Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 662 of 707

Fig. 3. Flexible impeller pumps utilize an elastomeric impeller that pushes the liquid from the inlet to the outlet port. Fig. 4. Flexible linear pumps utilize an elastomeric liner that has an eccentric cam turning within it. As this rotates, it pushes the liquid from the inlet to the outlet port. Fig. 5. Air-operated diaphragm pumps utilize air pressure acting on a manifold valve to provide alternate reciprocating motion to opposed diaphragms. When one diaphragm is pushing liquid out, the opposite diaphragm is pulling liquid in. worthy is the fact that the greatest amount of wear occurs when the pump is devel- oping its greatest amount of pressure as the plating filter is approaching maximum reduction of flow due to dirt pickup. Therefore, oversizing the filter will reduce the frequency of this occurrence. The flexible impeller (Fig. 3) and the liner impeller (Fig. 4) are both self-priming. They develop pressures up to 20 psi but require relatively fre- quent impeller or liner replacement when used continuously. Also, they cannot be used on abrasive solutions or where dry-running capability is required. Air-operated diaphragm pumps (Fig. 5) do not have rotating seals, impellers, or other internal parts. They depend on a pulsing, intermittent reciprocating motion acting on an elastomeric membrane to form a liquid chamber between two check valves and thus produce low flow rates at high pressure. The air supply can be reg- ulated to produce certain performance requirements. Because of their self-priming feature, capability to run dry, and ability to handle extremely viscous liquids or mate- rials with a high solids content, they are widely used in waste treatment and in oth- er industrial applications. However, since these pumps pulsate, the filter and pip- ing require pulsation dampening. Another common self-priming pump design is the progressive cavity design (Fig. 6). This design uses a rotor, which has a helix turning inside a stator with a similar helix at a set pitch. Liquid is passed from one chamber to another along the length 661

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