Valve to select higher pressure water source

Question

I'm trying to build an irrigation system which runs off city water supply and pumped water from a rainwater storage tank.

To keep it simple, the idea is to have the sprinkler timer open a valve from the city water, as well as turn on the tank pump. Each path has a backflow preventor, and they meet in the middle at a T before going to the sprinkler.

Using a pressure reducer, I keep the city water lower than the pumped water, so if the pump is on (tank not empty), its water is what goes to the sprinkler. However I'm not sure that will work on its own. If one is 30PSI and the other 20PSI, will the 30 win out entirely, or will the city water contribute too?

I'm thinking what might be necessary is a valve which selects the higher pressure water source, turning off the lower pressure one. A diagram might be something like this:

Where the central piece can slide left/right to block off the lower pressure, allowing the higher pressure to exit.

Does such a thing exist? Is there a better option?

Answer 1

Alternative method...you already have some sort of a water-level switch that enables the pump.

Why not add a relay to that which adds some additional logic to the city-water valve?

Just set it up such that the city-water valve is controlled by both the timer and the tank-water level.

Answer 2

This would be called a shuttle valve...

They are quite common in hydraulic control systems.

The only use I can find for irrigation is small 1/8" and 1/4" units for agricultural irrigation controls. Usual implementation is a stainless steel ball that rolls back and forth between two brass seats.