A term used to describe a situation in which a dispensing pump is maintained "at the ready," capable of immediately dispensing fuel.
In a suction system, the pumping unit inside the dispenser "pulls" product from an underground storage tank when the pump is turned on. When the transaction is completed and the pump is turned off, product remaining in the pipe line-between dispenser and tank-would normally flow back into the tank, an undesirable scenerio. It would mean that the next time the pump was operated the first drop of product out of the tank would have to move all the way through the empty piping before it could be dispensed. Such a situation would not only create a delay, but would also make the pumping operation more difficult.
Ideally, then, the piping and pumping mechanism should remain filled with product, even when the pumping unit is not in operation. This is accomplished through use of a check valve in the piping system. When the pump is turned off, the valve closes and prevents product in the line from flowing back into the tank. Both suction and pressurized systems use check valves for this purpose.
This practice is referred to as "keeping the pump primed." If a check valve failure occurs, and product in the line does flow back into the tank, the system is said to have "lost prime."
See also Check valve.