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Pump NPSH

Net positive suction head

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Section summary
1. Definition of NPSH
2. NPSH Calculation
3. Effect

1. Definition : What is pump NPSH

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.

2. NPSH Calculation : How to calculate the pump NPSH

NPSH required - NPSHr

The NPSH required for a given pump is given by the pump manufacturer. It can be read on the pump characteristic curve.

Pump NPSH

Figure 1 : Pump NPSH, required and available

NPSH available - NPSHa

The NPSH available needs to be calculated from the actual suction pipe layout of the pump. The following case is considered below :

Pump circuit

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]

With :
- 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.

NPSH calculation example

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

3. Effect

An NPSH available too low will lead to operation issues for the pump, especially a decrease in flowrate and the risk of cavitation.



Source

[Midoux] Mecanique et Rheologie des Fluides, Midoux, 1993
[Sihi] Basic Principles for the Design of Centrifugal Pump Installations, 2003