Installing a new diaphragm pressure tank and a new shallow well jet pump. The pump comes with the pressure switch on the pump and wired to the motor. Is it OK to leave it on the pump or should it be installed on the tank T?
Leave it where it is.
Generally it can be left on the pump. But if there is significant distance between the pump and the tank (more than a few feet), put the switch at the tank.
If there is significant distance between the pump and the tank, the inertia of the water in the pipes can cause problems with the switch.
When the pump starts, there will be a pressure surge at the pump before the water starts moving, and this pressure surge can cause the switch to immediately shut off. Then the pressure drops back down and the pump turns on again, and the cycle repeats.
When the pressure reaches the cut-out point of the switch, the switch opens to stop the pump. But due to the inertia of the water in the pipe between the pump and the tank, there is a sudden pressure drop at the switch, which causes the switch to close and re-energize the pump. Then the surge shuts the pump off and the cycle repeats.
The further the switch is from the tank, the greater the swing of the pressure surge will be, and the slower the switch will chatter, but the longer it will take the pressure to stabilize. The closer you are, the swing will be smaller and the chatter quicker, but the more quickly it will stabilize. When you get really close to the tank, the surge swing will be small enough that the switch won't chatter at all.
This repeated cycling can damage both the pump motor and the switch contacts. Placing the switch near the pressure tank prevents the surging because the tank acts as a buffer to absorb the inertia of the water.
Lest anyone say this is far fetched, I have actually observed this behavior. A neighbor installed a pressure tank in his house, but installed the pressure switch in the pumphouse (without a tank), around a hundred feet from the house. We observed this behavior; installing a small pressure tank in the pumphouse resolved the issue.
In my own water system, I had a shallow well pump connected to the pressure tank by about 6 feet of 3/4" pipe, running through three 90-degree turns. The pressure switch was mounted on the pump. I observed that when the pump would start, the pressure switch would chatter. I've since installed a submersible pump, and placed the switch within 2 feet of the tank fitting, and there is no more chatter.
I appreciate the time you took to answer my question. If the pressure switch doesn't work out on the pump, I'll try relocating it. Didn't want to void the warranty by taking the switch off the pump if it's not necessary. Guess I' ll just have to try it. Thanks again for your input.