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Failure Analysis Case Histories
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Stress
Corrosion Cracking of Copper Tube in Evaporator
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ENVIRONMENT:
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Refrigerant system |
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EQUIPMENT:
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Evaporator |
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MATERIAL: |
Two years |
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FAILURE MODE:
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Stress Corrosion Cracking |
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Background |
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An ethylene glycol solution was circulated on the
shell side of the evaporator. A refrigerant was circulated within the tubes. The
tubes were in place for 2 years and saw 1,354 hours of service. The failure
allowed the glycol solution to enter the refrigerant system.
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Acetic anhydride is
manufactured in the building where the evaporator is housed. The plant
occasionally uses ammonia to treat plant water. It is not known whether the
water in this system was ever treated with ammonia.
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As reported, the fracture occurred at a steel
support within the evaporator. The mating end to the fracture was not retrieved
from the evaporator.
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Description of Material |
| One smooth walled ¾” diameter copper tube approximately
13” long was received, Fig. At one end was a circumferential fracture. The
opposite end had been crimped shut. |
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Findings |
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The outside of the tube was of
a uniform copper color. The fracture end of the tube was removed and the
remaining tube split longitudinally. The internal surfaces of the tube were
bright with several spots of discoloration running linearly along one section of
the tube.
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Visual analysis of the fracture
end revealed substantial mechanical damage. The damage observed was consistent
with damage that would be expected to have occurred during tube removal caused
by rotation of the tube through a tight fitting orifice. Substantial mechanical
damage was also present on the circumferential fracture surface.
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The damage to the
circumferential fracture surface was too severe to make any definitive comments.
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| Small green deposits on the circumferential
fracture surface |
| The fractured end of the tube was removed and placed into
a scanning electron microscope (SEM) equipped with Energy Dispersive
x-ray Spectroscopy (EDS). The green spots were analyzed using EDS for elemental
composition, Table 1. These spots were comprised primarily of silicon and
magnesium with minor quantities of iron, copper, sulfur and calcium. A major
carbon peak was also present. The copper present in the spectrum was likely due
to the copper tube background. |
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