POWER September 2014 - 44

WATER
whether a plant should clean at the beginning,
during, or at the end of an outage.
Chemically cleaning at any point except
the very end of an outage leaves the tubes
vulnerable to some general corrosion. The
passivation step at the end of the chemical
cleaning is generally neither long enough
nor at a high enough temperature to create
a robust protective layer. It is often difficult
to ensure that the boiler gets really dry after
the cleaning or can be laid-up properly in
a wet condition. The superheater is always
back-filled during cleaning to minimize the
risk of contamination. So, unless it can be
drained and dried, this area will remain wet
until the unit is fired sufficiently to dry it
out. Therefore, the typical recommendation
is to perform the chemical cleaning at the
very end of an outage.
Heating the boiler for chemical cleaning
using an auxiliary boiler or steam from another
unit adds complexity and cost to the
cleaning. So, as a rule, it is best to wait until
the unit can be warmed using its own burners
or igniters and when the fans and instrumentation
associated with the fuel system (such
as flame scanners) are working properly and
have been fully tested. Many a utility has
waited for days with the chemical cleaning
vendor on site and ready to go while its staff
tried to get a fire in the boiler. For this reason,
some utilities have decided not to schedule
chemical cleanings during an outage at all;
instead, they take a weekend outage separate
from the overhaul for this purpose.
Similarly, pre-commissioning cleanings
are optimally performed as close to the steam
blow as possible, to minimize the opportunity
for corrosion to undo what was just done
with the cleaning.
The Actual Cleaning Process
Once the boiler is at the proper temperature,
the actual cleaning process can take between
16 and 72 hours to fully remove all the iron.
The shortest times are for the mineral acids
(HF and HCl); complex-forming solvents
such as EDTA require longer times.
The important thing is to make sure all deposits
have been removed during the cleaning
and that the chemistry has stabilized. With
HCl, the inhibitor that prevents the acid from
aggressively attacking the metal surface has
a finite time that it can protect the metal once
it is in the boiler. Therefore, with this solvent,
the cleaning solution must be drained
before the inhibitor breaks down, whether all
the deposit is removed or not. This has been
a problem in some boilers that were heavily
deposited, where drains have plugged, not
allowing the boiler to drain and resulting in
general acidic attack of the tubes.
In forced circulation boilers, one critical
44
2. Frac tanks lined up for chemical cleaning. Courtesy: M&M Engineering
piece is the flow of purge water through the
boiler circulation pump motors. This purge
water protects components in the motor
from the solvents, by providing a constant
outward flow past the motor cavity and
into the boiler. Purge water should be on
all boiler circulation pumps (in service
or not) as long as there is chemical in the
boiler. Since the purge water is designed to
provide water to the pump during normal
operation, the system is set up to overcome
normal boiler pressure. These high-pressure
pumps can produce high purge water
flow rates during the cleaning and can be
difficult to control. If the purge water flow
is excessive, the level in the boiler is constantly
rising to the point where solvent has
to be drained before it goes out of sight in
the temporary sight glass. Every gallon of
purge water dilutes the solvent and generates
more waste that will need to be disposed
of at the end of the cleaning.
Some utilities set up a separate purge water
system with pressure controllers and flow
meters just for the cleaning that can constantly
provide a slight positive purge water flow during
the cleaning process. This system can pay
for itself many times over in a single cleaning.
Once the chemistry in the boiler indicates
that the boiler is clean, next is a passivation
step. In the case of HCl, passivation is preceded
by rinses and neutralization of any remaining
acid in the boiler. In the case of EDTA,
passivation occurs after cooling the boiler to
160F and after the pH of the solvent is raised
with ammonia (for diammonium EDTA) when
oxygen is added. This also complexes any remaining
copper on the boiler tubes.
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The degree to which the boiler tube surfaces
are truly passivated is a function of the solvent,
the procedure, and the time and temperature at
which the passivation is performed.
What to Expect After Cleaning
When the cleaning is finished, the chemical
cleaning solvent is rapidly and completely
drained from the unit. Usually, two full boiler
volume rinses follow, with partial rinses
in addition to these in some cases. Conductivity
is used as an indicator to see how
well the solvent has been removed from the
boiler. In some cases, the last rinse is treated
with chemical to raise the pH to a normal
boiler pH range, and the boiler is fired to
180F to 200F so that it can be drained hot
and dried out.
As noted above, optimally, the boiler will
be started as soon as rinses are complete and
the normal boiler piping can be restored. Depending
on the design of the boiler, it may be
difficult to flush out all the iron oxides that
were released by the cleaning but were not
fully dissolved. This can contribute to " black
water " samples and high iron levels in the
boiler following the cleaning. Some utilities
have used boiler dispersants for a time to
promote the suspension and removal of any
remaining deposits in the boiler through the
normal drum blowdown.
Dealing with the BCCW
There will typically be between three and
four boiler volumes of chemical cleaning
wastes (BCCW), which includes the solvent
and all rinses following a chemical cleaning.
The waste and rinses are temporarily stored
POWER | September 2014
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POWER September 2014

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