Metal Finishing Guide Book

2012-2013

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between the pump impeller and the motor, a suitable seal is necessary to prevent leakage during the rotation of this shaft. A magnetically driven impeller or vertical cantilever are perhaps the only truly seal-less pumps. Other pumps, which use a liner, or section of hose, are seal-less; but, since these components may fail through usage, fatigue, and abrasive wear, the system, like any other, is subject to eventual leakage. It is always desirable to replace seal components before leakage occurs. Unfortunately, one never knows just how much longer a seal will last before replacement is necessary. They may operate from a few minutes to a more realistic several years. A lip-type seal consists of a molded, rubberlike material, which has a squeegee action in snugging itself around the shaft. A mechanical seal consists of two mirrorlike lapped surfaces, one rotating with the shaft, the other stationary in the pump, which are held together by a light spring pressure, preventing leakage. The preferred arrangement is an outboard mounted seal, so that exotic or nonmetallic seals are eliminated. A packing stuffing box consists of a suitable cavity, with the rotating shaft in the center, around which a compressible-type material may be inserted in alternating rings and held in place and adjusted by tightening the packing gland. Both the mechanical seal and the stuffing box seal are available with provision for water lubrication or recirculation of the solution being pumped. Usually, water from an external pressure water line is desirable, because it assures cooling and lubrication of the seal components. It reduces wear by keeping filter aid and dirt out of the seal area. The water also prevents the solution from crystallizing on the seal faces during shutdown periods. Even while the pump is running, crystals may form as plate-out might occur with electroless solutions. On double-seal pumps, care must be taken through the use of a check valve, or siphon breaker, so that no solution is pumped into the water system during an unexpected failure. Also, a regulator should be installed in the water line to control the pressure, because it will vary from low when the plating room is in operation to high during the weekend when no other water is being used. If the water pressure and flow to the seal are not regulated, it is possible to actually draw water through the packing into the plating tank, especially when the filter is clean, because a negative pressure exists at this point. This could cause chemical imbalance and even overflow of the plating tank. Solutions requiring deionized water for the seal use a double-seal arrangement, with an additional small pump recirculating the deionized water in the seal area. When selecting the type of seal to use, consider the fact that a stuffing box seal or lip-type seal wears slowly, giving warning that replacement will be necessary by gradually increasing constant leakage. A mechanical seal is more trouble-free on a day-to-day basis and yet may fail without warning; thus, there is a need for preventive maintenance. (See piping instructions to minimize solution loss.) Certain types of packing are more suitable for acid, and others are more suitable for alkaline solutions. The construction materials in a mechanical seal, such as the type of carbon and ceramic, along with what type of elastomer, also vary. Therefore, it is important to give the type of service to the manufacturer to assure suitable materials of construction. Some seal wear has to be expected, and periodic replacement of components is necessary. Whenever replacing the seal or packing, the pump shaft should be inspected. If worn or scored, it must be replaced. 790

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