Fire Flow with a closed system and PRVs.

Question

Hello, 
 I have a closed system for a subdivision that is being designed. I used PDD's, and because of the elevation differences I have used PRV's to limit the pressure in the lowest part of the system. When I try to run a fire flow analysis, it won't get even 1 gpm because my pressures within this low elevation area too low. I thought PRV's did not allow flow to reverse through them and therefore I should at least be able to get fire flows outside of that low elevation zone with the PRV's because the valves would shut.

Scott Kampa · Answer

Hello Craig, 
 
Based on the description of the issue, you may need to set up multiple zones in the model, such as a high pressure zone and a low pressure zone. You can then assign the appropriate elements to the zone. 
 
The automated fire flow analysis takes into account not only the pressure at the individual fire flow node, but also at all nodes in the same zone. So if a fire flow is added to a node in the high pressure zone, but results in a low pressure value in the low pressure zone, the fire flow may not be met. 
 
You can find more information on this, plus other good troubleshooting steps, in the following link: communities.bentley.com/.../understanding-automated-fire-flow-results 
 
Regards, 
Scott