During the outage season PSA inspectors typically discover cracking in
closure (crotch) plates around furnace wall openings. Cracking does
not just develop in openings in the immediate lower furnace, but in
upper furnace ports and sootblower wall openings further back
in the flue gas pass. Generally the rate of crack propagation is
slower in the upper furnace elevations, but with the aging population
of boilers in the industry tube failures can develop in more obscure
locations. This makes it important to thoroughly inspect
all furnace openings.
Cracks typically develop in the closure plate transverse to the tube,
propagating toward the tube over time. When it runs into the weld
between the closure plate and tube the crack may turn to follow the heat-affected zone of the weld
but generally will not penetrate the tube.
Other times, the crack will cross the weld and extend into the tube.
This crack had not crossed the membrane weld.
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This crack had propagated close to the tube.
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Once identified,
cracks should be removed by
one of several methods. The
choice of method used to remove or stop the crack will depend on its
location and orientation relative to the tube.
This photo shows a crack that was
removed by burr grinding the closure plate.
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"Burr
grinding" used at a relatively
low speed of 600 rpm does a good job of cutting closure plate
material and leaves a smooth, round surface which minimizes further crack
development and propagation.
Another method is use of an “air arc” to
cut the material. Air arcing should be followed by additional burr
grinding.
Lastly, use of a drill to “drill stop” the end of the crack will
work, but only if the end of the
crack is removed during the drilling process.
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Perform nondestructive testing
following the removal of any cracks. We prefer wet florescent magnetic particle
(WFMT) testing on ferrous materials and dye penetrant (PT) testing
on non-ferrous materials such as composite tubes.
In this case, WFMT
testing was repeatedly utilized as the tube was burr ground until the
crack was entirely removed. Upon completion, the
remaining wall thickness was measured to determine whether a pad weld
repair or Dutchman section would be used to complete the repair. |
This photo
illustrates a WFMT repair of propagation of a crack into a tube.
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