Technically anywhere in the system will work. However, to ensure the pump does not cycle it would be best to have it installed somewhere after the pump and before the first Tee.
Originally Posted by krsmitty
What is the expansion tank for/do?
The accumulation (aka expansion) tank is a small pressurized tank that usually is about half full of water and the other half has a pressurize bladder. The idea is to have a small reservoir that does two things, one as the pump pumps it keeps you from seeing little surges in the water flow, secondly if you have heard your pump cycling on/off repeatedly, as in a faucet is cracked open or you have flushed the toilet, it helps keep your pump from turning on all the time. This is great at night, as you may get one or two bathroom runs during the night before the pump turns on, waking up the light sleepers.