Low pressure section steam turbine bucket

Abstract

A bucket for use in the low-pressure section of a steam turbine engine is provided. The bucket has a vane length of at least about 45 inches. The bucket is comprised of a dovetail section disposed near an inner radial position of the bucket, a tip shroud disposed near an outer radial position of the bucket and a part span shroud disposed at an intermediate radial position. The intermediate radial position is located between the inner and outer radial positions. The bucket is comprised of a titanium-based alloy having between about 2% and about 6.25% by weight aluminum, up to about 3.5% vanadium, up to about 2.25% tin, up to about 2.25% zirconium, between about 1.75% and about 5.0% molybdenum, up to about 2.25% chromium, up to about 0.7% silicon and up to about 2.3% iron, with the balance being titanium.

Description

BACKGROUND OF THE INVENTION[0001]The present invention relates to high strength buckets for use in the last stage of steam turbine engines. Specifically, the invention relates to the application of certain titanium-based alloys for use in making high strength, last stage turbine buckets having vane lengths of about 45 inches or greater.[0002]It is generally recognized that the performance of a steam turbine is greatly influenced by the design and performance of later stage buckets operating at reduced steam pressures. Ideally, the last stage bucket should efficiently use the expansion of steam down to the turbine exhaust pressure, while minimizing the kinetic energy of the steam flow leaving the last stage.[0003]The service requirements of steam turbine buckets can be complex and demanding. Last stage buckets, in particular, are routinely exposed to a variety of severe operating conditions, including the corrosive environments caused by high moisture and the carry-over from the boil...

AI Technical Summary

Benefits of technology

[0010]Accordingly, a need exists in the art for a last stage bucket having longer van...

Problems solved by technology

The service requirements of steam turbine buckets can be complex and demanding.

Last stage buckets, in particular, are routinely exposed to a variety of severe operating conditions, including the corrosive environments caused by high moisture and the carry-over from the boiler.

Such conditions can lead to serious corrosion and pitting problems with the bucket material, particularly in longer, last stage turbine buckets having vane lengths of 40 inches or greater.

The development of larger last stage turbine buckets, e.g., those with vane lengths of about 40 inches or more, poses additional design problems due to the inertial loads that often exceed the strength capability of conventional bucket materials such as Ti6Al4V and iron-based alloys.

Steam turbine buckets, particularly last stage buckets with longer vanes, experience higher tensile loadings and thus are subject to cyclic stresses which, when combined with a corrosive environment, can be very damaging to the bucket over long periods of use.

As a result, water droplet impact erosion of the bucket material often occurs in the last stage.

Such erosion reduces the useable service life of the bucket and the efficiency of the steam turbine as a whole.

In the past, it has been difficult to find bucket materials capable of meeting all of the mechanical requirements for different end use applications, particularly mechanical designs in which longer vane buckets, i.e., those having vane lengths about 40 inches or more, have been employed.

Invariably, the longer buckets have increased strength requirements and, as noted above, suffer from even greater erosion and pitting potential.

The higher stresses inherent in longer vane designs also increase the potential for stress corrosion cracking at elevated operating temperatures because the higher strength required in the bucket material tends to increase the susceptibility to stress cracking at operating temperatures at or near 400 degrees Fahrenheit (F).

The effects of pitting corrosion and corrosion fatigue also increase with the higher applied stresses in last stage buckets having longer vane lengths.

Many times, an alloy selected to satisfy the basic mechanical design requirements of other turbine stages simply will not meet the minimum mechanical strength and erosion resistance requirements of last stage buckets.

These prior art physical attachment methods almost invariably lead to some degree of degradation in the weld heat affect zone over time, and thus the bucket may ultimately fail at those points.

In addition, the risk of defective welds always exists and can result in costly scrapping of entire bucket assemblies in some cases.

Thus, a portion of the insert can remain unhardened and vulnerable to premature failure or erosion under harsh operating conditions.

It has been found, however, that the bi-metallic structure disclosed in the '395 patent does not solve all of the pitting and corrosion problems inherent in last stage turbine buckets, particularly those with vane lengths of about 40 inches or greater.

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