Combustors: The repair of "hot oven"
Craftsmanship skills in the combustor repair shop
The combustor is the heart of the engine. Here thermal energy which will be converted downstream into kinetic energy is generated. The stresses that a combustor can actually withstand vary according to manufacturer and type, but in every case they will be extreme: pressures of between 27 and 29 bar and burning temperatures of over 2000°C. The gas is "cooled down" to 1400°C to 900°C in a very short distance before the gas jet exits the combustor, encounters the turbine blades and sets them in motion.
To withstand the high operating temperatures and pressure loadings, the manufacturers have developed a variety of construction methods. On the PW4000 and V2500 types individual component groups in a combustor – the inner liner, outer liner and dome with rings for the fuel injection nozzles – are built as single components. When it comes to the materials used, mostly nickel-base alloys, once again each manufacturer adds its own touch to the ingredients. Every engine type has different repair requirements and possibilities.
Heat, pressure and vibration all take their toll on the combustor, even if these days material composition and construction methods are quite sophisticated. Patches showing signs of damage are replaced or recoated. Metalworking trades and craftsmanship skills are in demand for the cleaning, milling, drilling and welding in the combustor repair shop at Lufthansa Technik in Hamburg. Burns feature prominently in the test results, but unusual signs such as dents can also occur. Everything is documented with photographs. Following a visual check and fact-finding, a repair proposal is drawn up by staff in production planning and, if necessary, agreed with the manufacturer.
The first stage involves dismantling the combustor into its constituent parts, removing protective coatings, cleaning the parts and checking for cracks, which is repeated after the repair by way of inspection. In the course of the repair process heavily burnt sections are milled out. After that, plates made from the same material are welded in or "patched–. Cracks are milled out and then welded. The finishing work which follows has to be performed equally carefully, as the new contour must match the original contour exactly.
Lufthansa Technik develops its own procedures for the repair of combustors. Some of the repair methods used for the V2500 combustor reflect the expertise and analysis of the engineers. To restore the fuel nozzle rings to a condition suitable for use, they have developed a coating that provides new protection for the part and also a procedure under which the damaged ring can be removed and replaced by a part developed in-house – a Spare Part Alternative Detail (SPAD). Because it is certified as a development organization, Lufthansa Technik is able to manufacture this SPAD in-house and weld it on. For the customer, the combustor repair brings obvious savings compared with the price of new parts.