Friction loss in pipe is a measurement or calculation of loss of flow or pressure due to the interaction of the fluid with the walls of the pipe. These losses need to be determined for piping systems, because pumps must be specified with enough power to overcome losses and provide adequate flow rates. Friction loss varies depending on the pipe materials, length and the liquid flow rate or velocity.
Piping systems consist of straight or curved sections of pipe, connections, valves and other fittings. As a system increases in length and complexity, the fluids running through it interact with the walls of the pipe and various fittings, creating drag. Excessive amounts of drag will eventually cause a loss of flow at the customer or destination if not overcome by added pumping power.
Materials used in pipes vary widely, from cast iron to plastics, and each type has a particular roughness on the interior walls. Pipe manufacturers issue tables that show the expected friction loss per length of pipe for different materials and pipe diameters, with the diameter measuring the distance across the inside area of the pipe. A measured or estimated roughness factor is used when calculating friction loss, with unfinished iron pipe having a higher roughness factor than smooth plastic pipe.
Friction loss tables show values of feet of water head per 100 feet of pipe (the value is the same in meters) because pump capacity and power requirements are normally sized in feet of water. This value estimates how many vertical feet a pump can push water with a given motor size, which allows designers to relate pump power directly to a piping system. The effect of friction loss increases with the length of the system and the number of fittings and curved sections, and can only be reduced by using smoother pipe, but cannot be eliminated.
Losses also increase as the velocity, or speed, of the fluid increases in the pipe. Increasing the system pressure is a better solution than increasing flow rates, which is why municipal water systems use elevated water towers. Maintaining a supply of water in an elevated tank not only provides extra water for periods of high demand, but it also maintains a more constant pressure to overcome system losses.
Friction loss is important in all commercial piping systems, because higher drag equals higher pumping costs. There are some applications where adequate pumping capacity is critical, such as fire fighting. A fire pumping truck must provide large quantities of water through long sections of hose, often to upper floors of a building. As the demand for water increases, the pump truck must supply adequate pressure at the nozzles to maintain fire-fighting capacity. Hose nozzles are designed as long thin tubes because this converts the hose pressure to high flow at the nozzle without the need for high velocities in the hose.
Air is also a fluid, and friction loss occurs in heating and air conditioning systems as air moves through the ducts that connect to various rooms. System designers use friction loss calculations to determine the fan power required for adequate airflow in buildings. Inadequate fan and duct sizing can result in poor airflow in some areas of a home or building, or inadequate heating or cooling on higher floors of a multi-story building.