NPSH

NPSH - Net Positive Suction Head.

A pump displaces the water in its suction side and ther by creates a vacuum. The atmospheric pressure then have to be capable of pushing the water up the pipe to fill the void.

The NPSH required are therefore the required atmospheric pressure available to get the water there.

In the 10m example this would typically be 10m + friction in the pipes and fittings.

say 10 + 0.2 m = 10.2m.

The NPSH available depends on pressure at the site (Atmospheric pressure - altitude x factor) and the sg pumped media)

The NPSH available in your example of 10 m would therefore be very close to the NPSH required (at sea level).

What I have seen in practise (with water) is that the system may work some days and wont on another (due to atmospheric changes). The impeller will cavitate quickly. Dropping the pump by a meter or so will be much better.

If you supply us more detail a better calculation can be done. (type of fluid,your altitude, inside diameter length and type of pipe, fittings, pump details etc)

flow 440m3/hr@10 m head,sp gravity-1.00 eff. 77 %,NPSH required 1.60 m.and my negative suction is -3.5 m .So plz. let me know how we can componsate the negative suction by help of NPSH required given by pump manufacturer.

The 1.6 m indicates that the pump require 1.6 m positive pressure at the suction side for effective operation.

If at your Altitude the atmospheric pressure can delver 9m lift (example)

The NPSH available may be 9 - friction & losses - 3.5 = 9 - 0.5 (example) - 3.5 = 5 m

The system should work properly with water provided an adequate suction pipe size and a moderate altitude.

It has been a long time since last working with pumps my definitions in post 1 is actually not consistent.

Dear Hendrik,

Thanks for your immediate respondse.

Regards

Vishal

there are many ways to compensate for losses in a pump. However some are more straight forward than others. Some ways to increase NPSH_A include:

-increasing the pressure at the inlet to the suction line.

-decrease the temperature of the pumping fluid. This reduces the vapor pressure of the liquid.

-decrease the flowrate of the fluid.

-increase the height above the pump centerline to the open surface in the vessel where the fluid is coming from.

NPSHr is given by manufacturer and the system must has at minimum this value as NPSH available, in other words, NPSHa > NPSHr in order to avoid pump cavitation.

If the NPSHr is 1.6m means that won't be problems if the NPSHa is at minimum the atmospheric pressure at sea level (10.3mH2O). So the next step is to determine the negative suction; to do so use this:

H = Patm*10.2 - NPSH -Hf-Hv-Hs

Where, H : Suction head (m)

Patm: Atmospheric pressure (bar)

NPSH: Available (m)

Hf: Friction losses

Hv: Fluid vapor pressure @ operation temperature (m)

Hs: Safety factor (0.5m is recommended)

Hope this helps.

BSChE.

Perhaps an even simpler explanation will help? Centrifugal Pumps, for the most part, dont suck, they blow. The weight of the atmospher (14.7PSI at sea level or about 40'TDH) allows the fluid to flow into the eye of the pump impeller where energy is added to create lift measure in head feet ( divide by 2.31 for PSI). Pumps require a certain amount of that atmopshere to get things moving (usually anywhere from 2-10' of that head) which is Net positive Suction Head Required. Some pumps have "back vanes" that can aide in reducing the NPSHR, and of course there are self priming pumps designed to pull a certain amount of suction lift to get things started. The Hydraulic Institute in the USA is the place to go for detailed data sheets on the subject.

Joe T.

Hola Joe. You are absolutely right. The problem with installing a pump is that there is a relationship between the height of the pump center line and the height of the water reservoir surface. You can sit the pump above the level of the water reservoir up to 10.2 meters ideally or close to 22 feet. But you have hydraulic losess in the pipes, housings, elbows etc. In addition you have hydraulic designs wich vary so you actually can not afford installing the pump too high above the water surface and never could install it above 10 meter( if you want no external aid) because that is the value of the atmospheric pressure. Beside this if the npsh is very narrow the pump will star cavitating... thats why lowering the water temperature will increase the npsh since the water will not boil... Why will you want to install the pump above the reservoir level? well maybe there are physical restrains or because you want to pump the water a few meters higher. But If you install the pump at the water level will never have problems. My recomendation is to analyze the pump performance graphics and set the pump at the place more convinient to your application. This metod is more accurate and practical than any other because you are actually working with the specific data for your pump. Also consulting the Hydraulic Institute in USA will always be an excelent choice.

Regards. Luis