Selection of pipe sizes. The pipe size selected for a particular installation depends mainly on the cost of the pipe and fittings and the cost of energy needed pumping the fluid. The cost of the pipe and the annual capital charges increase about the 1.5 power of the pipe diameter, while the power cost for turbulent flow varies with the -4.8 power of the diameter. Equations have been presented giving the optimum pipe diameter as a function of flow rate and fluid density, but these can be converted to the optimum velocity, which is nearly independent of the flow rate. For turbulent flow of liquids in steel pipes larger than 1 in. (25 mm) in diameter, the optimum velocity is