By Matt Tones, Lou Mattina & Jim Drago, P.E. Garlock Sealing Technologies
The nuclear power industry uses elastomers, or rubber, for many sealing applications such as airlock doors, hatches, pool gates, drywell heads, vibration dampening and others. Made from long-chain polymers from which they derive their names, elastomers also contain other materials and various additives to promote curing and desired properties. Among the most common elastomer materials are nitrile butadiene rubber (NBR), polychloroprene (Neoprene) and fluoroelastomer (FKM, Viton).
Most users have three principal criteria for specifying elastomers—polymer type, price and hardness. However, selecting an elastomer seal without knowing and understanding the terminology of the manufacturer can lead to the wrong type of material for a particular application. To prevent this from occurring, first define the application in terms of the material’s compatibility with the media being sealed, the required grade and compressibility. A simple acronym, TAMPSS, can be helpful in identifying the pertinent information (see Table 1).
Finished elastomer products are a combination of various ingredients even though they are identified by the primary polymer used in the compound. In and of themselves, elastomeric polymers are not suitable for industrial applications, but they are necessary to make finished products elastic. Table 2 lists common elastomers and the typical services for which they are used.
Figures 1 and 2 show the comparative resistance of these elastomers to temperature and gamma radiation.
An elastomer compound is basically a recipe for blending the ingredients it contains. In addition to the primary polymer, these compounds typically include inorganic clay and carbon black fillers, pigments, plasticizers and processing aids that produce the requisite chemical reactions to yield a useable material. Blended into a uniform, uncured mixture, these ingredients stabilize the finished elastomer for conformance to specifications for hardness, tensile and tear strength, elasticity, compression and creep. This “green” mixture is heated, milled, calendared, extruded or molded into sheet form or functional shapes.
Elastomers are generally classified into three grades: utility, commercial and premium. Because there are no industry standards with regard to formulation, products can vary significantly from one manufacturer to another. It should be noted that a product’s chemical resistance is largely determined by the polymer content of the material. Figure 3 illustrates how the polymer content, in this case nitrile, can affect the material’s fuel and oil resistance.
Notwithstanding these variations, you can still assure you are getting the elastomer you thought you were. First and foremost, have an open discussion with your supplier regarding the details of the application, including intended service, desired performance and viable materials. Then supplement this discussion by researching the relevant decision-support data.
Despite its somewhat misleading title, ASTM D2000-08, “Standard Classification System for Rubber Products in Automotive Applications” is applicable to both the processing and power generation industries.
From the scope of ASTM D2000: “This classification system is based on the premise that the properties of all rubber products can be arranged into characteristic material designations. These designations are determined by types, based on resistance to heat aging, and classes, based on resistance to swelling in oil. Basic levels are thus established which, together with values describing additional requirements, permit complete description of the quality of all elastomeric materials.”
The purpose of ASTM D2000 is to “provide guidance to the engineer in the selection of practical, commercially available rubber materials, and further to provide a method for specifying these materials by the use of a simple ‘line call-out’ designation.”
ASTM D2000 line call-outs for elastomers are like the vehicle identification number on a car. Alpha-numeric designations divulge information about the product, including up to 23 categories by material type and performance properties. Figure 4 shows the overall organization of the line call-outs.
The first six positions in the numbering scheme provide basic information. The first indicates whether the data is expressed in Metric SI or English units. The second identifies the material grade, important in the overall cipher of the code since it establishes the level of performance the material must meet.
The third and fourth positions assign a letter pair indicating the material’s heat resistance and swell-in-oil respectively, both of which are critical to the specification. The material’s ability to maintain its properties under these conditions will dictate the need for a premium, commercial or utility grade product. These letter pairs direct the user to the tables that comprise over 80 percent of the content of the ASTM D2000 standard. The tabulations are contained in “Table 6 – Basic and Supplementary (Suffix) Requirements for Classification of Elastomeric Materials.” A dedicated table for each letter pair—for example AA, BC, BK, CH and so on—provides the properties of the material.
The fifth position designates the hardness expressed as Shore A- Scale points, and the sixth position, tensile strength. Supplementing this basic line-call-out information are upwards of 17 different categories of properties such as heat, fluid, tear and abrasion resistance. These properties are designated with alpha-numeric codes ciphered in Table 6. Figure 4 contains the full list.
Using the Line Call-out
Here are a few rules of thumb: 1. The longer the call-out, the higher the quality of the material (a long number indicates extensive specification and testing). 2. Low tensile strength indicates low polymer and high filler content. 3. High elasticity indicates high polymer content, lower filler content and higher tensile strength.
The alpha pair of positions three and four indicate polymer type (see Figure 4). Users rarely develop their own line call-outs, but rather work with their suppliers and the TAMPSS guidelines to select a suitable material for their application.
Once the selection is made, the manufacturer supplies the D2000 line call-out specifying the material generically and excluding sub-standard materials. Specifying tensile strength alone, for example, will eliminate low-polymer-content, utility-grade materials.
Specifying oil swell will eliminate grades containing myriad unnamed and incompatible polymers. Special resistance to specific media can be added as a special “Z” requirement (Figure 4).
The properties of elastomer sheet gaskets are not always expressed in the same terms as those for compressed fiber and PTFE sheet gaskets. Table 3 shows the corresponding properties for elastomer sheet gaskets.
Media compatibility is a key determinant of an elastomer’s suitability for an application. There are a number of independent sources providing information on the chemical compatibility of elastomers. Manufacturers offer tabulated information with “acceptable-depends-unacceptable” performance rankings. Unless stated, these do not take into account elevated temperatures which can exacerbate the effects of a chemical. Nor do these tables reflect the application. For example an elastomer used to line a vessel has far more exposure to service conditions than a rubber gasket in a flange. The best approach to selecting the optimal elastomer for a given application is to refer to the published literature and consult with the manufacturer’s application engineers. Table 4 shows independent sources of information on the chemical compatibility of elastomers.
Given manufacturers’ substantial investments in proprietary formulations, it is highly unlikely consistent, industry-wide standards for elastomers will ever be developed, posing a quandary for users seeking suitable materials for their applications.
Not all elastomer compounds are created equal, even if they share the same generic nomenclature. Indeed there can be significant variations within the same types and grades of elastomers from different suppliers. Therefore specifying elastomer seals for nuclear plant applications calls for both qualitative and quantitative due diligence.
Qualitatively, it is important to establish the kind of relationships with suppliers that are conducive to collaboration in the process. Quantitatively it calls for thorough review of the available information on elastomers and their properties. Combining these approaches will greatly reduce the risk of using the wrong material for an application, and that alone can have far-reaching implications for a nuclear facility.
Authors: Matt Tones is senior manager of applications and engineering services for Garlock Sealing Technologies. Lou Mattina is manager of materials engineering. Jim Drago, P.E., is manager of business development. Viton is a registered trademark of DuPont Performance Polymers.
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