Hello,
I am trying to model a booster station and tank that will supply water and fire flow for a new subdivision. I have the piping to the tank no problem, but the distribution lines are giving me trouble. THe tank has a 16" line that will supply it, and the same 16" line will service two existing communities. The two existing communities will need a PRV since the elevation difference is so great.I tried modelling that with a 16" line coming down from the tank and then splitting before going through separate PRVs to their communities. I also have an 8" line coming from the tank to service the new subdivision which will go through a PRV coming out of the tank before splitting in several directions. I am having trouble with the fire flow. Each node needs 1,750 GPM plus baseline demands. The tank is at such a higher elevation than all of the fire nodes, yet it shows that many do not meet the flow demand. I am not sure what I have done wrong, because in reality each node would have plenty of pressure and flow. Can someone please look and help me with where I have gone wrong? I have attached the model below.
Thank you,
WaterCAD Final.7z
Hello Joseba,
It appears that many of the fire flow nodes do pass. There are a some that have some available fire flow but less than the needed fire flow. An example of this is H-29. This particular node appears to have adequate residual and zone pressures, but you are using a system pressure constraint as well. These is a pressure of zero at H-23, which is keeping additional flow from being included.
There are other fire flow nodes where the available fire flow is zero. In cases like this, it is often that you have low pressures in the model in a steady state run. If the pressure at a fire flow node or at a node in the same zone are already below the pressure constraints, no fire flow will be added.
In your model, the low pressures at H-23 and J-69 are the main culprit. If you compute a steady state run, you will see that the pressures at these two nodes are less than 15 psi. This is below your fire flow pressure constraints. These are downstream of one of the PRVs. There is a significant increase in elevation between the PRV and J-69. So while the hydraulic grade doesn't change much in a steady state run, the pressure will decrease by quite a bit as a result.
I would review the elevations and PRV settings, to make sure that they are accurate. If they are, the fire flow run is likely telling you the pressures in this particular zone are already quite low even without the additional fire flow. Some adjustment to the PRV may be needed.
You can find more information interpreting automated fire flow results here: Understanding Automated Fire Flow Results. This can help with troubleshooting results and provide some understanding on where the results are coming from.
Regards,
Scott
Answer Verified By: Joseba Criswell