NPSH

[JSPCGY]

Dear All,
We are installing a vertical turbine fire pump as NFPA 20 and we are providing the required submergence as required by NFPA 20.

We know that NPSH is an important consideration in pump selection.

I did not provide vendor NPSHa and neither I asked vendor about NPSHr.

I was told that NPSH is not applicable to fire pumps as long as we provide the required submergence.

Do I need to specify the pump NPSHa and compare it with NPSHr from Vendor? Did I make a mistake by following this ?

Please provide your comments.
Please advise where does NFPA 20 talk about NPSH for vertical turbine pumps?

Thanks

 

[BigInch]

I think you're OK. For a vertical turbine pump, NPSHA is usually the same thing as submergence depth. NPSHA for vertical turbines is defined as the equivalent height of the free surface of the fluid as it enters the pump intake. For vertical turbine pumps, you should also make sure that the minimum depth (at lowest fluid level in the sump) to prevent vortexing is also provided, as you should also for providing the minimum distance from the intake to the bottom of the sump and clear distances to the sides of the sump, and between pumps. All may have an effect on the net NSPHA available at the intake depth.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[rmw]

I second everything BigInch has said. If your pump vendor hasn't furnished their recommended sump dimensions, then by all means obtain this information from them and follow it. I have seen cases as has BigInch where adherence to minimum submergence was negated by poor sump design. The approaching velocity of the fluid as it enters the pump is critical in a vertical pump installation.

rmw

 

[Artisi]

I would suggest for a vertical turbine pump in an open sump the NPSHa is atmospheric pressure on the sump water surface + submergence minus any entry losses.

Again, sump design and correct installation is paramount for trouble free operation.

 

[BigInch]

Much better.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[JJPellin]

I will make a dissenting comment. I have never seen a problem with NPSHa in an fire water pump. They tend to be designed very conservatively, for obvious reasons. But we have a few big vertical turbine pumps in cooling tower service where the minimum submergence is not adequate to provide the NPSH margin we require. In other words, it is possible to satisfy minimum submergence but not satisfy NPSHr. The pumps I am referring to cavitate constantly and there is nothing we can do to stop it since deepening the sump is not an option. Ask the pump vendor for the NPSHr and take a few minutes to do the calculation. It is overwhelming likely that you won't have a problem. As already noted, the sump design is more likely to be an issue.

Johnny Pellin

 

[stanier]

You should be more concerned with vortexing and submersive swirls. Check out ANSI/HI 9.8 or US Army Corp technical papers on the subject.