NPSH - Fire pumps?

[Bagge]

Hi,

I am sprinkler designer from Europe. I am writing to you because I hope maybe you can give me some feedback on the following topic.

First of all - we use the European Insurance Rules "CEA 4001". In these codes the following reads:

"The NPSH required by the pump shall not exceed 5m at flows up to that defined in table 17". (Qmax).

And furthermore elsewhere in the rules, the following also reads:
"The suction piping, including all valves and fittings, shall be designed in such a way as to ensure
that the available NPSH at the pump inlet exceeds the required NPSH by at least 1 m with the
maximum demand flow and maximum water temperature"


Now - I can not understand the reason for this requirement. As long as you have an NPSH-available larger than the NPSH required by the pumpe plus a safety value by "+ 1", it should be okay - or?

Do you have a similar requirement in the NFPA?

 

[cdafd]

http://en.wikipedia.org/wiki/NPSH

NPSH is an acronym for Net Positive Suction Head

Fluid can be pushed very hard to get it down a pipe. The only limit is the ability of the pipe to handle the pressure. However, a liquid cannot be pulled very hard because bubbles are created as the liquid evaporates to a gas. The lower the pressure created, the bigger the bubble, so no more liquid will flow into the pump. Rather than thinking in terms of the pump's ability to pull the fluid, the flow is limited by the ability of gravity and air pressure to push the fluid into the pump. The atmosphere pushes down on the fluid, plus, if the pump is below the tank, gravity helps also. Until the fluid gets to the pump, these are the only two forces providing the push. Friction losses and vapour pressure must also be considered. Friction losses limit the ability of gravity and air pressure to push the water towards the pump at high speed. Vapor pressure refers to the point at which bubbles form in the liquid. NPSH is a measure of how much spare push you have before the bubbles form.

http://www.pumpschool.com/applications/NPSH.pdf

 

[cdafd]

http://www.cea.eu/uploads/DocumentsLibrary/documents/1237304692_cea4001s.pdf

 

[Bagge]

Thank you very much for your answer.

But I know all that - and that leaves the same question:

Why is it relevant to set up a maximum for the required NPHA by the pump (16,7 ft), as long as you make sure, that the available NPSH are bigger than the requried NFPA. In this case the required NPSH could be 23 ft, as long as the available NPSH are bigger.
In other words - is it relevant to talk about a to high NPSH in the pump? Are there any other reason to keep the NPSH required by the pump low - other than the above mentioned?

 

[Bagge]

I'm sorry - "NPHA" should of course be "NPSH" in the above.

 

[cdafd]

sorry did not get a chance to look at the book yet, just wanted to post a little background

 

[IFRs]

Often the pump is being fed by a water main that is used for other purposes. The owner of the feed line wants to have some pressure left after your fire pump kicks in so they are limiting you to 5M of head. That way they can still fight another fire or service other customers. If they did not have this restriction, the other guy's fire could take all the water pressure and your fire would just burn.

 

[Cidona]

IFRs: I don't know (posting here in hope of learning) if your theory would be appilicable to Bagge's situation as it wouldn't seem restricting the NPSH is providing a safety factor. If you tank was only 16m in height and your friction loss etc at Qmax was 15m and you have 1m NHSP, that you'd be complying with the standard even though in that situation your safety is only 1m. However what Bagge seems to be asking (please correct me if I'm wrong)is if has a system with a NPSH of 16m, what's the problem with that if he had a tank that was 100m (just to deal with the extreme to simplfy theory)? He would have more safety factor in situatio b than a but would not be ok if the 5m restriction is intending to restrict the calc in this regards.
Is it posible that the 5m restriction is mentioned in the 'Characteristics of the Pump' type chapter rather than the calc section , where it is restricting the type of pumps that can be installed rather than a restriction on the calc? i.e. That they are trying to prevent installation of a pump that requires 5m NPSH.
From "The Fire Pump Handbook", by Milosh T. Puchovsky and Kenneth Isman, 'There are two types of NPSH that need to be concidered: the NPSH required and the NPSH supplied. The NPSH required by the pump is determined by the pump manufacturer and is a function of both the speed and the capacity of the pump. Curves indicating the NPSH versus flow can be obtained from the manufacturer. The NPSH supplied is the pressure head at the pump inlet that causes water to flow through the pump and into the eye of the impeller. The NPSH supplied is a function of the water supply.
For any pump installation, the NPSH must be at least equal to th NPSH required for the operating conditions specified. If the NPSH supplied is less than the NPSH requried, some bubbles can collapse within the pump. This phenomenon is referred to as cavitation.'

My guess would be the section saying 'The NPSH required by the pump shall not exceed 5m at flows up to that defined in table 17".' is discussing the required NPSH by the pump.. While the "The suction piping, including all valves and fittings, shall be designed in such a way as to ensurethat the available NPSH at the pump inlet exceeds the required NPSH by at least 1 m with themaximum demand flow and maximum water temperature" section is discussing the supplied NPSH.

 

[Cidona]

Oops, that line towards the end was meant to include the word 'supplied' to read 'For any pump installation, the NPSH supplied must be....

 

[Bagge]

@Cidona: Thank you very much for your good answer (!)

You are absolutely right about the 5m restriction - that it refers to the pump manufacturer rather than the calc. section. But why does the pump manufacturer have to keep the value down at 5m?
My problem is that I have a pump manufacturer who can offer me a cheep pump - but unfortunately the NPSH are 6m (maybe the reason for the lower price). I have a NPSH-supplied value at 8.5m - there are not any risks for cavitation.
So that is the reason why the restriction for 5m is irritating for me, and the reason I ask, because I can't see the reason for it.

 

[Cidona]

I don't know why the restricion is there. I would think some of the Fire Pump manufactuerers would be able to give some insight on that as they woould have respresention on the committees when formulating these requirments.
The NPSH(R) on any of the pump curves I have here seem to at worst be at 15' (4.572)at 150% the rated flow of the pump, which with the thickness of the lines on the graph, may be the 5m. But not in excess. That may well just happen to be to the ones I've done, and not nesscessarily a requirement. While the requirement doesn't seem to be mentinoed in NFPA, NFPA does require that the Fire Pumps be listed. It may be part of the listing criteria of the Fire Pump. Again, the pump manufacturers might have some insight. Or maybe somewhere can tell you if they have used pumps with NPSH(R) in excess of 5m (16'5").
Would seem unlikely you'll be able to use the cheaper pump since it is certainly a requirement of the standard you are designing to. Do you know if it is a 'Listed Fire' pump? You've got me curious now to know if such an animal exists.

 

[Cidona]

By mistake had left a line out of the paragraph in the quote above from the Fire Pump Handbook. it should have read 'For any pump installation, the NPSH supplied must be at least equal to the NPSH required for the operating conditions specified. If the NPSH supplied is less than the NPSH required, some of the incoming water will vaporize and form bubbles. these bubbles can collapse within the pump. This phenomenon is referred to as cavitation.'