Pipe systems are generally used to transport fluids. When flowing through a pipe the friction causes the pressure energy of the fluid to fall and the internal energy of the fluid to increase. The decrease in internal energy is generally referred to as flow loss, which manifests itself as pressure loss in the fluid. In case losses occur a distinction is made between the internal friction in the fluid and the friction between the fluid and the wall or resistance.
The following general concepts of fluid mechanics are discussed in connection with the losses: