Pumps and turbines can be categorized in different ways. Based on the kind of fluid being pumped, pumps can be classified as water pumps, oil pumps, air pumps, etc. The most common type of pumps is the centrifugal pump whose name is derived from the centrifugal force generated in the pump, which in turn generates the pump head. For long-distance pipelines, it is not possible to place all the pumps near the pipeline inlet because that would cause enormous pressure in the pipe and the pumps, requiring unreasonably thick pipe wall and unreasonably strong pump casing. Positive displacement (PD) pumps are those that use their prime moving part to forcibly push the fluid out the pump chamber (cylinder) and into the discharge pipe. Ordinary piston and plunger pumps are for single-action: pumping the fluid only during the first half of each reciprocating cycle when the piston/plunger moves forward.