POWER November 2015 - 41

OPERATIONS & MAINTENANCE
without filler metals. A DMW made using
nickel-based filler metal lessens the effects
of the thermal expansion differences between
stainless steel and ferritic steel.
Relocate DMWs. Stresses and temperatures
are the critical factors in the lifespan of a DMW;
performance can be improved by controlling
these factors. The weld joint can be relocated to
a position where it is exposed to lower temperatures.
Frequent inspection and maintenance of
tube hangers, supports, and spacers can be performed
to reduce secondary loads.
Bake Tubes. Condensate in high-temperature
circuits creates major problems during
startup periods. The tubes should be baked
for long enough to evaporate the condensate
before increasing the heat input. Reduce the
thermal gradient between the fluid and metal
during startup periods. Although load cycling
plays a major role in thermal fatigue, once
the component reaches equilibrium, thermal
fatigue will not be a significant factor.
Use Rifled Tubing. The use of rifled tubing
in areas susceptible to underdeposit corrosion
can provide better flow mixing to avoid
potential corrosion issues. Load cycling significantly
increases the susceptibility of waterwall
tubes to corrosion fatigue. Fast startups
increase transient stresses because different
parts expand and contract at different rates,
breaking protective oxides and exposing bare
tubes to the corrosive environment.
Improve Welding Techniques. Pad welds
should be avoided in regions susceptible to
corrosion fatigue. The residual stresses from
welding exacerbate corrosion fatigue. Additionally,
poor weld profiles should be eliminated
to reduce stress concentration. Lack of
penetration in attachment welds can increase
metal temperatures and stress concentration.
Maintain Proper Water Chemistry. It is
critical to ensure that water chemistry is within
range for pH and oxygen content, especially
during startups or load shifts, to reduce the
risk of corrosion fatigue. Boiler cleanliness
must be maintained to reduce the risk associated
with phosphate hideout. Use trisodium
phosphates in place of mono- or disodium
phosphates to bump phosphate readings. The
addition of trisodium phosphates does not
cause acid phosphate corrosion, but the addition
of mono- and disodium phosphates can
promote acid phosphate corrosion.
Avoid heavy blowdowns, which will significantly
affect sodium phosphate ratios and aggravate
the situation in units susceptible to acid
phosphate corrosion. Perform periodic deposit
weight density testing to know how dirty the
boiler is. Boiler cleanliness will significantly reduce
a majority of waterside issues. ■
-Rama S. Koripelli, PhD (rkoripelli@
davidnfrench.com) is technical director
for David N. French Metallurgists
November 2015 | POWER
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POWER November 2015

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