I set all the the fire flow alternative to have an upper limit of 3,500 gpm., however when I run the model, the hydrant in the model produces 3500 gpm, but the actual field hydrants (ie fully open) only produces 1320 gpm? Anyone know why?
Hello,
The upper limit fire flow is used to prevent the fire flow analysis from producing results that are unrealistically large and also to help the fire flow analysis stop before calculating these very high flows. The automated fire flow run will add fire flows to a node until either one of the pressure constraints is reached or until the fire flow upper limit is reached. These results are basically telling you that the pressure are sufficient to at least get up to 3500 gpm of flow.
The following link has more information on this field: Understanding the Fire Flow (Needed) and Fire Flow (Upper Limit) fields for an Automated Fire Flow analysis. There is no set value the upper limit fire flow should be set to.
Regards,
Scott
So in that case does that suggest that my model is not calibrated correctly, since the model is suggesting fireflow results possibly greater than 3500 gpm but my field is suggesting otherwise?
Just to add - the fireflow reported in the software is not the same as the flow that you would get out of the hydrant if you simply open it. Automated fireflow simply incrementally increases the demand (outflow) at a fireflow node until the pressure drops to the pressure constraint or reaches the upper limit. In reality, simply opening the hydrant would limit the flow based on the opening characteristics, which you would instead model with an emitter coefficient. Essentially automated fireflow assumes that you would have a pumper truck that is able to pump water out of it, otherwise if you are simply opening a hydrant, you may need to set the upper limit equal to the expected hydrant flow.
To model the flow out of a hydrant when it is simply opened, you would use an emitter coefficient. The Flushing module available in WaterGEMS and WaterCAD can use emitter coefficients to model hydrants being opened (not pumped out of) to see if flushing velocity is met. See more in the following related articles:
How does the Automated Fire Flow analysis work?
Calculating available flow at a hydrant
Flushing in WaterGEMS/WaterCAD CONNECT Edition
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
I have the following field test. static pressure 84 psi, residual pressure 70 psi and flow of 1320 gpm. When I calculate the flow at 20 psi, i get 2999 gpm. Now I want to make sure my model gives me values close to 2999 gpm. When I run my fireflow test at that same hydrant, i am getting 3500 gpm and a residual pressure of 64 psi, which means if i wanted 20 psi that would put me way over 3500 gpm?
is that correct, or maybe I'm understanding this wrong?