What will happen if I have a waterline design having a 1& 1/2 inches to be connected to 3 in. pipe.?

Question

At the inflow connected to 1& 1/2 in. pipe, I have a 2 hp pump. The main 3 in pipe is going vertically upward to the ground floor up to the fifth floor.

Answer 1

Sorry, but we cannot tell you much about what will happen because there is not much information. The suitability of the
11/2 " pipe will depend on how much flow you desire and how much pressure is available in the 3 " Pipe at the point of connection. Also the 2 hp pump data does not really reflect the flow and pressure capability of the pump. Where (which floor) do you intend to put the pump and where is the 1 1/2 " connection ?
A 1 1/2 "pipe will handle about 40 gpm at a reasonable velocity and with pressure loss of about 3 psi per 100 equivalent feet. Equivalent feet takes into account the additional pressure loss Thur fittings.
Approx. small water pump horsepower = gpm x psi / 5500. Pump head is commonly expressed in feet (of water column). To get the equivalent water pressure (psi) divide feet by 2.31

Answer 2

Well, the velocity in the 1&1/2" pipe will be higher than the velocity in the 3" pipe. A quick look in a catalog I had handy, shows that 1&1/2" seems to be a pretty common port size for a 2hp pump, though there is some variation. It is likely that the pump is the limiting factor in this case, not the 1&1/2' pipe.