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No matter what design solutions engineers and construction contractors devise to contend with the growing number of water and air pollution regulations, they can't do very much without pumps. Gravity sometimes may play a role in water and wastewater systems, but pumps are without a doubt the prime movers in the business of fluid handling. And as water pollution control technology continues to develop to meet the needs, it is becoming more evident that non-metallic pumps are particularly suitable for many applications in the field.
Unfortunately, the quantity of published information covering the design features, materials of construction, and applied experience of non-metallic pumps used for water pollution control purposes is meager compared with what is available in published form on metallic pumps and other equipment. Plastic pumps have been around for decades (our company developed thermoplastic pumps in 1950), but the bulk of experience with them has been acquired in the chemical and other process industries, where they have been applied in numerous cases for handling corrosive and hazardous fluids.
As a result of government regulations affecting water quality and waste disposal, municipal facilities now are seen as chemical operations requiring the same degree of knowledge and sophistication associated with manufacturing and processing plants. With this in mind, a review of significant information on plastic pumps, and their design and application, should be of interest and value. Three important aspects are covered.
An additional area of increasing importance to engineers and managers in the water/wastewater field is the control of undesirable atmospheric emissions. The incorporation of thermoplastic pumps and other components in the design of scrubbing and odor control systems has solved a number of problems, and will be the subject of a future article in WEM.
Materials of Construction
Let's look first at materials of construction. Several years ago a study aimed at knowledgeable engineers and others involved with wastewater treatment revealed that a large majority had used non-metallic pumps for some purpose. But generally they were unaware of the range of potential applications for them, and the specific reasons for selecting non-metallic rather than metallic pumps.
Corrosion resistance was identified as the major attribute of non-metallic pumps. But their abrasion resistance and ability to avoid metallic contamination of the product they are pumping, thus preserving its purity, also are important characteristics. Of even greater significance is the inert chemical nature of the thermoplastics, which are suitable for use over the full pH range. This property simplifies the choice of the specific material, and extends the usefulness and service life of a given pump in a variety of applications.
Several other attributes of plastic pumps, in addition to their being chemically inert and resistant to abrasion, are worth taking into account. For instance, they are light, being 25 to 50 percent the weight of the metallic items they can replace. Since the plastic parts will not rust or seize, they are easy to service, and their initial cost is lower than pumps fabricated with exotic alloys.
The plastic materials of construction for rigid wet end components such as casings and impellers, which received the most mentions in the study were
Clearly, choosing a material of construction for specialized water or wastewater treatment pumping applications should be based on checking the corrosion resistance of the material in terms of the fluids to be handled and the anticipated temperatures. In many cases, a pump manufacturer may have experience in dealing with identical or similar service conditions. When it comes to handling waste streams containing unknown or varying chemicals and concentrations, engineering and operating personnel can be confident in the ability of thermoplastic pumps to be up to the task because the wetted parts are so chemically inert.
Spotlight on Pump Design
Some so-called plastic pumps on the market are misnamed. They are basically metallic pumps with non-metallic casings and impellers. For best results, plastic pumps should be designed to take maximum advantage of the unique properties of the plastic material. If the application requires non-metallic parts to be in contact with the fluids being pumped, the following points are important:
Non-Metallic Pump/Tank Systems
The traditional below-grade concrete sump with a mounted pump of some style is no longer the best way to contain and deal with hazardous liquid wastes. Regulations now require these to be lined with corrosion-resistant coatings to prevent chemicals, oils and other materials from leaching into adjacent groundwater. But concrete sumps are difficult to seal completely, and keep sealed. Coatings are typically 125 mils thick since sump service is considered immersion service, and anything less might not last long and also be inadequate. Regular inspections are necessary to ensure continued integrity of the coating. When chemicals penetrate it through constant immersion, patching is possible but difficult, and seldom acceptable. In many cases the coating must be completely stripped off and the concrete surfaces recoated.
Packaged non-metallic pump-tank units now available often can provide economical solutions to some of the problems described above. These are standard or customer-engineered self-contained tanks containing not only pumps, but level controls, control panels and related piping as well. Wetted parts machined or fabricated from a number of thermoplastic or thermosetting materials, for instance the five families of plastic compounds discussed earlier, make the systems suitable for handling a broad range of corrosive or otherwise hazardous materials up to temperatures of 275 F. Installation usually involves only electrical and influent/effluent connections. In most cases they are free-standing and require only a concrete pad, but some have been installed in existing concrete basins.
To sum up, plastic pumps and plastic sump systems have established a dependable service record over the last few decades that points to their suitability for many fluid handling duties. They are particularly capable where corrosive and hazardous liquid wastes or chemicals have to be contained and pumped. As a result they are being applied increasingly for such tasks in the water/wastewater field. Their development continues.
A thermoplastic resin will repeatedly soften when heated and harden when cooled. Decomposition occurs only at higher temperatures.
A thermosetting resin cannot be melted or remolded without changing its chemical structure.