The API 609 deifinition reads as,"The term trim refers to shafts, seating surfaces on the body and disc, and internal keys, pins, screws and bushings in contact with the contained fluid are also considered trim".Why is the disc not treated as a trim part? Why is it the seating surface alone of the disc is treated as the trim? This one is about a butterfly valve intended to be used in SOUR service. The specification calls for Inconel 625 trim. The body and disc are of ASTM A216 WCB casting. An acceptble material that may be used in accordance with NACE MR 0175.What would the use of Inconel impart in this case. Will the disc have to be of Inconel, either solid or overlayed fully. If that is the case then even the body internal will need to be overlayed.The additional material attributes required for the materials used for the trim components must be also from erosion and wear, in addition to corrosion. In this context will it be enough to have only the seating surfaces of Inconel as it says in the API 609. A butterfly valve with an unprotected (bare) WCB disc will rust and leak at the sealing surface. You can sometimes get away with not coating the body as the sealing surface may be isolated from the media.Inconel is applied by welding. It typically takes a few layers of weld before the top layer is mostly the corrosion resistant material. The first weld layer will be a mix of WCB and Inconel.In this case, you need to challenge the speficiation and question what they are expecting. Otherwise, if you just want to coat the OD of the disc, make sure your quote clearly identifies what you are planning to do.

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