Question
I purchased a home with an installed irrigation system.
It uses a pump fed by a pond.
There are about 20 heads on the system ...yes 1 zone.
I added a controller and a pump relay (originally just a timer at the pump) but now want to break the system into 3 zones.
I'm concerned that if one of the zone valves fails it could cause the pump to fail.
It has been suggested to use a "flow switch", or pressure switch or a pressure relief valve. Help!
I've even contacted a sprinkler manufacturer and didn't get a clear answer.

The output pipe from the pump is 1 1/2
The pump is a sta-rite, I don't know the model off the top of my head but it has enough capacity to run all of the 20  heads.(1/3 acre)

Answer
You could be in for a lot of problems if you try to split a system running off a pump.  You would have to install a pressure tank and a pressure switch at the very minimum.  The thing is that the pump is already supplying enough water to run 20 heads.  Cutting that to 7 or so means that you have to do something about the excess flow and pressure that the pump puts out.