While chemical inertness is the major reason for specifying a plastic pump, users can take advantage of other benefits as well
Reprinted from PUMPS AND SYSTEMS By Ken Comerford
A variety of nonmetallic materials are available to meet your needs. Not so long ago system designers would consider the use of plastic pumps only if available metal pumps were completely unsuitable. Plant engineers had learned to live with costly downtime and excessive maintenance due to corrosion. Extensive research activities concentrated on published corrosion rates for a long list of metals and exotic alloys in an endless array of chemical solutions at varying temperatures. Specific metal selection for a given application was left to the user’s discretion, based on what were considered to be “allowable” rates of attack.
Unlike metals, with their anticipated corrosion rates in mils penetration per year, plastics are either inert to or not suitable for a given fluid. Selection, therefore, tends to be less complicated. In a recent study of plant engineers and consultants who indicated they were using or specifying plastic pumps, the number one reason given for their selection of a specific pump was superior corrosion resistance.
Although there are many other reasons for specifying plastic pumps, chemical inertness is the major one. No other reason comes close. The benefits that accrue because of this resistance explains why plastic pumps are being used today in almost every facility handling corrosive, hazardous, toxic, and other aggressive liquids, and slurries.
CURRENT DESIGN PARAMETERS
Today plastic pumps are being produced for flows in excess of 2,000 gpm, with heads to 280 ft, and for use at temperatures from below freezing to 275° F. Recent developments have made it possible to incorporate many design features directly related to the unique properties of plastics into pumps that conform to ANSI specification B71.3 for horizontal process pumps.
With the successful introduction of these pumps, it was only natural that vertical centrifugal designs would follow. This is particularly significant in light of the need for deep sumps in the wastewater field. The advantage of lightweight, chemically inert plastics has stimulated a host of creative engineering approaches, and these sump pumps are now available in lengths up to 20 ft.
ADVANTAGES OF PLASTIC PUMPS
Although the main advantage of plastic pumps is corrosion control, there are many other benefits in applications that do not exceed their temperature and pressure limitations. • extended service life • superior abrasion resistance • freedom from contamination • low maintenance • broader range of usefulness • low weight saves money • low cost
PLASTIC MATERIAL OPTIONS
Although the materials handbooks list hundreds of nonmetallic formulations with varying properties, for pump users the choice for most applications narrows down to eight rigid plastics and eight elastomeric materials. Choosing a specific material for use in an application begins with a review of data published in handbooks and “corrosion” tables. However, experience with actual conditions is important. For this reason, we strongly recommend that pump specifiers consult with manufacturers before recommending nonmetallic materials for a new application. There is literally no substitute for experience when it comes to material selection.
Take the basic consideration that limits the use of plastic pumps — operating temperature. Although handbooks and specification sheets prepared by material suppliers may show suitability of some plastics at temperatures to 500° F, plastic pump manufacturers will generally not recommend their product for continuous duty at temperatures above 275-300°F. The dynamics of pump operation, which include turbulence, start-stop, wet-dry, abrasive particulates, and similar situations, suggest that textbook laboratory test data be tempered by actual experience.
The rigid plastics most widely used for pump construction are broadly divided into two groups — thermoplastics and thermosets.
Thermoplastics have linear molecular chains that flow over each other and separate when heated, then solidify into predetermined shapes upon cooling. Reheating permits reforming without significant change in properties. Thermosets, when heated, form permanent crosslinks between linear chains, creating a rigid structure that cannot flow again. Some thermosets are molded from liquid components that react to certain chemicals at room temperature to create tightly crosslinked chain structures. In either case, once the reaction is complete the plastic cannot be reformed or remelted.
Thermoplastic materials are the group most widely used for corrosion and abrasion resistance. Because they are homogeneous in structure, they offer greater resistance to a broad range of aggressive solutions. They also provide greater purity and can be used with ultrapure water, pharmaceuticals, and foods.