Pump NPSH

Can you explain more detailly.

Regards,

Vinod.P

the existing PVC pipeline has a 2in diameter and a total length of 7m and 2 90 degree elbows. the pump will be installed 4meters at the top of a horizontal cylindrical tank. a foot valve is installed at the suction inlet of the pipeline. flowrate is about 25m3/hr. the liquid is sodium hypochlorite. based on my computations, the NPSH is around -4m. i want to check if my calculations are correct. or if its correct, would there be a pump that has a NPSH requirement of less than -4m. thank you very much for your interest on my problem. im new in this field by the way, any help is very much appreciated.

what you want is a centrifugal pump that pumps air as well as your fluid. It starts with air, pumps that to produce enough suction head for the liquid to reach the inlet and then pumps the fluid.

Differences in density make this type of pump hard to make, but easy to describe, which is why in practice you do not see them. So you can make the pump in the low portion of a U trap with anough fluid in the U that by the time the pump runs out of fluid the fresh fluid has come over the lip of the U and floods the inlet, with some safety factor

Hello

You can use a membrans pumps for your application , . Pump with an inlet and outlet of 3or 4" . After that you will neef only air for use your pump .a good membran pumphave around 7 or 8 m of NPSM , so it's seems good for you . See Graco pumps ,Wilden Pumps ,

If you want more explication , I am here for trying to help you

Bye

The following are factors influencing NPSH:

· NPSH available should be more than NPSH required.

· NPSH required is a design factor normally given by pump OEM.

· NPSH available is the suction head, normally measured as the height between the 'Eye of the impeller' till the suction level. In this the details like: sp gravity of the liquid, pipe and pipe fitting frictional loss etc, are to be included.

Check:

1. The existing pipe size is adequate to supply the need for the pump. If pump suction port/flange is more than 2in (your existing line), you are sure to pinch the NPSP available.

2. As Sodium hypo is the liquid and if the pump under consideration is 'all polymer centrifugal type', take care of dry running. If metallic pump is being considered, take care of free Chlorine, which might liberate under suction condition.

Consider suitable elastomer for foot valve, since simple rubber may swell in hypo.

Guest (original poster):

There are a great many centrifugal suction lift pumps available and you can find them with a Google search. You might also want to read this site:

Basic theory of pumps

The NPSH of a suction lift pump is still positive as explained in the above referenced web site.

Checked that site. Typed NPSH in the search box and it gave no matches!! Doesn't exactly fill you with confidence.

Also saw in at least one of the sites given in this thread the statement that NPSHav cannot be negative. This is not so. E.g. if a pump is 40' above water surface in the suction tank, NPSHav is about -6' (assuming normal atmospheric pressure and ignoring losses). This just shows that if the pump you're hoping to use has a NPSHr of say 10' (typical figure) you're at least 16' out.

Cheers......Codey

You just click of the blue-lettered link in my response and it takes you to a page entiled "Basic theory of pumps", which has a sketch (when you read and scroll down) explaining that NPSH starts at 0 psia. Look at that sketch and understand it! The pump impellers create a vacuum at the suction suction eye of the impeller which means that theoretically the pump can lift 33.9 feet of water. It is more practical to assume that the pump can only lift about 25 feet because the impellers can not create a perfect vacuum of 0 psia.

I repeat again, NPSH is always positive, hence the name Net Positive Suction Head.

Ignore the "Search" box on that page. Just read and and understand the page itself.

Hello mbeychok - I think I understand NPSH pretty well.

When you say NPSH is always +ve you don't distinguish between NPSHav (provided by the setup) and NPSHr (required by the pump). I agree NPSHr is always +ve.

I also agree with your practical figure of about 25' lift - this is 33.9' from the atmosphere minus typical pump NPSHr.

If you have a pump 40' above water surface in an tank, what do you think the NPSHav is? It's no good saying it wouldn't pump so it's irrelevant, we only know it won't work because NPSHav < NPSHr.

Cheers....Codey

Codey:

Thanks for your last reply. I think that now we are both agreed and on the same page.

Regards, Milt Beychok

OK Milt, thanks and regards to you too

Codey

Short answer is no, on usual definition of NPSHR, which is an absolute measurement. It would mean the pump working with a lift greater than height provided by the pressure on the suction tank, ~ 10m if atmospheric, as I'm sure you know.

Having said that, in principle it's possible to have a 4m lift, but it looks doubtful in this case.

First, don't forget fluid SG. Sodium hypochlorite with 15% av. Cl₂ has SG 1.28.

Not clear whether your -4m means there's a 4m static lift, or whether pipe losses are included. But assuming it's static lift (worst case) I make NPSHav ~ 0.87m fluid (absolute). I don't think you'll get a centrifugal pump to do that.

Velocity comes to 3.54 m/s, which is pretty high for pump suction in a small pipe, even without NPSH issues.

Not sure about air-operated diaphragm pump either (with existing pipe). Even if it's double-acting peak velocity is higher still.

Looks as if system needs uprating. If you do, why not install the pump lower, with flooded suction?

I am a pump salesman in Detroit, MI. You need to provide a bit more information and a sketch, if possible. There are pumps called "self Priming" that should do the trick. email me at askjoet@yahoo.com directly

You might want to take a look into these two links:

http://www.eng-tips.com/viewthread.cfm?qid=59805

http://www.mcnallyinstitute.com/11-html/11-12.html

regards

If the sodium hypochlorite is a dosing chemical, as may be used perhaps in water chlorination for bacterial protection, then rather than a centrifugal pump it might be better to use a peristaltic pump instead. Peristaltic pumps lend themselves far more readily to flow adjustment by changing their speed, as the relationship between speed and flow is fairly linear, and they will operate quite happily on low NPSH so long as the correct tube material and thickness is specified. Try contacting a few manufacturers with details of this application and see what they say?

Alternatively, for sodium hypochlorite there are package dosing sets available off-the-shelf from a number of manufacturers that might save the bother of selecting inappropriate equipment for this application?