Preparing System Curves 5

[PBW2]

We are having a discussion in our office on the development of pump system curves; in particular for lift stations with triplex pumps. In developing the system curve for a triplex setup, with two pumps running there seems to be a couple of schools of thought:
1. Ignore the head through the individual discharge pipe and determine head for the common header and forcemain based on total (2 pump) flow.
2. Determine head for discharge for (1) pump based on pumps flow and add head for additional flow (2nd pump) through the common header and forcemain. The head for the individual pump discharge remains constant above the pumps capacity.

Also the question on how to handle additional flows from separate pump stations on a common forcemain has come up. The consensus is to add the flow as additional head to the system curve. However, what if there are (5) pump stations? Is it realistic to determine the system curve based on all (5) running at capacity? Anyone have any code references.

Thanks in advance for comments/suggestions/discussion.

 

[semo]

Not getting into the latter discussion in this thread, Rcooper was close in the method; but, as mentioned earlier the system and pump curves were confused and probably interchanged.

The system curve is the system curve and based solely on the piping/valves/fittings and flow.

The pump curves are based on the individual pumps; but, combinations can be obtained by adding the pump curves (even if they are different pumps). These curves are based on the pump output without reference for the piping. You need the system curve to find the duty point.

For parallel pumps, the flows can be added for each head value and a new pump curve produced for the combined pumps. If they are the same pump you can double the head values on the individual pump curve (which is where Rcooper was coming from).

For series pumps, you double the head value for each flow and a new curve can be plotted.

From this new curve, you can find the duty point by plotting the system curve over the pump curve. Typically the Hazen-Williams equation is used; but, the methods outlined above work fine for calculating the system curve.

The duty point won't be the same as the combined duty points for the individual pump curves. It will be less.

 

[PBW2]

Thanks for the discussion and all of the great points! What prompted the original post is the following situation:

(3) submersible pumps same size in parallel in a wetwell
Individual discharges (6") pumping to a common header then into the forcemain (12").
For the present one pump will pump 1000 gpm.
For future flow dual pumps will pump 1800 gpm.

In developing the system curve at buildout (future) there were three thoughts

1. Determine the head on the individual discharge pipe based on 0-1000 gpm flow (one pump capacity) and add the head through the common header and forcemain based on the future flow 1000-1800 gpm (dual pump).
2. Determine the system curve through the discharge pipe/header/forcemain from 0-2400 gpm. Wouldn't this give a false "head" for flows in the discharge pipe above the pump capacity?
3. Ignore the discharge pipe and determine the system curve based on flows through the forcemain.

Opinions?

 

[BigInch]

Don't ignore any pipe, fitting or valve with significant head loss and determine the system curve for each flowrate of interest now and in the future.

Why not download EPAnet and play with that for awhile and it should become apparent which losses you can ignore and which ones you can't.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

Still using Hazen Williams???

http://virtualpipeline.spaces.msn.com