Dear All,
I am normally used to have parabolic system head curves when the network in question is a single transmission line, which makes sense since the headlosses are a function of V^2/2g. However, I am getting a shape that resembles a third degree curve when I use WaterCAD's system curve feature in a network. Can anybody advise on why this is happening? May this be due to the Hardy Cross method of correction on the headlosses, which introduces further polynomials?
Thanks and regards,
Amine Salameh
Hi Amine,
We get a lot of questions about system head curves in WaterCAD, and most of them come down to the subtle but important differences between how WaterCAD computes them versus how an engineer would compute a system curve by hand.
As you mention, when you manually compute a system curve for a transmission line, you generally end up with a parabolic system curve (assuming you are using the Darcy-Weisbach formula where headloss is proportional to v^2). The process is pretty straight forward provided you have a single pipe, only one pump station, and no intermediate demands. But if you add some of those complications the calcs get much more difficult.
With WaterCAD you can easily tackle more complex systems - systems which would be practically impossible to solve by hand (e.g. multiple pumps, loops, demands, tanks, etc). WaterCAD does this by computing the hydraulics for the network once for each point on your system head curve, as if it were a regular WaterCAD simulaiton.
So, say you want a system head curve containing 5 points from 100 to 500 l/s (plus a point at 0 l/s), as shown below. WaterCAD would compute the model 6 times; once where there was no flow at the pump location, once when there was 100 l/s at the pump location, once where there was 200l/s at the pump location and so on.
Example System Head Curve:
Flow Head
0.00 29.7 100.00 30.5 200.00 32.6 300.00 36.3 400.00 41.5 500.00 48.1
Note that I say 'at the pump location' because, for these calculations, WaterCAD isn't using any pump information that you may have entered. Instead, behind the scenes, WaterCAD is merely applying a demand immediately upstream of the pump location and an inflow immediately downstream of the pump location, and then figuring out the resulting head difference. In other words, the pump is temporarily replaced in the calculation by two 'virtual' (i.e. not really there in your model) nodes which I will call a 'suction node' and a 'discharge node'. I have attempted to illustrate how this works in the attached figure (note though that you don't ever actually see these 'virtual' nodes in WaterCAD).
Now because WaterCAD is solving your entire network to work out the system head curve, there are many reasons why your resulting curve might not be parabolic. For example, there could be another pump station in your system which turns off at some point due to a pressure control; or a flow control valve might start to influence the system curve above a certain flow; or the supply of all the system demands might mean flow needs to travel a much longer distance into a storage tank, etc. (Actually, systems where the demands are supplied and where there are no storage tanks - sometimes called 'closed systems' - are a particularly special case, but that is probably a topic for another thread). Even the concept of 'static head' is difficult in a complex network because if you have, say, a second pump station in the network, your 'static' head at the first pump station will most likely depend on whether the second pump is on or off! But in the end, what WaterCAD gives you is a real system head curve because, according to your model, it is truly the head a pump would need to provide to deliver a certain flow.
Hopefully this has helped shed some light on the system curve calcs. One thing you may like to try is to try and manually recreate the results of the system curve calcs by replacing your pump with a 'suction node' and a 'discharge node' (as shown in the attached figure). This might help you understand the calculations for your particular system.
Regards,
Mal Sharkey
Product Manager Bentley
Very good discussion, Mal.
The most common reason for the pump curve not to be parabolic is water consumption between the pump and the downstream tank. When this consumption occurs, it lowers the system head at low flow. In fact, if the consumption between the pump and tank is greater than the pump out put(such that flow goes from the tank toward the pump), then there is an inflection point in the pump curve. If you want to check this out manually, put a single large demand between the pump and tank. WaterCAD/GEMS, of course, handles this automatically.
If you want to learn more about pump curves, get a copy of the paper
Ormsbee, L. and Walski, T. "Developing System Head Curves for Water Distribution Systems," Journal AWWA, Vol. 81, No. 7, July 1989 .
You realistically won't get a sensible result if you run a WaterGems System Head Curve on with Flow Control Valves in-line, and representing the inlet valves to the tanks. I would also recommend that you don't model each of the 13 tanks as separate model tanks, and rather combine these into 1 large model tank.........you also won't get sensible results if you have tanks that are hydraulically close.
From your descripton of the inlet valve, a TCV is perhaps more representative of what is there. With the TCV you can either input k values directly, or define a Valve headloss coefficient vs Valve Opening Ratio curve. To borrow one of Tom's phrases, "Model what you have".
Hello,
(I am a newcomer to this business, please forgive my ignorance.)
Could anyone direct me to information on how to produce a System Head Curve for a system with a pump station that has multiple pumps using WaterGems?
Thanks,
greerj