Accurate and Dependable Choice for In-Service Oil and Fuel Analysis

The basis of modern oil analysis programs refers to the use of optical emission spectroscopy (OES) for measuring the parts per million (ppm) levels of contaminants, wear metals, and additives in oil samples. Whatever an oil lab may need to measure, multi-elemental analysis is considered to be the core of an in-service oil analysis program. This article offers an overview of Rotating Disk Electrode Optical Emission Spectroscopy (RDE-OES), its applications, and the SpectrOil Series family of products, which combine the recent innovations for increased reliability and performance with 30+ years of experience in laboratory and military applications.

Early Background

Sometime after World War II, the Denver and Rio Grande Railroad, now defunct, started analyzing diesel locomotive engine oil by considering the spectral lines emitted by an in-service oil sample when excited by a powerful electric arc using carbon electrodes. Initial tests established the fact that monitoring the elements attributed to wear and contamination offered early warnings of chronic equipment failure.

Elemental spectroscopy is considered to be the backbone of an oil analysis program as it is extensively applicable to a closed loop lubricating system such as those present in gas turbines, gasoline and diesel engines, transmissions, compressors, gearboxes and hydraulic systems. In practice, an oil sample is occasionally taken from a system and then examined. The resulting data, when compared to earlier analyzes and allowable limits, may specify a sound mechanical system showing only normal wear, or it could even point out a possibly severe problem in its early stages. With this advanced warning, steps could be taken to correct the situation before serious injury or damage occurs.

Spectroscopic oil analysis functions by detecting the fine particles produced by relative motion of metallic parts in an oil-wetted system. Contaminants are also detected and badly degraded lubricants, or lubricant mix-ups, are identified by the concentration of additive elements. Multi-element analysis, along with knowledge of the materials used for building the engine, often allows identification of a particular component in distress. Table 1 demonstrates that typical metal elements can be examined by spectroscopy and their typical sources.

Table 1. Typical source of elements analyzed by spectroscopy.

Principles of Spectroscopy

Spectroscopy is a method for detecting and then quantifying the existence of elements in a material. Spectroscopy considers the fact that each element has an exceptional atomic structure, and when subjected to the addition of energy, each element releases light of particular colors or wavelengths. If this light is dispersed by with the help of a dispersing element, such as a prism, a line spectrum will result. The collected light can be examined and each element contained in the sample identified since no two elements have the same pattern of spectral lines. Furthermore, the intensity of the emitted light is proportional to the quantity of the element existing in the sample, allowing the concentration of that element to be determined.

These spectral lines are unique to the atomic structure of just one element. For the hydrogen atom, with an atomic number of 1, the spectrum is objectively simple (Figure 1). On the other hand, the spectrum of iron containing an atomic number of 26 is much more complex with a number of emission lines in the visible spectrum corresponding to the many possible electronic transitions that may happen (Figure 2). If more than one element exists in the sample, spectral lines of distinctively varied wavelengths will appear for every single element. It is necessary to separate these lines in order to identify and then quantify the elements present in the sample. Generally, only one spectral line among many is chosen in order to determine the concentration of a particular element. This line is chosen for its intensity and freedom from spectral line interference of other elements. An optical system is needed in order to accomplish this.

Figure 1. Emission Spectrum of Hydrogen.

Figure 2. Emission Spectrum of Iron.

Rotating Disc Electrode Optical Emission Spectroscopy (RDE-OES)

Spectrometers that look at the multitude of spectral lines from an excited or heated sample are known as optical emission spectrometers. All optical emission spectrometers comprise of three key components.

1. Excitation Source – introduces energy to the sample.
2. Optical System – separates and then resolves the resulting emission from that excitation into its component wavelengths.
3. Readout System – identifies and measures the light that has been separated into its component wavelengths by the optical system and then presents this information to the operator in a usable fashion.

An electric discharge is a typical method employed in the excitation source in modern spectrometers. The source has been designed to impart the energy produced in an arc, or spark, to the sample. For oil analysis spectrometers, a huge electric potential is set up between a disk and rod electrode with the oil sample in the gap between them. An electric charge stored by a capacitor is discharged across this gap producing a high temperature electric arc, which vaporizes a portion of the sample, developing a plasma.

A plasma is a hot, highly ionized gas which releases intense light. The light given off as a result of this process comprises of emissions from all the elements present in the sample. It is now possible to separate these emissions into separate wavelengths and then measure using a properly designed optical system. Temperatures in the 5000 to 6000 °C range are attained and even difficult to excite elements emit sufficient light to be readily detected.

Oil has been sparked, or “burned,” between a rotating carbon disk electrode and a carbon rod electrode since the early days of spectroscopic oil analysis. The sample is placed in a sample cap, the disk is then partially immersed in the oil sample and the disk rotates as the burn proceeds (Figure 3). This needs about 2 or 3 ml of sample, based on the exact cap being used.

