WEARING RINGS

20-foot suction lift. The pump consists of a split casing, wearing rings, and rotating element. SPLIT CASING.— The casing (fig. 4-7). is horizontally split at the shaft centerline. This enables easy removal of the upper casing half for inspection and maintenance. The casing is divided into three chambers; two suction and one discharge. The upper half of the casing contains a flange that may connect the pump to an air eliminator valve. Two external seal lines on the upper casing feed fuel from the discharge chamber to cool the mechanical seals. The lower half of the casing contains bearing housings, a suction flange and a discharge flange that connect the pump to the piping system. Drain holes and drain plugs are provided at the bottom of both flanges for draining the pump. WEARING RINGS.—There are four replaceable type wearing rings (two rotating and two stationary) installed within the pump casing. The two rotating are installed on the impeller. The two stationary are installed in the pump casing between the suction and discharge chambers. The stationary rings are held in place and prevented from rotation by the tongue-and- groove construction. When the pump is assembled, the rotating wearing rings ride inside the stationary rings. (Check the appropriate technical manual for the correct clearance between the stationary and rotating rings.) Wearing rings serve two purposes: (1) owing to their unique construction and close tolerances, they minimize leakage between the discharge and suction chambers and (2) they allow for the wear created between the impeller and pump casing. Fuel passing through the pump has a tendency to recirculate from the discharge chamber back to the suction chamber. As the fuel passes through the narrow clearance between the wearing rings, a partial seal is made by the rapid rotation of the impeller. This seal minimizes the leakage between the discharge and suction chambers. After prolonged use of the pump, the clearance between the wearing rings gradually increases due to wear. This is caused by the friction created by the rapid rotation of the impeller and the fuel passing between the wearing rings. As the clearance increases, the sealing effect decreases resulting in the loss of the rated capacity of the pump. Figure 4-7.—Centrifugal pump casing. 4-11