Customer saved an estimated 50% in cost compared to new replacement blade carriers and reduced outage duration by 6 months.
The repair scope offered was to recondition the stationary blades of 29 – 32. During the incoming inspection and sample removal, it was noticed that the blades had been manufactured from a cast iron*, which is not a very weld friendly material. Also, after grit-blast cleaning of a few blades and removal of the deposits, it showed that the erosion and corrosion of the airfoils was well beyond the stage to perform a reliable repair. About 25 – 30% of the airfoil shape was deteriorated, and a couple of the airfoils had broken off at a length of about six inches from the tip during the removal of row 29 and 30 due to fatigue from deterioration.
The customer was notified that the blades of row 29, 30 and 31 needed to be replaced. This was recommended to the customer at the initial site visit, and RTS offered a material upgrade to 403 , and an additional protective coating for the replacement blades.
The customer opted on the recommendation of RTS to replace the blades of row 29, 30, 31, and to restore the airfoils of the row 32 blades by spray metal build-up, the coating material also added substantial airfoil erosion protection.
The original blade material was a ductile cast iron with a high contents of carbon ( 3.85% ), cast- iron or spheroidal graphite iron. This material can be weld repaired to an extent ( small areas ), but the weld repairs to large areas will reduce the ductility and impact strength will be reduced considerably. Since the material of the airfoils had been eroded and corroded to such an extent ( Approx. 30% ), a weld build-up repair of row 29 and 30 was not feasible. The blade condition of row 31 was slightly better, but a weld build-up repair would still present problems.
*Cast iron is a relatively brittle material; it does not possess much ability to stretch. In its hardened state, the carbon is in a free form of graphite flakes in the metal structure, this means a fracture can easily spread along the graphite flakes, which have no mechanical strength. From an engineering point of view, and concerns about the reliability and integrity of the blades during operation in mind, the logical and safe solution is to replace the blades with an upgraded material, and apply an erosion/corrosion protective coating.
Blade Carrier Restoration Gallery