Calculating NPSH

3 min read

The most common problems encountered in a pumping system are located on the suction side of the pump. Both inlet design and NPSH are especially important parts of successful pump performance. This article will address NPSH.

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Defining NPSH 

NPSH stands for Net Positive Suction Head. It is the amount of Suction Head left after various losses have been deducted from a starting point. 

Positive Suction Head can be defined as pressure that causes liquid to enter the suction side of a pump. A pump does not suck liquid – a downward force is exerted on the liquid which allows it to enter the inlet of the pump. This ‘downward force’ is atmospheric pressure. 

NPSH has two distinct elements: 

  1. NPSHa – Available 
  2. NPSHr – Required 

Net Positive Suction Head Available (NPSHa) 

For every pump system, there is a value for the pressure available at the eye of the impeller usually expressed in metres (NPSHa). This is a function of the piping system inclusive of all friction losses, static head and velocity head. 

What’s absolute pressure? 

The starting point is atmospheric pressure, which is measured in “absolute” terms. “Absolute” means starting from a perfect vacuum: i.e. at sea level, atmospheric pressure will be about 1bar absolute pressure (abs), 14.5 psi abs, 100kPa abs, or 10.3 metres head of water.

Normal readings of pressure start by taking atmospheric pressure as the zero point and are referred to as “gauge” pressures. 

Calculating NPSHa 

We use the following formulae: 

NPSHa (flooded suction) = Ha + Hs – Hv – H 

NPSHa (suction lift) = Ha – Hl – Hv – H 

Ha = absolute pressure head on liquid surface (frequently atmospheric pressure) 

Hs = liquid level above the pump centre line 

Hl = liquid level below the pump centre line 

Hv = liquid vapour pressure (absolute) at the pumping temperature 

Hf = friction losses in the pipework

Why are 4 pole pumps the preference for cooling tower applications? 

In the below example, we illustrate how the NPSH requirement is always higher for 2 pole selections. This means 4 poles selections are the superior choice for cooling tower applications where when the tower and pumps are located on the same level.

Here’s what we were given: 

Altitude – 6-42m 

Atmospheric Pressure – 100.85kPa >>> 10.29m 

Temperature – 28°C 

Flooded Suction = Ha + Hs – Hv – Hf 

 

4 Pole 

2 Pole 

Designation 

CWP-L35-3,4 

CWP-L35-3,4 

Flow (L/s) 

45 

45 

Head (m) 

24.5 

24.5 

Pump Model 

BB125-32 

BB80-16 

Pump Speed (RPM) 

1450 

2900 

NPSHr (m) 

1.82 

4.38 

Ha (m) 

10.29 

10.29 

Hs (m) 

0.635 

0.635 

Hv (m) 

0.5 

0.5 

Hf (m) 

6.3 

6.3 

NPSHa (m) 

4.125 

4.125 

NPSHa > NPSHr + 1 

4.125 > 2.82 

4.125 < 5.38 

Note:  

  1. Vapour Pressure is found in standard tables or given by the client. 
  2. Keep all figures in the same units. 
  3. Atmospheric pressure varies with altitude. 

Examples of this variation: 

Altitude 

Atmos. Pressure (kPa absol.) 

Equivalent Head (m) 

(ft) 

(m) 

 

 

0 

0 

101.3 

10.37 

500 

152.4 

99.2 

10.19 

1000 

304.8 

97.8 

10 

1500 

457.2 

95.7 

9.82 

2000 

609.6 

94.4 

9.64 

2500 

762 

92.3 

9.45 

 Net Positive Suction Head Required (NPSHr)  

Each individual pump has a hydraulic requirement at the suction inlet (Net Positive Suction Head Required). This value is given by the pump manufacturer and will be expressed on the pump performance curve.

Positive displacement pumps also have specific NPSHr, and this can be shown graphically or in a table. NPSHr is the lowest NPSH at which a pump will operate satisfactorily without cavitation. 

NPSHa Must Always be Greater Than NPSHr 

Once you have calculated the NPSHa, you need to compare this result with the NPSHr on the pump curve. Is the NPSHa greater? It should be greater by at least 1 metre. 

Follow this rule to get it right: 

NPSHa > NPSHr + 1 

Installation done already? 

Sometimes we find that NPSH was not checked by the contractor before pump selection. You can still reduce/avoid cavitation in a system like this by: 

  • Reducing the friction losses of the system by modifying the suction pipework, e.g. increasing the line size and in some instances, reverting back to traditional y-strainers. This will factor in higher pressure drops through suction diffusers. 
  • Increasing the difference between the pump suction and cooling tower water level. You could elevate the cooling towers further, or install the pumps on a lower level.

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