I recently received a model from someone else and noticed that the fire flow available for select hydrants was greater than the field measurements for hydrant testing.
i.e. the fire flow alternative has an upper limit of 3,500 gpm. For the hydrants we opened in the field, each of them report an upper limit of 3,500 gpm:
However, in the field these hydrants (fully open) were only able to produce anywhere from 914-1085 gpm. What is this an indication of? An inappropriately calibrated model? Hazen Williams coefficients that are too large? Incorrect tank levels?
Let me throw in my 2-cents worth.
The key sentence in the link that Mr. Walksi referred to is: "In a Fire Flow Analysis, the results are based on distribution system capacity."
There are two types of fire flow tests: One which measures the amount of flow a hydrant is capable of providing (a "single hydrant test"). The other, a "two-hydrant test", measures the amount of flow the system itself is capable of providing. They are two very different tests and can yield very different results.
If you want to replicate a single-hydrant flow test, try setting the hydrant status to "open", then run your model and see what happens. I'm guessing that the results will show a flow of significantly less than 3500 gpm.
I would suggest that you not blindly use the Fire Flow Analysis option unless and until you are confident that you fully understand what it is telling you.
I don't believe that I was understanding at first that main capacity and "distribution system capacity" were one in the same. I hope you understand that I am asking questions because I am trying to learn and I appreciate help from everyone.
I did as you suggested and I did see a flow through the open hydrant of less than 3,000 gpm, 1,411 gpm to be exact. Assuming my emitter coefficient is correct, than this is the flow that I can expect to see out of this free flowing hydrant?
Yes. You should also look at the calculated residual pressure to make sure that it meets minimum criterion (I would be surprised if it did not). You may want to also double check your emitter coefficient.
Yes it looks like it does:
So essentially this is the pressure and flow that would be available to fight a fire if the hydrant was free-flowing. That being said, and perhaps this is not the place for this question, can a fire truck pump more flow out of the hydrant? Such that if the model says 1,500 gpm can be provided at this hydrant without dropping the pressure below the limit or increasing the velocity past the limit then a fire truck could pump 1,500 gpm out of the hydrant? Or is there another limiting factor?
Yes. Typically a pumper engine would be able to get more out of a hydrant. You would have to calculate the engine pump suction hydraulic losses and the NPSH available vs required to see how much you can get.
You can neve be sure you know that kind of combination of hoses. pumps, nozzles, etc. that the fire department will use. All the water department can do is provide adequate flow at the required residual pressure.
The flow that can be delivered can be limited by the number of available hydrants. Insurance rating people use 1500 gpm when evaluating system in the US.
With regard to free-flowing hydrant flow., how often does teh grass in front of the hydrant catch fire.