An eductor is a type of jet pump. Unlike other pumps, a jet pump has no moving parts. A sim-ple jet pump, illustrated in figure 13-15, consists of a jet supply line, a jet or nozzle, a suction line, a suction chamber, a diffuser, and a discharge line.

Figure 13-15.-Simple jet pump.

In a jet pump, pumping action is created as a fluid (water, steam, or air) passes at a high pressure and velocity through a nozzle and into a chamber that has an inlet and outlet opening.

The operating principle of a jet pump is as follows: Upon starting up, the rapidly moving jet fluid pushes on and gives sufficient motion to the air (or whatever substance may be in the suction chamber) to carry it out through the discharge line. Displacement of the air from the suction chamber creates a partial vacuum within the suc-tion chamber, causing fluid to flow through the suction line. The fluid entering the chamber from the suction line is picked up by the high-velocity fluid, thus providing continuous pumping action. Eductors are designed to pump large volumes of water. Figure 13-16 illustrates a portable educ-tor used for emergency dewatering of a flooded compartment. In modern ships, fixed eductors have replaced fire and bilge pumps as a primary means for pumping bilges, deballasting, and dewatering compartments. Eductors allow cen-trifugal fire pumps to serve indirectly as drainage pumps without the risk of becoming fouled with debris from the bilges. The centrifugal pumps pressurize the firemain, and water from the fire-main actuates the eductors. The eductors in modern combat ships have a much larger pump-ing capacity than fire and bilge pumps. They are installed as part of the piping in the drainage system and are flanged to permit easy removal and disassembly when repairs are necessary.

Figure 13-16.-Portable eductor.

Because of their simplicity, jet pumps gen-erally require very little maintenance. Since there are no moving parts, only the nozzles will show wear. The erosion action will cause the nozzles to become enlarged; in this case they are generally renewed. Occasionally the nozzles are removed; the strainers, if fitted, are cleaned; and a special reamer is inserted in the nozzles to clean out any rust or scale that may have accumulated.

CAUTION: Improper starting or securing of an eductor can cause rapid flooding of the space being pumped. Always follow the procedure on the posted operating card.