Most gas turbines have axial flow compressors with multiple rows of rotor blades and stator vanes that number into the thousands. Failure of any blade or vane can lead to severe collateral damage in both the compressor and turbine section of the engine.
Root cause analyses (RCA) often find foreign and domestic object damage and operational issues as among the usual suspects. However, shim protrusion and liberation and stator vane fretting wear are also becoming common reasons for failure. Other factors that can accelerate fretting wear include flow disruptions from casing geometry changes, air extraction points, rotor blade clocking, and higher frequency of start and stop cycles due to the demand for plant flexibility to accommodate solar and wind power.
Protruding and missing shims are regularly found during boroscope inspections and overhauls on GE gas turbine compressors. In some cases, a protruding shim can create a flow blockage of more than 15%. Blockage results in a force pulse which produces an alternating force or stimulus on downstream rotating blades. Since shims are generally located near horizontal joints, this produces one or two pulses per revolution. At these locations, force impulse on the rotating blades is the result of the area change in the stationary vane flow passage. This is exacerbated by trailing edge vortices that can upset the flow path following the protruding shim.
As the force profile is narrow, there are multiple harmonics which can contribute to circumferential force distribution on rotating blades. If any harmonic frequencies are resonant with blade natural frequency, blade vibratory response is amplified at this resonant condition.
At a resonant condition, blade vibration stress is directly proportional to the magnitude of the stimulus or alternating force on the blade; it is inversely proportional to the damping of the bladed system, and proportional to the resonant response factor and steady gas bending load on the blade.
The resonant response factor is the attenuation in response due to the phase relationship between the alternating force, or stimulus on the blade, and the particular mode shape. This method for predicting blade response can be used to evaluate the dynamic response of a compressor blade due to protruding shims. But it is important to understand that blade natural frequencies are dependent on several variables. They should not be predicted as discrete frequencies but as a range.
Temperature affects frequency due to the dependency of Young’s modulus on material temperature. Furthermore, manufacturing tolerances can affect frequency due to vane envelope tolerances and dovetail fit variations. This must be addressed by calculating all frequency ranges for the fixation point at the top and bottom of the contact area in the dovetail slot across the full range of operating temperatures.
This information can then be used to construct a Campbell diagram to illustrate the relationship of blade natural frequencies with the stimulus frequencies from the rotational speed of the compressor. As an example, consider the evaluation of the row 15 rotor blade of a 7FA compressor. One potential resonance was found with eight pulses per revolution (there is a stimulus force that occurs eight times for every revolution of the rotor) at the first tangential mode of vibration. Blade vibratory response was calculated as a function of different protruding shim conditions and assuming various values of damping (Q). Damping can be variable and sensitive to various factors such as dovetail fit. However, previous research and testing suggests that the damping for this blade design should be in the range of Q=200– 300 (Morgan P. Hanson. Effect of Blade- Root Fit and Lubrication of Vibration Characteristics of Ball-Root-Type Axial- Flow-Compressor Blades. Cleveland, Ohio; Lewis Flight Propulsion Laboratory, National Advisory Committee for Aeronautics, June 1950).
The blade vibratory stress at the eight pulses per revolution resonance would be in the 20,000 to 30,000 psi range if there was only one shim protruding 0.75 inches. This response is about 2.5× the predicted response at resonance without a protruding shim. For the case without a protruding shim, prediction was based on a random variation of stator forces (namely vane variable spacing and orientation) with a maximum variation of 1.5%. For two shims protruding at 180 degrees separation, the response is in the range of 40,000 to 60,000 psi. The blade failure mode due to stimulus from the protruding shims is high cycle fatigue.
A Goodman Diagram is typically used to evaluate the fatigue strength of a material under the combined influence of steady and alternating stresses. It shows the potential blade response due to protruding shims is higher than the deteriorated endurance strength for the material. High cycle fatigue is a possibility depending on the operational profile and the number and height of the protruding shims. Since the resonant frequency is 480 hz, fatigue cracking could occur in as little as ten million stress cycles which is less than six hours of operation on resonance.
Other major problems for compressors are loose vanes due to fretting wear in the casing T-slot groove, and severe wear on the vane attachment hooks. The average and maximum wear is always greater in the upper half (UH) casing and vanes. The worst location is at the horizontal joint on the right side looking downstream.
The reasons: (1) Aerodynamic forces push all the vanes over against the left side keeper bar in the UH, thus allowing the right-side vane to be the last one in the pack, making it rather loosely held. (2) This right-side vane in the upper half is notched to accommodate the keeper bar. Therefore, it does not have a full attachment hook in the base. This results in accelerated fretting wear. Typically, the average upper half vane looseness is nearly three times that of lower half vanes.
Variations in vane spacing and orientation must be kept small to have acceptably small harmonic excitations on downstream rotor blades. Platform lift creates a flow disturbance which will impact performance. An analysis was performed assuming a vane lift of 0.04″ in half the rows of vanes can result in a 1⁄3% loss in efficiency. Left unresolved, tip rock can lead to vane clashing with the compressor rotor blades, vane liberation, and compressor failure. Vane fretting wear and shim migration into the flow path are addressed by preventing vane movement. The lower half casing is where many vanes can be found solidly locked in place due to corrosion and deposits from the flow path.
One way to lock vanes is to pin them together. This procedure uses dowel spring pins inserted into holes drilled into the circumferential faces of the vane bases. Other vanes are attached, and more pins are added until a half ring vane segment is formed in both the upper and lower casing haves. This forces the vanes into an evenly spaced condition that reduces the average harmonic stimuli on rotor blades. In addition, this greatly reduces fretting wear, the potential for liberation of vanes and shims, and protruding shims that can lead to rotor blade high cycle fatigue stimulation and failure. This process has proven reliable and has successfully been implemented on more than 200 turbines over a period of more than 17 years.
Other solutions include: • A casing patch ring or weld repair and re-machining of the casing, as well as a complete replacement of the compressor vanes with a shim retention method. However, this approach does not address tip rock, i.e., wear of the hook fit and vane base due to fretting, which can occur over time • Replacing individual vanes with a multi-vane segment with a shim retention method, i.e., multiple vanes are machined in to a common or continuous solid base.
Across the large, worldwide fleet of gas turbines, there will be a statistically small number of compressor failures due to variability and wear impacting Goodman stress capability. When potential blade stimuli are identified, such as protruding shims and vane looseness, it is a best practice to eliminate them. The key to success of any solution is to have and maintain sufficient dampening over the wide variety of operating conditions.
Taking care of shim protrusion, shim liberation, vane fretting wear and tip rock will improve the general health and performance of the fleet. ■
Robert Traver is Senior Engineer at CTTS, a company established in 2019 to provide on-going fleet support for compressor vane pinning. Rodger Anderson is a Consultant who recently retired from DRS Technologies. For more information visit: cttsus.com.