The maximum amount of liquid that can move through a tube, according to the inside diameter and the resistance of the inside surface. Many practical problems in flow through pipes and channels have been solved by using simple empirical laws that embody a friction factor and that account for energy loss. The calculations for determining these factors are far too complex for a "simple" explanation here, as it involves viscosity, length of pipe, diameter of pipe, velocity of fluid, etc. It is important, however, for a pump to be allowed enough water through the suction pipes to be able to operate without CAVITATING from insufficient supply of water, as the water also provides cooling for the pump impeller and seal. As an example of the importance of flow rates, a medium head, 1 hp pool pump will need a 1 1/2 inch suction line, or a much shortened life expectancy should be anticipated.