SPECTRO-Focus PMI e Okt2011 Web

Positive Material Identification (PMI)by Spectrochemical AnalysisThe essential tool for infrastructure integrity testing in the process industries

Explosions, fires and other incidents at oil refineries, petrochemical plants and similar installations receive wide and often sensational publicity, particularly when there is loss of life or personal injury. Apart from the human cost, the financial losses to operators and insurers

can run to many millions of dollars. Often described as “accidents”, these incidents are not infrequent-ly traced to the use of piping, valves, and similar components made of inappropriate materials. Often the presence or absence of a particular alloying ele-ment in a steel component can be critical to its per-formance, but can be impossible to detect by a phy-sical inspection of the item. Over the last 20 years Positive Material Identification (PMI) or Verification has become accepted practice in the process and equipment supply industries. Modern spectroscopy-based analyzers are the answer to PMI in the de-manding process plant environment.

A few examples of situations that can arise, sometimes with disastrous re-sults, from the use of inappropriate materials will illustrate the need for PMI. Hydrogen gas is frequently en-countered in petroleum and chemical processes, and at high temperatures and pressures can cause High Tem-perature Hydrogen Attack (HTHA) in steel components that if undetected over time can lead to component fai-lure, with fire and explosion the like-

ly result under these conditions. At elevated temperatures (above about 400°F (204°C)) hydrogen atoms can readily diffuse into carbon steels. The diffused hydrogen reacts with the car-bon in the steel to form methane gas, that can accumulate at grain bounda-ries, weakening the steel and leading to cracks, fissures and ultimately to component failure. The use of steels containing alloy elements such as chromium, that have a stabilizing ef-

fect on the iron carbides, can effec-tively prevent HTHA. There are well documented cases where HTHA has led to catastrophic component failure due to the incorrect installation of a low alloy steel component. Ultra-pure water would not normally be thought of as particularly corrosive, but under certain conditions can be responsib-le for a mechanism known as Flow Accelerated Corrosion, or FAC. This has been experienced particularly in power generation, where ultra-pure water and steam are frequently en-countered. The factors giving rise to FAC are complex, but essentially when hot water or steam with a low oxygen content flows along a carbon steel pipe, the usually passive layer of iron oxide that normally forms on the surface can be dissolved. Over time, metal is gradually eroded, leading to weakening of the pipe and ultimately to failure. FAC has been responsib-le for several high-profile accidents in the nuclear power industry. Like HTHA, it can be prevented by the use of steel alloyed with chromium. These two examples might suggest that chrome steels are the answer to many problems on the chemical plant, but the situation is more complex. Type 316 stainless steels are widely used in the construction of chemical plants, and all contain chromium. However their mechanical strength and durability depends also on the carbon content of the steel. 316 stainless steel contains up to 0.07%, whereas 316L contains maximum 0.03%. This small absolute difference is enough to give the alloys clearly dif-ferent intergranular corrosion behavi-

THE ROLE OF PMI

Positive Material Identification (PMI) by Spectrochemical Analysis

or. Welded seams formed in the low carbon 316L are more durable than with the higher carbon alloy. PMI has a role not only during the construc-tion, operation and maintenance of process plant, but at all stages of the plant equipment supply chain. Incor-rectly identified raw materials, poor record keeping or missing documen- tation could all result in in appropriate components being used. Even if the correct materials are used, the trace-ability and validation of the materials used in formulation and synthesis plantis a required part of GMP (Good Manufacturing Practice) in the very tightly regulated pharmaceutical and food industries.

THE CHaLLEngE The obvious way of achieving PMI is elemental analysis of the material.Many analytical techniques require a sample to be taken for laboratory analysis, which may cause delay or be difficult or impossible while a pro-cess plant is in operation - shutting down an oil refinery just to take a metal sample is hardly a realistic pro-position. The ideal technique should be fast, simple to use on site and preferably non-destructive, so as not to compromise the integrity of the component under test. The ability to test components while the plant is operational, which may mean sur-face temperatures of several hundred degrees, would be an added bonus. Minimal sample preparation should be required. Small, hand-held X-ray Fluorescence (XRF) spectrometers can satisfy these requirements in most cases, and are often marke-

THE SPECTRO xSORT The design, performance and simp-le operation of the SPECTRO xSORT hand-held X-ray fluorescence spec-trometer make it ideal for PMI in the petrochemical and process sectors. XRF spectrometry is a well proven technique for metals analysis, popular since its introduction in the 1950’s. The technique works by irra-diating the surface of the sample with a beam of X-rays. This induces fluore-scence in the atoms in the sample, which is then re-emitted as X-rays of a lower energy. Each element emits X-rays of a different and unique ener-gy or wavelength, whose intensity is proportional to the concentration of that element in the sample. Detec-tion systems have been developed that can discriminate between the energies emitted, measure their in-tensities and hence determine the concentration of the different ele-ments in the sample. This technology is known as Energy Dispersive X-ray Fluorescence, or ED-XRF. The xSORT has been optimized for fatigue-free on site analysis. Some relevant de-sign considerations are:

