetic energy of a moving fluid) and elevation head (the potential energy of a fluid as a result of its height above a reference point). NPSHa represents the margin by which the total head in a system is greater than the liquid’s vapor pressure at a given temperature, (which indicates how close the liquid is to vaporizing). It indicates how close the fluid is to its flashing conditions under the conditions at the suction end of the pump.
NPSHa is determined by the suction system: tank pressure, liquid level, suction piping losses and the vapor pressure of the liquid at pumping temperature. This is what the system delivers to the pump inlet. For an open-tank system, NPSHa can be approximated by adding the atmospheric head and the static suction head, then subtracting the friction losses in suction piping and the vapor pressure head of the liquid.
NPSH is the total head at the pump suction, expressed in feet of liquid, above the vapor pressure of the liquid being pumped. It represents the energy available to move liquid into the pump without flashing to vapor.
For a pump to operate without cavitation, NPSHa must exceed NPSHr by a sufficient margin. NPSHr is the minimum NPSHa that is required for a pump to achieve a specified performance for a specified flowrate and pumped liquid (Figure 1).
FIGURE 1. This centrifugal pump cutway shows the flow of liquid from suction side to discharge [2]
In the case of a system with high NPSHa, such as pumping cold water (low vapor pressure) from an elevated tank (high static head) through a short suction line (minimal friction losses), the margin of NPSHa over NPSHr is large, so liquid would be unlikely to flash when entering the pump. In contrast, a situation where NPSHa is low, such as when pumping a hot or volatile liquid (higher vapor pressure) from below grade (no static head) through a long suction line (higher internal pipe friction), the margin between NPSHa and NPSHr decreases and flashing becomes more likely.
The Hydraulic Institute (www.pumps.org) implemented a 3% reduction in head at a constant flowrate caused by a reduced suction head as the NPSHr of the pump. This is because this value was the smallest head drop that could be consistently and practically measured [1]. Manufacturers’ pump curves often show plots of NPSH3 (essentially a specific level of NPSHr) and pump flowrate.
Differential head
When a liquid enters the rotating element of a pump (a region known as the impeller eye), its pressure drops due to Bernoulli’s fluid-mechanics principles, as the liquid entering axially is being turned radially outward by the impeller vanes (Figure 1). The liquid actually sees its lowest pressure inside the pump, rather than at the suction flange. This pressure drop is the reason why NPSHr is important: the pump needs enough total head at the inlet to overcome the internal pressure drop without the liquid flashing.
After the low-pressure zone, the pump impeller imparts energy to the liquid by converting mechanical rotational energy into liquid velocity. The pressure rise at the discharge point is the total differential head (the difference between suction head and discharge head).
If the NPSHa is too close to the NPSHr, not only will cavitation at the inlet be a concern, but the pump may not be able to maintain sufficient differential head.
References
1.Gaydon, P., The Basics of NPSH and Pump Operating Regions, Hydraulic Institute blog post, December 2022, www.pumps.org.
2. Fantagu, Centrifugal Pump image, Wikimedia Commons, https://commons.wikimedia.org/wiki/File:Centrifugal_Pump.png.
3. Sarver, J., Finkenhauer, B. and Liu, Y.A., Pump Sizing Made Easy, Chem. Eng., January 2018.