Sizing a pump for parallel operation

[wdwrkrME]

After determining an operating point of 6,000 gpm and 100 ft of head for a pumping system with 3 identical pumps running in parallel, what is the best way to start sizing a pump? I was told that I can start by dividing the total flow by 3 and add 25%. This will be the flow that should intersect the system curve (a moderately rising system curve. From this, I can go and select the pump. Is this common, or is there a better way?

 

[Artisi]

Select 3 pumps that can achieve 2000gpm at 100ft total head operating at BEP with adequate NPSHa/r margin.

That is of course you are sure that the total head calculation has included all losses on both the inlet and discharge side of the pump.

Actually the best method is to superimpose the system curve/s over the pump curve especially if there is the possibility of a varying total head due to any variance in inlet conditions. For example, if the inlet condition is variable because of any change in the static head, either postive or negative.

 

[BigInch]

DO NOT ADD 25% to the flow, unless you have a possible error in the system flowrate of +25%. Each of your 3 pumps should be selected with a BEP of 1/3 the system flowrate to have any chance of ever operating at BEP conditions.

What you're really talking about is how well the system design flowrate specification is known today. How much error is in your specified system flowrate right now? Do you really have 25% error? If you have that much error in the possible flowrate, you need to re-evaluate that immediately and probably select a larger pipe diameter too. If you are designing now for a future flowrate 25% higher than the present flowrate, it may be better to design now for 100% of the today's flowrate and make provisions for changing the pumps later, when you actually need to pump that extra 25% higher flow. 5 or 10 years operating at the correct flowrate (at BEP) now will usually pay for respecifying pumps later on, based on the energy savings alone. With a possible error of 25%, an optimized design for pump flow, head, power and pipe diameter is not possible. IMO, the system has not yet been designed and optimized properly.

Making provisions for errors in the required pump discharge head usually makes more sense, because there is more potential for errors in the hydraulics of the system due to head losses from fittings, pipe configuration, estimated lengths of pipe, errors in elevations, etc., than there is from possible errors in flowrate. It is easier to trim or replace a pump's impeller for head adjustments than it is to change out the pump's size for flow busts. Using that method, you could size everything properly now to correct head and flow at BEP. Just be sure you can fit an impeller in the pump that will give you 10% more head, if you need it because of screwed up hydraulic calcs.

BigInch's Laws for Pump System Design:

1.) Know your flowrate and head requirements and design for BEP.

2.) Optimize your pipe diameter to give lowest power cost.

3.) Its always easier to get 10% more system flow with 10% more pump head, then it is to get 10% more pump head with 10% more system flow.



**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

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

 

[ccfowler]

Why are you using three pumps in parallel? Is the system flow and pressure always constant, or are widely varying flow rates expected?

Usually, when a system has 3 pumps, they are three "50%" pumps with the intent of improving reliability by having two running with the third as a ready stand-by.

If the system operates over a wide range of flow rates, then the pump selection cannot be based on a single operating point. Instead, the pumps and controls must be capable of safe, reliable operation under all reasonably expected conditions including start-up and shut-down.

 

[GarySandStorm]

To answer the question, yes you do divide the total flow by # pump sets you wish to have running in parallel at once. However 25% seems a bit excessive, but if it is a command from higher ups who are we to say nay .. ~10% Like BigInch said should be good enough.

and a small useless note: when i do selections I also take into consideration motor v. pump speed, to adjust the pump curve to match the motor design speed using affinity laws.

eg, pump curve is rated 1450rpm, motor is 1430 rpm, then draw curve from 1450 to 1430. then use that to plot duty. maybe i am just too cautious but who's 306SS nuts are on the line lol

Hope this helps (^^,)
Good Luck

1: I still can't find a hardware store that sells left hand screwdrivers? >_> <_< >_>
2: Yo new boy, we going to need to lift this now, so go across the road to Hire it and ask for 2 Sky Hooks.

 

[dabluffrat]

Go onto any pump mfg website and use their selection tool.

Don't add 25% to each pump as previously mentioned, margin should be calculated into your 6,000 @ 100 ft gpm calc.

consider 2 pumps in parallel, and one stand by, as previously mentioned. operations will thank you later.

Pump curves considered should be continuously rising to shut off, not flat.

Pump selection should be at, or slightly to the left of, BEP flow.

Ensure adequate NPSHa vs NPSHr as mentioned. 3 ft is a good margin.

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