Weight, convenience and portability. Complete with battery pack, the xSORT weighs less than 4 pounds (1.64 kg) and has an ergonomically designed handle and grip. All that is necessary is to bring the instrument in contact with the surface of the test sample and press the trigger. Usually little or no sample preparation is re-

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quired, but if samples are uneven or corroded, results may be improved by grinding the surface with a suita-ble abrasive. The user interface and results are displayed on an optimally positioned touchscreen. The inst-rument is robust, its housing made from shock- resistant ABS plastic. When not in use it can be carried in a handy holster.

General Corrosion (flow influenced) - Photo cour-tesy of the Energy Institute, London

ted as a universal solution to PMI. It should be remembered however that they cannot handle every PMI task. For example, they do not mea-sure carbon, so could not distinguish between the two types of stainless steel in the example above. For this and the measurement of some other elements like nitrogen, Optical Emis-sion Spectrometry (OES) is the pre-ferred technique. The new SPECTRO xSORT hand held XRF spectrometer and the SPECTROTEST mobile OES metal analyzer, both from SPECTRO Analytical Instruments both employ the latest technology in their respec-tive techniques and together provide a comprehensive solution for PMI.

Analytical performanceThe two components that define the fundamental performance of an ED-XRF system are the X-ray source and the detector. The stabi-lity of the primary source of X-rays affects both the ultimate detection limit of the instrument and the pre-cision of the analysis. Some early hand-held EDXRF instruments used radioactive isotopes as the source of primary X-rays, but these have asso-ciated safety and stability problems.


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In the xSORT a miniaturized low power X-ray tube, a close relative to the ones used in SPECTRO’s high per-formance laboratory analyzers, en-sures exactly defined excitation and hence good precision. The detector used in the xSORT is an advanced Silicon Drift Detector, or SDD.

Compare to the Si PIN diode detec-tors used in many other instruments, the SDD displays better resolution (ability to discriminate between ele-ments) and can process information ten times faster, giving faster analy-sis. SPECTRO xSORT delivers metal (alloy) grade identification and mate-rial verification in only 2 seconds for most alloys. This speed of analysis makes the xSORT ideal for use in live plant situations, as it minimizes the time the unit (and the operator!) has to be near any hot surface, and reli-able measurements can be obtained from surfaces as hot as 930 degrees F (500°C).

The “light” elements like Mg, Al and Si, require a longer measurement time, the xSORT just requires addi-tional 10 seconds to identify various aluminum and magnesium alloys. The xSORT can measure these light ele-ments in air – it does not require the path of the X-Ray beam to be flushed with Helium gas or even under vacu-um during the measurement.

As an added safety feature, the SPECTRO xSORT recognizes if no sample is present a fraction of a second after a measurement is started by the operator. If not, the shutter is closed immediately and the analysis aborted. There are also LEDs on the side of the instrument to tell the operator and others in the vicinity when the X-ray tube is activated and a measurement is in progress.

Easy and Safe to UseIt is not necessary to be a skilled ana-lyst to carry out successful PMI with the xSORT. The easy to understand graphical interface allows operations to be selected directly on the touch-screen using a finger or a stylus. Cali-bration of the instrument is automatic using stored calibrations combined with SPECTRO’s ICAL (Intelligent Cali-bration Logic).

With any XRF instrument stored ca-librations must be periodically che-cked against a known standard. The xSORT is fitted with an automatic shutter that closes between each measurement, partly to protect inter-nal components and partly to protect the operator from possible exposure to X-rays. ICAL ingeniously uses the shutter itself (i.e. when it is closed) as the external standard and checks the calibration between measure-ments. No operator intervention is re-quired as any necessary adjustments are made automatically.

Similarly, it is not necessary to be an expert to interpret the results. xSORT can automatically compare the results of the analysis with a stored library of alloy compositions and identify the alloy, or it can verify the alloy against a stored specification. Even easier for some applications, it can give a simple Pass/Fail message based on a reference sample. Of course, the full elemental analysis can be displayed if required, and all these results can be stored or sent to an external printer or PC via a wireless interface. The role of the automatic shutter in protecting the operator has already been men-tioned.


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ExaMPLES OF PMI wITH SPECTRO xSORT The following results illustrate the ability of the SPECTRO xSORT to dif-ferentiate between different alloys. In each case, the average values quoted are based on three 2-second measurements, but a single mea-surement would suffice to identify the alloy. SPECTRO xSORT calculates and displays the calculated error on the result automatically if required. It is not necessary to display all the ana-lytical results, as the SPECTRO xSORT can identify the alloy automatically: As has been mentioned above, minor differences in composition can signifi-cantly affect the performance of dif-ferent alloys.

