GAS TURBINE TECHNOLOGY
Cracking the FA R0 problem1 March 2009
Cracking of the R0 (first row) inlet compressor blades has proved a troublesome issue for GE 7FA and 9FA gas turbines (which have 17 compressor and three turbine stages). But it is hoped that a redesigned blade – the product of an intensive two year long development programme undertaken by GE, and to be introduced shortly – will go a long way to addressing the issue. James Varley reports.
The new blade will employ similar material to that currently used (Custom 450 stainless), but with modified heat treatments and now referred to as GTD-450. The blade will have a strengthened airfoil and be laser shock peened in the critical area, the lower leading edge.
R0 cracking problems first came to light in 2001 when a 9FA machine at Black Point in Hong Kong experienced a catastrophic failure.
On line compressor washing was identified as a major contributor, causing the initial erosion, roughening and pitting from which the eventual crack propagated as a result of high-cycle fatigue. New recommendations were introduced curtailing the frequency of online washing (and also inlet fogging, considered another potential source of initiating erosion due to the ingress of water droplets) and proposing regular polishing of the blade leading edges.
But it also soon became clear that design evolutions of the FA machine, aimed at increasing efficiency – including modifications to the R0 blade such as lengthening it and flaring the tip end – had resulted in higher stresses in the leading edge, reducing tolerance to nicks and scratches.
Subsequently the OEM offered a modified blade option called the P-cut - designed to redistribute stresses away from the airfoil leading edge. But cracking has been encountered in the modified blades, with the P-cut shifting the origin from the leading edge to below platform. The P-cut could even be said to have exacerbated the problems, and is being phased out.
In fact the root cause of FA R0 cracking, which has been experienced on both 7FA and 9FA machines, is still under investigation, as are potential solutions. In the USA EPRI is just completing the second phase of its “GE FA compressor dependability study.” This aims to “continue and refine the phase 1 root cause analysis, evaluate several corrective solutions including erosion-resistant coatings and laser shock peening, and study the root cause and corrective actions for other FA compressor dependability issues.” EPRI estimates the benefit from preventing a compressor failure is about $10-20 million.
Root cause failure analysis in Phase 1 of the EPRI study included blade vibration monitoring in an operating turbine and detailed stress modelling. Results provided insights into the vibratory behaviour of the blade but could not conclusively identify a dominating root cause.
EPRI concluded that regardless of the source of the high-cycle fatigue responsible for crack initiation and propagation, “improving damage tolerance of the highly stressed blade leading edge would be beneficial.” One option EPRI examined was removal of material from the pressure side of the root-bearing surface, which shifts the maximum steady-state stress back from the leading edge. A second possibility investigated by EPRI was replacing the C450 stainless – used in the first five stages of the compressor – with a titanium alloy, which is lighter and more erosion-resistant. Modelling showed that the lower density and weight of the titanium would reduce stress in the leading edge.
Phase 2 of the EPRI study has consisted of three tasks:
• Verification and refinement of root cause analysis, including investigation of recent failures in machines without the P-cut, further examination of the source of excitation, and computational fluid dynamics flow analysis including struts and inlet guide vane settings over the load range.
• Identification of corrective solutions, including surface treatments such as titanium nitride, tungsten carbide, nano-structures and surface peening - with a critical review of GE’s new offering.
• Investigation of other FA compressor reliability issues, focusing on blades and vanes downstream of R0, particularly in the initial five stages. Also included was a review of various modifications intended to improve compressor dependability, ranging from modified on-line washing, advanced inspection, and component modifications to various pinning techniques.