STAGE 3: Severe sulfidation — There is evidence of liquid sodium sulfate under the protective layer, along with a buildup of scale on the blade surface, and perhaps even on blade root and in shroud areas. Mechanical integrity is significantly affected and the sulfur corrosion will continue even with no further exposure to contamination attacks.
STAGE 4: Catastrophic attack — There is evidence of large blistering scale with deep penetration of the blade base metal, eventually leading to component failure unless the degraded parts are immediately removed from the engine. Structural integrity is lost.
Minimizing the effects
While there are only limited things a pilot or mechanic can do to prevent the sulfidation attacks on the turbines, just being aware of the turbine’s vulnerability can justify shortening maintenance intervals. Shorter maintenance intervals are more costly, but the long-term cost savings more than offset that expense. Pratt & Whitney Canada (P&WC) recommends: performance recovery wash, desalination/desulfidation wash to slow down corrosion effects and borescope inspection to monitor engine condition. Since sodium (salt) is a major contributor to sulfidation, if the aircraft is operated from or near (within 20 miles inland of) a saltwater source, the likelihood for severe damage is greatly increased. The same is true if the aircraft is operated near high pollution risk areas. Since desalination and desulfidation wash use only clean water, it is nearly impossible to damage the engine by washing the engine too often. When in doubt, perform desalination and desulfidation washes more frequently.
A common myth is that changing engine operating temperatures can alleviate sulfidation problems. The truth is that changing engine operating temperatures only moves the location within the turbine area where sulfidation occurs. Lowering operating temperatures moves the sulfidation to the CT vane ring, whereas increasing the operating temperatures moves the sulfidation to the CT shroud segments and the power turbine vane ring. For this reason, flight crews are not encouraged to make any changes in the operating regime of the aircraft to decrease sulfidation. However, they should be aware that periods of high temperature during start-up, and high temperature/high power conditions during take-off and cruise all contribute to “premature hot engine distress.”
Flight crews should use “Engine Condition Trend Monitoring” procedures if available to record flight performance data that will help technicians during inspections.
Technicians should inspect other parts of the “gas path” including the intake, fuel, and exhaust systems that can also contribute to overtemperature conditions in the hot section. Installation of intake and exhaust covers while the aircraft is parked for extended periods, especially in high wind environments, is highly recommended. In addition to intake air quality, fuel quality and proper delivery functions are important to maintain recommended engine temperatures and avoid “gas path contamination.” Thorough and regular inspections of fuel quality, fuel storage, and delivery equipment, as well as fuel nozzle efficiency and fuel pump filter condition, help decrease the potential for contributing conditions to sulfidation. Fuel handling is a complex process. If you are not familiar with proper fuel handling techniques, purchase fuel from sources that are experts in proper fuel handling techniques.
Besides periodic inspections, technicians can perform motoring washes to clean and desalinate the compressor section of the engine. Performance recovery wash uses cleaning chemicals in the compressor with OEM-approved products like R-MC engine cleaner followed by clean water wash in the compressor and turbine. Desalination wash uses clean water in both the compressor and turbine to remove salt, air pollution, agricultural chemicals, and sulfur. In extreme cases, daily washes (or even after each flight) may be necessary when the aircraft is operated in high-risk areas.
Sulfidation exists in the blistering environment of a turbine engine hot section.
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