Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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into the filter chamber. The filter need not be filled completely, but most contain a sufficient volume of liquid so that, as the hose is lowered to approximately the same height as liquid in the chamber, the hose will gradually fill with solution. Shake the hose to make certain any air trapped in the top of the pump or in other high points is completely expelled. When the liquid level completely fills the hose, keep the tip of the hose at the same position, but close the valve between the pump and the filter chamber. Now insert the hose in the tank (since the valve is closed, virtu- ally no liquid will run out of the hose if a gloved hand is cupped over the end). Start the motor and wait until the motor has reached its proper speed; then slowly open the valve to the filter. This is a further precaution, which will enable the pump to cre- ate enough suction to handle the small amount of air that may still be in the line. When transfer pumping out a tank, it is advisable to connect a 90O hose barb or a strainer to the suction end of the hose so that it may be lowered as solution lev- el drops. This prevents cavitating the pump, which could occur if the end of the hose rested flat on the bottom or against the side of the tank. If the hose has a tenden- cy to curl, insert a length of straight, corrosion-resistant pipe into the end to accomplish the preceding purpose. Since the most difficult time to prime a pump is after most of the solution has been removed from the tank, operators often dump this remaining heel, which is a needless waste of solution. Plating tanks with sumps at one end minimize this loss when solution transfer is necessary. Small self- priming pumps, such as drum pumps, may be used to salvage the heel left in the plating or treatment tank. PUMP SEALS The available types of pump seals vary from no seal at all to lip type, packed stuff- ing box, and mechanical. Since conventional pumps have an interconnecting shaft 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 ver- tical 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 mir- rorlike 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 lubrica- tion 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 663

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