2.3 Relationships of Pipe Size and Friction

Pipe cross-sectional area and flow vary with the diameter (of the pipe) squared. As we have previously calculated, this means that twice the pipe diameter yields four times the flow, at a given friction loss.

The friction loss table illustrates this relationship of pipe cross-sectional area (diameter) and flow. Remember, the equation is F = V x A and area is proportional to diameter squared.

In the Friction loss for PVC class 160 Pipe table, look at a friction loss value of 0.21 to 0.24 psi per 100 feet marked by a (2) notation for each pipe. Compare those friction loss values to the flow rate column of the table to see that doubling the pipe diameter increases the flow rate by at least four times.

Of course, the values would be more correct using actual pipe inside diameters. The Pipe Size vs Flow Rate chart summarizes pipe sizes and flow rates as described above.

This explanation is a summary of the relationship between pipe area and friction.

One more item can be noted --- water velocities are greater as the pipe size increases.

This illustrates that for larger sized pipes more of the water flow is further away from the pipe walls and water can flow faster with the same friction loss as smaller pipe.