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In some regions, a significant portion of households in a water distribution network may use private pumps to store water in a tank, to alleviate low pressure periods. You may have a need to analyze the impact that these private pump stations and tanks have on the rest of the network.
Most engineers who work with water distribution do not get into the details of individual house pumps. One challenge is that it can be difficult to determine exactly which pumps are running. Often the best you can do is estimate the demand over time and spread it evenly among the model nodes. At any given time, that estimate will be wrong but over time, it can be better than any other estimate.
Pipe sizing is usually based on a reasonable worst-case peak demand. If you want to model the pumps, you could simply place a demand on nodes such that the sum of the demands equals the flow into the system. As the system gets smaller, the peak to average flow ratio increases.
Consider also looking at the factors that are used by pressure sewer pump companies like e-one to better understand a reasonable number of pumps to be running at once, then multiply it by the flow from a pump to see if the value is reasonable.
Concerning where to place these demands, the worst case tends to be when the flowing pumps are far from the source.
There are some theoretical methods to assign a probability of certain flows in a system, such as the Poisson Rectangular Pulse model but you will need data to calibrate that type of model and it may be difficult to obtain sufficient data for some systems. With that data you can estimate the probability that certain pumps will be running at any time.
Some questions to ask yourself:
As you can see there is a lot of judgment involved in designing this sort of system.