Metal working – Method of mechanical manufacture – Impeller making
Patent
1996-07-29
1998-05-26
Cuda, Irene
Metal working
Method of mechanical manufacture
Impeller making
2940207, B23P 1500
Patent
active
057550301
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION OF INVENTION
The present invention relates to a method of repairing turbine blades and is primarily concerned with repair of rotor blades having apertures therein through which apertures pass blade interconnecting members such as lacing wires, cover bands and seals or the like.
For the sake of convenience, all such discontinuities will hereinafter be referred to as lacing wire holes.
After prolonged use of a turbine, it is not uncommon, particularly with some designs, for damage to occur around lacing wire holes. In view of the inclement environment in which turbines have to operate the metal of a turbine blade surrounding the lacing wire hole may be adversely affected during use. The metal surrounding the hole cane be damaged due to relative movement between the lacing wire and the blade. Such relative movement can cause hardening to the metal of the turbine blade causing brittleness which can encourage the formation of cracks and, if no remedial action is taken, any cracks formed may extend further and further away from the lacing wire hole into the blade and eventually render the blade in an unsafe condition.
In many cases the condition of the metal surrounding the lacing wire hole may not be immediately apparent. It is envisaged that when a turbine is taken out of commission for a plant maintenance programme, the blades will be inspected and if any blades in the array show any sign of damage it may be that all blades will require attention, albeit the damage may not be visible.
The damage may occur for a variety of reasons. There is normally a clearance between the lacing wire and the lacing wire hole allowing the lacing wire to float in a dampening mode and the gap that exists can give rise to the problems mentioned above. Furthermore, if the lacing wire hole is formed with slightly incorrect dimensions this can lead to an increased gap between the lacing wire and the blade, the gap can trap contaminants and furthermore incorrect fitting between the lacing wire and the blade can lead to movement between the two parts causing a hardening of the metal in that region. The hardening and trapping of contaminants leading to the phenomenon known as stress corrosion cracking.
Applicants have been involved in many processes for the repair of turbine blades, British Patent No. 2,091,139B discloses a method of repair of turbine blades in which an outer leading edge part which has been cracked or eroded is removed and replaced by a new piece of hard material. Prior to the application of considerable heat to the blade that may occur during welding and prior to the application of any physical force to the blade, and subsequent heat treatment, any lacing wire hole within the vicinity of the repair such that it may be affected by the considerable heat or physical force is first filled with weld material. The blade is therefore homogeneous and does not have discontinuities which, can either during the application of considerable heat during either welding or subsequent heat treatment processes, lead to inbuilt stress in the blade. It has been customary in many repair processes carried out by the applicant to first fill the lacing wire hole with weld material and then subsequently reform the lacing wire hole by drilling, for example.
British Patent No. 2,114,921B deals with the same problem and rather than filling the lacing wire hole with weld material involves the filling of a lacing wire hole with a plug which is welded to the blade, the plug may stand proud of the blade or be formed with other formations to provide locating means for a drilling machine so that the lacing wire hole can be accurately reformed.
A still further process is disclosed in application publication No. 2,006,372; the filling of lacing wire holes by driving in a tight fitting plug which can additionally be provided with a heat sink and possible fluid cooling to minimise adverse affects around the lacing wire hole.
All the above mentioned processes have been concerned with filling the lacing wire hole to remove the discontinuity wh
Cuda Irene
Turbine Blading Limited
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