Titanium Steam Turbine Blading. Workshop Proceedings Palo Alto, California, 9–10 November 1988

433 Pages·1990·31.87 MB·English

by  R. I. Jaffee (Eds.)

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ACTA METALLURGICA
ENGINEERING FRACTURE MECHANICS
SCRIPTA METALLURGICA
Titanium Steam
Turbine Blading
Workshop Proceedings
Palo Alto, California
9-10 November 1988
edited by
R. I. Jaffee
Electric Power Research Institute
EPRI ER-653Ô prepared for the
Electric Power Research Institute
EPRI
Electric Power
Research Institute
Pergamon Press
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Copyright © 1990 Electric Power Research Institute
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Library of Congress Cataloging in Publication Data
Titanium steam turbine blading : workshop proceedings, Palo Alto,
California, 9-10 November 1988 / edited by R.I. Jaffee.
p. cm.
"EPRIER-6538 prepared for the Electric Power Research Institute."
ISBN 0-08-037301-1 (Pergamon) :
1. Steam-turbines-Blades-Materials-Congresses. 2. Titanium-
-Congresses. I. Jaffee, Robert Isaac, 1917- . II. Electric
Power Research Institute.
TJ737.T57 1989
621.1'65~dc20 89-39747
CIP
Legal Notice
This report was prepared by Robert I. Jaffee as an account of a
workshop sponsored by the Electric Power Research Institute, Inc.
(EPRI). Neither EPRI, members of EPRI, the author, the organ­
izations) named below, nor any person acting on behalf of any of
them: (a) makes any warranty, express or implied, with respect to the
use of any information, apparatus, method, or process disclosed in
this report or that such use may not infringe privately owned rights; or
(b) assumes any liabilities with respect to the use of, or for damages
resulting from the use of, any information, apparatus, method, or
process disclosed in this report.
Printed in the United States of America
The paper used in this publication meets the minimum requirements of
American National Standard for Information Sciences - Permanence of
Paper for Printed Library Materials, ANSI Z39.48-1984
ABSTRACT
After a long research and development period, beginning with the commercial
availability of titanium in early 1950's, the steam turbine manufacturers and
users have been developing titanium alloy blading for the long last row and steam
transition row applications. Commercial introduction of titanium blading into low
pressure steam turbines is just beginning in many countries ove^ the world. A
workshop was organized by the Electric Power Research Institute and held "Ή Palo
Alto, California on November 9 and 10, 1988, to provide a record of the status of
the development and to facilitate and accelerate the introduction of the titanium
blading into commercial use.
After an introductory paper that provided a historical background on the titanium
blade application in low pressure steam turbine, the workshop was organized into
the following sessions: A, Design and Application of Titanium Blades; B,
Manufacture of Titanium Blades; C, Properties and Characteristics of Titanium
Blades; D, Discussion of Workshop Issues and Conclusions. In total, there were 23
papers presented, incuding the final discussion. These are presented, with
records of discussions after each paper, in the present volume. Attendance at the
workshop was limited to 40 participants, session chairmen, and utility users. The
attendance included representatives from eastern and western Europe, Japan, and
the United States. As such, it is a complete statement of the science,
technology, and application of titanium steam turbine blading worldwide.
V
FOREWORD
The capability of low pressure steam turbines to produce power is limited by the
long last row blading and the strength of the rotor to support the blades. The
steel last row blades have reached their maximum length of 840 mm (33.5 inch)
operating at 3600 RPM or 1200 mm (48 inch) operating at 3000 RPM in blades made of
12% Cr martensitic steel. To go to longer blades without increasing the stress on
the fastening to the turbine rotor required going to an equally strong but lighter
material. When titanium alloys become available in the early WSO's the producers
of steam turbines turned to the new titanium alloys. These permit consideration
of 1000 mm (40 inch) blades operating at 3600 RPM and 1350 mm (54 inch) blades
operating at 3000 RPM. The longer titanium alloy blades permit high power to be
generated without increasing the amount of steam per unit of exhaust area. Thus,
power of steam turbines may be increased about 50 percent by the 1000 mm, 3600 RPM
or 1350 mm, 3000 RPM titanium blades. This development makes possible a new
generation of more powerful low pressure steam turbines.
The excellent corrosion resistance of titanium in condensing steam contaminated by
salt impurities offers the other major application of titanium blading alloys in
the low pressure steam turbine. The 12 Cr blading steel loses most of its
corrosion fatigue resistance in condensed saturated salts characteristic of early
condensation from steam contaminated by chlorides, sulfates, and other salts
through condenser leakage and démineraiizer breakdown. In contrast titanium
alloys are relatively immune to loss of fatigue strength in salt solutions
condensed at the steam transition, generally the L-l row. This provides a second
major application for titanium blading alloys in low pressure steam turbines. An
EPRI workshop, "Corrosion Fatigue of Steam Turbine Blade Materials", R.I. Jaffee,
Ed., Pergamon Press, 1983, provides documentation on the corrosion fatigue
performance of 12 Cr, Ti-6A1-4V, and other blading materials in this
application. The present volume provides information on the production and
properties of titanium alloy blades in the L-l steam transition application as
well as the last row application.
