MAINTENANCE MATTERS
On the other hand, the MORE Company’s PT6A ICA only allows a maximum fuel nozzle inspection and cleaning interval of 300 hours, and requires this to decrease if fuel nozzle condition is not satisfactory. Repair stations that inspect and clean PT6A fuel nozzles report that engines using the Pratt & Whitney Canada inspection interval occasionally have fuel nozzles that are not flowing properly at the end of the P&WC inspection interval, but that engines using the MORE Company inspection interval almost never have fuel nozzles that are not flowing properly at the end of the MORE Company’s inspection interval. Repair stations that inspect and clean PT6A fuel nozzles usually only charge a few hundred dollars to clean and inspect an engine set of fuel nozzles. In the long run, shorter fuel nozzle inspection intervals can save money.
SULFIDATION Pratt & Whitney Canada has written two service information letters (SILs) that are helpful in understanding sulfidation. They are SIL No. 1023 “Sulfidation Attack” (May 20, 1985) (i.e., Ref. 1), and SIL No. GEN PT6- 016 “Sulfidation Attack” (March 9, 1998) (i.e., Ref. 2). When iron or steel is attacked by oxygen, they corrode. We call this corrosion rust. When high-nickel alloys or high-cobalt alloys are attacked by sulfur, they also corrode. We call this corrosion sulfidation. Sulfidation “is caused by the condensation of an alkali metal salt, usually sodium sulfate, on the surface of the part” (Ref. 1). “Most aviation turbine fuels contain traces of sulfur in sufficient amounts for sulfidation to occur if a source of sodium is present.” (Ref 1). Sodium (e.g., sodium chloride AKA salt) is found in the air above salt water up to an altitude of one mile, along with air pollution, volcanic dust and gasses and agricultural chemicals.
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Ref. 1 contains a map of the United States showing where sulfidation occurs. The Atlantic, Pacific and Gulf coasts have sulfidation problems caused by salt water. The North East (the portion of the United States that is both north and east of Saint Louis, MO) has sulfidation problems caused by air pollution. The South East (the portion of the United States that includes Louisiana through South Carolina) has sulfidation problems caused by air pollution. A volcano in Iceland erupted
five years ago, throwing enormous amounts of volcanic dust and gasses into the atmosphere. More recently, there have been additional smaller volcanic events. My local television weatherman said that both the Icelandic volcano and La Nina weather patterns were the cause of our severe winter weather. In summary, it is impossible to
fly anywhere in the Northern Hemisphere without encountering sodium to a greater or lesser degree from one or more of these sources. “A proven method of minimizing sulfidation is the turbine desalination wash using plain water” (Ref. 2). More frequent wash intervals are appropriate for heavier contamination. Ref. 2 says that chromium aluminide, platinum aluminide and Sermaloy J (silicon aluminide) have improved sulfidation resistance. Based on my experience with parts manufacturer approval (PMA) replacement parts, Sermaloy J seems to be the better coating choice in small and medium PT6A turboprop engines. The washes that I mentioned previously while discussing compressors have a two-fold benefit in that these washes benefit both the compressor and the hot section.
“It has been suggested that the
power level at which an engine is operated may have an effect on the rate of sulfidation” (Ref. 2). “PWC does not recommend attempting to
resolve a sulfidation problem through changes in operating regime” (Ref. 2). Two real-world examples will help illustrate this point. The maximum turbine inlet temperature (CT vane inlet temperature) on a PT6A-20 engine is 1,821°F (994°C). The maximum inter-turbine temperature (CT blade outlet temperature) on a PT6A-20 engine is 1,382°F (750°C). The power turbine blade exhaust temperature is approximately 1,000°F (538°C). “Researchers have found that sulfidation attack takes place when condensate-coated surfaces operate at temperatures in the range of 1,250° to 1,800°F” (Ref. 2). Hence, most of the hot section parts will be exposed to some degree of sulfidation. The PT6A-21, PT6A-28, PT6A-34 and the PT6A-114A have the same compressor, combustion chamber liner, CT blades, PT vane ring and PT blades. PT6A-21 and PT6A-28 have the non-air cooled CT vane ring, whereas the PT6A-34 and PT6A-114A have the air- cooled CT vane ring. The maximum allowable operating temperatures for these four engines are significantly different, and this significantly affects where sulfidation usually occurs. PT6A-21 maximum temperature is 1,283°F (695°C), and the most frequent sulfidation location is the CT vane ring. PT6A-28 maximum temperature is 1,382°F (750°C), and the most frequent sulfidation location is the CT blade airfoil. PT6A-34 maximum temperature is 1,455°F (790°C), and the most frequent sulfidation location is the CT shroud segments. PT6A-114A maximum temperature is 1,481°F (805°C), and the most frequent sulfidation location is the underside of the CT blade platform. These examples show that sulfidation will continue to occur, and significant changes to the engine operating temperature will only change the location where the sulfidation will occur.
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