ASTM D6728 Standard Test Method for Determination of Contaminants in Gas Turbine and Diesel Engine Fuel by Rotating Disc Electrode Atomic Emission Spectrometry
4. Summary of Test Method
4.1 A fuel test specimen is excited by a controlled arc discharge using the rotating disk technique. The radiant energies of selected analytical lines and a reference are collected and stored by way of photomultiplier tubes, charge coupled devices, or other suitable detectors. A comparison is made of the emitted intensities of the elements in the fuel test specimen against those measured with calibration standards. The concentration of the elements present in the fuel test specimen are calculated and displayed.
5. Significance and Use
5.1 Operating experience of gas turbines and diesel engines has shown that some of the ash-forming substances present in a fuel can lead to high temperature corrosion, ash deposition, and fuel system fouling. Ash-forming materials may be in a fuel as oil-soluble metallo-organic compounds as water-soluble salts or as solid foreign contamination. Their presence and concentration varies with the geographical source of a crude oil and they are concentrated in the residual fractions during the refining process. Although distillate fuel oils are typically contaminant free, ash-forming materials may be introduced later in the form of salt-bearing water or by contact with other petroleum products during transportation and storage. Specifications of gas turbine and diesel engine fuels and the significance of contamination and trace metals are detailed in Specifications D2880 and D975.
5.1.1 Pre-conditioning of the fuel before it reaches the gas turbine or diesel engine has become a prerequisite for installations that use heavy petroleum fuel, and also for sites that use light distillate fuel oils. On-site fuel analysis to determine the extent of contamination is an integral part of a fuel quality management program. It is used first to determine the extent of the required treatment, and later, the effectiveness of the treatment. It starts with the delivery of the fuel, continues throughout fuel handling and ends only as the fuel is injected into the turbine or engine.
5.1.2 Fuel contamination specifications vary among the different gas turbine manufacturers. However, without exception, each requires that contaminants must be as low as possible. In most power generation installations, it is the owner who has the responsibility of verifying fuel cleanliness in compliance with the turbine manufacturer's warranty specifications. This leads to an on-site analytical instrument performance requirement of below 1.0 mg/kg for several elements.
6. Interferences
6.1 Spectral - Most spectral interferences can be avoided by judicious choice of spectral lines. High concentrations of some elements can have an interfering influence on the spectral lines used for determining trace levels of contaminants. Instrument manufacturers usually compensate for spectral interferences during factory calibration. A background correction system, which subtracts unwanted intensities on the side of the spectral line, shall also be used for this purpose. When spectral interferences cannot be avoided with spectral line selection and background correction, the necessary corrections shall be made using the computer software supplied by the instrument manufacturer.
6.2 Viscosity Effects - Differences in viscosity of fuel samples will cause differences in uptake rates. Internal references of the instrument will compensate for a portion of the differences. Without a reference, the analysis will be adversely affected if the test specimen has a different viscosity from the calibration samples. The hydrogen 486.10 nm spectral line shall be used for light fuels, and the CN 387.10 nm spectral line shall be used for heavy fuels as an internal reference to compensate for viscosity effects.
6.3 Particulate - When large particles over 10 µm in size are present, the analytical results will be lower than the actual concentration they represent. Large particles may not be effectively transported by the rotating disk electrode sample introduction system into the arc, nor will they be fully vaporized.