The power generation application for titanium alloy blades is illustrated by the
development of 1000 mm, 40-inch blades for the 700 mm Chubu Hekinan units. Papers
representative of the efforts of the three Japanese turbine manufacturers and
their suppliers are described. The 1360mm, 3000 RPM titanium titanium blade being
developed by Alsthom in France and its suppliers also are described. The steam
transition L-l blade application is covered by papers by Asea Brown Boveri and
Westinghouse.
Developments in Eastern-bloc nations have included applications of last row blades
in nuclear as well as fossil low pressure steam turbine applications by the
Leningrad Metallurgical Zavod (LMZ) are described, as well as water droplet
erosion testing at the Skoda Works. Supporting technology at universities and
research laboratories is illustrated by the papers from the Technical University
at Hamburg-Harburg. In the U.S., G.E. has been active mainly in development and
testing for the last row application. Westinghouse undertook a major L-l steam
transition blade development in conjunction with EPRI, and contributed a paper
documenting the development, including an analysis of the failure of a row of
titanium blades from aero-elastic causes. In the U.K., design aspects are covered
by a paper from N.E.I. Parsons, and manufacturing and characteristics by a paper
by G.E.C. Turbines and Generators. The forging industry was represented by papers
by Boehler, Thyssen, and Kobe.
vu
I would like to acknowledge with thanks the help and support proved to me by the
Electric Power Research Institute in the development of titanium alloy blading for
low pressure turbines, including sponsoring the present meeting. In particular, I
would like to thank Mrs. Georgia Inglis, who served ably as workshop secretary,
and who provided invaluable help in preparing the present volume of proceedings
for publication.
R.I. Jaffee
Electric Power Research Institute
viii
TITANIUM STEAM TURBINE BLADING
R. I. Jaffee
Electric Power Research Institute
3412 Hi 11 view Avenue
Palo Alto, CA 94304
ABSTRACT
The use of titanium alloys for the long last stage and L-l stage of low-pressure
steam turbines has been in development for over 30 years, and appears to be enter­
ing commercial use. The titanium blades are used in the steam transition L-l row,
because they are immune to pitting corrosion by the corrosive early condensates at
the Wilson line. In the last stage row, lighter titanium blades permit longer
blade length and larger exit annulus areas than 12Cr steel blades. The last row
can be designed to increase the thermal efficiency by 1-2% or the power by about
50%, reducing the number of steam flows by one-third to one-half. A disadvantage
of titanium alloys is their low damping capacity, which requires blade designs
that maximize mechanical and aerodynamic damping. The water droplet erosion
resistance of titanium alloys is sufficient not to require protection in the L-l
row, but the last stage rows operating at supersonic tip speeds require the use of
erosion shields.
HISTORICAL PERSPECTIVE
During the decade of the 1950s, titanium was in its early stages of development,
and steam turbines were becoming of ever-increasing size in order to take advan­
tage of economies of scale. Thus, a confluence of both developments at that time
was inevitable. Compared to 12Cr blading steels, titanium alloys offered as good
or better fatigue strength, 40% lower density, and corrosion resistance to the
acid chlorides characteristic of the early condensation from steam containing
chloride impurities. The development of 12Cr blades for 3600-rpm (60-Hz) machines
reached an apparent impasse at 31- to 33-1/2- inch (775-840-mm) length, beyond
which the centrifugal stresses become too high for the blade and the disc steeple
holding the blade.
Steam turbine sizes beyond 1000 MW could be achieved through the use of a cross
compound instead of the tandem compound arrangement of LP turbines, which
1
effectively doubled the number of steam flows and increased capital cost. This
was the scheme utilized by Brown Boveri for its series of 1300-MW units built for
TVA and AEP. The only way to achieve very large capacities in tandem compound
units was through titanium alloy blades. The LMZ in the U.S.S.R. built 1200-MW
tandem compound steam turbine units with 1500-mm (60-inch) titanium blades for
3000-rpm operation. In Western countries, development of titanium blading slowed
down considerably in the 1970s, when the trend to larger units turned around in
favor of smaller 600-800-MW units, because of greater emphasis on perceived better
reliability of smaller subcritical units and greater capital economy with the
tandem compound arrangement.
During the 1950-1960s, the steam turbine manufacturers installed many titanium
alloy test blades in L-0 rows in operating units. Generally these test blades
were placed in relatively small machines with blade lengths of about 23-28-inch
(575-700-mm) long. These installation of titanium blades in otherwise steel rows
was possible because the dynamic vibration characteristics of titanium blades are
approximately the same as 12Cr steel blades of the same geometry. The frequency
of vibration of a blade is proportional to the square root of the modulus of elas­
ticity divided by the density, (/E/p), and is the same for titanium alloys and
12Cr steel to within a few percent. As of 1977, when EPRI supported a survey of
the status of titanium blading in steam turbines Q ), thirteen cases of titanium
test blade and four cases of full rows, mostly in last rows, were cited. To our
knowledge, all of these test and production rows are still in operation, some
being in service for at least 25 years without any reports of failure.
