Many older heavy duty gas turbine models have a very simple compressor casing design for attaching the stationary blades. It consists of grooves machined into a cast iron casing into which are slid blades with square bases. These bases have extending ears on two sides that engage undercut grooves in the main groove. This design works well, however, when the blade groove is very close to an extraction slot this results in a thin ligament in the casing which eventually cracks. This allows blades to liberate into the flow stream which results in major engine damage. One engine, the GE frame 5 with compressor cast iron casings has a tendency to crack in the blade attachment groove at the horizontal joint in row 10 where the air extraction is taken. The casing hook tends to bend due to the aerodynamic forces on the blades. An analysis shows how the blade forces are transferred to the weak casing ligament. This results in a crack at the thin ligament. The bent and cracked casing hooks are generally visible through a borescope inserted into the extraction cavity from the air pipe flanges. If this situation is not repaired, the cracks can lead to both casing material and blade liberation into the compressor flow stream. A quick and low cost repair has been developed to restore these engines to a reliable operating condition.

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