The amount of water a pipe will deliver at any one pressure is a direct result of the size of the pipe, the smoothness of the inside surface, and the pipe length. The procedures used in selecting pipe size are affected by whether there is only one building or hydrant being serviced by the line, or two or more buildings.
Smoothness
Friction between the inside walls of the pipe and the moving
water creates a force that opposes the flow of water through the pipe. The
greater the roughness of the inside of the pipe, the greater the force caused by
friction, and the higher the pressure required to push the same amount
of water through the pipe. Consequently, pipes with smooth inside
surfaces can
deliver more water than pipe walls with rough surfaces using the same pressure.
Pipe connections - elbows, reducers, tees, unions, etc. - as well as valves, can add greatly to the friction.
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