Failure Knowledge Database / 100 Selected Cases

Burst of Steam Turbine Rotor in Fossil Power Plant
June 19th, 1974, Gallatin, Tennessee, USANITTA, Akito (Central Research Institute of Electric Power Industries)
KOBAYASHI, Hideo (Tokyo Institute of Technology)(Summary)
An intermediate pressure (IP) - low pressure (LP) steam turbine rotor of the Gallatin No.2 unit in theTennessee Valley Authority (TVA), which was located in the center of the State of Tennessee, USA,experienced a brittle fracture at about 3,400 rpm during a cold start on June 19th, 1974 after 106,000 hoursof service. Consequently, the rotor fractured into dozens of pieces, and some of the pieces flew off to hit aboiler building.

Unfortunately, at the time, a pre-service boresonic inspection* was not required for thisrotor, and thus the rotor had been kept under service operation without noticing the existence of a large
cluster of manganese sulfides (MnS) that had become segregated near its bore. A creep-fatigue crack
growing from the MnS cluster combined with temper embrittlement due to its long-term service led to theburst of the IP-LP rotor.

In response to this accident, US electric power companies conducted boresonicinspections of all of their own rotors. Also, the Electric Power Research Institute (EPRI) developed theSAFER (Stress And Fracture Evaluation of Rotors) code for deciding whether to run or retire a steam
turbine rotor, and the code was employed for the operation management of rotors.

This accident caused abig shock in the USA and other countries. Also, the accident gave us many lessons (e.g., the requirement
for nondestructive evaluation methods) and resulted in a worldwide advance in steam turbine technology.
Therefore, this accident can be considered as one of the most significant accidents resulting in valuableknowledge and lessons that were transferred to future generations.
*Note : boresonic inspection = ultrasonic inspection conducted to the inside of a bore

1. Component
The burst component was an IP-LP turbine rotor made of Cr-Mo-V steel (ASTM A470, Class8).

Therotor was forged from an air melted ingot in 1954, followed by austenitization heat treatment conducted at955℃.

The unit was base loaded at 225MW from May 1957.

The steam conditions of the burst rotor

were 13.8MPa and 566℃, and its rotation speed was 3,600rpm.

2. Event
On June 19th, 1974 during a cold start after a long shutdown for repairing the TVA Gallatin No.2 unit,
the IP-LP rotor burst at approximately 3,400rpm. The rotor had been in operation for 106,000 hours fromits operational start in May 1957 (see Fig. 1).
The following paragraphs describe the results from Fault Tree Analysis.
(1) Fault tree diagram for mode, mechanism and process of fracture (Fig.2)
The burst rotor forged from an air melted ingot in the 1950's had a MnS segregation zone near its bore.
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Failure Knowledge Database / 100 Selected Cases

However, at that time, the pre-service boresonic inspection was not required for the rotors austenitized at955℃.

Thus, its service operation was continued for over 100,000 hours without noticing the existence ofthe MnS segregation zone, which resulted in crack growth due to creep-fatigue interaction. Also, temper
embrittlement occurred over the long period of operation.

Therefore, the rotor burst by brittle fractureduring a cold start after a long shutdown for repair.
(2) Fault tree diagram for design and manufacturing errors (Fig. 3)
At that time, the pre-service boresonic inspection was required only for the steam turbine rotors made ofa Cr-Mo-V steel that had been austenitized at 1100℃.

Because the austenitization temperature of therotor was 955℃, the pre-service boresonic inspection was not conducted. As a result, the rotor wasoperated for a long period without noticing the existence of a large MnS cluster that had become segregatednear the bore during ingot manufacturing.

This MnS cluster caused the burst accident.

(3) Event tree diagram for the steam turbine rotor burst attributed to the lack of the requirement for thepre-service boresonic inspection (Fig. 4)
The existence of a MnS segregation zone near the bore of the rotor forged from an air melted ingot of a
Cr-Mo-V steel was not detected because the pre-service boresonic inspection was not required for rotorsthat were austenitized at 955℃.

