Preparing System Curves
Preparing System Curves
(OP)
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.
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.





RE: Preparing System Curves
What you're asking about, "head through the individual discharge pipe". What does that mean?
And the part about "add head for additional flow (2nd pump) through the common header and forcemain". No idea of what you're trying to suggest.
If all pump stations can operate together, it is realistic to design the system for that condition. If all pump stations won't operate all together now, in five years, THEY WILL.
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RE: Preparing System Curves
You then plot the pump curves on the system head curve for each pump and find the point(s) of intersection. These are the operating points for each of the pumps running individually.
When more than one pump is running ( in parallel) you must add the pump curves together for those pumps which are on. Two identical pumps running in parallel will roughly double the flow, though not quite as you'll see when you do this.
good luck
RE: Preparing System Curves
The intersection heads are identical to RWF7437, but you are looking at the influence of the system on an individual pump. The advantage of this method is you can see how the efficiency, NPSHr, flow rate compared to BEP flow rate and power all compare for all pumping conditions. To identify the total flow rate with three pumps running you would find the intersect of the system curve/3 and the pump curve, and multiply this by 3.
Another advantage of this method is you can compare the performance of various different pumps easily on a single graph.
RE: Preparing System Curves
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RE: Preparing System Curves
RE: Preparing System Curves
RE: Preparing System Curves
That was said earlier:
"Two identical pumps running in parallel will roughly double the flow, though not quite as you'll see when you do this."
RE: Preparing System Curves
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RE: Preparing System Curves
Calculating individual flowrates and pressure drops is a bit difficult and you can construct a system curve without doing this by using Parabola Method. However, you should be cautious that parabola method assumes a constant friction factor for all flowrates (or Reynolds numbers). I gave a detailed procedure in one of the past threads in piping forum. This procedure deals with what cwg mentioned in his post.
The starting point of your system curve depends upon the static component of the pressure drop and this is your pressure drop corresponding to zero flow (in other terms, this head is necessary minimum value for all flowrates)
Once you draw the system curve with the maximum possible flowrate, draw the compound pump curves on it (keeping head constant and adding up flowrates, for similar pumps) and the intersection gives you the operating point for any setup. The operating flowrate, i.e the intersection of pump curve and system curve, divided by the number of working pumps gives you flowrate through each pump.
RE: Preparing System Curves
RE: Preparing System Curves
Not to detract from your parabolic curve approximations, which I have used from time to time, but you yourself do recoginze some limitations in its application.
I suggest that exact system curves are relatively easily put together (with a spreadsheet), especially if using the very accurate and computationally efficient and easily implemented Churchill pipe friction equation. Using Churchill, it is quite easy to construct system curves, even for a pipeline system flowing multiple products.
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RE: Preparing System Curves
No doubt that you are correct. However, I am more vulnerable to pure mathematical figures than to scattered graphs
RE: Preparing System Curves
h=Aq^2 + Bq + C
where A is the turbulent headloss component, B is the lamina component and C is the static head
RE: Preparing System Curves
Its not too confusing and, since you're a nice guy (in fact we're both nice guys), I'll share my system curve spreadsheet with you all. This one is using an old method I had implemented of a 1-cell contained 3x iteration of Colebrook-White friction factor equation. Use the rapidshare link below to download it if you like.
http://
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RE: Preparing System Curves
That is very good. I never tried that out of sheer laziness though I had my first piping spreadsheet 8 years back. Thanks for your continuous efforts towards accuracy. If the email id on virtualpipeline is yours, I would like to send you a spreadsheet I prepared that has a comparison of friction factors (equations) and pipe sizing by 2K and 3K methods. You will get katmar's version of Churchill as a bonus and that equation seems to be a perfect fit even in transition region. Let me know.
Thanks again.
rcooper,
I would like to know more about Bq and its simultaneous occurance along with Aq2. Can you please explain?
RE: Preparing System Curves
Yes Churchill works through all flow regimes. And yes, my e-mail is on my web page in the contacts section. I would be pleased to hear from you.
If you would like to share with everyone, www.rapidshare.com makes it quite easy to do so. You don't even have to make an account if you don't want to, or even log in for that matter.
http://virtualpipeline.spaces.msn.com
RE: Preparing System Curves
Take a system where you pump from a tank, through some pipes, down through a pressure sand filter and through some pipes to a reservoir.
Your tank has level So
Your reservoir has level S1
Therefore you static head is S1-So=C
Your pipe work friction and fittings loss is given by (K+fL/d)v^2/(2g) - assuming one pipe diameter and the friction factor does not vary with velocity. i.e. for turbulent flow the headloss is proportional to the squart of the flow rate.
The flow through the sand filter is lamina, so the head loss is proportional to the flow rate, so the headloss for this section of the works is given by Bq.
So if you have a system which involves both turbulent flow and lamina flow the head loss is proportional to both q and q^2.
As I said above, this happens if you are flowing down through a bed of sand. If you are flowing up through a bed of sand, the sand could fluidise, at which point the headloss through the sand is independant of the flow rate.
RE: Preparing System Curves
BTW, how do you guys manage these wide posts without having to change the resolution? I have IE6 and there is no horizontal scroll bar appearing.
RE: Preparing System Curves
IE has no hscroll. You have to use full screen view, or grab the right edge and stretch IE wider.
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RE: Preparing System Curves
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.
RE: Preparing System Curves
(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?
RE: Preparing System Curves
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
RE: Preparing System Curves
http://virtualpipeline.spaces.msn.com