During a planned major outage, last stage blades were found with cracking in the blade attachment. The blades were of a free-standing design, which means they are not coupled to adjacent blades by means of a shroud, snubber or tie wire. The cracking was clustered in groups and were not present 360 degrees around the row.
TG Advisers scanned the blade with blue light technology to create a solid model. TGA then meshed the solid model to complete a finite element analysis (FEA) of the blade. Using the FEA software, TGA completed a series of calculations including blade natural frequencies and blade root stresses.
The blade natural frequencies on large blades are important to ensure that the blade is well tuned, meaning it will not be excited by a multiple of running speed.
Analysis Findings: The results of the blade frequency calculations showed that the blade was well tuned against multiples of running speed. However, a key finding was that the stresses in the blade attachment, when vibrating at its first natural frequency, was highest at the location of observed cracking.
Free-standing blades are susceptible to aeroelastic vibration, both at high flows (unstalled flutter) and low flows (stall flutter) and certain combinations of LP exhaust pressure. It has been shown that adjacent free-standing blades with approximately the same natural frequencies are most susceptible to unstalled flutter. An industry best practice to reduce exposure to aeroelastic vibration on free-standing blades is to frequency test each blade and mix-tune the blades. This simply means to ensure there is a multiple hertz natural frequency variation between adjacent blades. This can be accomplished passively using manufacturing tolerances or actively by the manufacture of two different part numbers with slightly different geometry to produce the desired frequency variation.
Since the cracking was found during a routine major outage, the design change options were limited to return the unit to service as soon as possible. Ultimately TG Advisers recommended the following:
- Mixtuning of blades – It was confirmed that the blades were not originally mixed tuned.
- Material change – This upgrade offered improved strength.
- Shot peening of blade attachments – This helps resist cracking by introducing a compressive stress layer on the surface.
To date, there has been no issues with the upgraded blades.
If you have a turbine with large free-standing blades, ensure the blades are mix-tuned when assembling or replacing blades! Periodically audit flow and backpressure conditions to ensure they are compliant with allowables. Complete thorough inspections of the blade root and rotor attachments when possible. In addition to major outages, on many designs inspections of the L-0’s can be completed through the condenser after peak run seasons.