Why solid particle erosion is a concern

For industrial gas turbines in general, larger particles are well controlled by appropriate inlet air filtration systems. However, certain fuels, such as low grade liquid fuels or certain types of syngas, may cause the formation of ash as part of the combustion process. Despite advances in gas cleanup procedures, syngas produced from coal gasification contains traces of fly-ash particles with diameters ranging from 1 μm to 10 μm. Another issue is the particle formation during combustion, especially when using low grade fuels or coal derived syngas.

Aircraft engines air filtration has been studied in some detail, specifically for cases of sand or ash ingestion into jet-engines. Less attention has been paid to ground-based applications, since in most of these cases, improved filtration can be utilized to adequately avoid or control blade erosion, while fouling still remains an issue.

In recent years, there has been renewed interest in this topic and the associated prediction of solid particle behavior for ground-based applications, because of practical limits of filter material efficiency, size, and cost. Specifically, to determine a proper filter material and design for a ground-based machinery application most cost effectively, one must be able to predict the size, type, and weight of solid particles which would cause the machine operational damage through fouling and erosion. In the gas turbine’s compressor, fines and particle ingestion damage is mostly related to fouling (buildup of solid matter on the blades) and erosion through abrasive metal removal from the blades’ leading and trailing edges (rounding of the blades); i.e., blades are effectively sandblasted which leads to a deformation of the airfoil shape. The net consequence of the above is a general loss of compressor pressure ratio and overall gas turbine efficiency. Some of these fouling losses are recoverable through cleaning of the blades, but erosion losses are generally not recoverable.

In the combustor, the local flame temperature can be as high as 4,500°F. At these temperatures, sand will become a liquid silicone oxide (often called slag) that will deposit on the combustor injector and liner walls. These slag deposits will often lead to plugging of cooling holes on the liner, thus causing inadequate local flame cooling and wall hot spots, leading to premature combustor liner metal failure. In the hot turbine section, the sand and dirt particles generally create two separate problems: Solid particles can easily plug up film cooling holes/slots, and thus, create film-cooling deficiencies.

Also, the hot sand and silicone exhausting the combustor and entering the nozzle section can cause silicone deposits. Due to the high impact pressure and temperature of the sand particles, the silicone melts out of the sand and forms a solid film layer on the surface of nozzles and buckets. This film decreases the heat transfer (heat carry) ability of the metal and results in local overheating of the blades.

A final issue for gas turbines to consider is the blade cooling air that is bled for the compressor discharge and is then used as an internal cooling flow in the gas turbine hot section blades. Any solid matter, such as sand particles, that is carried into this cooling flow, will deposit in the very fine blade cooling passages and plug them. Consequently, there will be insufficient cooling flow on the hot section blades, and premature high temperature blade metal failure will be the result.