Civil engineer here modeling in WaterCAD.   I have a 510 Zone HGL system interfacing with a 600  HGL Zone system.   I currently have a PRV to separate the systems.  Line size is 16".   In most cases the systems operate independently and there is no flow across PRV.   However if I run a fireflow test in the 600 Zone side, the pressures dip so much in the 600 Zone that some flow (350gpm) is flowing from the 510 to the 600 zone through the PRV to supplement the flow.   This is what I want, but I'm not sure what physical valve setup this really is?   The in the watermodel, the PRV is selected and the HGL to maintain is at 510 as in the PRV is ensuring the 600 Zone is at least 510 HGL that would be supplimented by the 510 zone below if needed.  Perhaps there is a better way to accomplish this?   This is an interim condition on the 600zone.  Ultimately they will be an isolation valve here likely.

So long story short, looking to see how I can phsically specify what to use here.  I need the lower 510 zone to supplement the 600 zone to reach my needed fire flow.

To me it's more a Pressure Regulating Valve (PRV) that you need. You can get self regulating types that will act in accordance with what is sensed in the downstream pressure, which seems to be your situation. If set to regulate downstream pressure to 510 (ft head?) then the valve would be closed while the downstream pressure was above that value. But if the pressure in the 600 zone fell below that, then the valve would begin to open to try and adjust it back.

Just on the TLAs (Three Letter Acronyms), as noted above there can unfortunately be more than one meaning - a PSV is not what you need (Pressure Safety Valve), but PRV is also used to mean Pressure Relief Valve.

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