One of the most popular steels for chemical plant construction is Type 304 stainless steel, the standard aus-tenitic chrome nickel steel alloy. This alloy is considered to be indestruc-tible and is characterized by high cor-rosion resistance, making it especially suitable for vessels and piping that will contain aggressive liquids at high temperature and pressure. However it is not the universal solution: hot acetic acid causes accelerated cor-rosion of Type 304 steel. Type 316, with a higher molybdenum content, is more resistant to this corrosion. SPECTRO xSORT can differentiate be-tween these alloys easily.

Corrosion by Acidic Production Chemical (Scale inhibitor) - Photo courtesy of the Energy Institute, London

The difference in the molybdenum content can be clearly seen.

AlSI 304

Ele-ment

Measurement time: 2 Sec

Certified Value [%] Average Value [%] 2 * SD [%]

Mn 1.48 1.32 0.130

Cr 18.37 18.50 0.152

Mo 0.20 0.21 0.016

Ni 8.09 8.14 0.178

Cu 0.40 0.35 0.126

AlSI 316

Ele-ment


Certified Value [%) Average Value [%] 2 * SD [%]

Mn 1.78 1.64 0.066

Cr 16.56 16.70 0.048

Mo 2.11 2.17 0.076

Ni 10.38 10.50 0.260

Cu 0.17 0.11 0.030

[Note: The concentration values in the tables below are expressed as percentages, and the preci-sion of the measurement is expressed by SD or 2*SD, also in percent concentration. SD is the Standard Deviation of a number of measurements and indicates the spread of the data about the average (or mean) result. The smaller the SD, the better the precision of the measurement. 2*SD, also known as “two sigma” is twice the standard deviation and indicates that 95% of readings will fall within this range. In the examples given these values are calculated on the basis of three separate measurements, but the SPECTRO xSORT can also use the data collected during a single measurement to calculate an SD. Clearly any concentration reported that is of the same magni-tude as the measurement error is going to be unreliable, which leads to the concept of Limit Of Detection or LOD, the lowest concentration that can be reliably measured. This is conventionally three times the SD. xSORT can be set up to either display the < symbol, or not to report a result, if it detects a concentration lower than the LOD.]


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Alloy C-22

Ele-ment



Cr 21.32 21.40 0.340

Mo 13.02 13.10 0.280

W 2.87 2.80 0.260

Fe 3.63 3.88 0.058

Co 0.59 0.54 0.034

Ni 57.30 57.70 0.320

Alloy C-276

Ele-ment



Cr 15.70 15.80 0.128

Mo 15.70 15.80 0.360

W 3.25 3.24 0.182

Fe 5.25 5.34 0.032

Co 0.23 0.27 0.074

Ni 59.00 59.00 0.420

The significant elements in these alloys are “heavy” elements, easily determined by XRF. The “lighter elements” are more difficult to measure, and it is here that the superior performance of the SPECTRO xSORT becomes apparent. AISI

321 is a general purpose austenitic stainless steel with added titanium to reduce intergranular cor-rosion. With its high signal throughput, SPECTRO xSORT can reliably determine titanium and differ-entiate between 321 and 304 in only 2 seconds.

AlSI 321

Ele-ment



Mn 1.52 1.23 0.178

Cr 17.45 17.60 0.122

Mo 0.36 0.38 0.032

Ni 9.42 9.50 0.144

Ti 0.63 0.67 0.036

V 0.13 0.11 0.022

Cu 0.30 0.20 0.080

Most chlorine environments require alloys with a higher content of chrome and molybdenum. A high level of tungsten, as in alloys C-22 and C-276 results in good pitting and crevice corrosion resistance.

304 steel, while offering good corrosion resistance, is difficult to machine. 303 austenitic chrome nickel steel is excellent for machining has a lower corrosion resistance. The cause of this can be found in the different sulfur contents. 304 chrome

nickel steel contains a maximum 0.03% sulfur, whereas 303 chrome nickel steel contains a minimum 0.15%. The measurement of sulfur is even more of a challenge, but with a slightly longer measuring time, xSORT can identify the higher sulfur content and separate the 303 and 304 grades.

AlSI 303

Ele-ment



Si* 0.63 0.82 0.260

Mn 1.87 1.72 0.066

S* 0.38 0.30 0.042

Cr 17.35 17.80 0.046

Mo 0.58 0.59 0.026

Ni 8.64 8.69 0.056

V 0.11 0.08 0.002

Cu 0.51 0.46 0.046*Note that only grade identification is possible with S and Si. If more than grade is required using S and Si then other instrumentation is necessary.


