When a fluid pump pushes a fluid into a distribution network, frictional forces resist the flow and appear to push back against the fluid exiting the pump outlet. The frictional forces originate from the viscosity of the fluid and the movement of the fluid against the walls of the piping, tubing, and other components of the fluid distribution system.

The pump output pressure must be greater than the backpressure of the fluid in the distribution network in order for the fluid to flow.

See also PEI/RP700.