Figure 3. RDE spectrometer sample stand showing oil sample being “burned”.

A fresh disk and a newly sharpened rod are needed for each sample in order to eliminate sample carryover. This method is known as rotating disk electrode (RDE) optical emission spectroscopy (OES), or uniting the two, RDE-OES. Alternatively, it is referred to as RDE-AES, which stands for rotating disk electrode atomic emission spectroscopy.

The light coming from the plasma is separated by the optical system, in a spectrometer, into the discrete wavelengths that comprise it. An optical device known as a diffraction grating is used for separating the discreet wavelengths. The diffraction grating is a concave mirror with extremely fine lines on its surface that leads to incident polychromatic light to be separated into component wavelengths and then focused on an array of light detectors.

Figure 4 presents the key components of an oil analysis spectrometer using a polychromator optic, based on the Rowland Circle concept. Light from the excitation process, or the “burn,” leaves the fiber optic cable and passes through the entrance slit, where it is concentrated on the diffraction grating by a lens. The entrance slit introduces light containing the elements present in the oil sample and defines the shape of the spectral lines at the focal curve after it is diffracted by the grating. The grating here is to separate, or “diffract,” this light into its component wavelengths. The spectral lines can be photographed or electronically quantified by charge coupled devices (CCDs) or photomultiplier tubes (PMTs).

Figure 4. Schematic of a Rotating Disk Electrode Optical Emission Spectrometer for Oil Analysis.

The readout system of a spectrometer is usually controlled by an industrial grade processor and software. A clocking circuit and amplifier periodically read the charge on a CCD chip, or Photo Multiplier Tube, and transform it from an analog to a digital (ADC) signal in order to measure the light that has fallen on a pixel. The charge accumulated on a pixel is transformed to an arbitrary number defined as “intensity” units. The total intensities for each element are compared to calibration curves stored in memory, at the end of the analysis, followed by converting them into the concentration of the element present in the sample (Figure 5).

Concentration is generally expressed in parts per million (ppm). This information can either be printed, or displayed on a video screen. After completing the analysis and recording the results, the system is ready for the next analysis. The analysis results may be stored on the hard disk, left on the screen or sent to an external computer.

Figure 5. Readout system of an RDE-OES.

SpectrOil Family of Spectrometers: The Market Leading RDE Spectrometers

RDE-OES spectrometers for used oil analysis are still the choice for elemental measurement. It has been over five decades since the technology was first adapted by railways and the military because of the instrument’s ruggedness, stability and reliability. R&D continues with the method, with key improvements in limits of detection, and long term stability, a result of new electronic hardware, software and optics. The newest innovations in these areas are integrated into the SpectrOil 100 Series spectrometer, illustrated in Figure 6.

This system has a very tiny footprint, and is easier to use than ever before, making it the ideal solution for coolant, lubricant and on-site fuel labs within power generation, mining and commercial laboratories. The SpectrOil is available in a range of application specific configurations illustrated in the following table.

Figure 6. The SpectrOil 100 Series RDE Spectrometer.

Table 2. The SpectrOil 100 Series models and calibration ranges in ppm.

The SpectrOil M is a sturdy and transportable system mainly used for military applications as specified by the DoD JOAP program (Figure 7). SpectrOil 110E is customized to the specific requirements of engine monitoring, while the SpectrOil 120C realizes the requirements of ASTM D6595, and is the default option for commercial customers.¹ The SpectrOil 120F for fuel analysis complies with the requirements for ASTM D6728, and an extra L/D program is available to meet the specific low vanadium and alkali element limits needed by GE Power systems gas turbines, per the GE- MTD-TD-002 specification. These systems are also employed for coolant analysis and washdown water analysis.²

Figure 7. SpectrOil M Series elemental analyzer.

Advanced Software for Stability and Accuracy

SpectrOil Version 8 software includes latest innovations such as advanced signal processing and background correction capabilities for major performance improvements. The system offers an intelligent technique of measuring peak signals, resulting in a lower LOD by 2x for a majority of elements (Table 3). Active alerts allow a user to know when results could be impacted because of instrument drift. New Integrated Standardization is a more efficient workflow that saves time when measuring real samples. Finally, better instrument self-monitoring means enhanced long term stability, so standardization is required less often and results are steady over time (Figure 8).

Figure 8. Measurement of a 0.2 ppm Vanadium base oil standard over 70 days at a power generation field laboratory.

Table 3. Typical 2-Sigma LOD (ppm) for SpectrOil calibrated with the CS-24 commercial oil analysis program. The Version 8 software and latest optics technology provide exceptional signal to noise and system performance.

