Patent Application
20050238492
The present invention relates to a control stage for a high temperature steam turbine and particularly relates to buckets of the control stage having internal cavities within the airfoils of the buckets to reduce the weight of the bucket and creep damage in the turbine rotor.
The control stage of a steam turbine, i.e., the first stage of the turbine downstream of the control valves and steam inlets, creates unique loading on the control stage buckets. Typically, there are four inlets for admitting steam to the control stage, with a control valve for each inlet. The inlets are generally arranged in quadrants. Consequently, steam is emitted over segments of arcs and is therefore not uniformly applied on the first stage bucket airfoils. Because of this unique non-uniform flow and, hence, loading on the control stage, and over long-term operation at high temperatures, creep damage can and does occur in the turbine rotor, particularly in the coupling between the buckets and the rotor.
Typically, the buckets are secured to the rotor by dovetails. For example, the buckets may have first are second dovetails extending in an axial direction. Alternate dovetails on the rotor are received between the first and second bucket dovetails. The remaining rotor dovetails are received between the bucket dovetails of next-adjacent bucket. The rotor is typically formed of a material which does not have the high creep or rupture strength characteristic of the creep or rupture strength of the buckets end, accordingly, creep damage can and does occur about the rotor dovetails. Conventionally, the dovetails of the bucket and rotor are secured to one another by insertion of axially extending crush pins in openings formed in both the bucket and rotor dovetails. Creep strains around the pinholes of the dovetails, particularly in neck regions of the rotor dovetails, have been demonstrated. Accordingly, there is a need to control the loads or stresses in those regions to prevent creep strain from causing cracks to develop in the rotor dovetail.
In accordance with a preferred aspect of the present invention, the airfoil of each bucket is provided with an interior cavity thereby reducing the weight of the bucket and subsequent centrifugal loading on the rotor during operation. More particularly, the reduced weight of the bucket reduces the creep strain on the rotor dovetails during long-term centrifugal and high temperature loading on the turbine rotor. Particularly, the interior cavity of the bucket extends through the bucket platform and substantially throughout the entire length of the airfoil, terminating adjacent the airfoil tip. The cavity is closed adjacent the tip of the airfoil and remains open at the platform. The cavity may have a generally airfoil-shaped interior surface corresponding generally to the external profile of the airfoil with due consideration being given to the necessary strength of the airfoil. The cavity may, however, be otherwise shaped. By reducing the weight of the bucket, the long-term creep damage on the rotor dovetail is minimized or eliminated.
In a preferred embodiment according to the present invention, there is provided a reduced weight control stage for a steam turbine comprising a steam turbine bucket having an airfoil, a platform and a first dovetail for attaching the bucket to a generally correspondingly shaped rotor dovetail, the airfoil having an interior cavity extending between the platform to a location adjacent a tip of the airfoil enabling a reduction in the weight of the bucket for reducing creep damage along the rotor dovetail.
In a further preferred embodiment according to the present invention, there is provided a reduced weight control stage for a steam turbine comprising a rotor having a plurality of dovetails projecting radially outwardly of the rim of the rotor and extending generally axially, a plurality of buckets each having an airfoil, a platform and first and second dovetails projecting radially inwardly of the platform and defining a generally axially extending space therebetween for receiving one of the rotor dovetails, each airfoil having an interior cavity extending through the platform to a location adjacent a tip of the airfoil enabling a reduction in the weight of the bucket for reducing creep damage along the rotor dovetail between the first and second bucket dovetails.
Referring now to the drawings, particularly to
To minimize or eliminate the creep damage in the turbine rotor during long term operation under high temperature and centrifugal loading, the steam turbine buckets 14 have their airfoils 24 formed of reduced weight. To accomplish this, cavities are formed in the interiors of the airfoils 24. For example, referring to
In order to form the cavity 26 within the airfoil, the airfoil is hollowed out using an Electro-Discharging Machining (EDM) or Electro-Chemical Machining (ECM) processes. For example, an EDM tool 32 (
Referring to
From the foregoing description, it will be appreciated that with each of the buckets having a substantial mass of material removed, i.e., hollowed out to form the cavity 26 within the airfoil of each bucket, as well as the platform, the buckets are reduced in weight. This reduces the centrifugal loading on the rim of the rotor and particularly on the reduced necks of the rotor rim dovetails 22. Consequently, creep loading adjacent the neck portions of the rotor dovetails 22 is minimized. In a particular preferred embodiment, each control stage bucket for a steam turbine is reduced in both weight and volume in comparison with the identical bucket without the interior cavity formed therein as in the present invention. Particularly, the weight of the control stage bucket is reduced by about 10%. This reduction in weight significantly reduces the centrifugal loading on the rotor. The volume of each control stage bucket is likewise reduced by about 10%. This is particularly significant in the control stage of a steam turbine where non-uniform flow or partial-arc flow occurs on the first stage buckets. This essentially pulsating flow increases the likelihood of increased creep strains which are more than offset by the reduction in weight of the buckets enabling reduction or elimination of the creep strain damage.
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.
