ASTM D4418 Standard Practice for Receipt, Storage, and Handling of Fuels for Gas Turbines
6. Significance of Contaminants
6.1 Contamination levels in the fuel entering the combustor(s) must be low for improved turbine life. Low contamination levels in the fuel in the turbine's in-plant fuel system are required to minimize corrosion and operating problems. Providing fuel of adequate cleanliness to the gas turbine combustor(s) may require special actions by the user. These actions might include special transportation arrangements with the fuel supplier, particular care in on-site fuel storage and quality control procedures, and establishment of on-site cleanup procedures. Each of the four classes of contaminants defined in 3.2 has its own significance to system operation.
6.1.1 Water will cause corrosion of tanks, piping, flow dividers, and pumps. Corrosion or corrosion products in close-tolerance devices, such as flow dividers, may cause plugging and may stop flow to the turbines. Free water is potentially corrosive in sulfur-containing fuels, it may be particularly corrosive. Free water may contain dissolved salts that may be corrosive, and may encourage microbiological growth.
6.1.2 Particulate Solids may shorten the life of fuel system components. Life of fuel pumps and of various close-tolerance devices is a function of particulate levels and size distributions in the fuel. High levels of particulates can lead to short cycle times in the operation of filters, filter/separators, centrifuges, and electrostatic purifiers. Since such separation devices do not remove all the particulates, certain quantities will be present in the down-stream fuel.
6.1.3 Trace Metals refer both to those metals present as metallic compounds in solution and to metals present in particulates like rust. They are dissolved or suspended either in the fuel hydrocarbons or in free water present in the fuel. The significance of several individual trace metals with respect to hot corrosion is discussed in 6.1.4 through 6.1.5. Although lower levels of trace metals in a fuel will promote longer turbine service from a corrosion standpoint, the specification of excessively low levels may limit the availability of the fuel or materially increase its cost. Table 1 suggests levels of trace metals that would probably yield satisfactory service.
6.1.4 Ash is the noncombustible material in an oil. Ash-forming materials may be present in fuel oil in two forms: (1) solid particles, and (2) oil- or water-soluble metallic compounds. The solid particles are for the most part the same material that is designated as sediment in the water and sediment test. Depending on their size, these particles can contribute to wear in the fuel system and to plugging of the fuel filter and the fuel nozzle. The soluble metallic compounds have little or no effect on wear or plugging, but they can contain elements that produce turbine corrosion and deposits as described in 6.1.5.
6.1.5 Vanadium and Lead - Fuel contaminants might include soluble compounds such as vanadium porphyrins, metallic soaps, or tetraethyl lead that cannot be removed from the fuel at the gas-turbine site.
6.1.5.1 Vanadium can form low melting compounds such as vanadium pentoxide which melts at 691°C (1275°F), and causes severe corrosive attack on all of the high-temperature alloys used for gas-turbine blades. If there is sufficient magnesium in the fuel, it will combine with the vanadium to form compounds with higher melting points and thus reduce the corrosion rate to an acceptable level. The resulting ash will form deposits in the turbine and will require appropriate cleaning procedures.
6.1.5.2 When vanadium is present in more than trace amounts either in excess of 0.5 ppm or a level recommended by the turbine manufacturer, it is necessary to maintain a weight ratio of magnesium to vanadium in the fuel of not less than 3.0 in order to control corrosion.
6.1.5.3 An upper limit of 3.5 is suggested since larger ratios will lead to unnecessarily high rates of ash deposition. In most cases, the required magnesium-to-vanadium ratio will be obtained by additions of magnesium-containing compounds to the fuel oil. The special requirements covering the addition and type of magnesium-containing additive, or equivalent, shall be specified by mutual agreement between the various interested parties. The additive will vary depending on the application, but it is always essential that there is a fine and uniform dispersion of the additive in the fuel at the point of combustion.
6.1.5.4 For gas turbines operating at turbine-inlet gas temperatures below 650°C (1200°F), the corrosion of the high-temperature alloys is of minor importance, and the use of a silicon-base additive will further reduce the corrosion rate by absorption and dilution of the vanadium compounds.
6.1.5.5 Lead can cause corrosion, and in addition it can spoil the beneficial inhibiting effect of magnesium additives on vanadium corrosion. Since lead is only rarely found in significant quantities in crude oils, its appearance in the fuel oil is primarily the result of contamination during processing or transportation.
6.1.6 Sodium, Potassium, and Calcium - Fuel contaminants might also include fuel-insoluble materials such as water, salt, or dirt, potential sources of sodium, potassium, and calcium. These are normally removed at the gas-turbine site, unless such contaminants are extremely finely divided.
6.1.6.1 Sodium and Potassium can combine with vanadium to form eutectics that melt at temperatures as low as 566°C (1050°F) and can combine with sulfur in the fuel to yield sulfates with melting points in the operating range of the gas turbine. These compounds produce severe corrosion, and for turbines operating at gas inlet temperatures above 650°C (1200°F), additives are not yet in general use that control such corrosion.
6.1.6.2 Accordingly, the sodium-plus-potassium level must be limited, but each element is measured separately. Some gas turbine installations incorporate systems for washing oil with water to reduce the sodium-plus-potassium level. In installations where the fuel is moved by sea transport, the sodium-plus-potassium level should be checked prior to use to ensure that the oil has not become contaminated with sea salt. For gas turbines operating at turbine inlet gas temperatures below 650°C (1200°F), the corrosion due to sodium compounds is of minor importance and can be further reduced by silicon-base additives. A high sodium content is even beneficial in these turbines because it increases the water-solubility of the deposits and thereby increases the ease with which gas turbines can be water-washed to obtain recovery of the operating performance.
6.1.6.3 Calcium is not harmful from a corrosion standpoint: in fact, it serves to inhibit the corrosive action of vanadium. However, calcium can lead to hard-bonded deposits that are not self-spalling when the gas turbine is shut down, and are not readily removed by water washing of the turbine. The fuel-washing systems, used at some gas turbine installations to reduce the sodium and potassium level, will also significantly lower the calcium content of fuel oil.
6.1.7 Microbial Slimes caused by microorganisms can plug filters and other close-tolerance openings. Some organisms can cause corrosion as well as produce slimes. Under anaerobic conditions, hydrogen sulfide, which may cause corrosion, can be generated by biological action. Biocides are available for controlling the growth of microorganisms, but their effect on trace metal levels and other fuel properties should be considered. Since water is required for the growth of the microorganisms, one way of controlling their growth is to eliminate the presence of water through tank-stripping operations or other separation techniques.
7. Keywords
7.1 contaminants; fuel handling; fuel storage; gas turbine fuels
