We had the power go out this season during a heavy rain, and my backup sump pump was almost going off continuously. When I ran outside to make sure the pipe was directed away from the house, I noticed the flow of water completely fillled the exit pipe and came out at a good rate.
Now I was thinking about installing a 2nd backup pump. IF both pumps ever go off at the same time, i.e. if one pump is overwhelmed and the last line of defense is needed, will this be too much flow for the 1.5"exit pipe?
What happens when you have two equal pumps, HD watchdog specials, doubling the rate into the same pipe?
Im trying to avoid putting another exit pipe in thru the siding.
The computation of flow through a single pipe driven by two pumps requires some relatively complicated analysis. The flow does not double, because the losses in the pipe increase as the square of the velocity, and the total flow is limited by the pumps needing to overcome the total losses in the system. You can either do the mathematics, which requires a lot of detail (exact pipe diameter, length of pipe, number of bends in the system, roughness of the pipe, elevation difference between the pump intake and the outlet, and the pump output curve) or you can assume you will get about 1.5 times as much flow as from a single pump. Probably close enough.