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. |
