How to select a centrifugal pump such that NPSHa minus NPSHr is greater that 3.5m, however the NPSHa i have got is only 3m ! What can i possibly do to select a proper pump ?

I would be grateful for your suggestions.

why you need to have 3.5m safety margin? I put pumps with 1 m safe margin, them are working well. NPSHa can be increased by increasing the pipe diam, eliminating the elbows, reducer etc. fittings and decreasing the suction lift.

Mr. Johnny my client has this specific requirement of having a marging in excess of 3.5m. Also the liquid I am pumping has high concentration of dissolved gases.

You need to exercise caution! If the liquid is in any way 'hot', see if it's possible to reduce the operating temperature so as to gain NPSH from the vapour pressure. Also, as little as 5% entrained air or gas will cause centrifugal pump performance to reduce by about 50% as well as having to cope with all sorts of random forces that will reduce reliability. What are the application details so that readers of this post can give suggestions as to possible pump types?

This application seems more complex than the applications I experience. More details on the fluid to be pump will be critical to do the NPSH calculation. And manufacturer's inputs are needed since the NPSHr are based on inhouse test. (e.g. on clear water, 40 C).

Install the pump atleast 0.5 m below the Suction Source

The higher margin requirements are based on vaious operational parameters of the pump and system and a therefore not to be altered. The change in systtem flow,Temperature & pressures will have change in actual NPSH requirement. Is the pump in closed system, variable speed or system requirement? Incase of limitations on the suction, detum and process corrections the pump selection is to be done with Low-npsh or enducer type pump suitable for the application.

you can use a vertical pump API VS6 type ....

You may also consider reducing the RPM that you are working with to gain some NPSHr by the pump. Far more detail is needed to determine what options could be considered for your application.

Consider the following:

A. Increase your suction piping size, reduce run length and number of fittings to minimize friction losses.

B. Increase the suction tank/reservoir height if practical.

C. Fit a higher Nss impeller (the practicality of this depends on the pump Ns)

D. Go to a slower speed (As Debra noted). This may necessitate a 2 or more stage pump


I agree with other comments that you need to be much more specific on your application. What I provided are general possibilities. What I would employ in practice depends on a lot of things such as the fluid pumped, the required rated point and desired operating range.

as Simon said. Beside that you can use Enducer (impeller) that is installed on pump shaft ,and will generate an opposite effect that will balance the NPSH margine, but it need to be fabricated from high alloy for longer life cycle.

Also if your system is a closed tank ,you can pressurise the tank with suitable gas (will increase pressure on suction side), if not you can install another small pump on suction side to balance the NPSH margine.

Moreover ,you can infuence the designer to lower the pump set level for 1-2 Meters to achieve more Positive suction conditions, or as Simon said you can increase the tank level; instead you can set up the system (adjust Float switch) to keep Min. liquid level Value in tank as much high as possible, but make sure that the in take-tank filling rate will keep enough fluid inside it to match that creteria.

Avoide the Tees and binds in suction pipe.

Regards