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.
In a case where you need to flow multiple nearby hydrants for a given fire, consider one of the following approaches:
1) Use one single node to represent all of the hydrants that will be flowing, and use automated fireflow. 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. If all other fireflow nodes use a global Needed Fireflow, choose "true" for "specify local fireflow constraints" on the junction representing the three hydrants (or check the box in the table at the bottom of the fireflow alternative) and enter the specific Needed Fireflow of 3000 gpm. If you have separate hydrant elements in the model, remove them from your fireflow selection set, (so that the automated fireflow routine only computes the junction, instead of the hydrant elements). You can do this by selecting the hydrants, right click in the drawing, choose "remove from selection set", then choose the fireflow selection set. Open the fireflow alternative to refresh the list of fireflow nodes based on that selection set. Also make sure that the junction in question is included in the fireflow selection set.
2) If lateral losses may be significant, consider using a hydrant instead. For example if three hydrants need to flow at the same time, each with a needed fireflow of 1000 GPM, combine the hydrant nodes into one, located near the middle of the three and adjust the lateral pipe to be the equivalent of the three lateral pipes. Place the 3000 gpm needed fire flow on that new hydrant.
If your hydrant is connected to the water main via a pipe element (at a tee), then adjust that pipe's property (the pipe element, not the separate "lateral" element)
If your hydrant is placed in-line with the water main (not at a tee), then ensure that the property "Include hydrant lateral losses?" is set to "true" for the hydrant, then enter the equivalent diameter pipe (representing the three lateral pipes).
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.
4) If you need to run an EPS to account for change in storage as a result of the multiple hydrants being opened at the same time for a specific amount of time, you will need to enter a demand pattern with a 1.0 multiplier only at the times when the fire occurs. You can either model the separate hydrant elements, or combine into a single one per option 1 or 2 above. See more details here: Running a fire flow analysis in EPS (Extended Period Simulation)
5) If you have advanced programming knowledge, are a member of the Bentley Developer Network (BDN) and are willing to invest the time needed to learn the WaterCAD/WaterGEMS schema, it may be possible to develop a tool to do this using Waterobject.NET . See also: Multi Fire Flow (forum discussion)
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)
Multi Fire Flow (forum discussion)
Understanding Automated Fire Flow Results