NPSH safety margin criteria

[Yobbo]

L.S.,

I looked in several pump books for a good safety margin for the NPSHA compared to the NPSHR-3%. Different sources tell me to keep a fixed safety margin of 0.5 meter, others of 1 to 1.5 meter. I heard about criteria from the Hydraulic Institute, where even a factor is being applied to the NPSHR to calculate the NPSHA, but I haven't got them at my disposal. So far I applied 1 meter, but as I am confronted with a case, where I think I should have more reliable ground I am looking for a sound set of rules concerning this issue.

Could anyone of you shed a light on this subject or refer me to the right literature?

Karel Postulart, The Netherlands
Nuon Power Generation

 

[Artisi]

Sorry no rules, you have to make your own decisions based on the likelyhood of the pump operating into the safety margin and its ramifications on pump output, the pump and it's tolerance to operating with NPSHa being less than NPSHr and damage etc likely to take place etc etc. Depending on pump style, size, materials of manufacture etc margins could be from 1M to 5M or more.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[JJPellin]

The best example of a solid reference for the value of a larger margin is in "Pump User's Handbook" by Bloch and Budris. They propose NPSH margin ratios much larger than other systems would suggest. In an oil refinery, I find that these large margins are often impossible.

In our plant, we require a fixed margin of 3 feet for hydrocarbon and 5 feet for water. But, we also require that the NPSHa is calculated using a conservative method. The level is defined at the bottom nozzle or bottom tangent (depending on the vessel arrangement) of the suction vessel. We have had very good results with our method.


Johnny Pellin

 

[rmw]

If faced with the issue, it would depend where on the NPSHr curve I was operating. If operating far out on a pump curve where the NPSHr curves typically begin to turn up, I would use a healthier margin. If operating on some portion of the pump curve where the NPSHr curve was relatively flat, I'd feel comfortable with less margin - but only if I didn't think the system had the capability to run the pump out on the curve where NPSHr would get you in trouble.

Also, if you have a situation where you can test your pump to verify the NPSHr curves, then once verified, you can margin accordingly. I've done that in manufacturing situations where we had to use the same pump(s) repeatedly. We tested and knew whose NPSHr curves were accurate and whose were not so good.

rmw

 

[ione]

Interesting (at least to me) stuff in the attached paper. As you can see the Hydraulic Institute criterion, which relies on 3% drop, and which has become a standard in the industry, appears to lead to misleading results in some applications. As stated above by others, there are not magic numbers, which can satisfy all applications.

 

[BigInch]

I'd make a safety margin based on how far away you are with the actual product and actual temperure from cold water, keeping in mind that sonic energy related to bubble formation and colapse has a lot to do with cavitation. The highest margins seem to correspond to less dense, relatively low vapor pressure products, such as hydrocarbons. A percentage of NPSHR as a safety margin seems to make more sense than a fixed number. Some references state cavitation can actually begin at as much as 25% higher than actual NPSHR.

 

[stanier]

http://www.pump-zone.com/pumps/cent...fugal-pump-can-you-provide-too-much-npsh.html

http://books.google.com.au/books?id...ed=0CDAQ6AEwAw#v=onepage&q=npsh grist&f=false

 

[QualityTime]

The previous posts have given you some good information and guidelines. If you are pumping water and you really want to be sure extra safe there is no damage to your impeller consider making the impeller out of stainless steel

 

[DerPumpeMeister]

It is best to speak with the manufacturer of the pump you are using. They should have a recommended NPSHa/NPSHr margin. There is no criteria that works for all pumps under all conditions. In fact, I regularly design pumps that do not have a required margin between NPSHa and NPSHr; they can and do operate safely and reliably under the NPSHr-3% curve. Keep in mind that NPSHr, incipient cavitaion and minimum starting liquid levels are not the same and the pump manufacturer should have values for all three.

http://www.EngineeringGuideBook.com

 

[Yobbo]

Gentlemen,

Thank you for all of your reactions. What I will do is try to get a hold of Grit's book about cavitation. Alas there are no general rules that can be applied. It does surprise me though that so many different books present different rules. I think the best when opting for a certain ( type of ) pump is to let the manufacturer, who designed it, give instructions how the high the NPSHA should be compared to the NPSHR over the whole region of operation.

A last additional question. What inaccuracy can be expected while calculating the NPSHA? I heard that an inaccuracy of about 20% is not exaggerated.

With best regards,

Karel

Karel Postulart, The Netherlands
Nuon Power Generation

 

[BigInch]

It should be relatively easy to get within 10%, if you make the effort.

 

[ione]

The answer is it depends.

1. The vapour pressure which enters the NPSHa is strictly related to the fluid temperature and so the closer to reality is the temperature used in the design phase the more accurate the value of NPSHa computed.
2. The minor losses due to fittings in the suction line can be calculated with different methods (Crane, 2-K method, 3-K method) and this will also effect the value of NPSHa computed.
3. Head loss due to friction are strictly related to the pipe’s roughness, and the correspondence between roughness value assumed and real roughness will affect the value of NPSHa computed accordingly.

I consider the accuracy applied when calculating NPSH available reflects to the safety margin required.

 

[BigInch]

That's ione good way to look at it. I like it. You'd certainly better try for better than the NPSHa safety margin you're allowing.

 

[zumaquero]

My first comment here.

I just was trying to calculate NPSHa for a diesel pump and had some problems to select the correct value of the vapor pressure. Is ther any data I can use always for ambient temperature? I didn't find a standard but only some "no believable" data. Most of them said 0,3 kPa, is that correct?

Anyway, I saw in most of the booked I read, a security margin of 0.5 m, but normally they talked about water, not hydrocarbons.

Thank you

 

[ione]

zumaquero,

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