Reliable and accurate positive material identification (PMI) analyzers that can be taken to remote locations or deep within large industrial plants are today’s essential kit in asset integrity inspection.
Optical emission spectroscopy (OES) technology has been relied upon for years to deliver the full chemistry of a sample with excellent precision and accuracy. However, the rise of handheld laser induced breakdown spectroscopy (LIBS) analyzers that boast their ability to test for carbon has challenged this perception. But can they provide the precision and accuracy that’s so critical in this field?
We understand the industry’s need to meet the evolving regulations and industry standards, including American Petroleum Institute (API) standards and the new PHMSA “Mega Rule,” which highlights the importance of selecting the right analysis methods.
API Standards: Oil and gas professionals working to meet the API standards can use analysis technology in their operations. Precise analysis is particularly important when addressing standards such as the API 5L, which provides standards for seamless and welded steel line pipe used in transporting gas, water and oil and is applicable to the natural gas and oil industries, and API RP 578 that provides guidelines for material control and verification programs on ferrous and non-ferrous alloys, with guidelines on how to carry out non-destructive testing (NDT).
PHMSA ‘Mega Rule’: As we all know, the PHMSA’s “Mega Rule” recently came into effect. With these changes, those responsible for gas transmission pipelines laid before 1970 now need to determine the material strength of these lines by verifying the specific material properties and the Maximum Allowable Operating Pressure (MAOP).
Heavily reliant on NDT and PMI principles, the testing must be extensive enough to ensure at least a 95 percent confidence level in material properties, following valid statistical methods, and the equipment used in the analysis must follow typical NDT guidelines.
Correct Grade Verification
Steel grades are ultimately determined by their composition and when performing material verification analyses, you need to ensure you can measure the right elements at the right levels. Many elements are added to achieve specific properties.
Carbon: Carbon plays a crucial role in the behavior of steel. It contributes to strength and brittleness and affects workability and weldability. Using the wrong grade of steel in relation to carbon content can cause mechanical failure. Determining the correct grade of carbon or L-grade steel is also important when it comes to high temperature processing, such as welding. If the component can’t withstand the temperature being applied, it will be destroyed, which is costly.
Carbon Equivalent (CE) Content: To predict the behavior of the material when processed at high temperature, the carbon equivalent concept is used. This uses the percentage of other alloying elements to give a “carbon equivalent” value that is used instead of the pure carbon percentage when evaluating material properties. In practice, the CE value is obtained by using a carbon equivalent equation.
The carbon equivalent formulae have been created to give a best estimate indication of a carbon content that would relate to the hardenability for the steel being measured. The formulae are also used to determine related properties, such as toughness and strength.
Other Elements in Steel/Iron: Beyond carbon and carbon equivalence, there are other elements that have a significant impact on behavior and must be verified and carefully controlled. These elements include manganese, phosphorus, sulfur, chromium, niobium, copper, molybdenum, nickel, silicon, titanium, vanadium, boron, tungsten, tantalum, hafnium, nitrogen and boron at low ppm.
What Can OES and LIBS Deliver on the Elements That Really Matter?
We’ve seen that there are many elements that must be measured to comply with regulations and correctly determine metal grade. Which analyzer should you choose for in-the-field grade determination and regulatory compliance?
Handheld XRF: Handheld XRF is not able to analyze carbon or boron, this is because the technique doesn’t resolve lighter elements of the periodic table well enough. Although widely used for hundreds of applications, XRF is not suitable to comply with the PHMSA “Mega Rule.”
Handheld LIBS: Handheld LIBS is excellent at some alloy grade identification. Historically, LIBS was unable to detect carbon, however today some instruments have this capability. The API RP 578 has been amended to include LIBS as a measurement technique however, even though LIBS is capable of measuring Sn, As and B, the detection limits are too high to be useful for most of the applications.