The long-term operation of the rotor having a big flaw of MnS inclusions

near its bore resulted in creep-fatigue crack initiation and propagation from the flaw.

Finally, a brittle

fracture during a cold start was caused by the crack growth in combination with the temper embrittlementthat had resulted from the long period of operation.

3. Course
The burst rotor forged from an air melted ingot in the 1950's had a MnS segregation zone near its bore.
At that time, the pre-service boresonic inspection was required only for the steam turbine rotors made of aCr-Mo-V steel that had been austenitized at 1100℃. Because the austenitization temperature of the rotor
was only 955℃, the pre-service boresonic inspection was not conducted. Consequently, the service
operation of the rotor was continued for over 100,000 hours prior to the burst accident without noticing theexistence of the MnS segregation zone (Fig. 5). As shown in Fig.6, a crack grew due to creep-fatigue
interaction during the operation period.

Also, the rotor material was degraded by the temper

embrittlement that occurred during the long period of operation.

Consequently, the brittle fracture of therotor during a cold start was caused by the superposition of mechanical damage and material degradation.
Finally, the rotor fractured into thirty large pieces, and some of the pieces flew off to a hit boiler building.

4. Cause
The pre-service boresonic inspection was required for rotors that had undergone austenitization
treatment at 1100℃ which had previously experienced some accidents.

However, the inspection was not

required for rotors in which the temperature of the austenitization treatment was shifted to 955℃ as acountermeasure to the previous accidents.

If the inspection had been required for all rotors, the burst

2Failure Knowledge Database / 100 Selected Cases

accident could have been avoided because the large MnS segregation zone of the rotor would have been
detectable by the pre-service inspection.

5. Immediate Action(1) The pre-service boresonic inspection was made mandatory for all rotors after this accident. In
particular, inclusions such as MnS segregated easily in the middle part of air melted Cr-Mo-V steel
ingots that had been manufactured in the 1950's.

Previously, a bore had been machined into thecenter of a rotor to locate and remove the inclusions.

However, it is difficult to remove inclusions

perfectly as seen in this accident. Therefore, the pre-service boresonic inspection is essential for
improving the reliability of rotors.
(2) In the case of a hollow rotor with a bore, the tangential stress due to centrifugal force is the largest at
its bore surface.

On the other hand, in the case of a solid rotor without a bore, the maximum

tangential stress is in the center of the rotor, and its value is smaller than that in a hollow rotor.

Forexample, for a rotor with a very small bore diameter, the maximum tangential stress in a solid rotor ishalf the value in a hollow rotor as shown below.

Therefore, a solid rotor in which the maximum

stress is smaller has an advantage with respect to creep damage.

Improvements in steel

manufacturing technology since the age of air melting include the development and application of newtechniques such as vacuum melting.

Recently, solid rotors are available with reduced amounts ofinclusions as a result of the high quality forging process.
The maximum stress applied in a rotating cylinder such as a rotor is shown as follows:(a) Rotating hollow cylinder
The tangential stress σθ is the highest on its inner surface as expressed by the following equation.
(σθ )max = (σθ )ra =γω２

3 – 2ν

g4(1 – ν)

rb 2

1+

1 – 2ν

ra

3 – 2ν

rb2(1)

where, ra is the inside radius, rb is the outer radius, γis the specific weight, ω is the angular velocity(constant) , g is the acceleration of gravity, and ν is the Poissson's ratio.

When ra → 0 in Eq.(1), thefollowing equation can be obtained.

(σθ )max =γω２

3 – 2ν

g4(1 – ν)(2)

rb2(b) Rotating solid cylinder
Both the tangential σθ and the radial stress σr are the highest in its center as expressed by the followingequation.
(σθ )max = (σr )max =γω２

3 – 2ν

g8(1 – ν)

rb 2

(3)

Comparing the maximum stresses given by Eqs. (2) and (3), (σθ)max in the rotating solid cylinder isfound to be half the value of that in the rotating hollow cylinder.

