What Is PMI? How to Do It?

An Introduction to Positive Material Identification Analysis www.bdpipefitting.com

PMI (Positive Material Identification) testing is the analysis of materials to determine the chemical composition of a metal or alloy at particular (usually multiple) steps of alloy manufacturing or in-process alloy installation. Knowing the exact composition and grade of an alloy enables suppliers, plant workers, and other responsible parties in the chain of custody of components to match alloy specifications that are chosen for their specific properties such as heat resistance, corrosion resistance, durability, etc. Having the right alloy in the right place is essential in places like petroleum refineries and chemical plants, because the right alloy with the right properties is often all that stands between a safe, efficient operation and lost time and revenue.

Stainless steel grades differ by chemical composition. The key differences between the various grades are mainly the amount of Fe (iron), Cr (chromium), Ni (nickel), Mo (molybdenum), and Cu (copper). Some grades are very different from each other and have different properties; for example, 400 series stainless steels are magnetic while 300 series stainless steels are not. Other grades are more similar with very similar properties, and it is much harder to tell the difference between them without the aid of technology. For example, 304SS has 18-20% Cr and 8-10.5% Ni, whereas 316SS has 16-18% Cr and 10-14% Ni. Although these alloys are very close in composition, they respond differently to corrosive chemicals and higher temperatures making it necessary to be certain you are using the correct alloy for the job intended.

Currently most alloy manufacturers, petroleum refineries, and other businesses responsible for alloy grade verification use technology to quickly and easily separate various grades of stainless steel. The most common, portable, and easy-to-use tool for this purpose is a handheld XRF (x-ray fluorescence) analyzer. These instruments are highly accurate at determining the chemical composition of alloys, and thereby their grade. Moreover, they do so within a matter of 5 seconds or less for most stainless steel grades, without significant sample preparation, and safely. They pay for themselves in terms of reducing materials mix-ups and getting the best price for scrap identification, and they do not require any advanced training to operate. An operator can be trained to use one within a few hours.

The need for PMI testing in the Oil and Gas and Refining Industries came to light as early as 1992 when OSHA determined it was necessary to enforce PSM or Process Safety Management of Highly Hazardous Chemicals (HHC)to help avoid accidents, injuries and death. Recognizing the need for guidance for increased safety against these HHC releases, OSHA created an instructional document in 2007 describing specific protocol for implementing a National Emphasis Program (NEP) in the hopes of reducing or eliminating these types of hazards in the work place at petroleum refineries and other chemical processing plants.

Due to the inherent danger in analyzing materials that are in process, it is necessary to use instrumentation that is non-destructive. This testing qualifies the analysis as non-destructive testing, or NDT. Non-destructive testing (NDT) indicates that the material is not altered in any way and is essentially in the exact same condition it was prior to testing with no visible marks or structural damage left from the analysis. The instrument of choice for non-destructive testing where carbon levels are not needed for evaluation is handheld XRF.