The following guidelines should be followed to ensure optimum engine performance.
• Compressor turbine blade tip clearance should be maintained close to normal limits. Compressor turbine blade clearance can be improved by maintaining vane ring lug and groove geometry and minimum side clearance of lugs.
• The same power turbine first-stage vane ring class needs to be maintained as was installed on initial engine build.
• The interstage sealing rings should be serviceable to prevent pressure leakage.
• Inspect the compressor bleed valve for proper operation.
• Ensure no pressure leaks are present in the gas generator assembly area.
The following conditions should be maintained by rework on the affected components. Accomplish rework by hand stoning and lapping on locally manufactured lapping plates.
Flatness and sealing must exist on the flat sealing land between the small exit duct and the leading edge of the CT vane ring’s outer ring. This should be lapped to obtain a good sliding fit.
Flatness and sealing must exist on flat sealing land between the CT vane ring and lockplate. Lap as required.
A free sliding fit must exist between the lugs of the CT vane ring and the slots in the Number 2 bearing cover flange and CT shroud housing. Hand stone the lugs if necessary.
A free sliding fit also must exist between the combustion chamber liner and small exit duct. Rework by hand as required.
Note that excessive clearance between lugs and slots will cause tangential play and destroy blade tip clearance. Replace or return to an overhaul facility for rework those components with excessive clearance.
Bulge on small exit duct.
Zero grind fit
One of the most time consuming parts of performing a hot section inspection is the grinding of compressor turbine shroud segments. If the segments are replaced, grinding may be necessary to remove high spots and/or any slight eccentricity of the segments in order to obtain correct turbine blade tip clearance.
Although many operators are performing their hot section inspections themselves, some of them are choosing to send their hot sections to a certified repair station for inspection. These repair stations, like Heritage Turbines, are able to provide a zero grind hot section for the customer. Basically, the CT vane ring, turbine disk, small exit duct, and number 2 bearing cover are sent in for inspection. Once all inspections and repairs are accomplished, the unit is placed on a lathe for all necessary grinding. Working in a controlled environment on a lathe, they are able to obtain very tight tolerances between the turbine disk and CT shroud segments so that when the operator installs the components on the engine, no grinding is needed. The compressor turbine still needs to be checked for clearance, but usually no additional grinding is necessary.
Stan Bernstein of Heritage Turbines explains that this is a valuable service for the customer. He states, "Grinding the hot section segments in the field can be a very time consuming endeavor. Consecutive steps of grinding and measurement are performed until the desired tolerances are obtained. With a zero grind hot section, the customer installs it, and it is ready to go with no grinding necessary."
When it comes to packing the hot section for shipment, do not skimp on the material. Bernstein states "It is vital to protect the components for shipment. All of these items are very expensive, and skimping on packing can easily add up to thousands of dollars worth of damage."
Taking measurement on compressor turbine for fit.
Once all items are inspected and all defects are corrected, the units are reassembled. Be sure to maintain proper clearances and free sliding fits as directed in the maintenance manual.
Once the engine is reassembled, the an engine performance run should once again be performed to check that all engine parameters are within limits and to ensure proper operation.
The PT-6 engine is an engine with a proven history of reliability. A hot section inspection — along with other good practices, such as engine washes and fuel nozzle checks — can help ensure your engine gives you years of safe and efficient operation.
This article on PT-6 hot section tips was written by Standard Aero's Kelly M. McCallum, and originally ran in the September 2001 issue of AMT. Due to several reader inquiries on the article, we have...
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