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. |
