I ran a fire flow analysis based on maintaining 20 psi of pressure in each pressure zone. However, when I apply the available fire flow under maximum day conditions, the pressures in the zone go negative. For example, the fire flow analysis indicates that Hydrant X can provide 2,000 gpm of available fire flow in Pressure Zone 1. However, when I add a demand of 2,000 gpm to Hydrant X, surrounding junctions have negative pressures. How can I solve this issue?
I will try to explain what I think is happening without looking at the model first.
"The fire flow analysis indicates that Hydrant X can provide 2,000 gpm of available fire flow in Pressure Zone 1. However, when I add a demand of 2,000 gpm to Hydrant X, surrounding junctions have negative pressures."
Say Hydrant X is in Zone A, and the fire flow constraint "Pressure (Zone Lower Limit)" is set to 20psi. The fire flow analysis will increase the fire flow applied at Hydrant X until one of the constraints are violated, or the "Fire Flow (Upper Limit)" is reached. If the fire flow analysis shows that Hydrant X has 2,000 gpm of available fire flow then none of the constraints were violated at 2,000 gpm.
If you apply the 2,000 gpm at Hydrant X when running the same scenario as steady state with Calculation Type set to Hydraulics Only, and there are surrounding junctions with negative pressures, then my guess is the junctions with negative pressures are not in the same zone as Hydrant X.
If this does not make sense, or does not seem to explain what you are seeing, then please provide the model files and steps to reproduce and we will be able to provide an explanation specific to your model.
You can upload the model files using a method listed in the following link.
Sharing Hydraulic Model Files on the Haestad Forum
Also, I recommend taking a look at the following article.
Understanding Automated Fire Flow Results
Bentley Technical Support