Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 665 of 707

siphon breaker, so that no solution is pumped into the water system during an unex- pected 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 over- flow of the plating tank. Solutions requiring deionized water for the seal use a dou- ble-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 pre- ventive 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 period- ic 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. 664

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