Failure Knowledge Database / 100 Selected Cases

Brittle Fracture of Turbine Rotor in Nagasaki
October 24, 1970 in Nagasaki-shi, Nagasaki
Masayuki Nakao (Institute of Engineering Innovation, School of Engineering, the University of Tokyo)
Engineers were condu cting perfor mance and s afety test s of a n ew lar ge-capacity turbi ne at a s hipyard whenbrittle fracture occurred in the 50-ton rotor, 1,778 mm in diameter (maximum) and 3,590 mm in length, due tomicrostructural f laws a nd a strong not ch ef fect. Its fr agments sc attered in the ar ea s urrounding t he shipy ard,killing 4 and injuring 61 people. This incident happened in the time when larger-capacity turbines were in needfor generating more energy to meet the rising demand, and manufacturers were shifting towards fully domestic
development instead of joint developments with western countries.
1. Event
Engineers were conducting performance and safety tests of a new large-capacity turbine at a shipyard whenbrittle fracture occurred at the 50-ton rotor, 1,778 mm in diameter (maximum) and 3,590 mm in length. Itsfragments scattered in the area surrounding the shipyard, killing 4 and injuring 61 people.
2. Course
While a traditional turbine had fo ur shafts in three chambers, high pressure with single-shaft, intermediatepressure w ith sing le-shaft and low pr essure wi th double-shaft; the new turbine r educed the nu mber ofshafts into three, one for the hi gh-intermediate pressure chambers and two in the low pressure chamber asshown in Figure 1.

Figure 1. Schematic Diagram of 330,000-Kilowatt Unit [1]1Failure Knowledge Database / 100 Selected Cases

The n ew turb ine also integrated retaining rings at th e root of turbine blades connected to the r otor of anelectric gen erator, a nd the retaining rings became la rger in diameter. The h igh-speed roto r h ad speci ally
designed rotor blades to obtain high efficiency and increase output.

Figure 2. Fractured Turbine Rotor [1]In or der to test the per formance and the saf ety of th e assembled new turbine, the rotor drive s ystem wassubjected to oversp eed runs at 120 p ercent of the maximum r otational spee d. The overspeed te st wa s toincrease the turbine speed by 20 % from the normal operation speed of 3,000 rpm to 3,600 rpm.

Figure 3. Location of Rotor Fragments [1]2Failure Knowledge Database / 100 Selected Cases

A brittle fracture occurred in the rotor when the turbine speed was in creased to 3,540 rpm, 118 percent of
the maximum rotational speed. No ab normal bl ade shaft vibration was monitored durin g the tes t pr ior tothe rotor fr acture under the tes t temperature of 40 – 50 degrees C. (The turbine was not su bjected to testwith high-temperature and pressure steam. The shaft was rotated by a machine.) The fractures started fromthe r otor bor e and br oke t he r otor into f our pi eces, a lmost q uadrisectioned ( Figure 2). The four r otor
fragments was scattered in all directions (Figure 3).
The t est si te wa s l ocated at Nagasaki Bay, surrounded by the o cean and mountains. O ne rotor frag ment
weighing 9 to ns flew 880 m eters toward s the water (“ S” i n F igure 3 ), o ne we ighing 11 to ns flew 1500
meters and landed at 200-meter elevation in the mountain (“M”). Two fragments remained in the laboratory- one flew across the test room damaging equipment and injuring people (“W”), and the other struck thefloor (“H”).
3. Cause
Photo 1 and Figure 4 show the fracture of the turbine rotor in detail. Failure of the turbine rotor was causedby fractures initiated from a flaw at the rotor bore in the following manner:(1) The casting of the turbine rotor produced a microstructural flaw such as a por e, crack, or inclusion inthe rotor bore. Air an d die lubri cant wer e trapped i n t he c avity, r esulted i n microporousity in th ecastings and columnar dendrites across in the bore region of the rotor.
(2) The casting process failed to eliminate the formation of microporousity. The cooling velocity was slow
in the rotor bore during the final heat treatment, which caused brittleness in the casting.
(3) The tangential stress increased in the region of the rotor because of the larger rotor size.
(4) Brittle fr acture oc curred in the rotor bor e as the in creased t urbine speed app lied a gr eater tange ntial
stress to the bore region. Ductile fracture was initiated at high stress points, resulting in the breakup ofthe rotor.