SpectrOil spectrometers offer very accurate and stable elemental analysis measurements for oils and fuels, making the method the first choice for laboratory managers and reliability professionals who require fast results without sample preparation, glassware, gases, or advanced training. SpectrOil analyzers are a natural fit for sites where time sensitive samples are produced and erratic sample volumes (mostly five samples, occasionally 100) are common. A point to be noted is that Formula 1 Race teams depend upon this technology to aid their cars, where time reliability, sensitivity and sub-ppm changes in wear mean the differences between winning and going home (Figure 9).

Figure 9. McLaren and Mobil 1 use the SpectrOil 100 Series on the global F1 circuit (Ref: Mclaren).

Laboratories with steady volumes can gain from the lower cost of sample processing with high stability and accuracy; also automatic sample processing can be warranted. RDE spectrometers have always been hard to automate because of the need to restock the graphite electrodes after each analysis. The practical solution to RDE spectrometer automation is the SpectrOil M/R D2R2, which uses an integrated autosampler and two graphite disk electrodes as shown in Figure 10.

A robotic arm in the sample changer automatically adds each of the 48 oil samples in sequence, at a rate of 80 samples per hour, without the necessity for sample dilution.

Figure 10. Robotics in SpectrOil M/R spectrometer.

The whole automation system mounts to the spectrometer sample stand and realizes all the functions of sequentially adding and removing oil samples and switching graphite electrodes. It is self-contained and runs autonomous of the spectrometer’s operating software. Although operation is automatic, it also has the ability to manually sequence through each of the robotics’ operations. The newer signal processing and optics hardware make this solution very attractive to commercial laboratories and exclusive support laboratories with steady volumes.

Case Study: Railroad Field Laboratory

Railroad and power generation companies with older EMD high horsepower diesel electric engines depend upon spectrometric oil analysis to detect abnormal bearing wear in its initial stages. These fleets standardize on a low zinc “locomotive oil” to sidestep premature wear of bushings and bearings. A commercial oil laboratory fitted a SpectrOil in a field lab at a railroad switching yard to support everyday maintenance operations. In a summary report of the previous three SpectrOil oil analyzes, Ag (silver) was detected at 2 ppm.

This level is a warning alarm, prompting an investigation. In this type of engine, wrong oil containing a zinc-based additive package can cause severe wear issues. Several components, such as wrist pin bearings, have silver coatings that corrode and wear when zinc is present. The initial stages of the corrosive action caused by the zinc additive are shown by the increase in the copper, iron, and silver wear metals. The SpectrOil analysis of other elements – phosphorus, magnesium, and zinc helped offer a clue to the root cause of the wear that the locomotive was topped off with wrong engine oil.

A suggestion was made, centered on the analysis, to drain and flush the system, as well as detect correct top-off oil requirements. This fault would not have been exhibited this early by any other condition monitoring method such as vibration analysis, ultrasound, thermography or performance monitoring. Without oil analysis, the wear issue could have resulted in a bearing failure and a key overhaul, costing more than $150,000. Nuclear power generation facilities that utilize large diesel engines for emergency backup power seek the low detection levels of wear that the SpectrOil can also provide.

Table 4. Spectroscopic results in ppm for an EMD medium speed diesel locomotive. A SpectrOil 110 E system is ideal for railroad field labs.

In conclusion, the newest innovations in Optical Emission Spectroscopy, shown in the SpectrOil product line, provide top quality elemental analytical performance available today. The new optics and analytical capabilities, integrated with the proven reliability and stability of the SpectrOil technology make it the perfect solution for elemental analysis in used oil and fuel analysis.

References

1. ASTM D6595 Standard Method for Determination of Wear Metals and Contaminants in Used Lubricating Oils or Hydraulic Fluids by Rotating Disc Electrode Atomic Emission Spectrometry.

2. ASTM 6728 Standard Test Method for Determination of Contaminants in Gas Turbine and Diesel Fuel by Rotating Disc Electrode Atomic Emission Spectrometry.

3. Rhine, W.E., Saba, C.S., and Kaufman, R.E., “Metal Detection Capabilities of Rotating Disc Emission Spectrometers,” Lubrication Engineering, Vol. 42, #12, p 755

4. Lukas, M., and Giering, L.P., “The Effects of Metal Particle Size in the Analysis of Wear Metals using the Rotating Disc Atomic Emission Technique,” presented at the International Symposium on Oil Analysis, Erding, Germany, July 1978.

5. Anderson, D.P. & Lukas, M., “Diesel Engine Coolant Analysis. New Application for Established Instrumentation,” Presented at 1998 Technology Showcase, JOAP International Condition Monitoring Conference, Mobile, AL, April 20-24, 1998.

6. Lukas, M. & Anderson, D.P., Machine and Lubricant Condition Monitoring for Extended Equipment Lifetimes and Predictive Maintenance at Power Plants, Proceedings of Power-Gen International, Jakarta, Indonesia, 1996.

This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.

For more information on this source, please visit AMETEK Spectro Scientific.