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Case Study #10 — Fastener Failure Workshop

Nuclear Power Plant
 

Industry:

Nuclear Electric Power Plant
Cooling Tower

Problem:

The fasteners bolting on impellers that drive water through the cooling tower were suddenly breaking, with the impellers falling off. This caused dangerous overheating, which subsequently triggered an automatic shutdown of the reactor, and thus the plant.

Injuries:

None.

Damages:

The damage to the cooling system was minimal; however, the losses incurred while the facility was down and unable to supply power were significant. Until the root cause was determined and corrected, a restart could not be contemplated.

Observations:

All of the fasteners bolting the impellers were broken. This is a hex bolt and nut application with machined bearing surfaces on the impeller accommodating the fasteners. There were no noticeable outer irregularities on the bolts, such as wrenching marks of the hex flats. The fracture surfaces were fairly flat, but they were craggy in appearance.

Investigation:

The hex bolts in this application were head marked "410," as in Type 410 stainless steel. There were no manufacturer's identification marks. The hex nuts appeared to be of a similar type material, but were not marked. We were told that the fasteners were provided by the contractor who built the cooling system. Samples of the failed specimens were submitted to an independent lab for testing analysis. The hardness of the bolts was HRC 42-44, and the nuts were HRC 35-37. Both products were of Type 410 stainless steel. With the magnification of a scanning electron microscope, the bolt's fracture surface had corrosion products, and the rock candy appearance of intergranular failures.

 

During an interview with the maintenance supervisor, it was learned that the water supply going into the cooling tower was primarily from the runoff of melted snow from the nearby mountains. To prevent algae from growing, an algaecide of granular chlorine was to be dumped into the water once a month. We then analyzed the water, and it showed a very high level of chlorine.

 

Further interviews discovered that recently, after being chastised for not strictly following written maintenance procedures, workers on each of three shifts dumped chlorine in the water, not just on one shift as the supervisor intended.

Conclusion:

While the dilute condition of the normal algaecide treatment had proven safe, the overload of chlorine became a serious problem. Stress Corrosion Cracking (SCC) began with the chlorine attack of the stainless, and this led to the failures.

Recommendations:

The 410 stainless worked, but the A286 stainless would be a safer choice — and with the manufacturer's identification marks. In addition, a thorough review of the maintenance procedures for clear instructions is required.

 

As appeared in Fastener Technology International magazine.

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