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Sunday, July 27, 2008

Chemical Pump and Sealing Pump

The vertical in line pumps, although relatively new additions, are finding considerable use in chemical and petrochemical plants in the United States. An inspection of the two designs will make clear the relative advantages and disadvantages of each.

Chemical pumps are available in a variety of materials. Metal pumps are the most widely used. Although they may be obtained in iron, bronze and iron with bronze fittings, an increasing number of pumps of ductile iron, steel, and nickel alloys are being used. Pumps are also available in glass, glass lined iron, carbon, rubber, rubber lined metal, ceramics and a variety of plastics, such units usually being employed for special purpose.

Sealing the Centrifugal Chemical Pump
Although detailed treatment of shaft seals is presented in the Sealing of Rotating Shafts, it is appropriate to mention here the special problems of sealing centrifugal chemical pumps. Current practice demands that packing boxes be designed to accommodate both packing and mechanical seals. With either type of seal, one consideration is of paramount importance in chemical service, the liquid present at the sealing surface must be free of solids. Consequently, it is necessary to provide a secondary compatible liquid to flush the seal or packing wherever the process liquid is not absolutely clean.

The use of packing requires the continuous escape of liquid past the seal to minimize and to carry away the frictional heat developed. If the effluent is toxic or corrosive, quench glands or catch pans are usually employed. Although packing can be adjusted with the pump operating, leaking mechanical seals require shutting down the pump to correct the leak. Properly applied and maintained mechanical seals usually show no visible leakage. In general, owing to the more effective performance of mechanical seals, they have gained almost universal acceptance.