Non-circular pins for closure group assembly

Abstract

A set of bladed buckets is provided for a turbine wheel having a male dovetail. The turbine wheel includes axial projections projecting outwardly from both sides of the wheel and formed on substantially the entire periphery of the wheel, except for an interruption by a notch formed by removal of portions of the male dovetail at a bucket loading location. A closure group includes a bladed closure bucket and a pair of adjacent bladed buckets, each having a root portion, a platform and an airfoil. The root portions include a pair of radially inwardly extending laterally spaced tangs, and retaining grooves having a non-circular profile. The adjacent bladed buckets are disposed on each side of the bladed closure bucket. A retaining key has a non-circular axial cross section, and is configured to fit into an opening defined by opposing retaining grooves in the bladed closure bucket and bladed buckets.

Description

BACKGROUND OF THE INVENTION[0001]The invention relates generally to a closure element in a turbine wheel, and more specifically to a closure element having a non-circular axial cross-section.[0002]Steam turbine blades, or buckets, are often designed for installation on a turbine wheel in a tangential direction, axial direction or radial direction. The buckets are typically attached to the turbine wheel using external circumferential dovetails, with a male dovetail on the wheel periphery (margin) and a complimentary female dovetail in the base or root of the bucket. In order to load these buckets onto the wheel, a notch which locally removes the male dovetail portions is cut on the periphery of the wheel, leaving a generally rectangular core portion. Each bucket is then initially located over the core material in the notch and then displaced tangentially onto and around the wheel. The last bucket or block to be assembled to the wheel is called the closure bucket, block or blade. Once...

AI Technical Summary

Problems solved by technology

Limitations of material stress capability mean that only a lightweight block, which has no airfoil, can be used as the closure block, causing reduced performance.

Buckets for other stages may also be subjected to high temperatures and great stresses.

Because the closure block has no airfoil, there is an opening in the steam path with detrimental effects on performance (i.e., efficiency) of the wheel.

The reason behind the inability to support an airfoil on the closure bucket is the fact that the retaining pin passes through the core material in the highly stressed dovetail region of the wheel.

The closure bucket typically does not have a dovetail to provide support because a dovetail would be useless in the notched space that the closure bucket occupies.

The applied loads on the closure, pins and adjacent buckets are thus not uniform.

Consequently, creep and permanent deformation of the closure bucket and / or the adjacent buckets may occur after a period of operation at high temperatures and high centrifugal loads.

For example, such high temperatures and loadings may occur in the reheat section of an intermediate stage turbine.

As a result, the closure bucket may tend to elongate at its base or root in response to these high temperatures and stresses over time, with the result that the slot or hole for receiving the pins may elongate in a radial outward direction.

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