Metal working – Method of mechanical manufacture – Impeller making
Patent
1995-02-08
1997-03-04
Cuda, Irene
Metal working
Method of mechanical manufacture
Impeller making
298897, 228119, B23P 1500
Patent
active
056067968
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of repairing turbine blades.
In use, turbine blades are subject to at least one of erosion damage and cracking due to high stress in the blade.
The tip of a turbine blade in a rotor assembly is the part of the blade which experiences maximum velocity and is hence the part subject to the greatest erosion.
The present invention relates primarily but not exclusively to blades used in steam turbines. Erosion or damage of steam turbine blades generally occurs following contact between the turbine blades and either particles carried by the steam or water droplets.
Damage can occur from metal or other solid particles, for example boiler scale, entering a steam turbine with the steam flow and striking the blades. Water droplet damage is generally restricted to the final stages of a turbine. However, all stages of a turbine can be affected by particle damage.
The rate of erosion of turbine blades is dependent upon the relative speeds between the blades and the particles or water droplets causing the erosion and it is generally the leading edge at the outer end of each blade that is most prone to erosion by impact.
At low pressure end of a steam turbine the steam is at a lower temperature having been cooled during passage through the turbine and at the dew point water droplets precipitate giving rise to "wetter" steam. The transfer of energy by impact in the turbine blades of the steam is therefore greater at the lower pressure end than at the high pressure end of the turbine. Once again it is predominantly the outer ends of the blades which are especially subject to erosion.
To reduce the rate of erosion, it is usual to provide a shield at the leading edge of the outer end of the turbine blade, such a shield being made from a material harder than the blade material.
Steam turbine blades may be made from many suitable materials, a typical example being a 12-13% chrome steel and the shield may be made from stellite or a suitably hardened steel. The shield may be brazed or welded to the blade.
After a period of use it is often found that the shield has been considerably eroded and one accepted method of repair is to first remove the old shield and secure a new shield in place either by brazing or welding the new shield to the blade or building up a new shield depositing fused metal in a welding operation. The blade is then machined in the area of the repair to return it to its original aerofoil profile.
Cracks in turbine blades due to stress in the blade usually occur at stress concentrations and notches found near discontinuities in the blade aerofoil or root section.
Turbine rotors generally speaking have two types of blades:
(a) Free-standing, secured to the rotor only at the root but which have no connection between adjacent blades;
(b) "Tied" blades which are additionally connected to each other by cover bands connecting each blade by tenons at the end of the blade secured to a shroud passing round the ends of all the blades, or "lacing wires" which pass through holes in the blades, each blade being brazed or otherwise secured to the lacing wires. Some turbine blades are provided with snubbers, snubbers being projections from the lower and upper surfaces of the blade to connect with similar projections from the upper and lower surfaces of adjacent blades.
Repair of the two types of turbine blades involving welding or brazing steps may lead to deformation of the blades. In the case of free-standing blades the blade is allowed to deform during the application of heat from the welding or brazing stage and thereafter may be mechanically returned to its original shape, i.e. by bending. With blades which are tied to each other by lacing wire, snubbers, etc., the application of substantial thermal energy to the blade during a welding or brazing operation does not deform the blade to such an extent since unrestricted movement of the blade is not permitted due to the presence of the lacing wires, etc. Since the blade is not able to freely move considerable stress is imparted to the blade
REFERENCES:
patent: 5210946 (1993-05-01), Monroe
patent: 5351395 (1994-10-01), Crawmer et al.
patent: 5448828 (1995-09-01), Willems et al.
Cuda Irene
Turbine Blading Limited
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