Patent Application
20120093634
This invention generally relates to turbine technology and, more specifically, to turbine buckets or blades formed with integral tip shrouds.
Tip shrouds located at the tips of turbine buckets or blades dampen vibrations and support the tip areas of the airfoil portions of the buckets. Tip shrouds also form the radially outer boundary of the hot gas flow path through the turbine stage. As such, tip shrouds span from one bucket to another and make contact at their oppositely-facing circumferential edges. Forward and aft edge portions of the tip shroud typically overhang the airfoil and it is these areas that are often exposed to higher temperatures and high bending stresses. These overhung areas can also creep, and even creep to rupture, before the useful service life of the entire bucket is consumed, thus limiting the life of the bucket.
Tip shrouds are often formed integrally with the bucket airfoil and may also support integral seal teeth designed to prevent hot gas leakage around the outer edges of the shrouds. The bucket or airfoil and tip shroud are usually formed by casting, and the tip shroud is typically machined to its final figuration.
The overhung forward and aft edge portions of the tip shrouds are often scalloped, removing overhung material, to reduce mass in these areas but this reduces the tip shroud outer flow path coverage, and thus reduces efficiency. In addition, the material for the bucket is generally selected as the lowest-temperature and lowest-cost alloy that can withstand the stresses and service life requirement for the airfoil portion.
It would therefore be desirable to provide a tip shroud configuration that is less prone to creep and thus more favorable in terms of meeting the predicted service life of the bucket.
In accordance with a first exemplary but non-limiting embodiment, the invention provides a turbine bucket comprising an airfoil portion; and a tip shroud at a radially outer end of the airfoil portion including a first radially inner tip shroud component formed integrally with the airfoil and composed of a first metal material, and a second radially outer structural tip shroud component composed of a second metal material bonded to the inner tip shroud component.
In another exemplary but non-limiting aspect, the present invention provides a turbine bucket comprising an airfoil portion; and a tip shroud at a radially outer end of said airfoil portion, said tip shroud including a first radially inner component formed integrally with said airfoil and composed of a first metal material, and a second radially outer component composed of a one or more metal materials bonded together to form the second radial outer component, which is bonded to said first metal material, said second metal material made up of one or more components having a higher-temperature capability, and/or or a lower density than said first metal material.
In still another aspect, the present invention provides a method of forming an integral turbine bucket tip shroud comprising a) forming an airfoil portion of a turbine bucket with a first integral, radially inner tip shroud component; and b) bonding a second radially outer structural tip shroud component to said first integral, radially inner tip shroud component.
The invention will now be described in connection with the drawings identified below.
With reference now to
In any case, it is to be understood that both components 116A and 116B are structural in nature, i.e., the inner component 116A is not merely clad or coated with another material. In fact, the outer component 116B adds strength to the tip shroud 114.
Pre-bonding treatment of the tip shroud component flat interface surfaces 117A, 117B may include surface roughening, nickel-flashing or other suitable techniques for enhancing the bonding between the tip shroud components. It will be understood, however, that one or both interface surfaces could be formed with a radial tab adapted to seat in one or more recesses in the respective opposed surface to resist shear along the bond line.
Once the inner and outer tip shroud components 116A and 116E are bonded together by, e.g., diffusion brazing, the outer, higher-temperature capable component 116B supports and strengthens the lower-temperature capable tip shroud component 116A, greatly increasing temperature and structural capability, and increasing the creep and, or low-cycle fatigue capability of the tip shroud. Diffusion brazing is beneficial because it offers bond-line strength close to the component parts themselves.
In the configuration where the outer tip shroud component 116B is made from a lower density material, it reduces pull or G forces on the lower tip shroud 116A and on the entire bucket. With this embodiment, the stress of the bucket airfoil is reduced and the service life improved. In addition, the tip shroud size in the overhung areas (20 and 22) can be increased to form a continuous circumferential surface only interrupted by the gaps between bucket tips. This improved coverage reduces tip losses, and improves tip clearance, increasing performance.
One or more shroud seal teeth 118 may be incorporated into the outer shroud component 116B, further improving tip shroud performance.
It will be appreciated that other bonding techniques may also be employed. Inspection of the bonded interface can be performed after finish-machining of the bonded tip shroud components. The bond line may be inspected for any possible voiding, while the inner bonded area, away from the outer surface bond line, can be non-destructively inspected by, for example, ultrasonic inspection or witness holes to ensure full bonding.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