3Failure Knowledge Database / 100 Selected Cases

6. Countermeasure
All electric power companies conducted the following countermeasures immediately after the accident:(1) Rotor replacement based on the results of boresonic inspection(2) Requirement of boresonic inspections for all rotors
After several years of R&D following the accident, EPRI introduced the following countermeasure:(1) Development of the SAFER code in EPRI and use of the code in electric power companies forthe decision of Run/Retire for a rotor
Manufacturers have conducted the following countermeasures based on the results of technologicalprogress after the accident, although they have no direct relation with the accident:(1) Development and application of vacuum melting process(2) Development and application of a solid rotor

7. Knowledge"The nondestructive evaluations should definitely be required, even if there is no firm technical reason
for the evaluation."

8. On the SideA steam turbine rotor is a large component rotating at high speed. Therefore, if it bursts, it is possible
for large fractured pieces to fly off like missiles and cause serious damages as seen in this accident.

Table

1 lists typical examples of burst accidents of steam turbine rotors experienced peviously in Japan andforeign countries.

An accident with casualties is also included among them.
Table 1

Typical past burst accidents of steam turine rotors.

Accident YearCountryUnit

TypeCauses

1950

HollandNijmijen

LP Rotor

Brittle Fracture

1951

GermanSiemens

LP Rotor

Brittle Fracture

1954

USA

Ridgeland No. 4

LP Rotor

Brittle Fracture

1970

Japan

MHI

LP Rotor

Brittle Fracture

1972

Japan

Kainan No. 3

LP Rotor

Bad Installation+ Resonance

1974

USA

Gallatin No. 2

IP-LP Rotor

Creep-Fatigue+ Embrittlement

In Japan, a power plant is maintained based on the concept of preventive maintenance.

Particularly, inthe case of a high speed rotating component such as a steam turbine rotor, a component with a crack-likeflaw is removed immediately after the flaw is detected.

Figure 7(3) shows an example of a high-pressure

turbine rotor in which a thermal fatigue crack grew throughout the entire heat groove.

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In this case, theFailure Knowledge Database / 100 Selected Cases

cracked portion was removed (called a "skin cut") and the curvature of the heat groove was increased bymachining in order to reduce stress concentration at that part.

Also, this example shows the importance ofassessing the lifetime to crack initiation caused by thermal fatigue. The amount of thermal fatigue life
consumption, which is called the Low Cycle Fatigue Index (LCFI), has been usually controlled to limit therate of temperature variation during transient operations such as startup and shutdown.

In this case, thecrack initiation life is defined as the length of time from fabrication to the point when the LCFI reaches
100%.

However, the value of LCFI was only 23% when the thermal fatigue crack shown in Fig.7 wasdetected and caused the accident(4).

This shows that a more accurate assessment of thermal fatigue life isnecessary for operating a rotor reliably. As an aside, the SAFER code was introduced to Japan in themiddle of the 1980's.

However, the original function of the code for crack growth analyses was not used,

and another function of damage analyses was added to the code to assess the crack initiation life.

Thus,

preventive maintenance has been applied in Japan while the break down maintenance approach is used inthe USA.

In other words, the difference of culture between nations appears on the concept ofmaintenance.

9. Information Source(1) L.D.Kramer and D.Randolph, "Analysis of TVA Gallatin No.2 Rotor Burst : Part Ⅰ-MetallurgicalConsiderations," Proc. 1976 ASME-MPC Symposium on Creep-Fatigue Interaction, pp.1-24(1976).
(2) D.A.Weisz, "Analysis of TVA Gallatin No.2 Rotor Burst : Part Ⅱ-Mechanical Analsis," Proc. 1976
ASME-MPC Symposium on Creep-Fatigue Interaction, pp.25-40(1976).
(3) J. Hoashi and T. Terayama, The Thermal and Nuclear Power, Vol.20, No.10, pp.1128-1144(1973).
(4) K.Kuwabara and A.Nitta, CRIEPI Report, No.74049(1974).

