Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation
|Quoting specifications without full knowledge of what that specification really covers ... only promotes a false sense of security that eventually backfires. Take for instance spiral wound metallic gaskets (refer to "Machine Design - Case Study No. 8: Color Coding Communication Limitations ..."); these gaskets can be very dependable. There are many different specifications which cover every aspect of that gasket. In a new application where flange leaks were a problem, it was decided to go to the old dependable spiral wound gasket which was used extensively elsewhere, though in a less severe environment. The specification quoted was "API-601 with a stainless steel winding and '---' filler material". The problem here is that the specification doesn't make specifying the retaining ring obvious. Naturally, the vendor just went with the usual carbon steel outer ring. In this case the "usual" backfired, because these gaskets were exposed to a corrosive environment outside of the pipe flange, and the outer ring was being corroded away.
Using color coding can make life simple when training personnel to use the correct materials ... it can also make it hell. Take the spiral wound metallic gaskets in the example above. Every mechanic knew that yellow coded spiral wound gaskets were used up in another area. When first used down in another part of the plant the outer retaining ring corroded away and the gasket would begin to leak. The word was given to get 316 stainless steel gaskets because the "yellow ones are failing." Of course it would have helped to know what exactly was failing (refer to the first paragraph in "People - Case No. 4: Words of Wisdom ..."). Since the vendor heard "API-601 made out of 316 stainless steel", the vendor complied and supplied exactly what he was told. The new gaskets came in, and the winding which use to be 304 stainless steel (and coded yellow) was now 316 as requested (and coded green. by accepted unified standards). Since the outer ring material was never specifically addressed, the vendor supplied the default material which was carbon steel. Again the gaskets were failing (by way of the outer carbon steel retaining ring corroding away), and again no mention was given as to what exactly was failing .... just the gaskets, period. So now the word is out that even "the green ones are failing." When a proper failure analysis was done, it was overlooked that even with an upgraded outer ring made out of 316 stainless steel, the vendor would still color code the gaskets green according to accepted standards. When the gaskets were scheduled to be installed (more than 50) and they arrived on the site, the job was almost canceled because "those damn green ones leak!" Education about the infamous outer ring not being specifically addressed in the general color coding came to light, and the 50 gaskets were installed.
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