Blade blending is a targeted repair procedure for the blades of gas and steam turbines to remove minor nicks, dents, or rubs while maintaining aerodynamic configuration and protecting the integrity of the blade. Blade blending allows the facility to avoid changing parts unnecessarily and to return the unit to service earlier. Check out https://www.youtube.com/watch?v=_KiOsJlGfF0 to learn more.
For junior engineers or operators, it may distinguish between a short outage and avoidable tripping of the unit due to failure. It also allows for an important experiential learning opportunity to engage the crew to link the aircraft inspection results with root causes of damage from foreign object damage or tip rubs.
Blade blending is the controlled removal and reshaping of a small, damaged area, which may be located on a blade’s leading edge, trailing edge, or tip. During blending a technician will blend the defect into the remainder of the metal surrounding the area of damage to prevent stress concentrations occurring at the edge of a sharp notch. The blade blending process has objectives of recontouring the airfoil to a smooth profile, reestablishing mass balance across the rotor, and preserving efficiency.
The blending works an engineer, or technician completes is considered acceptable work in the case of both gas and steam turbines when it adheres to manufacturer limits of depth, length, and contour while maintaining the structural integrity of the blade. Once the blending work is completed the amount of metal removed will still be a minimal, often measured in tenths of a millimeter, while reducing stress concentrations to a significant extent.
Blending will only be completed where a careful inspection of the asset has determined the damage is minor damage and stable. Teams depend on visual inspection, borescopes for hard-to-access rows or rows that remain hot and non-destructive testing to verify there are no hidden cracks. To know more about NDT, click here.
Once these pass, technicians will blend the turbine blade in a controlled, safe manner.
Small dings behave similar to stress risers. Under high centrifugal forces and thermal cycling, those small notches can become cracks. The blending process removes that notch or stress riser and redistributes the stress over a larger area, reducing the likelihood of propagation of the defect. In addition, blade blending is beneficial in protecting aerodynamic performance, which directly supports efficiency and heat rate of the unit.
Power plants combining blending with vibration analysis and trending often see the return of the same defect associated with foreign object damage, rubbing, or inlet filtration prior to a trip or extended outage. Fleet experience in new and used fleet shows that a blended nick correctly blended will stabilize, while an untreated notch will likely become a crack within a few starts mostly on higher cycling units.
Successful execution of turbine blade blending typically involves controlling and possessing good metrology. Crews determine which reasonably removes material and checks the restored profile, so the restoration is repeatable and consistent from job to job.
Blending is not a black and white solution for repair. There are circumstances where replacement is the safe option if a crack exists, if the defect exceeds the allowable depth or length, or if the damage falls near the cooling hole or fillet platform. A repeated blends may be acceptable if the airfoil is not too thin, decline the margin against creep and fatigue. The situation with hot gas-path inspections gives us one point of inspection that drives replacement decision point due to the elevated temperatures causing degradation of the material.
In addition, time remaining to next outage, OEM or engineering limits, and owners/subsequent role of the unit on the grid should all be considered when making the determined call. When you are beyond limits to repair, replacement maintains reliability, prevents secondary rotor work, and removes costly unplanned downtime.
