Nonmetallic Sealless Pumps: How To Pick A Winner

Heightened awareness of environmental, safety and maintenance issues in the last few years has made process-fluid leakage a paramount concern throughout the chemical process industries (CPI). Sealless pumps are becoming the often-preferred choice for pumping hazardous and toxic fluids because of their ability to prevent or minimize leakage problems. After years of being considered expensive niche products for specialty applications, sealless pumps are finally entering the limelight.

Significant trends in pump design, with respect to the handling of corrosive fluids, point to the use of sealless configurations to minimize leakage, and the specification of nonmetallic, chemically inert materials of construction to ensure resistance to acids, caustics, salts and other aggressive fluids. To pick the optimum sealless pump for an application, one must become familiar with the technical and economic issues involved in the pump-selection process, the available pump types, and the various nonmetallic materials that are suitable for the service.

Defining a sealless pump
The Hydraulic Institute (Parsippany, N.J.) defines a sealless pump as one in which the impeller shaft is completely contained in a sealed, pressurized vessel (called the containment shell) that contains the process fluid. Leakage of the pump fluid into the surrounding environment is prevented by the exclusive use of static, rather than dynamic, sealing technology. Only two pump designs meet this definition: the magnetic-drive pump (MDP), in which the impeller shaft is driven by a magnetic- coupling arrangement, and the canned-motor pump (CMP), which features a rotating magnetic field within the motor stator. The Hydraulic Institute definition is therefore valid, but unnecessarily limiting. For the purpose of this article, a sealless pump is defined as one that does not use packing or mechanical seals to isolate the process fluid.

This broader definition permits consideration of a larger group of pump designs. There is, however, no intent to imply that these other sealless-pump designs eliminate the risk of hazardous or toxic emissions. This discussion is based on a search of commercially available designs with a focus on ones whose fluid-contacting parts are made of nonmetallic materials. No implication is made that this search has been all-inclusive and that all commercial products have been reviewed.

A wide variety of nonmetallic materials are available for the construction of wetted pump components used in corrosive-fluid applications. These include thermoplastics, thermosets and elastomers. A general knowledge of the characteristics of each class of material is helpful in selecting the proper one for a particular application. Table 1 (below) summarizes the significant physical characteristics of the rigid plastics and Table 2 compares the elastomers used most often in aggressive fluid applications.

Detailed data on the relative corrosion- resistance and chemical-inertness characteristics of these plastics are readily available from many sources, including chemical engineering textbooks, published articles, product catalogs and research data from the plastics manufacturers. However, this venue of information does not substitute for experience. When seeking advice on material selection, supply the full conditions of service, and consult with your pump supplier on critical applications. Do not hesitate to ask for the performance characteristics of materials in actual installations.

Heightened awareness of environmental, safety and maintenance issues in the last few years has made process-fluid leakage a paramount concern throughout the chemical process industries (CPI). Sealless pumps are becoming the often-preferred choice for pumping hazardous and toxic fluids because of their ability to prevent or minimize leakage problems. After years of being considered expensive niche products for specialty applications, sealless pumps are finally entering the limelight.

Significant trends in pump design, with respect to the handling of corrosive fluids, point to the use of sealless configurations to minimize leakage, and the specification of nonmetallic, chemically inert materials of construction to ensure resistance to acids, caustics, salts and other aggressive fluids. To pick the optimum sealless pump for an application, one must become familiar with the technical and economic issues involved in the pump-selection process, the available pump types, and the various nonmetallic materials that are suitable for the service.

Defining a sealless pump
The Hydraulic Institute (Parsippany, N.J.) defines a sealless pump as one in which the impeller shaft is completely contained in a sealed, pressurized vessel (called the containment shell) that contains the process fluid. Leakage of the pump fluid into the surrounding environment is prevented by the...