How do you choose between all the various TYPES of gasket materials

Nov05PUMPS&SYSp54-55

10/16/05

12:12 PM

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From the voice of the fluid sealing industry

SEALING SENSE Q. How do you choose between all the various TYPES of gasket materials? A. Many variables affect the selection of a gasket for a pump or piping system. One is the gasket design itself; another is the type of material or materials used in the specified design. Gaskets are produced from a wide array of materials including rubber (elastomers), compressed fiber (CNA), PTFE, flexible graphite and metal. Metal can also be combined with one of the above non-metallic materials to reinforce the gasket. Each material, or combination of materials, offers specific properties essential to effective sealing under a given set of operating conditions. These include temperature, pressure, and nature of the fluid being sealed. The operating conditions dictate which material will provide the best service in the application. So how do you choose which gasket is best for the pump or piping system?

Flange Design Bolting and flange designs vary from pump manufacturer to pump model and have a significant impact on the performance of the chosen seal design. The quantity, size, grade, lubrication and spacing of fasteners used to apply load to the gasket should be considered when selecting gasket materials and designs. Thin, less rigid flanges and/or flanges designed with minimal bolting might yield improved performance with a non-metallic or non-reinforced gasket due to its better conformance under low load. On the other hand, tongue-and-groove flanges with heavy bolts can apply very high gasket stresses, potentially damaging soft gasket materials that are not designed for extreme loads. Minimum, Effective, and Maximum Stress calculations should be performed to determine the gasket suitability for the application. Gasket manufacturers can provide information regarding these values for a specific material to aid in the decision-making process.

Pressure and Temperature OEM Design Compatibility There are conditions under which many of the gasket types listed above will seal adequately, but selection is restricted to only one or two because of the pump or piping design. The first factor to consider is the type of seal installed by the pump manufacturer. A pump designed with an O-ring groove limits the design and choice of materials that can be used. For example, it is very difficult to switch from an elastomeric O-ring to another material. O-ring flange designs typically use a deep groove, a surface that does not work well with flat sheet gasket materials. Raised face and full-face flanges are much more universal, allowing the user to apply materials and seal designs that yield specific benefits in the application. The finish applied to the sealing surface by the manufacturer, as well as field conditions, should also help determine the selection of materials and designs. A coarse or rough surface finish (above 250 RMS) requires the use of a more conformable, non-re

inforced material to fill flange voids and imperfections. More rigid or reinforced materials can be utilized in conjunction with smoother flange finishes. In any case, the original OEM design should be considered as the baseline for duplication or improvement. Pumps and piping systems can operate at a very high temperature and/or pressure. These conditions eliminate some of the choices. While specific chemical resistance advantages and improved conformance to surface irregularities can be realized with some products, introducing elevated temperatures will increase creep and creep relaxation of these materials. Metal gaskets or reinforcements such as expanded steel, perforated or“tang,” and corrugated core can offer more stability at elevated temperatures, pressures and thermal cycling. Elevated temperatures and increased pressures have individual degrading effects on sealing materials. These negative effects increase exponentially when both conditions are present simultaneously. Most gasket manufacturer recommendations are based on collective consideration of all the variables. However, those who specify should use these recommendations as guidelines and make decisions based on experience and safety factor requirements.

Chemical Resistance The nature of the fluid being sealed is another important factor in gasket selection whether gas or liquid. The gasket material must be chemically resistant to the medium to be sealed.

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NOVEMBER 2005

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