Submerged cargo pumps are centrifugal pumps driven by a 3-phase electric motor, the stator of which is inside a leak-proof container to which the electric supply leads are connected, the stator and drum forming a sealed unit. The rotor fits inside the stator drum and drives the impeller immediately below, so that the pump and its driving motor are completely submerged inside the cargo tank.The pumps, as illustrated, suck through the centre. The liquid discharge section forms a jacket around the pump stator and acts as a cooling agent.
The bulk of the discharged liquid is sent up on deck through the pump discharge pipe, but a small proportion of the liquid is fed back to the pump through the top ball bearing, between the rotor and stator, and through the lower ball race to the impeller, thus cooling and lubricating the bearings and rotor. For this reason, it is absolutely essential that the pump is never run without liquid passing through it.
The pump motor is cooled by the liquid discharge and is protected by the following devices:
(a) Low current cut-out. This indicates that the pump has lost suction. It stops the pump, and an alarm is sounded.
(b) High current cut-out. This indicates (apart from starting up) that the pump is seized or is seizing up, or that the impeller has been jammed by a foreign object. It stops the pump and sounds an alarm.
Though the pump is a centrifugal pump, it has a small axial propeller called an inducer beneath the impeller to assist (or boost) the suction pressure.
Electric supply—pump leads are well insulated leads, each phase being in a separate stainless steel conduit pipe, sealed at both ends, each conduit pipe being firmly secured and therefore earthed to the cargo pump discharge pipe. The leads pass through a tight gland at the pump end to the motor terminals and terminate on deck in a watertight junction box near the tank entrance. Similar leads connect the junction box to the distribution panel in the engine room. Before using the pump, the whole system should be megger tested and the pump not used if the megger reading is less than 2 megs.
Of such importance is the continuous flow of liquid discharge for cooling and lubricating purposes that the pump must be stopped at once if it does not pick up suction right away. The starting surge current is so large that it generates a lot of heat and after a pump has failed to pick up suction, it is essential to wait one hour before trying a re-start, so as to give time for the heat generated by the surge current to dissipate. If a pump is stopped after running for some time, it may be re-started immediately because the heat generated in the starting surge current will have been dissipated already.
