Static Suction Lift vs NPSHr vs NPSHa

[VAMechE]

I would like some clarification/advise/sanity check on a project I am reviewing at work. I was handed a project (already in construction) for a municipal water pumping system for one of our hospitals. I will describe the system first (starting at intake end) then ask my question.

The intake in the hudson river is a screened inlet (with about 1 ft of loss at the system design point of 150gpm) attached to long radius elbow and then ~1800' of 8" DR-11 HDPE pipe to the pump station on the banks of the river.

Mean low tide is about 15.5' below the centerline of the pump.

Where the line enters the pump house there is 2 8" x 4" Wye's (one feeds each redundant pump). a 4" gate valve, a 4" basket strainer, another 4" gate valve, then a 4"x3" concentric reducer on the inlet of the pumps.

The pumps are Goulds 3796 3x3-13 MTi's with a cut down impeller of 12.875". The pumps are in parallel and one will be in standby while the other is active. Design flow rate is 150gpm at a TDH of 140' The pump spec sheet lists and NPSHa of 10ft, a NPSHr or 4ft (at 150gpm, but the curve does not go below 150gpm). I found that on the goulds website they listed "static suction lift" at 20ft.

I calculated somewhere between 3' and 4' of friction loss's for the intake. Add the 15.5' for vertical lift and you end up with about 19' required on the intake side. When I first saw the design I was worried about the pumps being able to pull a prime. I am still a little worried about that.

So my questions is what do I design to? NPSHa (33-19=14 which is greater than 10') looks OK. NSPHr (14 >> 4), or static suction lift? I am guessing that static suction lift has nothing to do with the friction loss's but I would like a clarification on that!

Any help/advise is greatly appreciated!

 

[TenPenny]

Static suction lift has nothing to do with friction, that's where the 'static' comes in. Friction becomes part of the npsh calc, but not the static suction lift calc. You should also check the priming time - there is a calculation sheet available for that pump, if the priming time is too long, the liquid will tend to heat up, possibly too much.

You will also have to consider the lowest river level that you expect to have to prime the pump at - this might be the mean low tide level, or maybe not, depending on your system.

If you have any doubts about the pump operating properly, have your Goulds guy run it by the guys up in Seneca Falls - they're pretty careful on 3796s, and can give you all the pointers you need to make sure it'll work, or what you might need to do to your piping. They're not interested in a quick sale but a bad application.

 

[VAMechE]

TenPenny,

Thanks for the reply! I spoke with the Emerick Associates in Cohoes and they gave me some more info on the pumps as well. The info they gave me provided a priming time curve that I didn't have previously, I'm a little concerned about that as well, since there could possibly be about 75' of 8" pipe that could be empty. The info on the priming has a note that it is for a length of 3" equal to static suction lift (which is only 20ft). The difference in volumes is pretty large between 3" pipe and 8" pipe. A quick calc shows 8" Dr-11 at about 7.8 gal/ft and 3" at about 1.4 gal/ft. Not only is the distance over 3X longer than in the chart, but it is 5X more volume per foot! Would this mean if it shows 20 seconds for priming a 3" pipe 20' long that it would take 5 minutes (300 seconds)?

 

[BigInch]

More or less. In any case, definitely that's too long to be running while waiting for prime. How can you have 75' of empty pipe? You can't be filling 75' of pipe while "priming".

Can you put a small submersible booster out there somewhere to fill that before starting the mains, then shut it down when you have flow established?



**********************
"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/

 

[TenPenny]

If you're going to have 75 ft of 8" pipe that's empty, you're looking at a pretty big volume of air to get rid of...remember that during this priming cycle, the only liquid the pump will see is what's in the casing...

Ask them for a copy of page 725.4A200 page 4 of 4 (or whatever the current issue is), which gives the priming time calculations for other pipe sizes...

How often do you expect to need this pump to prime?

 

[VAMechE]

I was just trying to wrap my head around the concepts of static suction lift and NPSH but I can't reconcile why you would only be able to pull a suction lift of 20' when the NPSHr is 4 ft????? I would think that if the NPSH was 4' you could pull a prime up to 29' (33'-4'). Does the NPSH only apply to water once it's flowing? I.E. instead of pulling water up hill you would be able to run the pump from a closed vessel that was only 4' above vacuum?

 

[VAMechE]

TenPenny,

I don't think that the pipe will be empty very often, but I am trying to figure out all my worst case scenarios. Realistically it will be empty upon installation and and if for some reason during maintenance it is necessary to open the pipe up upstream of the first gate valve (or maybe if someone forgets to close that valve when cleaning one or both of the strainer baskets).

75' of pipe is a guess from looking at the elevations. It might actually only be 30', but I will have to verify the actual path of the directional drilling.

 

[BigInch]

A typical practical limit to a suction lift is around 23-25 feet.

Yes, rest assured that somebody will forget to close that valve.

NPSHR applies at all times.
Prime Time for pumps,

http://www.deanpump.com/chemb1.htm

**********************
"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/

 

[Artisi]

One option you have is to install a NRV near to the pipe inlet or a foot valve in the strainer - this will ensure the pump is always primed following a shut down.