Last month we examined two of the three factors to be considered when determining the pressure requirements of a pump - household working pressure and lift pressure. The third factor is pressure drop due to friction loss. Friction loss is the energy lost due to friction when two objects move in relationship to one another. When you drive your truck down the road at 30 mph, the pressure caused by the friction of the air against your truck uses up a certain amount of horsepower. As your speed increases, more horsepower is required to overcome the friction. This is friction loss, and it works against you in a piping system just as it does on the highway system.
As the water travels through the pipes of a water system, friction causes the pressure slowly to drop so that the pressure at the outlet is always lower than the pressure at the inlet (if the system is level). This is called the friction loss or pressure drop, and it varies depending on several factors: