It is obvious that an undersized pipe has very high energy costs. If the pipeline is too small, the fixed costs may be higher than for the next larger pipe size due to the requirement and cost of a larger pump. As a general rule of thumb, a water velocity in the pipe of 5 feet per second is the maximum allowable in most circumstances except suction pipes on centrifugal pumps, which should be between 2 and 3 feet per second. Table 1 shows the flow rate for various size pipelines when the velocity is 5 feet per second. There also are differences between types of pipes.
Smooth pipe has less friction loss and, hence, a lower operating cost than rough pipes.
Plastic pipe, such as PVC, is the smoothest, followed by aluminum, steel, and concrete, in that order. Therefore, pipeline cost comparisons should be based on total operating costs rather than on only initial installed purchase price. More precise figures from manufacturers' specifications should be used for design purposes. The least cost of operating a system also extends to selecting the proper size valves and fittings. The economic analysis is similar to that of the pipeline.