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Case Study #6 — Fastener Failure Workshop

Petroleum Refinery
 

Industry:

Petroleum Refinery
Propane Gas Generation Unit

Problem:

An explosion occurred at the gas generation unit after it had been rebuilt. The fasteners used on the various pipe flange joints were being called into question.

Injuries or Fatalities:

Three refinery workers lost their lives in the explosion.

Damages:

Three light trucks were destroyed and there was over US$1 million in damages to the gas generation unit.

Observations:

Double end studs and their accompanying hex nuts are the common method of holding pipe flanges together. The part of the structure that was still intact had these stud/nut combinations, called stud bolts.

Investigation:

Interviews and physical evidence showed there was a large propane gas leakage. Propane is heavier than air and can stall a motor vehicle. This leak stopped three trucks. The last truck to arrive, driven by a newer worker who tried to restart the truck, ignited the gas.

 

Leaks at pipe flange joints are normally caused by gasket problems, improper clamping with the stud bolts, or both. The gaskets were found to be normal. Independent lab tests showed the hex nuts met specs, but the studs did not. The studs, marked "B7," were improperly heat treated and were too soft. The threads on the studs were necked-out by overstretching. In addition, the manufacturer's ID symbols, required by ASTM A 193, were missing from the ends of the studs.

Conclusion:

The studs had a normal blackened heat-treated appearance and were assumed fit for service. When the generation unit was assembled, the nuts were tightened as usual. The studs most likely were stretched even more when gas pressure rose during operation. Unfortunately, without full traceability to the manufacturer, the refinery could not prove the source of the products. We did, however, work with them to qualify their stud inventory, and to improve their fastener purchasing and quality assurance practices so a tragedy like this would not happen again.

Recommendations:

It's a step in the right direction for purchasing to order fasteners correctly by specification, but to not verify conformance could be a deadly practice, as this case shows. Precautionary checks on products using independent labs can prove to be a valuable investment.

 

As appeared in Fastener Technology International magazine.

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