Sometimes you will find water distribution networks that must be supplied from two different source points, as could be the case for those that have a water storage tank, floating on system, and a pumping station.
Under this condition, from the point of view of designing the water network, several operating conditions are usually considered, one of which is that which establishes that both the storage tank and the pumping station are simultaneously providing water to the network, and the water consumption corresponds to the maximum.
Whatever the network’s demand hypothesis is, the condition will be the same. That is, it will be expected that the pumping station will contribute with a percentage of the demand while the rest will be served by the water stored in the tank.
In AQUEDUCTS, in order to facilitate the process of selection of the piezometric head in this second source, or, in other words, the selection of the pumping equipment, we have included the option of creating the system head curve considering an additional supply point to the water network.
To explain this feature, we will continue using the water distribution network example that we have already designed considering that the supply is made from this point.
And, for example purposes, we will assume that at this point the water storage tank is located.
You will see at the System Head Curves dialog, accessible from the Design button options, this box in which specifying the existence of a second water supply point to the network, allows the selection of one of the existing demand nodes in the network.
You can also specify here the demand percentage that will be supplied from it, with respect to the mean flow of the system.
For now I’m going to close this dialog to return to the drawing area and thus create a new pipe section that will represent the discharge pipe from the pumping system.
Let’s create a new node.
I’ll call it P. E, to identify it later in the list, and I’ll assign the ground elevation automatically.
It’s important to note that, for the purposes of generating the system head curve, I have kept the type of this node as a simple demand node, although I know it should really act as a fixed head node for the demand hour I am calculating.
If I perform the calculation, since the node does not have any demand, there is no significant change in the results.
100 percent of the water network demand is entering from our water storage tank, while from here nothing enters to the network.
So let’s go to the System Head Curves dialog to generate the corresponding curve.
I activate the box and select the node from the list.
I’m going to assume that about 35% of the total flow of the network will be supplied from it.
Click the Generate button to see what we get.
I will change the number of points to 10 and remove this row from here to see the chart better.
And there is the system head curve of the pumping station node, taking into account that the minimum pressure in the network must be 20 meters.
As we have done on other occasions, we can ask for values from this curve.
Let’s see what flow will be delivered if the piezometric head is 502 meters.
7.41 liters per second.
Let’s verify it by modifying the network.
I will now assume that here we have another source or fixed head node,
where the piezometric head, for the moment of the water network’s maximum demand, is 502 meters.
I redo the calculation to get a flow that would be quite similar to what the curve has told us.
Of course, we see that the pressures are above 20 meters, with the lowest being 20.45.
Also note that now 19 liters per second enter to the network from the water storage tank, about 70 percent of the total.
In the tank, the piezometric head that we had fixed was 502.35 m, a value similar to the one obtained for the second feeding point.
In this way, you have a curve that would allow you to evaluate characteristic curves of pumping equipment in order to establish what would be the ideal combination for the different flows to be supplied.
For example, for the case that we are evaluating, the maximum possible consumption of the network, you will know that the characteristic curve of the pump you select must intersect with this curve at this point approximately.
In a similar way, you can evaluate other demand conditions with this same curve, allowing you to define if it is necessary to use equipment of variable speed or of different characteristics that, associated in parallel, allow you to manage the range of expected flows.
As you can see, it is a tool that will facilitate the preselection process of water pumping equipment.