10. Primary Scenario
01. Misjudgment
02. Narrow Outlook
03. Poor Guidelines
04. Planning and Design
05. Poor Planning
06. No Inspection
07. Steam Turbine Rotor
08. Air Melted Cr-Mo-V Steel Forging
09. Bore
10. Oversight of MnS Segregation
11. Usage
12. Operation/Use
13. Operation of Machine
14. Failure
5Failure Knowledge Database / 100 Selected Cases

15. Fracture/Damage
16. Creep-Fatigue
17. Crack
18. Burst of Rotor

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Failure Knowledge Database / 100 Selected Cases

566℃，3600rpm

225MW・Base-load operation from May, 1957
・Brittle fracture during a cold startin June, 1974 after 106,000h service
3400rpm

Fig. 1

Overview of the burst TVA Gallatin No.2 IP-LP turbine rotor.

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Failure Knowledge Database / 100 Selected Cases

Burst of rotor
or GateCold start

Ductile fracture

Ductile fracture

or GateTemperembrittlement

or Gate

No temper
embrittlement

Crack growth

Crack arrest

or GateLong term
operation

Time-independent fracture

Time-dependent fracture
or GateIntegration

No integration

and Gate

Fatigue

CreepSCC

or GateHigh stressregionLow stressregionOtheror Gate

Inclusions in an ingot

No inclusion in an ingot

or GateAir melting

Fig. 2

Vacuum melting

Fault tree diagram for mode, mechanism and process of fracture.

8Failure Knowledge Database / 100 Selected Cases

Burst of rotor
or GatePeriphery

Boreor Gate

DesignMaterial

Inspection

Manufacturing

Repair / replacement

or GateNo requirement of boresonic inspection

Requirement of boresonic inspection
or GateAustenitization at 955℃

Fig. 3

BoreAir
melting

Austenitization at 1100℃

Fault tree diagram for design and manufacturing errors.

MnS
segregation

1100℃austenitization

Ultrasonicinspection

Creep-fatigue

CrackgrowthTemper
embrittlement
Yes

Results
Burst(Brittle fracture)

Yes
NoYes
NoNoNoYes

No
Yes

Initial flaw

Initial flaw

NoNo problem

No

Fig. 4

Initial flaw

Initial flaw

Yes

Yes

Crack

No problem

Event tree diagram for the steam turbine rotor burst attributed to not requiring
the pre-service bore sonic inspection.

9Failure Knowledge Database / 100 Selected Cases

Long-term service without noticing theexistence of a large MnS segregation
zone near bore

Bore

Detected MnSsegregation zoneNo
BoresonicinspectionYes

Boresonic inspection
1968 recommendation : Require UT/MTinspection to a Cr-Mo-V forging rotor
austenitized at 1010℃
Forging of air melted
Cr-Mo-V steel in 1954,
955℃ austenitization
Fig. 5

MnS segregation zone existing near bore.

Total strain range

427℃,
Hold-time = 23h,
Nf = 76cycles

Link-up of MnS by intergranular cracking
427℃,
Hold-time = 23h,
Crack initiationat N(=12)/Nf = 0.16

Metal temp. :413~427℃

MnS

Fatigue life Nf (cycles)

MnS

Fig. 6- Temper embrittlement
during operation- Creep-fatigue interaction

Crack growth by creep-fatigue interaction from MnSs.

10Failure Knowledge Database / 100 Selected Cases

Wheel- Thermal fatigue crack detected in April 1971
after 53,980h operation- Operation of the rotor with such crack for
11 month from the last inspection

Thermal fatigue crack- Reuse after removing the crack ("skin cut")- Larger curvature (i.e. reduce the stressconcentration factor : Kt=3.65 ⇒ Kt=2.75)

Heat groove
Thermal fatigue crack-Grew in the whole at heat groove-The maximum depth = 37.5mm

Fig. 7Cyclic softening

Thermal fatigue crack initiating at the heat groove of HP turbine rotor.

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