Positive Material Identification (PMI)

Accidents like fires and explosions at petrochemical plants, oil refineries and similar installations receive significant attention, especially when there is personal injury or loss of life. In addition to the human cost, the financial losses to insurers and operators can run to millions of dollars. These incidents are often traced to the use of valves, piping, and similar components made of improper materials.

Sometimes, the presence or absence of a specific alloying element in a steel component can be very crucial to its performance, but this cannot be possibly detected through a physical inspection of the item. Over the last two decades, process and equipment supply industries have been using the Positive Material Identification (PMI) technique to deal with this issue. However, latest spectroscopy-based analyzers provide a suitable solution to PMI in the challenging process plant environment.

High Temperature Hydrogen Attack

Hydrogen gas is often encountered in chemical and petroleum processes, and at high pressures and temperatures can cause High Temperature Hydrogen Attack (HTHA) in steel components. If this is not detected over time, it can possibly lead to component failure, fire, explosion, etc.

At high temperatures, hydrogen atoms can easily diffuse into carbon steels, forming methane gas. This gas can build up at grain boundaries and weaken the steel, leading to fissures, cracks and eventually to component failure. However, HTHA can be effectively prevented by using steels that contain alloy elements such as chromium.

Flow Accelerated Corrosion

General Corrosion (flow influenced)

Figure 1. General Corrosion (flow influenced)

Corrosion by Acidic Production Chemical (Scale inhibitor)

Figure 2. Corrosion by Acidic Production Chemical (Scale inhibitor)

Although ultra-pure water is not particularly corrosive, it can lead to Flow Accelerated Corrosion (FAC) under certain conditions. FAC has been responsible for a number of major accidents in the nuclear power industry.

Factors that promote FAC are very complex, but when hot steam/water with a low oxygen content flows along a carbon steel pipe, a layer of iron oxide that forms on the surface is dissolved. Over time, the metal is steadily eroded, weakening the pipe and ultimately leading to failure.

Similar to HTHA, FAC can be prevented by using steel alloyed with chromium. Figures 1 and 2 show the flow influenced corrosion, and corrosion caused by acidic production.

These two examples may indicate that chrome steel is the perfect solution to many problems on the petrochemical plants, but the situation is more complex than previously thought.

316/316L Stainless Steel

Type 316 stainless steels contain chromium and are extensively utilized in the construction of chemical plants. However, their durability and mechanical strength also depends on the steel’s carbon content.

316 stainless steel contains up to 0.07% carbon, while 316L contains no more than 0.03%. This slight difference is sufficient to give the alloys different intergranular corrosion behavior.

Welded seams formed in the 316L stainless steel are more durable compared to the higher carbon alloy.

Positive Material Identification

PMI plays an important role at all stages of the plant equipment supply chain. Missing documentation or incorrectly identified raw materials could result in inappropriate components being used.

Even if the right materials are utilized, the traceability and validation of the materials employed in formulation and synthesis plants is a required part of Good Manufacturing Practice (GMP) in the food and pharmaceutical industries.

Elemental analysis of the material is the obvious way of achieving PMI. Many analytical methods involve time- and labor-intensive processes. The ideal technique should be simple, fast and non-destructive and must not compromise the integrity of the component under test. The capacity to test components while the plant is operational would be an additional benefit.

In most cases, compact, hand-held X-ray Fluorescence (XRF) spectrometers can meet these requirements; however, they are not suitable for all PMI tasks.

XRF Spectrometry

xSORT handheld XRF spectrometer

Figure 3. xSORT handheld XRF spectrometer

SPECTRO Analytical Instruments has developed the SPECTROTEST mobile OES metal analyzer and the SPECTRO xSORT hand held XRF spectrometer (Figure 3) that use the latest technology in their respective techniques and provide a complete solution for PMI.

XRF spectrometry, introduced in the 1950s, is a popular technique for metals analysis. It works by irradiating the sample’s surface with a beam of X-rays. This process induces fluorescence in the atoms in the sample, which is again emitted as X-rays of a lower energy.

Each element produces X-rays of a unique wavelength or energy, whose intensity is relative to the concentration of that element in the sample. Present detection systems can distinguish between the energies produced, quantify their intensities and thus determine the concentration of the different elements in the sample. This technology is known as Energy Dispersive X-ray Fluorescence, or ED-XRF.

The design, performance and easy operation of the SPECTRO xSORT handheld X-ray fluorescence spectrometer make it suitable for PMI in the process and petrochemical sectors. The instrument has been optimized for fatigue-free on site analysis.

SPECTRO xSORT XRF Spectrometer

The X-ray source and the detector are the two components that define the basic performance of an ED-XRF system. The stability of the primary source of X-rays affects the accuracy of the analysis as well as the ultimate detection limit of the instrument.

In the xSORT XRF spectrometer, a small low power X-ray tube ensures precisely defined excitation and hence excellent accuracy. The spectrometer uses an advanced Silicon Drift Detector (SDD), which offers better resolution and can process information faster, giving faster analysis. For most alloys, the xSORT allows metal grade identification and material verification in just 2 seconds.

With easy-to-use graphical interface, operations can be selected directly on the touchscreen using a stylus or a finger. Automatic calibration is facilitated through stored calibrations coupled with SPECTRO’s Intelligent Calibration Logic (ICAL). The xSORT is equipped with an automatic shutter that closes between each measurement, partly to protect operators from potential exposure to X-rays and partly to protect the internal components.

The xSORT can easily differentiate between different alloys. The difference in the molybdenum content can be clearly seen. It calculates and displays the calculated error on the result automatically.

The SPECTRO xSORT automatically compares the results of the analysis with a stored library of alloy compositions and identifies the alloy, or it can validate the alloy against a stored specification. Based on a reference sample, it can give a simple Pass/Fail message.

SPECTROTEST Mobile OES Analyzer

SPECTROTEST mobile OES analyzer

Figure 4. SPECTROTEST mobile OES analyzer

The SPECTROTEST (Figure 4) is a mobile metal analyzer that is based on the principle of optical emission spectrometry (OES) .

In this technique, an electric spark or arc is used to excite the atoms in the sample, where each element produces light of characteristic wavelengths in the visible and ultra-violet regions of the spectrum. Using a diffraction grating, this light is separated into its different wavelengths, and again individual intensities are determined with a suitable detector.

Similar to the xSORT, the SPECTROTEST utilizes ICAL procedures and can detect and verify alloys automatically in a few seconds.

Conclusion

PMI has become an accepted practice in the process and equipment supply industries. The SPECTROTEST mobile OES analyzer and the SPECTRO xSORT hand-held ED-XRF analyzer provide a perfect solution for PMI in this demanding sector.

This information has been sourced, reviewed and adapted from materials provided by SPECTRO Analytical Instruments GmbH.

For more information on this source, please visit SPECTRO Analytical Instruments GmbH.

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