POWER November 2015 - 43

FUNDAMENTALS
2. In with the new. Installing this replacement steam header will extend the plant's operation
for years to come. Courtesy: Brandon Bell, PE
the state of materials (Figure 3). When used
effectively, these techniques are able to track
the progression of either thermal fatigue or
creep, which helps plants plan in advance for
replacement of components rather than waiting
for material failure and then scrambling
to correct the problem. Some commonly
used nondestructive examination techniques
follow.
Liquid Penetration. A common nondestructive
examination technique used for
detecting surface cracking on a material is
called liquid penetration. This technique
is very versatile, as it does not require specific
material properties for the metal being
tested.
Prior to the material being tested, it must
Thermal fatigue cracking tends to be a
localized phenomenon that can be identified
using conventional nondestructive examination
methods. Once thermal fatigue cracking
is identified, welding can easily repair
it. Creep damage typically is identified using
metallographic examination methods.
If a material has been identified to be in the
tertiary stage of creep, simple repairs are not
possible, and replacement of the material is
required (Figures 1 and 2).
Analyzing Boiler Materials
To avoid forced outages resulting from thermal
fatigue and creep, nondestructive examination
techniques can be used to determine
be cleaned of any contaminants and allowed
to dry. A low-tension liquid with a visible
dye is then applied to the surface of the material,
at which point the capillary effect will
draw the liquid into any discontinuities in the
metal. Any excess liquid is removed from the
surface prior to inspection.
Under a white or fluorescent light, the material
is inspected for the presence of the liquid
penetrant. The presence of the liquid penetrant
indicates voids in the surface material, either
from cracking or porosity of welds.
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POWER November 2015

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