O&M

Fatigue life improvement of R0 compressor blades: A comparison of laser peening and low plasticity burnishing

Low plasticity burnishing can be applied to new or existing R0 blades to extend component life while increasing reliability, confidence and margins of safety.
Fatigue life improvement of R0 compressor blades: A comparison of laser peening and low plasticity burnishing
(Photo courtesy of Lambda Technologies Group.)

By Michael Prevey, President of Surface Enhancement Technologies at Lambda Technologies Group

EPRI (Electric Power Research Institute) reports that 7FA gas turbines have a rotor life of about 144,000 hours, or 5,000 starts. The primary life-limiting factor is high cycle fatigue cracks propagating from erosion damage at the leading edge of the R0 compressor blade. The erosion reportedly initiates from a number of factors, including fogging and compressor washing, foreign object damage, and corrosion.

Significant operation and maintenance costs are associated with preventing these fatigue failures. Blending of the blade edge is required once erosion damage reaches 0.008in. (0.2mm). This occurs approximately every 500 starts and necessitates blade replacement, requiring extended equipment downtime.[i]

Initial efforts to decrease the frequency of blending and blade replacement have included 1) thickening the leading edge of the blade to provide more material for removal 2) changing the material of the blade to improve resistance 3) and laser shock peening to apply residual compressive stress to the leading edge. All of these solutions provide some improvement, but have limitations. Material changes and thickening of the blade provided a moderate improvement in fatigue life while degrading engine performance. Laser peening effectively imparts compressive stress to the blade, but is damaging to the surface, producing nearly four times the roughness of LPB at the critical boundary layer. It is only applied to new blades and doesn’t allow for further repair once the blending threshold is met.

Solution

Lambda Technologies developed a solution to extend the fatigue life and improve the damage tolerance of the R0 blades. Using low plasticity burnishing (LPB®), Lambda Technologies’ engineers applied a deep, stable layer of engineered compression to the leading-edge region of the blades. The goal of this project was to match or exceed the compression of laser-peened blades and improve damage tolerance without damaging the surface.

Figure 1. Fatigue testing setup. Photo courtesy of Lambda Technologies Group.

R0 blades were processed with LPB® and laser peening, respectively. LPB® was applied in a single pass, while R0 processing specifications currently require 6 passes for laser peening. Erosion damage was simulated at 0.025in. (0.635mm) to imitate damage well over the threshold allowed in service.

Fatigue testing, surface roughness, and residual stress measurements were performed on the blades to compare the surface enhancement processes. The fatigue setup can be seen in Figure 1, where the leading edge is loaded in cantilever tension. The blades were tested at EPRI estimated maximum operating stresses.[ii] All measurements and testing were performed by Lambda Research, Inc.

Results

Figure 2. Residual stress measurement results. Photo courtesy of Lambda Technologies Group.

Through-thickness compression was achieved on the LPB-processed R0 blade, effectively providing infinite life for blades with damage <0.025in. (0.635mm), including those that reach the current blending threshold.

Residual stress measurement results are shown in Figure 2. LPB produced residual compression that was nominally 50% higher in magnitude at all depths and locations measured compared to laser peening. Linear elastic fracture mechanics (LEFM) predicts the higher magnitude compressive residual stress will result in better fatigue performance for the blade in service.

Fatigue testing was performed with only LPB processed and baseline blades due to a limited number of laser-peened samples. Damage tolerance was improved by a factor of three compared to the baseline blades. The results of this testing are shown in Figure 3.

Figure 3. Fatigue testing results. Photo courtesy of Lambda Technologies Group.

In a 2015 report on the use of compressive layer surface treatment on the 7FA R0 blades, EPRI stated, “Low plasticity…burnished blade’s residual stress measurements met or exceeded the compressive layer depth and magnitude of the originally applied laser shock peened compressive patch.”[iii]

One 7F turbine owner and operator installed several LPB blades in their turbines as part of a field test in 2015 and, “ran the hell out of the units.” The blades performed well and are still in operation today.

Further uses and applications

LPB has been in production on many applications since the early 2000’s. While many of these applications are related to turbines, the process is not limited to turbomachinery applications. To name a few, LPB has also been used to treat stress concentrations in welds, to eliminate stress corrosion cracking in pipes, and to improve the throughput of pumps. Some applications in the power industry include:

  • Mitigating erosion damage in compressor blades of 7FA and 9FA gas turbines
  • Eliminating the chance of cracking from fretting fatigue damage on rotor through bolts in 501G-class gas turbines
  • Improving fatigue resistance to erosion and high-cycle fatigue in low-pressure steam turbine blades
  • Mitigating foreign object damage (FOD) initiated cracking in seventh-stage compressor blades of 501F gas turbines
  • Alleviating stress concentrations in first-stage compressor blades of Taurus 70 gas turbines

Selecting low plasticity burnishing over laser peening for R0 blades provides a 50% improvement in magnitude of compression. LPB provides a significant decrease in the number of overhauls required in all applications and, as a result, reduces the amount of time a unit needs to be shut down for repairs. The process can be applied to new or existing blades to extend component life while increasing reliability, confidence and margins of safety.

About the Author: Mike Prevéy joined Lambda Technologies in 2010 as a Project Engineer. In 2014, he was appointed to Engineering Supervisor. In 2018 he was promoted to Operations Manager. In 2023, he was promoted to the position of President of Surface Enhancement Technologies, part of the Lambda Technologies Group.

Mike is responsible for overseeing the surface enhancement division of Lambda Technologies Group. His degrees include a BS in Mechanical Engineering from University of Dayton and an MA in Business Administration from Xavier University. He holds four patents related to Lambda’s low plasticity burnishing (LPB®) process and continually making advancements in Lambda’s surface enhancement efforts.

References

[i] EPRI (2019). Cracking the FA R0 problem. Modern Power Systems. https://www.modernpowersystems.com/features/ featurecracking-the-fa-r0-problem//featurecracking-the-fa-r0- problem-412766.html

[ii] EPRI (2015). Improving compressor airfoil damage tolerance: evaluation of compressive layer surface treatment. Gas Turbine Advanced Components and Technologies https://www.epri.com/ research/products/3002006059

[iii] EPRI (2015). Improving compressor airfoil damage tolerance: evaluation of compressive layer surface treatment. Gas Turbine Advanced Components and Technologies https://www.epri.com/ research/products/3002006059