44 maintworld 4/2014

SOMETIMES ONLY the trade name (grade) of a metal is required, but sometimes the concentration of important elements like carbon is also needed. The main technologies used for PMI are XRF and OES. Detailed technical discussion of PMI is beyond the scope of this article. We will however discuss the pros and cons of the various techniques.

The two main reasons to use PMI are global sourcing of material and the quality assurance requirements for pro-ducts of most customers. The sourcing of components and subassemblies is very global-driven by the need for cost reduction. A typical example is a cast component. The buyer wants to make sure the cast component (imported from overseas) is manufactured from the ma-terial specified in the attached material certificate. There are, unfortunately three possibilities here: the certificate tells the true story, the certificate tells the true story of a different cast compo-nent, or the certificate has been “further processed”.

Experience shows that the more glo-bal sourcing a fabricator uses, the more problems they are likely to experience due to materials getting mixed-up. It is also common that the material verifica-tion is taken seriously only after an inci-dent has already occurred. PMI done on a purchased component before it is used in the production can save a lot of money and production capacity.

The Traceable Quality Assurance of fabricators’ products is required more

and more by their customers and autho-rities, especially for critical components like pressurized vessels (production or repair). Local regulations stipulate certain testing and documentation of the materials used. There are a growing number of industries where material mix-up cannot be tolerated at all with classical examples bring petrochemical and nuclear plants.

For global petrochemical companies it is practically impossible to accept components or process equipment for critical parts of the process unless PMI has been done. PMI is carried out many times, along with Ultrasonic testing of the welding seams. Sometimes only 10 percent of the products are tested but in critical processes 100 percent PMI is required. This means that every pipe, valve, flange, bend, bolt, nut and welding seam is verified and verification is docu-mented. In certain cases the verification of the alloy grade is enough; in other cases however, a full analysis of the main alloying elements is required.

One of the trends during the last de-cade has been the PMI of old refineries and chemical plants. In plants built be-fore 1980, PMI was seldom done during

construction. Consequently, this means that many of these plants are currently having PMI carried out - while plant is in full operation. This creates specific challenges for the PMI because the me-asurements have to be made on surfaces that are operating at several hundred de-grees. Normally only XRF can be applied in these cases.

Depending on the policy of the com-pany, PMI can be done by the company itself or it can be contracted to an out-side inspection company. The trend is to outsource these activities, but this all depends on how frequently and urgently PMI is needed.

The Standards to be FollowedIt is worth mentioning here that even though Foundries and Metal manufactu-ring plants use XRF and OES in their la-boratories as standard instruments, this is not normally regarded as belonging to the PMI area; PMI is for confirmation and (big) laboratory analysers are used for verification.

Most of the NDT methods used in the industry are standardized. In PMI no real standards have been created. Ho-wever, one of the front-runners in PMI,

Esko KantonenDirector, Sales,

Asia - Pacific, Bruker Elemental, Esko.

Kantonen @bruker-axs.se

John PattersonPrincipal, Portable

Analytical Technologies LLC,

jihpatterson @gmail.com

PMI as Quality Assurance Tool for ProductionPMI (Positive Material Identification) means verifying mainly metallic material based on the chemical composition. PMI is used mainly when field measurements are needed and in the laboratory people still speak about complete elemental analysis. A new method called LIBS is also briefly discussed in this article.

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46 maintworld 4/2014

American Petrochemical Institute, has created their own recommended practi-ce API (American Petroleum Institute) RP – 578 (2nd edition). It is not an actual standard but a recommended practice and is a widely used comprehensive do-cumentation on the subject.

Of course the biggest Petroche-mical companies (Exxon, BP, Indian engineers) have their own internal recommended practices as well. These standards tend to specify equipment for PMI on a performance basis rather than specifying a certain brand to be used.

Portable EquipmentThe first PMI units (portable elemental analysers with software able to tell the alloy grade name) came in the early 1980s. The units were quite bulky; the weight was close to 10 kg and used radio-active isotopes, which limited their use in some locations. Also the analysis of light alloying elements was impossible or quite clumsy. But despite this, they were portable and could analyse a large object without the need to take a sample, thus

destroying the component.The real step forward in portability

and usability came when the miniatu-rized X-ray tubes and solid-state X-ray detectors came to the market. This allo-wed handheld XRF (HHXRF) analysers to be built as compact pistol-like units. The next step forward happened some 6 years ago when detector technology allo-wed the convenient analysis of light (Mg Al Si, S, P) alloying elements. This made the analysis of many Ti alloys and Al/Si bronzes possible, and also helped to ex-pand the use of the technology to alumi-nium alloys, especially in the aerospace industry. At the same time the software has developed so that the documentati-

on of the measurements is done more or less automatically in tamper-proof files. This also greatly improved the rugged-ness of the analyser.

Mobile OES & LIBS AnalysersThe role of mobile OES technology is much smaller than that of portable XRF, but the real benefit of OES (compared to XRF) is that low levels of carbon can be detected and separated. In essence this means that low carbon stainless steels can be separated from standard grade (304/304L, 316/316L). The actual use of OES analysers is more demanding under field conditions, but in return higher accuracy can be achieved for some ele-ments.

In the past year LIBS (Laser Induced Breakdown Spectroscopy) has been of-fered as a handheld instrument for PMI analysis. This technique offers relatively fast analysis when light elements are required to determine the grade name. While the technique offers relatively simple operation, the analytical per-formance has not yet been proven. In

The material verification is taken seriously only after an incident has already occurred.

Typical user application is the receipt inspection of material. Analyser shows the full analysis of the main alloying elements.

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addition, LIBS potentially offers the ana-lysis of very light elements like carbon and possibly beryllium. However, these elements require special consideration and may not be analysed without a shield gas. Prior to using this technique the user should analyse known samples to determine that the analyser is capable of providing the necessary precision and accuracy.

PMI ApplicationsIt is worth mentioning that coatings can be measured with an HHXRF unit as well. The coating thickness (normally in the range of one to several tens of micro-

meters) can be measured as well as the concentration of different elements in a specific coating depending on the ca-libration. The limitation is that the ana-lytical task gets a bit difficult if the same element is in the coating and in the base material, or if the coating is too thick.

Some of the more recent applications of HHXRF include FAC (Flow Accelera-ted Corrosion) and Sulfidation (Sulfidic) Corrosion Failure analysis (as defined in API 939C). The FAC requires the analy-sis of 500 ppm Cr levels and the Sulfidic Corrosion analysis requires analysis of 1000 ppm levels of Si in steels. Both me-asurements are possible with the latest generation analysers.

Thus, it appears that more and more PMI analysis will be required in the futu-re as new materials are being developed. The awareness of the unexpected costs caused by material mix-up is encoura-ging fabricators and users to pay close attention to the material being used in all applications and to have the ability to prevent mix-ups from happening. n

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Typical mobile OES analyser.

There are a growing number of industries where material mix-up cannot be tolerated at all.

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Integrate our license free DMS and Spectralyser software easily with your existing CMMS!

44-47_MaintWorld_4-2014_PMI.indd 47 14.11.2014 11.32