Photo 1. Fracture Origin on the Surface [1]Engineers failed to u nderstand t he microstructural fl aws at the rot or bo re. It w as a possible technicalmanagement oversight.
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Failure Knowledge Database / 100 Selected Cases
The test used an ultrasonic flaw detector to detect an internal flaw of the rotor. However, it only detectedpremature f law ec hoes ( Flaw t olerance: Less th an 5 mm in s ize). I t was not pr oven a t t ime th at thecollective effect of microporosites in a region is hi ghly destructive t o structure, a lmost e quivalent to thedestructiveness of a 6 mm flaw. It was still an unkn own phenomenon at time related to a micr ostructural
flaw.

Figure 4. Crack Growth and Rotor Fracture [1]4. Immediate Action
Engineers in vestigated wha t ha ppened, stu died the rotor fragments and an alyzed th e fracture mechanism
based on the material proper ties and the characteristics of the incident. The injured and the families of thedead were w ell compensat ed. Det ails on the inci dent we re ce ntrally c ontrolled so th at inacc urate
speculation was n ot circulated in the public. A new turbine was produced using a rotor manufactured by abetter casting method, and delivered to the customer in time before the due date that came two months after
the incident. No inconvenience was raised to the customer.
5. Countermeasure
The low ductility at the rotor bore resulted in a low tolerance for brittle fracture. In order to elim inate theimperfections and the grain segregation at the rotor bore that caused low ductility, engineers implemented a
new casting technology to manufacture the rotor. Instead of the vacuum silicon deoxidation method, theyturned to vacuum car bon deoxidation method for producing ingots. In ad dition t o r enewing t he s teelmanufacturing process for producing the base material, improvements were made to the heat treatment andthe ultrasonic flaw detection methods.
A new s pin rig was implemented to pr event r ecurrence o f the accident that r esulted in shattering offragments and casualties. The test equipment and turbine was placed in a pit on the ground and the rotor
was covered robustly so that fractured fragments would not scatter in the area.

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Failure Knowledge Database / 100 Selected Cases

6. Summary
The turbine f ailure r esulted in a de vastating ac cident with a fractured r otor and many casualties. Theincident required many technical improvements, responses to the customer, compensations to the de ad andthe injured, as well as the social and the legal actions (whether or not to handle the incident as a criminalcase and claim professional negligence resulting in deaths and injuries).
7. Knowledge(1) It is ess ential to understand all manufacturing processes and relat ed phen omena su ch as in got
solidification, ch anges i n t he crystalline st ructure during the for ging process an d in t he m echanicalproperties during forging and heat treatment (embrittlement in particular).
(2) Material pr operties ar e no t alway s the sa me. Th e same ingredients do not always obta in th e sa meproperties.
(3) It is cr itical to understand the mechanism that leads to a failure. In particular, the fracture mechanism
and conditions.
(4) The road to success wo uld be bu mpy but wor th the ef fort. The road to suc cess is a lways und er
construction, and the road to failure is always smooth. Every measure requires elimination of the root
cause to prevent recurrence of the failure. For this incident, the effective measure was implementationof an ingot-making method that does not produce imperfections.
It is al so cri tical to introd uce measures for m inimizing damage fr om failures. Suc h measures req uire
improvements in test equipment and environment as well as drills under hypothetical situations.
8. BackgroundA tur bine is a r otary engin e tha t extracts energy fr om fluid f low. I t is the main mechanical device of apower ge neration sy stem tha t ex tracts ther mal energy f rom pressurized ste am b y s ending steam in highpressure and temperature to the blades attached to a shaft (the rotor assembly). The blades react to the flow
so that they rotate and impart energy to the rotor. The generator connected to the turbine convert the energy
into electrical en ergy, whic h is supplied to communities. Elec trical energy is on e of the many ty pes ofenergy that is currently used i n the s tandard op eration of human business. The 1970’s w hen t he in cident
occurred w ere at the end of th e high-growth period of the J apanese economy. Low o il prices at timeenabled the manufacturers to develop larger turbines for generator systems that can produce more energyand su pport vital economic activ ities (The oil cr isis in 1973, three yeas after this incident , d rasticallychanged the en ergy situation in th e world). There wa s also a change in th e devel opment trend during
1970’s. Domestic development of turbine technology was becoming the industry trend at time, instead o fjoint de velopment b y Japanese an d the foreign manufacturers su ch as W estinghouse (U. S.), General
Electric (U.S.) and Siemens (Europe).
References
5Failure Knowledge Database / 100 Selected Cases[1] Yotaro Hatamura (Editor), Jissai-no Sekkei (Practical Design) Research Foundation (1996) Zoku-Zoku
Jissai-no Sekkei (Practical Design III), The Nikkan Kogyo Shimbun, LTD.

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