|1. Definition of
|2. NPSH Calculation|
Meaning of NPSH : the term pump NPSH stands for Net Positive Suction Head.
It represents the pressure drop in between the suction point of the pump and the minimum pressure that is reached at the pump inlet.
If the pump NPSH available is less than the NPSH required, the pump will not work properly. Either the liquid will not be pumped (not possible to prime the pump) or the operation will generate cavitation.
The NPSH required for a given pump is given by the pump manufacturer. It can be read on the pump characteristic curve.
Figure 1 : Pump NPSH, required and available
The NPSH available needs to be calculated from the actual suction pipe layout of the pump. The following case is considered below :
Figure 2 : Pump circuit
The NPSH formula is given below :
Equation 1 : NPSH calculation [Midoux]
Note : in many cases the velocity head u2/2g is negligible which simplifies the relation [Sihi]
- h1 = height from the level of liquid to centrifugal pump axis (m)
- p1 = pressure of the tank from which liquid is pumped (Pa)
- pv = vapor pressure of liquid pumped (Pa)
- u1 = velocity of liquid in the suction pipe (m/s)
- ΔHf = pressure drop by friction in the suction pipe (m)
- ρ = specific gravity of liquid pumped (kg/m3)
If the NPSH available is very low, it is important to select a low NPSH pump so that it can still suck the liquid without cavitation.
Refering to figure 2, a pump is installed at the outlet of a tank, to transfer water to another tank. It pumps 10 m3/h.
The tank is at atmospheric pressure, the pipe internal diameter is 60 mm, the height difference in between the level of liquid in tank 1 and the pump is 2 m. Water is at 20c.
STEP 1 : calculate the liquid vapor pressure
The vapor pressure can be either read from tables or calculated thanks to an Antoine law. For water at 20c : 2339 Pa.
STEP 2 : calculate the velocity in pipe
The velocity is 10/(π*0.062/4)= 3536 m/h = 0.98 m/s.
Note : in this example, the pipe is short and the velocity low, friction losses are neglected, if it is not the case and you wish to calculate the pressure loss, please refer here.
STEP 3 : calculate the NPSH
NPSH = 2+(101325-2339)/(9.81*998)+0.982/(2*9.81)+0=2+10.11+0.05=12.16 m
An NPSH available too low will lead to operation issues for the pump, especially a decrease in flowrate and the risk of cavitation.