I need some assistance on the type of pressure valve I need for a subdivision I am working on. I am a land development engineer in NC, USA. 

This situation is that my proposed subdivision will be served by an 8" city water main that will not meet fire protection (20psi).  The county has a 16" main across the street. 

We are going to tap onto the 16" main with a valve that will "open" only when the pressure on the city side goes below a certain set amount (20psi).  This will work in a fire situation (or when every one flushes at the same time). 

The flow will be metered by the county and charged to the city, who will then spread it out to the homeowners in the subdivision.

My question is this.  I am searching the internet for available suppliers but am getting confused on all of the options.  Am I looking for pressure relief valves, pressure sustaining valves or a combination of the two?

Sounds like a pressure-sustaining valve to me?

Your pressure drops, this valve opens up, which instantly boosts your pressure, which shuts the valve back off- feedback effect.  check how they handle that problem with the valves.

You need to be looking at a modified altitude valve.  This valve will sense the downstream pressure and open when it reaches a low setting, then close when it reaches an upper setting.

You probably need to be talking with the Ames/Watts company or other control valve manufacturer.  Giving them the specifics can help them design the solenoids that you need on the valve to correctly meet your conditions.

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