The suction line of a non-self-priming centrifugal pump must be primed prior to the pump's start-up (see Pump system). If the suction impeller is not flooded when the pump is at standstill, a foot valve or a swing check valve can be installed beneath the lowest water level in the suction line. The installation of such valves is however limited to small nominal diameters only, as surge pressures may occur when the foot valve or swing check valve suddenly close due to backflow following the pump's shutdown. It is therefore necessary to adequately size components such as the foot valve, pump casing, valves and piping, applying a safety margin in order to protect them against the maximum static pressure load. To protect the pump itself, a further swing check valve is usually installed in the discharge line. This configuration can however only operate satisfactorily if
the discharge-side check valve shuts before the valve on the suction side.
If a foot valve or a swing check valve are not installed in the suction line, the pumps are primed prior to start-up by venting the pump and the suction line. Smaller sized pumps with foot valve are sometimes primed manually by means of a feed funnel valve.
Despite their wet well installation, deep-well turbine pumps are also equipped with foot valves if their pump shafts run in water-lubricated rubber bearings (see Plain bearing) and if some of these bearings are situated above the normal well water level of pumps installed at large depths. Such bearings require water lubrication when the pump is started up, and the foot valve ensures that all the bearings remain flooded. In the case of self-priming pumps, the self-priming device must be filled with the fluid handled before start-up.