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Case Study #16 — Fastener Failure Workshop

Electric Power Plant
 

Industry:

Electric Power Plant
Pacific Island Paradise

Problem:

The fasteners holding the side pressure plates on a new type of steam turbine generator were breaking. When the fasteners broke, for safety purposes, pressure sensors would automatically shut down the plant, and all electric power to the island was lost.

Injuries:

There were no physical injuries at the power plant. However, the reason for the new plant was that when Hurricane Iniki hit the island, there were sustained winds averaging 150 miles per hour (241 km/hr) and gusts in excess of 200 miles per hour (322 km/hr). This phenomenally destructive force ravaged the island with damage. It caused a number of deaths, and it stripped the land of every blade of grass, turning a lush tropical landscape into a desolate smudge of brown. The storm also took away their power plant, and each time the new one went out because of fasteners, the mending fears of the residents were dealt with a relapse of bad memories.

Damages:

There was no damage to the plant, only to the fasteners.

Observations:

The pressure plates were rectangular steel about 2' (200cm) by 3' (300cm) with fasteners spaced about every 3" (75cm) apart around the periphery, with a thin gasket. All fasteners broke in the thread runout area adjacent to the extrusion angle leading to the full body diameter. A flat washer was used under the head of the screws, and they were of plain finish. The plates and the surrounding metal structure appeared to be painted with a type of hi-temp black paint.

Investigation:

This plant was the first of its type in service, and it ran without difficulties for a few days after being built. The manufacturer was contacted when the blackouts began. However, there was no interest in coming a great distance over small bolts breaking. They said to follow the maintenance guide, apply anti-seize compound to the threads and torque the bolts to the manual. Further, because of liability, the manufacturer prohibited changing the bolt from the specified product. The fasteners in question were 1/4"-20 x 1 1/4" Grade 8 hex cap screws with through-hardened SAE flat washers. The failure mode was obvious fatigue. The torque with lube was reasonable, and there were ample threads available within the tapped holes. However, the screws were nearly bottomed out into the extrusion angle because of the manufacturer's length selection.

Conclusion:

While the installation was acceptable, the screws were clearly too long, and this focused service loads to the thread runout area. The loads needed to be spread out.

Recommendations:

A good practice, where viable, is to distribute loads over as many un-engaged threads within the clamped joint as possible. The plant didn't dare change the screw, but the application was made successful by placing thick spacers under the heads. This effectively put a "spring" into the assembly, and was enough of a change for the island to keep its lights on.

 

As appeared in Fastener Technology International magazine.

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