Figure 9-6.—Stuffing box on a centrifugal pump.snug against the stationary seal face. The rotatingseal and all of the assembly below it are affixedto the pump shaft. The stationary seal face is heldstationary by the seal gland and packing ring. Astatic seal is formed between the two seal facesand the sleeve. System pressure within the pumpassists the spring in keeping the rotating seal facetight against the stationary seal face. The type ofmaterial used for the seal face depends on theservice of the pump. When a seal wears out, itis simply replaced.You should observe the following precautionswhen performing maintenance on mechanicalseals:. Do not touch new seals on the sealing facebecause body acid and grease can cause the sealface to prematurely pit and fail.. Replace mechanical seals when the seal isremoved for any reason or when the leakage ratecannot be tolerated.. Position mechanical shaft seals on theshaft by stub or step sleeves. Shaft sleeves arechamfered (beveled) on outboard ends to provideease of mechanical seal mounting.. Do not position mechanical shaft seals byusing setscrews.Fire pumps and all seawater pumps installed insurface ships are being provided with mechanicalshaft seals with cyclone separators. The glands aredesigned to incorporate two or more rings ofpacking if the mechanical shaft seal fails.A water flinger is fitted on the shaft outboardof the stuffing box glands to prevent leakage fromthe stuffing box following along the shaft andentering the bearing housings. They must fittightly on the shaft. If the flingers are fitted onthe shaft sleeves instead of on the shaft, ensurethat no water leaks under the sleeves.Stuffing Box Packing.— Although mostcentrifugal pumps on gas turbine ships havemechanical seals, you should be familiar withstuffing box packing.The packing in centrifugal pump stuffingboxes (fig. 9-6) is renewed following the PMS.When replacing packing, be sure to use packingof the specified material and the correct size.Stagger the joints in the packing rings so they fallat different points around the shaft. Pack thestuffing box loosely and set up lightly on thegland, allowing a liberal leakage. With the pumpin operation, tighten the glands and graduallycompress the packing. It is important to do thisgradually and evenly to avoid excessive friction.Uneven tightening could cause overheating andpossible scoring of the shaft or the shaft sleeve.On some centrifugal pumps, a lantern ring isinserted between the rings of the packing. Whenrepacking stuffing boxes on such pumps, be sureto replace the packing beyond the lantern ring.9-5
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