Stepping down a pipe size will increase the SPEED of the flow somewhat, as in ft per second, but it may also reduce the FLOWRATE, as in the gph. The latter is because the friction losses increase, if the head driving the flow is not increase to compensate for the extra friction the gph will be reduced
In a gravity system the thing that drives the flow thorough the gravity fed pipe is the difference in the levels of the water surfaces at either end of the plumbing, so a smaller bore pipe will cause a bigger level difference. The length of the pipe also has a direct influence on the fiction losses and therefore the level difference, a longer pipe requires more head, ie a greater level difference to generate the same flowrate.
The following chart http://www.mdminc.com/Friction_Loss_Chart.htm
may be of use as a guide to judging the size of pipe you require. If you are UK based divide the flow rates by 1.2 to get UK gph and assume your pipe will resemble sch40.
The speed of the flow is what determines how much sediment remains in suspension and how much sediment settles out, since smaller bore generally equals higher speed there should be less sediment deposited in 3" pipe than in 4" pipe
Posted on Jul 19, 2005, 9:36 AM from IP address 81.79.133.233