Materials

Past experience by operations provides a guide to materials specifications. It is unwise to assume that the vendor is completely knowledgeable about materials for the user's process. Stating operations' experience with materials or defining materials in the specifications, will help the vendor in his design and will prevent his having to gain experience the hard way at your expense. However, you do not have to make the specification so rigorous that you prohibit vendor experience with materials. By proper wording of the specification, you can invite comments that reveal vendor experiences in materials. Specifying a non-standard material will call the vendor's attention to a possible non-routine situation. A large number of forced outages are traced to vendor design, but they really reflect the lack of application knowledge, which could have been prevented if the purchaser communicated the specifications properly.

Some of the most obvious examples of problems with gas and materials are frequently found in refining or petrochemical applications. One is the presence of hydrogen sulfide. Austenitic stainless steel, normally a premium material, cannot be used if chlorides are present due to intergranular corrosion and subsequent cracking problems. The material choice is influenced by hardness limitations as well as operating stresses that may limit certain performance parameters.

Material and welding quality inspections, based on industry standards, are desirable. Again, API gives some guidance. Beyond this, the inspection method and acceptance criteria must be clearly stated. In preparing his bid, the vendor must understand exactly what is desired.

Bolting materials must be considered. While not strictly a material problem, bolting also includes the requirement for studs, the tapping requirements, bolt head style and many aspects, as well as plating type if allowed. In the various compressor standards, API provides guidance, but particular plant experience may add to or modify those requirements.

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