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Last updateFri, 31 Oct 2014 11am

PMI Analysis

materials_q_and_a_graphicQ: Why does the valve supplier refuse to accept my claim that its material is out of specification based on my PMI analysis?

A: Positive material identification (PMI), or alloy verification, and certified material test reports (CMTR) are common requests by customers to help assure they are receiving the proper materials in their valves and other chemical process equipment they purchase. However, it is important to understand the differences in these terms and the capabilities of the equipment used to provide this information.

PMI is usually provided by relatively inexpensive portable X-ray analyzers manufactured by a number of different companies. These instruments provide quick and easy verification of alloy identity for just about any size part. Sometimes customers will request that the supplier run such a test to verify they are supplying the correct alloys in their products and sometimes customers will conduct their own tests on equipment they receive. However, some users of these instruments expect more from them than they are actually capable of doing. Some of the instruments simply produce a readout indicating what alloy the part is, e.g. 304 or 316 stainless steel. However, some of the more sophisticated instruments will provide an actual readout of the part's composition, and this is where problems arise. Some users have the belief that the readouts from these instruments can be used to verify a supplier's CMTR or confirm whether the part meets the ASTM limits for the alloy.

Although some of these portable instruments do provide elemental analysis, their accuracy is certainly nowhere close to that of spectrographic instruments used in foundries or steel mills that cost hundreds of thousands of dollars and that are used to generate the CMTRs. For example, one portable analyzer supplier states that its instrument can analyze nickel in a stainless steel with a one-sigma precision of + 0.35%. Or in other words, the portable analyzer's result is only within this range 68% of the time.

To improve to 95% confidence (two sigma) the precision would be + 0.70%. For 99.7% confidence (three sigma), the precision would be + 1.05%. To look at it another way, if the portable analyzer produces a nickel reading of 9.5% for a 304 stainless steel, one can be 68% certain that the actual nickel content is between 9.15% and 9.85%, 95% confident that the actual nickel content is between 8.8% and 10.2%, and 99.7% confident that the actual nickel content is between 8.45% and 10.55%. This is hardly accurate enough to justify questioning a CMTR analysis. Some of the instruments are capable of running analyses at higher precisions, basically by running the analyses for longer times to collect more data. However, users typically do not use these higher-precision modes because it takes longer. Even when these higher-precision modes are used, the results are still not as accurate as the methods used to produce CMTRs.

In addition to reduced accuracy, PMI analyzers also are not capable of analyzing all elements of consequence in the alloys used in the process industries. For example, they do not provide information on carbon, nitrogen, phosphorus, sulfur, or silicon. This means they cannot verify the carbon content in carbon and alloy steels, nor can they distinguish between standard and low-carbon grades of stainless steel. They cannot verify the nitrogen content in many of the newer stainless steels, which are nitrogen-alloyed for increased strength and resistance to chloride pitting. They cannot verify that silicon, sulfur, and phosphorus contents have been met.

Therefore, if a customer wants all components in a valve certified to an ASTM-grade chemistry, this can only come from the CMTR analysis, which usually requires the valve be processed as a special order to ensure traceability. Despite what manufacturers of portable analyzers may claim and what customers may believe, these instruments can only provide alloy verification and cannot be used to check compliance to a CMTR or an ASTM specification.

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