[How to] Calculate NPSH - Net Positive Suction Head

Gud morning all.....!!! This post is delivered based on a request. Even though the request is comment form, i would like to deliver in full length, because while trying to learn about this on my own, i found its depth.

For getting all those that i'm delivering may be you need to go through some terminology, which will be useful to easily what i'm delivering.

What is NPSH & What does it mean exactly ?

NPSH - Net Positive Suction Head

It is just difference between Suction pressure and Vapour pressure.

NPSH = Ps - Pv.

Ps - Stagnation suction pressure at the pump inlet while pump is in running condition.

Pv - Vapour pressure of the solvent that is pumped.

What is Stagnation suction pressure ?

Stagnation suction pressure is the static pressure at a stagnation point there the fluid velocity is zero.

What is the relation between Pressure and Head ?

Head,h(feet) = 2.31 x P(psi) / SG.

P - Pressure, SG - Specific Gravity.

Don't get confused, i'll explain you from where i've established this equation.

Usually Vapour pressure shall be expressed in absolute scale, So suction pressure also be expressed in absolute scale, and we got a basic relation between Absolute pressure, Gauge pressure & Atmospheric pressure.

Absolute pressure = Gauge pressure + Atmospheric pressure,

Absolute pressure = Gauge pressure + 1.

All the pressure units were in psi.

What is Cavitation ?

Usually everyone knows cavities in nothing but holes and thats for solids, while coming for liquids cavities is nothing but just air bubbles.

These bubbles while drain the capacity of pumps,
Even they may effect the pump mechanical seal.

How to avoid the cavitation ?

By arranging the suction and discharge line in such a fashion that the pump should be possibly near to the suction receiver and the discharge line size should be 15-20% less than that of suction line, so that there may be chance for healthy pressure development in the discharged.

How liquid  Velocity effects the pressure ?

Mostly everyone thinks that if pressure increases the velocity shall increase, but as per me that not right, because if pressure increases it means whatever the flow that was initiated by a pump is not getting getting pumped through the line, so the velocity and pressure both are inversely proportional.

What is Head ?

Head is nothing but just pressure, let me clear you in detail, Usually the pump capacity depends upon the Impeller dia and impeller RPM. The pressure we get depends on the Specific gravity of the liquid. 

Conversions :

Feet of liquid = Inches of mercury x 1.133 / SG
                         = Pounds per square inch x 2.31 / SG
                         = Millimeters of mercury / ( 22.4 x SG ).

What are the types of NPSH ?

As per me there are two types of NPSH,

1. NPSHR - Net Positive Suction Head Required,
2. NPSHA - Net Positive Suction Head Available.

Going deep into the topic, i would like to elongate the NPSH as

NPSH = Static head + Surface pressure head - Liquid vapour pressure - Loss in the piping, fitting, valves.

Static head - it should be measure from the centerline of the pump suction to the top of the liquid level.

Vapour pressure of the product, for your convenience here i'll present you two graphs for absolute pressure vs temperature.

Vapour pressure chart for -60 to 240 F

Vapour pressure chart for -180 to 60 F

 Note: These graphs were grabbed from google images.

Loss of pressure due to fittings, valves & piping.

find the below charts, with resistance Coefficients.

Use the formula h = V^2 / ( 2 x g ), feet of liquid.

Friction fittings

Friction valves

Note: These images were grabbed from google images

Now lemme demonstrate the with some basic examples, because gathering information is just a beginning, but utilising them and analysing the case studies in much more important.

Example :

In 1.25" pipe size at 50 gpm, there will be 36.0 feet of loss for every 100 feet of pipe.

Friction loss in 1.25" to 1.5" pipe lines

 Pressure loss due to friction in 2.00" to 2.50"

Pressure loss due to friction in 3" pipe size

Pressure loss in 6" pipe size

Now lets start our show, and enter the calculation part, from here be careful and upto my expectation, many readers should have given up reading all the stuff 😆

Example :

Lets suppose the tank having liquid is at sea level and the liquid is 10 m above center line of pump impeller and coming to piping its a 15ft of 2" and a included 90° elbow bend, pumping 150 gpm water at 68°F, NPSHR is 12 ft.

Now, Static head = 10 ft,

Atmospheric pressure = pressure x 2.31 /SG = 14.7 x 2.31 / 1 = 34 ft absolute.

Vapour pressure at 68°F is 0.27 psi,

Pressure to head conversion = 0.27 x 2.31 / 1 = 0.62 ft,

Bring the friction charts into focus,

150 gpm flowing through 2" shows a loss of 38 feet for each 100 ft of pipe line, i.e, 38 / 15 = 2.53 feet of head loss.

The K factor for 2" 90° elbow bend is 0.6 ft,

Clubbing the whole feet of head loss will give us 2.53 + 0.6 = 3.13 ft.

NPSHA = Static head + Atmospheric pressure + gauge pressure - Head due to vapour pressure - friction losses 

= 10 + 34 + 0 - 0.62 - 3.13 = 40.25 ft

The NPSH required is 12 ft and we are able to generate 40.25 ft, so this is more than enough for pumping.

That's it.........!!!

This post is requested by Mr. Ram,

Hope everyone able to grab this, if any queries feed free to comment / message.

Comments are most appreciated.

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About The Author

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Hi! I am Ajay Kumar Kalva, Currently serving as the CEO of this site, a tech geek by passion, and a chemical process engineer by profession, i'm interested in writing articles regarding technology, hacking and pharma technology.
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