Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation



Machine Design - Case Study No. 8: Color Coding Communication Limitations for the API Standard for Spiral Wound Gaskets


Spiral Wound Metallic Gasket Color Coded

Color coding is a form of communication which must be understood by the people doing the work.

To the left are four spiral wound metallic gaskets, and all four are completely different. Yet logic would tend to make one believe that the two green gaskets are alike, and likewise, the two yellow gaskets are alike. This color coding conforms to the industry standard as adopted by the Fluid Sealing Association, Metallic Gasket Division. The critical information missing here, and which unfortunately is sometimes taken for granted, is the material of construction of the outer ring. For each pair shown on the left, there is one which has a carbon steel outer ring, and one which as a stainless steel outer ring. If the material of construction for the outer ring is critical, as was in this case, the color coding falls short.

The predominant colors green and yellow indicate the winding material, and the pink stripe indicates the filler material which goes between the windings. The two green ones have a winding made out of 316 stainless steel, whereas, the two yellow ones have a winding made out of 304 stainless steel. The filler (pink stripe) is identical for all four. The information regarding the material of construction of the outer ring is usually stamped on the outer ring itself, if you know what you are looking for.

The figure at the lower left shows a full view of a spiral wound gasket. The winding and filler material are on the inner portion of the gasket. The outer ring centers the spiral windings when it contacts the flange bolts (see figure to the left); it also serves as a radial support for the windings themselves.

This failure involved two problems: 1) The actual failure mode was corrosion of the carbon steel outer ring (see lower right figure), and 2) Lack of understanding regarding the limits of the color coding (refer to "People - Case Study No. 10: Color Coding Communication Limitations ...")

Looking at the lower right hand figure, when the carbon steel outer ring corroded away, the winding lost its radial support. The only thing holding the windings together was a small spot weld at the very end of the winding. When this spot weld failed, the winding essentially came undone like a spring. The unwinding action is exaggerated here for illustrative purposes.

Spiral Wound Metallic Gasket Full View Corroded Spiral Wound Gasket

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