During the 1970s period, a great deal of forced outage in L-l rows was being
experienced in the United States and Europe as a result of corrosion fatigue of
12Cr blades originating at pits induced in the airfoils and blade roots as a
result of corrosive condensates from salt impurities in the steam. At least one-
third of all forced outage was of this type. The impurities most frequently
originated from condenser leakage, with resulting contamination of the feed water
supplied to the boiler. The corrosion was exacerbated by dissolved oxygen
(aeration) in the water that occurred during shutdown periods. Two major projects
were initiated to study corrosion fatigue during this period. EPRI supported
project RP912 with Westinghouse to study the effects of various simulated conden­
sates on the corrosion fatigue of 12Cr steel (403 stainless), 17-4PH stainless,
and TÎ-6A1-4V titanium alloy, all of which were steam turbine blade alloys used or
considered for use in L-l blades subject to corrosion fatigue in aqueous salt
solutions representative of condensates from impure steam (2). A similar study
2
(FE-KKS 2.4/2F) was conducted in Germany by the Allianz-Zentrum für Technik in
cooperation with the German steam turbine makers, AEG, BBC, KWU, and MAN, in which
12/13% chromium steels, 17-4PH, duplex stainless steels, and Ti-6A1-4V were
studied for pitting corrosion and corrosion fatigue in aqueous solutions repre­
sentative of initial condensation during operation (3).
Results of these two studies and other work on corrosion fatigue of steam turbine
blade materials were reported in the proceedings of a 1981 EPRI workshop (4). The
results of all investigations are in essential agreement: the fatigue strength of
12Cr steel in all of its modifications are severely degraded to about 10-20% of
the value in air when tested in saturated NaCl solutions with pH of 4 or less,
particularly in the presence of dissolved oxygen, as shown in figure 1. Under the
same conditions, titanium alloys maintain above 90% of their corrosion resistance
in air, as shown in figure 2. The more corrosion resistant 17-4PH stainless and
duplex stainless steels have intermediate corrosion fatigue resistance between
12Cr and titanium alloys.
As a result of the realization that titanium alloys had superior resistance to
loss of fatigue strength under conditions that cause 12Cr blades to fail when
operated in impure steam, a new steam turbine application opened up in the late
1970s period, namely L-1 blades for operation in the steam transition region. BBC
installed two rows of 400-mm (16-inch) TÎ-6A1-4V L-1 blades in the AEP Mountaineer
1300-MW unit in. A report on this application after 12,000 hours of operation
showed successful use of titanium alloy blades (5). Steel blades had failed in a
BBC unit of the same type and size as a result of pitting from condensed NaCl
deposits on the airfoil base and subsequent corrosion fatigue cracking. A second
installation of titanium L-1 blades was made by Westinghouse in a 700-MW fossil
unit at Tugco's Martin Lake Station (6). A titanium alloy L-1 row and a 17-4PH
L-1 row were located at opposite flows of the LP turbine. Telemetry tests were
conducted to obtain stress and frequency data. The titanium Martin Lake No. 3 L-1
row was installed in December 1984 and is still running. A subsequent installa­
tion of four L-1 titanium blade rows in Martin Lake No. 2 failed shortly after
startup. A report on this failure will be presented in this workshop. A view of
the BBC and Westinghouse L-1 blades is shown in figure 3.
The use of titanium alloys and more corrosion-resistant steels was not the only
approach to solving the problem of L-1 blade failures in the steam transition row
of large fossil LP turbines. If the steam could be maintained pure enough,
through prevention of salt ingress from condenser leakage and other sources of
3
_ 500
Mean stress 245 MPa
CO # -· H 0
2
Hi 22% NaCI
~Air ΡΗ?Λ
pH9
2 300 [^
pH5
? 200 ; — l · ^ ^^
«S
| 100
5
10 102 103 104
Oxygen Content (ppb)
Figure 1. Effect of pH and oxygen content in water and saturated NaCl on the 107
cycles fatigue strength of 13 Cr steel at 80°C (3).
700
Û. 600
Σ «**· · t^.
<n 500
en
400 — ^^ »
55
300 — —·— Air (24°C)
"5 200 *— Air (100°C)
c
100 _ —·— 22% NaCl solution (100°C)
Φ
0 ; ' MÜi'l :■:-''' - t 11 !l!;l ' : !Miül S ' ',Ι'Γ.Ί
104 105 106 107 10e 109
Number of Cycles
Figure 2. Fatigue properties (R=-l) of mi 11-annealed TÎ-6A1-4V bar stock (7)
4