NPSH - Net Positive Suction Head

A definition and an introduction to Net Positive Suction Head - NPSH

The low pressure at the suction side of a pump can encounter the fluid to start boiling with

  • reduced efficiency
  • cavitation
  • damage

of the pump as a result. Boiling starts when the pressure in the liquid is reduced to the vapor pressure of the fluid at the actual temperature.

pump cavitation bubble formation

To characterize the potential for boiling and cavitation the difference between

  • the total head on the suction side of the pump - close to the impeller, and
  • the liquid vapor pressure at the actual temperature,

can be used.

Suction Head

Based on the Energy Equation - the suction head in the fluid close to the impeller can be expressed as the sum of the static and the velocity head:

hs = ps / γ + vs2 / 2 g         (1)

where

hs = suction head close to the impeller (m, in)

ps = static pressure in the fluid close to the impeller (Pa (N/m2), psi (lb/in2))

γ = specific weight of the fluid (N/m3, lb/ft3)

vs = velocity of fluid (m/s, in/s)

g = acceleration of gravity (9.81 m/s2386.1 in/s2)

Liquids Vapor Head

The liquids vapor head at the actual temperature can be expressed as:

hv = pv / γ         (2)

where

hv = vapor head (m, in)

pv = vapor pressure (m, in)

Note! The vapor pressure in fluids depends on temperature. Water, our most common fluid, starts boiling at 20 oC if the absolute pressure in the fluid is 2.3 kN/m2. For an absolute pressure of 47.5 kN/m2, the water starts boiling at 80 oC. At an absolute pressure of 101.3 kN/m2 (normal atmosphere), the boiling starts at 100 oC

Net Positive Suction Head - NPSH

The Net Positive Suction Head - NPSH - can be expressed as

  • the difference between the Suction Head, and
  • the Liquids Vapor Head

and can be expressed as

NPSH = hs - hv         (3)

or, by combining (1) and (2)

NPSH = ps / γ + vs2 / 2 g - pv / γ         (3b)

where

NPSH = Net Positive Suction Head (m, in)

Available NPSH - NPSHa or NPSHA

The Net Positive Suction Head available to the suction system for the pump is often named NPSHa. The NPSHa can be estimated during the design and construction of the system, or determined experimentally by testing the actual physical system.

pump npsh

The available NPSHa can be estimated with the Energy Equation.

For a common application - where the pump lifts a fluid from an open tank at one level to an other, the energy or head at the surface of the tank is the same as the energy or head before the pump impeller and can be expressed as:

h0 = hs + hl         (4)

where

h0 = head at surface (m, in)

hs = head before the impeller (m, in)

hl = head loss from the surface to impeller - major and minor loss in the suction pipe (m, in)

In an open tank the head at surface can be expressed as:

h0 = p0 / γ = patm / γ         (4b)

For a closed pressurized tank the absolute static pressure inside the tank must be used.

The head before the impeller can be expressed as:

hs = ps / γ + vs2 / 2 g + he         (4c)

where

he = elevation from surface to pump - positive if pump is above the tank, negative if the pump is below the tank (m, in)

Transforming (4) with (4b) and (4c):

patm / γ = ps / γ + vs2 / 2 g + he + hl         (4d)

The head available before the impeller can be expressed as:

ps / γ + vs2 / 2 g = patm / γ - he - hl         (4e)

or as the available NPSHa:

NPSHa = patm / γ - he - hl - pv / γ         (4f)

where

NPSHa = Available Net Positive Suction Head (m, in)

Available NPSHa - the Pump is above the Tank

If the pump is positioned above the tank, the elevation - he - is positive and the NPSHa decreases when the elevation of the pump increases (lifting the pump).

At some level the NPSHa will be reduced to zero and the fluid starts to evaporate.

Available NPSHa - the Pump is below the Tank

If the pump is positioned below the tank, the elevation - he - is negative and the NPSHa increases when the elevation of the pump decreases (lowering the pump).

It's always possible to increase the NPSHa by lowering the pump (as long as the major and minor head loss due to a longer pipe don't increase it more). Note! It is important - and common - to lower the pump when pumping a fluid close to evaporation temperature.

Required NPSH - NPSHr or NPSHR

The NPSHr, called as the Net Suction Head as required by the pump in order to prevent cavitation for safe and reliable operation of the pump.

