Flange surface roughness can affect the sealing quality of non-metallic and semi-metallic gaskets

The finish of the flange surfaces onto which a gasket is installed can greatly affect the gasket’s performance. Different gasket materials can interact in different ways with flange surfaces, depending upon how they are finished. Paying attention to flange surfaces before installing a gasket can greatly improve the gasket’s performance and extend its life.

Although non-metallic flanges, such as those made from reinforced plastic or other materials, are encountered in some applications, metallic flanges are by far the most common. Metallic flange surfaces can range in type from rough castings to a mirror-like smoothness produced by a lapped, or cold water, finish. What is important to note is that the level of friction that exists between the flange and gasket surfaces is a key determinant of seal quality. The degree of roughness of the flange surfaces can greatly affect gasket creep and relaxation and blowout resistance.

At Canada Rubber Group Inc. (CRGI), we supply two broad categories of gaskets: non-metallic and semi-metallic. Our non-metallic gaskets are fabricated from sheet rubber compounds that are especially formulated to service a wide range of applications. In contrast, our semi-metallic gaskets are produced by combining rubber compounds with a metal core. Semi-metallic gaskets are often referred to as spiral wound gaskets.

Non-metallic gaskets made from sheet rubber are more forgiving to flange misalignment. However, these types of gaskets are also more susceptible to creep relaxation and movement under load. In particular, flange surfaces which are unduly smooth may fail to create the friction needed to prevent creep relaxation and movement of the gasket when under load. This can lead to a loss of joint bolt tightness and leakage of the gasket.

To prevent creep relaxation and leakage of non-metallic gaskets, it is preferable to have a rougher flange surface that will “bite” into the gasket. For most types of non-metallic gaskets, flange surfaces should typically be finished to a range of 200 – 500 µin.

Semi-metallic gaskets, such as spiral wound gaskets made from PTFE and flexible graphite compounds, are particularly well-suited for severe service applications. These types of semi-metallic gaskets exhibit good conformability to flange surfaces. For spiral wound gaskets, flange surfaces should typically be finished to a range of 125 – 250 µin.

It should be noted that some applications require conformity to the ANSI/ASME B 16.5 standard. This standard covers pressure-temperature ratings, materials, dimensions, tolerances, marking, testing, and methods of designating openings for pipe flanges and flanged fittings. The ASME B16.5 standard also requires that flange faces (raised face and flat face) have a specific roughness to ensure that these surfaces be compatible with the gasket and provide a high quality seal.

CRGI supplies a broad portfolio of non-metallic and semi-metallic gaskets that service a wide variety of applications. Our sales specialists will be pleased to speak with you about your specific application needs. To find out more about the non-metallic and semi-metallic gaskets we can supply, please contact our sales department at crg@canadarubbergroup.com.

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