POWER September 2014 - 40

WATER
Boiler Chemical Cleaning:
Doing It Correctly
Like some of those tests your doctor is always after you to get, boiler chemical
cleaning is something that most of us would rather not think about
but that we all agree is necessary. Adding to our general discomfort with
the process are new Environmental Protection Agency regulations, which
make the disposal of chemical cleaning wastes more expensive. Here is a
review of what to do, when, and some things to watch out for.
David Daniels
E
veryone knows (or should know) that
boiler tubes containing deposits create
long-term reliability problems for the
boiler. Deposits insulate the water in the tubes
from the fire, causing the metal temperature
to increase dramatically between the deposit
and the tube metal. Long-term overheating of
the metal will result from prolonged operation
with heavy tube deposits. The tubes will
first bulge and then fail. Because the deposits
tend to be widespread, this generally means
that large sections of boiler tubing will be
damaged and require replacement.
The deposits also concentrate any boiler
water chemistry and contamination that collects
under them. The increased metal temperature
caused by the deposit increases the rate
of corrosion caused by any phosphate, caustic,
or chloride underneath the deposit. With the
exception of corrosion fatigue, all the waterside
boiler tube failure mechanisms occur under
deposits. Get rid of the deposits and you
also stop the water-side tube failures.
Determining When to Clean
The standard method for determining when
to chemically clean a boiler is to take a
boiler tube sample and have the deposit
amount measured-a deposit weight density
(DWD)-and the composition of the deposit
analyzed. But there are other conditions besides
the DWD that require that the boiler be
cleaned. These include:
■ One or more failures due to an under-deposit
corrosion mechanism, particularly hydrogen
damage. The first priority must be to prevent
further damage by removing the deposits via
a complete chemical cleaning.
■ Major contamination event or multiple
small events, particularly condenser tube
leaks. Contamination events increase the
amount of deposit in the boiler and its corrosiveness.
Chemical cleaning removes the
40
deposits and the contamination underneath
the deposits before they corrode to failure.
■ Replacement of boiler tubing. The rule of
thumb is to chemically clean if you are
replacing more than 10% of the surface
area of the boiler. This helps to create a
uniform layer of oxide on all the tubes.
■ A major change in the boiler fuel or burner
design. Changing fuels, such as from
coal to gas, or modification of the burners
can result in changes to the area of high
heat flux in the boiler. When implementing
such a major change, it is best to start
with a clean boiler.
■ A change in the chemical treatment regime.
Such changes would include moving from
one chemical treatment to another, say
from all-volatile treatment to oxygenated
treatment (OT).
Using Deposit Weight Density to
Determine When to Clean
The standard DWD test should not only provide
a deposit loading but also an analysis of
chemical composition of the deposit on the
tube. This chemical analysis of the deposit can
be done quantitatively, using an inductively
coupled plasma emission spectrometer (ICPES),
but it is more commonly determined
semi-quantitatively using electron dispersion
spectroscopy (EDS). Occasionally, X-ray diffraction
data is also provided to indicate the
chemical compounds that are present.
Optimally, the tube sample for DWD
should be about 18 inches long and from the
highest heat flux area of the boiler. This is
typically above the burners or on the underside
of the nose arch. The idea is to find the
tube in the boiler with the most deposit in a
high-heat area. You cannot use a tube that has
failed, because some or all of the deposit will
have been removed by the failure.
DWD is determined by removing the deposit
in a carefully measured area of the tube.
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The tube is split and the deposit on the firefacing
(hot) side is analyzed separately from
the insulation-facing (cold) side. As far as
chemical cleaning is concerned, the side that
counts is the hot side.
The change in the weight of the tube divided
by the water-touched area where the
deposit was removed produces the DWD
result. This can be expressed in gram/ft2
gram/meter2 (g/m2, SI, International System
of Units). The conversion is 1 g/ft2
to 10.76 g/m2
.
Currently, the most common method of
deposit removal for the DWD test is beadblasting
with glass beads (NACE TM019999).
The other method that is occasionally
used is to dissolve the deposits in a solvent,
typically, inhibited hydrochloric acid, HCl
(ASTM D3483-83 [2005] Test Method B). In
general, the solvent method produces slightly
higher DWD results on the same tube, as
some small amount of metal is removed with
the deposit.
When bead-blasting the tube, a layer of
deposits often will become visible, such as
a layer of copper. A good DWD report will
describe and show any anomalies found as
the deposit was removed.
Best practice is to grab a tube sample from
the boiler during each major outage or at least
one every two years. Each sample should be
from a similar elevation or area in the boiler.
Comparing the DWD results from year to
year shows the deposits are accumulating in
the boiler and can be used to anticipate the
need to chemically clean, though the deposit
formation is rarely linear. Figure 1 presents
a chart with chemical cleaning recommendations
by boiler operating pressure.
There are three general recommendations
on this figure. If the DWD result is in the
top area, a chemical cleaning should be performed
as soon as it can be scheduled. The
lowest area represents a relatively clean tube.
POWER | September 2014
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POWER September 2014

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