The required NPSHr for a particular pump is in general determined experimentally by the pump manufacturer and a part of the documentation of the pump.

pump efficiency bep

The available NPSHa of the system should always exceeded the required NPSHr of the pump to avoid vaporization and cavitation of the impellers eye. The available NPSHa should in general be significant higher than the required NPSHr to avoid that head loss in the suction pipe and in the pump casing, local velocity accelerations and pressure decreases, start boiling the fluid on the impellers surface.

Note that required NPSHr increases with the square of capacity.

Pumps with double-suction impellers has lower NPSHr than pumps with single-suction impellers. A pump with a double-suction impeller is considered hydraulically balanced but is susceptible to an uneven flow on both sides with improper pipe-work.

Example - Pumping Water from an Open Tank

When elevating a pump located above a tank (lifting the pump) - the fluid starts to evaporate at the suction side of the pump at what is the maximum elevation for the actual temperature of the pumping fluid.

At the maximum elevation NPSHa is zero. The maximum elevation can therefore be expressed by modifying (4f) to:

NPSHa = patm / γ - he - hl - pv / γ

          = 0

For an optimal theoretical condition we neglect major and minor head loss. The elevation head can then be expressed as:

he = patm / γ - pv / γ         (5)

The maximum elevation - or suction head - for an open tank depends on the atmospheric pressure - which in general can be regarded as constant, and the vapor pressure of the fluid - which in general vary with temperature, especially for water.

The absolute vapor pressure of water at temperature 20 oC is 2.3 kN/m2. The maximum theoretical elevation of a pump when pumping water at 20 oC is therefore:

he = (101.33 kN/m2) / (9.80 kN/m3) - (2.3 kN/m2) / (9.80 kN/m3)

    = 10.1 m

Due to head loss in the suction pipe and the local conditions inside the pump - the theoretical maximum elevation normally is significantly decreased.

Maximum theoretical elevation of a pump above an open tank at different water temperatures are indicated below.

Suction Head for Water as Affected by Temperature

Suction head for water - or max. elevation of a pump above a water surface - as affected by the temperature of the pumping water - is indicated below:

Temperature Vapor Pressure Suction Head 
(oC) (oF) (kN/m2) (m) (ft)
0 32 0.6 10.3 33.8
5 41 0.9 10.2 33.5
10 50 1.2 10.2 33.5
15 59 1.7 10.2 33.5
20 68 2.3 10.1 33.1
25 77 3.2 10.0 32.8
30 86 4.3 9.9 32.5
35 95 5.6 9.8 32.2
40 104 7.7 9.5 31.2
45 113 9.6 9.4 30.8
50 122 12.5 9.1 29.9
55 131 15.7 8.7 28.5
60 140 20 8.3 27.2
65 149 25 7.8 25.6
70 158 32.1 7.1 23.3
75 167 38.6 6.4 21
80 176 47.5 5.5 18
85 185 57.8 4.4 14.4
90 194 70 3.2 10.5
95 203 84.5 1.7 5.6
100 212 101.33 0.0 0

Pumping Hydrocarbons

Note that the NPSH specifications provided by manufacturers in general are for use with cold water. For hydrocarbons these values must be lowered to account for vapor release properties of complex organic liquids.

Fluid Temperature (oC) Vapor Pressure (kPa abs)
Ethanol 20 5.9
65 58.2
Methyl Acetate 20 22.8
55 93.9

The head developed by a pump is independent of liquid and a performance curve for water can be used for Newtonian liquids like gasoline, diesel or similar. Note that the required power to the pump depends on the liquid density and must be recalculated.

NPSH and Liquids with Dissolved Gas

NPSH calculations might be modified if there is significant amount of dissolved gas in a liquid. The gas saturation pressure is often much higher than a liquid's vapor pressure.

Related Topics

  • Pumps - Piping systems and pumps - centrifugal pumps, displacement pumps - cavitation, viscosity, head and pressure, power consumption and more

Related Documents

Key Words

  • en: pump npsh
  • es: NPSH de la bomba
  • de: Pumpen-NPSH

Search the Engineering ToolBox

- "the most efficient way to navigate the Engineering ToolBox!"

Engineering ToolBox - SketchUp Edition - Online 3D modeling!

3D Engineering ToolBox - draw and model technical applications

Engineering ToolBox - SketchUp Edition - add standard and customized parametric components - like flange beams, lumbers, piping and more - to your SketchUp model - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro. Add from the Sketchup Extension Warehouse!

Translate the Engineering ToolBox!
About the Engineering ToolBox!

close