OES: OES has for many years been seen as the gold standard of metals analysis, with low detection limits for traditionally difficult elements, such as carbon, boron and tungsten. Manufacturers have made OES instruments portable so you can carry them with you, making the measurements at height or in a ditch easy.
So, we must discount XRF as it can’t analyze carbon, but what about the other two technologies?
Head to Head: LIBS vs OES
Taking a series of industry standards of known composition, we tested the instruments side by side under laboratory conditions. The clearest result is that handheld LIBS can’t measure phosphorus, sulfur and boron at all.
In many cases, the LIBS trace is far from the red line that denotes the certified reference value. Although this looks alarming, in practice this is easily remedied by re-calibrating the instrument against a known reference prior to field testing. So, we can assume that the average value measured by handheld LIBS will have better accuracy in a real testing situation.
What is more problematic though is the lack of precision. In some cases, like with Carbon and Manganese, the standard deviation is more than an order of magnitude worse with handheld LIBS.
The problem with this is that this could easily put the material grade in the wrong category, especially with something like carbon where the differences between grades is very small. This could have serious consequences for product performance, especially where the grade is used to find the maximum operating pressure within a pipeline. For the full report, please visit: https://hhtas.net/LIBSvsOES.
Choosing the Right Analyzer
We know what needs to be measured to remain compliant and safe, and we’ve seen exactly how OES compares with handheld LIBS for key elements. This is what you need to consider before making your choice:
- Precision for correct grade interpretation
Some of our data implies that LIBS is not accurate which will not be the case in the field because handheld LIBS can be type standardized for accuracy. The issue is precision. The variation with each measurement on key element results makes it easy to mis-interpret the grade which is risky.
- Ability to measure a range of elements for CE
To have confidence in your results, you need to measure carbon equivalency. This means measuring many elements – not just carbon — to low detection limits. Handheld LIBS can’t measure elements such as boron, making it impossible to accurately calculate the standard PCM CE formula.
- Regulatory compliance
Many country-wide guidelines demand verification of a range of elements. Many elements that you need to measure for compliance — such as phosphorus and sulfur — can still only be measured with OES.
- Measurement time
In a busy production environment or during a plant shutdown, it’s important that measurements can be taken efficiently with no loss of performance. OES requires only a quick flush of argon through the optics, and you can begin measuring. Handheld LIBS with carbon needs a recalibration and warm-up time before analysis can begin.
Also, you’ll need to wait for the handheld LIBS analyzer to reach the optimum temperature when moving between different types of samples. For example, changing between low alloy steel and stainless steel can take up to half an hour, and if the ambient temperature fluctuates, you’ll need to compensate and carefully manage this to retain the confidence level of your measurements.
If the optics of a handheld LIBS get hot, they can adversely affect the results. This means you’ll have to pause testing and wait until they cool down. This interruption of testing can be significant and the same applies to cost when the instrument can’t be potentially used for 30-40 percent of the day.
OES technology has been relied upon for years to deliver the full chemistry of a sample with excellent precision and accuracy. While it’s great to see the advancements in handheld LIBS, it’s not a handheld OES and as such a direct replacement to portable OES instruments. You’ll still need to rely on OES technology if you need to measure carbon, phosphorus, sulfur, boron, arsenic and tin in low alloys and stainless steels, and nitrogen in duplex steels.
In material verification, you need to be able to measure a wide range of elements to very low detection limits. Using technology that’s not up to the job can lead to incorrect grade specification and catastrophic consequences.
When it comes to regulatory compliance, implementing a widespread in-situ testing program using less than ideal equipment is a waste of time and money — and that’s before considering the cost of non-compliance.
At the end of the day, are you willing to take the risk where lives are at stake, or would you prefer to back the technology that gives you the whole picture?
Tags: asset management, November December 2020 Print Issue, PHMSA Mega Rule
Michel Molderings is Product Manager for mobile OES at Hitachi High-Tech Analytical Science. Jordan Rose is Marketing Manager, Americas for Hitachi High-Tech Analytical Science.