A definition and an introduction to Net Positive Suction Head - NPSH
Low pressure at the suction side of a pump can encounter the fluid to start boiling with
- reduced efficiency
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.
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.
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)
hs = suction head close to the impeller
ps = static pressure in the fluid close to the impeller
γ = specific weight of the fluid
vs = velocity of fluid
Liquids Vapor Head
The liquids vapor head at the actual temperature can be expressed as:
hv = pv / γ (2)
hv = vapor head
pv = vapor pressure
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 expressed like
NPSH = hs - hv (3)
or, by combining (1) and (2)
NPSH = ps / γ + vs2 / 2 g - pv / γ (3b)
Available NPSH - NPSHa or NPSHA
The Net Positive Suction Head made available the suction system for the pump is often named NPSHa. The NPSHa can be determined during design and construction, or determined experimentally from the actual physical system.
The available NPSHa can be calculated 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:
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)
he = elevation from surface to pump - positive if pump is above the tank, negative if the pump is below the tank
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)
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.
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). This is important and it is common to lower the pump when pumping fluids 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.
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 impeller surface.
Note that the required NPSHr increases with the square 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 increasing the the elevation for a pump located above a tank, the fluid will start to evaporate at a maximum level for the actual temperature.
At the maximum elevation NPSHa is zero. The maximum elevation can therefore be expressed by (4f):
NPSHa = patm / γ - he - hl - pv / γ = 0
For optimal theoretical conditions we neglect the 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 height is therefore:
he = (101.33 kN/m2) / (9.80 kN/m3) - (2.3 kN/m2) / (9.80 kN/m3)
= 10.1 m
Due to the head loss in the suction pipe and the local conditions inside the pump - the theoretical maximum elevation is significantly decreased.
The maximum theoretical elevation of a pump above an open water tank at different temperatures can be found from the table below.
Suction Head as Affected by Temperature
|Temperature||Vapor Pressure||Max. elevation|
Be aware that the NPSH specification provided by the manufacturer in general is for use with cold water. For hydrocarbons these values must be lowered to account for the vapor release properties of complex organic liquids.
|Fluid||Temperature (oC)||Vapor Pressure (kPa abs)|
Note that the head developed by a pump is independent of the liquid, and that the performance curves for water from the manufacturer can be used for Newtonian liquids like gasoline, diesel or similar. Be aware that required power depends on liquid density and must be adjusted.
NPSH and Liquids with Dissolved Gas
Be aware that NPSH calculations might have to be modified if there are significant amounts of dissolved gas in the liquid. The gas saturation pressure is often much higher than the liquid's vapor pressure.