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How can I model a fire event where multiple hydrants are flowing at the same time?
The traditional automated fireflow routine in WaterCAD and WaterGEMS will apply and analyze fireflow at one node (hydrant) at a time. Each fireflow node run is independent. So, the automated fireflow routine currently cannot run multiple hydrants at once.
Consider one of the following approaches:
1) Use one single node to represent all of the hydrants that will be flowing. For example if you have three hydrants that need to provide 1000 gpm, consider the needed fireflow to be 3000 gpm, then use a junction in the vicinity of the hydrants in question as the fireflow node. You'll then need to ensure that you have enough hydrants to deliver that 3000 gpm.
1) Combine the hydrant nodes into one, located near the middle of the three and with a lateral pipe (or lateral property of the hydrant) set to be equivalent to the three laterals, so as to not skew the results. Place the 3000 gpm needed fire flow on that new hydrant.
3) Perform a standard steady state analysis (not automated fireflow) with the required fire flow applied to each hydrant, check the results (pressure), adjust the demand as needed, repeat.
When talking about multiple fires (not multiple hydrants for the same fire), the fires are usually far enough apart that distribution capacity is not the issue. The limitation is usually source/storage. The question would be "Do you have enough water in storage to make it through 2 hours?" In this case, configure a few EPS scenarios with two fires running at the needed fire flow and watch how the pumps and tanks behave.
It makes a difference if the fires are in the same pressure zone or different zones. If they are in different zones, there may be very little interaction between the fires if each zone has its own storage.
It would not be practical to run a fire flow analysis for an exhaustive combination of all pairs of nodes. Let's say you have 20,000 node model. Running every pair would results in 400,000,000 runs which would take a long, long time to run and you'd probably run out of memory along the way.
Fire flow analysis (forum discussion)
Understanding Automated Fire Flow Results