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ExaMPLES OF PMI wITH SPECTROTEST

As mentioned at the beginning, the only difference be-tween steel alloys 316 and 316L is their carbon content: the 316 alloy contains up to 0.07% carbon; the 316L maximum 0.03%. SPECTROTEST can easily differentiate between these two alloys.

AlSI 316

Ele-ment


Certified Value [%)

Average Value [%] 2 * SD [%]

C 0.042 0.045 0.002

Si 0.26 0.29 0.013

Mn 1.25 1.17 0.024

P 0.022 0.019 0.004

S 0.020 0.021 0.003

Cr 16.64 16.16 0.174

Mo 2.11 2.05 0.067

Ni 10.66 10.80 0.119

V 0.13 0.13 0.007

Cu 0.09 0.11 0.004

AlSI 316L

Ele-ment


Certified Value [%)

Average Value [%]

2 * SD [%]

C 0.019 0.022 0.001

Si 0.44 0.46 0.004

Mn 1.21 1.18 0.012

P 0.026 0.023 0.003

S 0.020 0.023 0.003

Cr 17.36 17.05 0.047

Mo 2.11 2.04 0.001

Ni 11.86 12.09 0.082

V 0.05 0.06 0.001

Cu 0.09 0.11 0.004

SPECTROTEST

As these results show, the xSORT will be able to handle most of the PMI tasks encountered in the chemical process industry. However as was mentioned above, in some cases elements outside the scope of the xSORT, like carbon or nitrogen, will need to be measured for a complete PMI. The SPECTROTEST is a mobile metal analyzer using the principle of Optical Emission Spectrometry (OES). In this technique, the atoms in the sample are excited using an electric spark or arc, where each element emits light of characteristic wavelengths, but this time in the ultra-violet and visible regions of the spectrum. This light is separated into its different wavelengths using a diffraction grating, and again individual intensities are measured with a suitable detector. The SPECTROTEST detector is also state-of-the-art: highly sensitive and fast

multiple CCD’s give fast analysis and generate high quality data that enables the same sophisticated approach to data handling. Like SPECTRO xSORT, the SPECTROTEST uses ICAL procedures and can identify and verify alloys automatically in a few seconds. Alloys can be differentiated not only by their metal content but on the basis of elements like carbon, nitrogen and boron, which are impossible to measure by hand-held XRF.The physical requirements of the optical system mean that SPECTROTEST is larger, weighing 64 lbs (29 Kg), and less portable, although the flexibly coupled up to 8 m long sample probe still allows access to confined spaces. The nature of the excitation process (arc or spark) also means that extra care may be needed when flammable vapors or liquids are likely to be present.

Stress Corrosion Cracking of a 316L SS Pipework - Photo courtesy of the Energy Institute, London


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Using SPECTROTEST, it is also possible to conduct onsite identification of austenitic steels alloyed with nitrogen or so-called duplex steels (those whose structure is approximately half austenite and half ferrite). As an alloying element

in austenitic steels, nitrogen stabilizes the austen-itic structure without decreasing the durability of the steel. In high alloy, chemically resistant steels, it also increases their resistance to corrosion to a certain extent, especially pitting corrosion. Among the duplex steels, the X2CrNiMoN 22-5-3 (1.4462) grade has found wide acceptance. The nitrogen content in duplex steels usually ranges from 0.10 - 0.22%.

Duplex Alloy 1.4462 (X2CrNiMoN22-5-3)

Ele-ment



C 0.013 0.019 0.001

Si 0.47 0.55 0.011

Mn 1.74 1.62 0.016

P 0.023 0.014 0.004

S 0.002 <0.002

Cr 22.41 21.64 0.046

Mo 2.89 2.64 0.020

Ni 6.01 5.88 0.076

V 0.15 0.12 0.001

N 0.103 0.139 0.019

PMI has become an essential part of good practice and risk reduction in the process industries. The SPECTRO xSORT hand-held ED-XRF analyzer and SPECTROTEST mobile OES analyzer provide a complete solution for PMI in this de-manding sector.

COnCLuSIOn

GERMANYSPECTRO Analytical Instruments GmbH Boschstr. 10 47533 Kleve Tel.: +49.2821.8 92-0 Fax: +49.2821.8 92-22 00 [email protected]

U.S.A.SPECTRO Analytical Instruments Inc. 91 McKee DriveMahwah, NJ 07430Tel.: +1.800.548.5809 +1.201.642.3000Fax: [email protected]

HoNG KoNG (Asia-Pacific)SPECTRO Analytical Instruments (Asia-Pacific) Ltd.Unit 1603, 16/F., Tower III Enterprise Square No. 9 Sheung Yuet Road Kowloon Bay, Kowloon Tel.: +852.2976 9162Fax: +852.2976 [email protected]

SPECTRO-Focus PMI e Okt2011 Web

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