THE VALUE OF PMI TESTING
JUST BECAUSE IT’S SPECIFIED DOESN’T MEAN IT’S TESTIFIED
In the rigid world of engineering many times we become complacent with specifications and designs. We trust that what is documented on our construction blueprints is what we actually have installed in our facility. However, this belief could not be further than the truth, and, in fact, could be considered a risky assumption.
As a failure analysis company with over 30 years of history conducting inspections and evaluations of high-temperature and high-pressure components, we have seen our share of material discrepancies. In fact, the number of cases where Thielsch Engineering have identified either improper weld filler material, or improper spool piece material would likely be alarming to the average power plant personnel. Fortunately, in many of these cases, the discrepancies were identified during routine inspections and catastrophic failures were avoided. Unfortunately, due to deregulation and the competitive nature of our world energy market today, these examinations are becoming less and less routine. Even with technical advisories being circulated by the manufacturers of various piping and turbine components warning of such discrepancies, these simple inspections are still not being performed as rigorously as they should be.
In a technical advisory issued by Siemens Westinghouse in October of 2000, urgent recommendations were made advising material verifications to be conducted at specified locations of reheat inlet piping due to a discovery of rogue piping material identified during a routine inspection. These material verifications should be conducted using either optical emission or X-Ray fluorescence spectrometer, otherwise known as PMI (positive material identification) testing. 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. PMI material inspection can be very beneficial for materials that are high-quality, such as a high alloy metal or stainless steel. A PMI test for material can also be used to determine the alloying content filler material used in welds.
In an effort to bring awareness to the industry and provide solutions for evaluation, this article will provide the details of several documented occurrences of the existence of these material discrepancies that Thielsch Engineering has been party to.
The following examples are cases where Thielsch Engineering identified either incorrect piping spool materials or incorrect weld filler materials. In some cases the improper material was identified using a PMI testing analyzer during routine outage inspections. In other cases, the piping material had already cracked and the rogue material was identified during a laboratory failure analysis, as is the scenario for our first case.