Jack,
I don't understand the physics behind the reason for using a vacuum relief valve, but I do use one upstream of my fertilizer injector just because it said to, in the installation instructions - to keep fertilizer from flowing back into the house water supply. They also recommend them if you bury the tape.
I would guess you would need one wherever you use a valve to shut off the flow of water. But I have no clue why. I shut off my water at the injector, so only use one vacuum relief valve.
Sorry I can't be more help, let me know what you find out. I've always been curious as to how the tape can suck air (and dirt) when the valve is off.
Steve
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