Obviously, proper operation of the fuel atomizer is critical to APU operation. That’s probably why a lot of maintenance personnel don’t mess with fuel atomizer inspection. “The nozzle is so critical to proper operation of the APU, a lot of operators don’t even mess with it,” says Clarke. “They just make it a 100 percent replacement item. They’ll send it in and put an overhauled nozzle in its place.”
But you don’t have to be afraid of the atomizer inspection. “Again, follow the maintenance manual,” Clarke tells AMT. “If a proper inspection is performed, there should be no reason to automatically replace a nozzle.”
One thing Clarke cautions about is the debris contamination. “Be very careful not to allow debris to get introduced in the nozzle when cleaning it. The manual tells you to apply 30 psi of air to the fuel inlet while cleaning it. It also instructs you to hold it upside down, face down, when you clean it. Gravity is your friend and allows debris to fall down and not re-enter the nozzle. You really need to follow the manual when cleaning these nozzles.” Clarke also warns that the same 30 psi of air that is meant to keep contaminants out of the nozzle when cleaning it can cause debris to re-enter the side of the nozzle because of a venturi effect. See photo on page 10.
During your combustion assembly inspection, be sure to look not only for carbon buildup, but for evidence of a piece of carbon breaking off. “When a piece of carbon breaks off, you’ll see a flat spot with points that stick up and are sometimes kind of white on the edge,” Clarke shares. “That’s an indication that a piece has broken off and has gone downstream.”
If a carbon piece breaks off, it will follow the same path the ignited air/fuel mixture does. It will exit the combustor assembly and go through the turbine nozzle and on to the turbine wheel and out the APU. But if the debris is large enough, it can get stuck in the turbine nozzle. This can lead to compressor stalls.
These are a few tips on 36-100 APU fuel atomizer and combustor assembly inspection. For more information about 36-100 APUs or for information on APU services Dallas Airmotive provides, call Jeff Clarke at (856) 455-0539 or e-mail firstname.lastname@example.org.
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