NPSH Margin in Vertical Turbine Pumps

[dhlinva]

I'm doing a design review for a seawater system using vertical turbine pumps in located in sumps within tanks. The system is to be designed to pump the capacity of the tanks in 9 hours. To do that requires 5000 gpm to be pumped. The BEP of the selected pumps is 3800 to 4000 gpm, but the builder determined early on (before I became involved) that it was possible to operate these pumps at the end point of the curve until we identified possible NPSH constraints as the tank levels decreased. Their solution was to install VFD's, but slowing down the pumps blew the time constraints. The builder then proposed increasing the maximum speed of the pumps 200 rpm to gain a minor amount of additional capacity on the high end.

In order to run the pumps at maximum capacity (to meet the 9 hour constraint) the NPSH margins being used in their simulation are almost non-existent - in the neighborhood of 1 to 3 %. ANSI/HI 9.6.1-1998 states that "NPSH margins are not normally a consideration for most standard vertical turbine pumps, since they generally have a Low Suction Energy and cavitation noise is not an issue." I calculate a suction specific speed of 12,800, but to tell the truth, based on what I've read, I'm not clear on whether this makes it high energy or low.

I would prefer a 5000 gpm pump operating at BEP. Then we wouldn't be looking at NPSH. The situation is complicated by fact that the pumps have been purchased and delivered. I would like an NPSH margin ratio of 1.2 to 1.3 and minimum of 5' (based on Hydraulic Institute recommendations), but I will admit that this amounts to a factor of safety because I'm not sure of what I'm dealing with when operating so far outside "normal" parameters. I'm not sure sure how to intelligently determine what NPSH margin is required without expensive, detailed analyses that might be required only due to a poor selection of pumps. I can't seem to get the attention of the builder with my technical concerns.

My concern is obviously for the longevity of the pumps. I'm not sure what indications of cavitation will be available to the operators with these pumps when located 55' down in a tank (other than loss of discharge pressure). Unless something similar to Goulds PumpSmart® in installed, I think the pumps will be routinely operated in cavitation.

There may be many things I've gotten wrong here. NPSH is not something I generally need to get involved with and have little experience with the finer details. I'd appreciate any input you guys might be able to provide. Thank you.

 

[dhlinva]

Wishful thinking or looking for someone to say "yes you have enough NPSH margin"? I'm not sure I get that one.

I know they aren't calculating for enough margin, but how can you say they don't have enough NPSH margin if you can't say what that margin should be? Okay, how do I say that? That's why I'm here. That's the only reason I'm here asking questions. I'm looking for something to hang my hat on and I see the NPSH argument as the strongest at this point. I can say I'm concerned about the margin, I can say I think with the unknowns involved that there should be more margin, I can say that ANSI/HI 9.6.1 recommends this or that margin, but if I can't say what the margin should be be with some authority, it weakens the argument immeasurably. This argument would have been easier to win if there weren't expensive, already purchased 55' long pumps involved. The train left the station a long time ago.

I'm trying to get the builder to face what I consider to be the facts. Until they agree with me that those are the facts, we don't move forward. They seem to be convinced in the face of everything that these will operate acceptably. I don't understand it, but further agreement without technical justification won't help persuade them.

I've goten some good tips though and I appreciate all the input.

 

[Artisi]

NPSH margins are an unknown quantity and are usually set based on experience or very rigid testing in ideal conditions, you can only be guided by experience or margins set out in publications such as Hydraulics Institute etc.

Besides NPSHa/r margins there are other factors to be considered, you also need to consider the operation so far out on the curve where pre-rotation, internal recirc etc are likely to be encountered, deflection of impeller and shaft and insufficient submergence likely to entrain air due to vortexing of the incoming flow plus the very poor hydraulic efficiency which equates to $$$ and can cost a fortune over the life of the pumps.

I realise you are in a bind - caught between supplier and owner - but you can only present what you have along with your recommendations, it's the owners final choice which way to go on this.

 

[BigInch]

As artisi says, you should be more concerned about the money spend running out on the curve over the life of the pump rather than some nebulous margin to allow you to do it. The whole point with pump selection as you know, is to select pumps with BEPs at your intended operation point. If they arn't selected properly, and you don't intend to select a new one properly, running within some margin still isn't going to make it right. If you're going to use it, use it knowing that its a wrong pump and stop looking for excuses to try to make it right. Also as artisi says, margins are based on experience with a particular installation with a particular product at a particular temperature and possibly at a particular flowrate and what works for one will be 20% too short on another.

I regress in stating my initial comment. Either change the pump, or change the operation spec to allow a longer pump out time and you might as well slow your flowrate down to the pump's BEP while you're at it. Those are the only ways to make it right. For some reason it appears that you just can't make the decision, so pass it along to someone who has the authority or economic interest to do so.

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"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein

 

[Artisi]

BigInch - well put, and as we don't have the full installation detail including sump design, placement of pumps in the sumps etc etc. we can only assume based on the info' supplied that the pumps are the wrong selection.

 

[BigInch]

I rest my case.

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"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein

 

[dickon17]

If you have a VFD, why not operate the pumps at a higher speed when the tanks are full and then slow down to meet reducing NPSH availability as the tanks empty?

 

[nationalpump]

As was stated by Artisi and BigInch these pumps are not the correct selection for the application regardless if they meet the NPSHr. Pumps are designed to run at BEP...not 140% BEP...where problems can be expected. So if you speed the pumps up you will still be damaging the pumps and reducing the time until failure.

 

[BigInch]

He's already running out the curve, why more speed?

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"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein

 

[dhlinva]

I think I was, but perhaps I wasn't clear enough in the beginning. The builder can only meet the time constraints with a 1% NPSH margin. Any increase in margin puts them outside the time they need to meet. I was interested in determining what the margin should be - only to put the pumps out of contention with as little argument from the builder as possible.

I never thought these were the correct pumps and have told the ship owner - the one who's paying for the ship. That these pumps have not been properly selected according to standard practice was and is obvious. The mfg. recommended the pump for operation at the endpoint. I thought I might be able to find something here to back-up what I'd already found. I think I'm just getting beat-up now & am no longer part of the conversation. There's no new information to be had here.

Thanks for the input.

 

[hbw]

Tell the builder and the ship owner that you should not operate the pumps off BEP. BEP is the rate that the impeller vane angles are designed for. You should either replace these pumps for 5000 GPM or lower the pumping rate to the BEP of these pumps ~4000 GPM.

One percent margin is acceptable unless the bid specifications say otherwise.

 

[BigInch]

Nobody's trying to beat you up. If you advised the owner that its the wrong pumps already... IN WRITING... and your company isn't responsible for performance, let go of it. You did your job. You can take a horse to water, but you can't make him drink, right?

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"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein

 

[Artisi]

1% margin on NPSH - that's reading from the published curve which you cannot take as gospel, if you have a test curve for NPSHr at the flow /head you want to pump at then maybe and it's only a maybe, it might be possible to run with a 1% margin. Following on from this, it is possible that it might be acceptable to run the pumps into marginal cavitation for short periods of time without too suffering too much damage but that's a decision for the owner and not a recommendation you should make. However, running so far out on the curve could introduce other problems which exceed the cavitation problems you are likely to experience as pointed out in a previous post.