Fulton Electric (FB-L) Steam Boiler - Manual (Page 33)

Electric IOM
THE PROTECTION FROM
CORROSION OF
BOILERS IN STANDBY
CONDITION
Prevention of corrosion in a boiler in
standby condition is more difficult than in an
operating boiler. This brief report has been
prepared to summarize the methods that
may be followed to prevent or minimize
deterioration of the internal surfaces of
boilers from corrosion during inoperative
periods.
Two sets of conditions must be met:
1. The boiler must be held in readiness to
operate at any time on short notice. This
may be designated as intermittent
standby.
2. The boiler will be continuously
inoperative for an indefinite period of
weeks or months. This is prolonged
standby.
Intermittent Standby
In general, the water level maintained in the
boiler under these circumstances
corresponds closely to that of operation, or
reduced firing, the temperature is held
closely to that of steaming temperature.
Circulation, however, is very slight if at all.
During operation the boiler water is
maintained uniformly in an alkaline state,
and by its rapid circulation, segregation of
any water containing oxygen is prevented.
During the standby period, however, some
loss of water occurs…slight leakage
through the blow down valve, slight
leaking…which is replaced by feed water.
IF the feed water is sufficiently oxygen free
and of suitable alkalinity (pH value),
conditions leading to corrosion will not be
developed. However, if the conditions are
anything but ideal, delayed intermingling of
boiler water and feed water due to lack of
circulation or low alkalinity, oxygen rich
water may form at the boiler surfaces and
initiate corrosion. In the case of
considerable length of time and of
appreciable make up of feed water to
replace losses, the boiler water alkalinity
may completely disappear, and general
corrosion will result.
No single rule can be given to assure
correct conditions in the boiler. The regular
boiler water tests must be made as carefully
on these boilers as on the operating boilers.
This is due to the fact that these boilers
while in standby can not be adjusted as
easily as the operating boilers, and as such,
they can be severely damaged by water
problems more easily.
If the alkalinity falls to low, it can be boosted
by putting a small amount of alkali solution
(preferably a caustic solution) directly into
the boiler at the point of feed water entry
with a pump or any other convenient
manner.
In the cases where quantities of oxygen are
dissolved in the feed water, a solution of
sodium sulfite (Na2SO3) can be fed to the
boiler by means of a pump or either
separately or in conjunction with the alkali
solution. The minimum amount of residual
sodium sulfite that should be maintained in
the boiler is 30 – 50 ppm as a residual.
If oxygen pitting and localized corrosion in
separate sections of the boiler, then this is a
display of typical corrosion due to feed
water segregation. Should this occur, then
the segregation of the feed water must be
stopped. This can be done by intermingling
feed water (either by boiler circulation, a
circulating pump, or through an injection
method.) Additionally, the requirement for
make up water can be brought under
control by stoppage of system leaks.
Finally, if several boilers are subjected to
intermittent use, they should be alternated
in turn in operating service. This will stop
the irregularities that may come about to
one boiler that experiences extended
standby conditions. This type of rotation
prevents nay one boiler from suffering from
potential standby problems. On the other
hand, do not alternate the boilers on a basis
that is more frequent than every week, or
you may experience problems from the
heating and cooling of the boiler, which will
result in leaks due to frequent expansion
and contraction of the boiler pressure
vessel.
The rules for treating a standby boiler are in
large, the same as those for an operating
boiler. The application of chemical
treatment to a standby boiler must be as
fully and more carefully performed.
Prolonged Standby
Two general procedures are available:
1. The boiler may be emptied and dried out,
and kept dry.
2. The boiler may be filled completely with
water. In this case the drums or the
boiler body, whether a water tube or a
fire tube boiler is being treated, are filled
to the steam stop valve or water supply
valve.
Draining and Drying
This method will allow excellent protection
from corrosion to the metal surfaces so long
as there is no moisture present in the boiler.
One method is to let the boiler open for free
circulation of air after drying. Another
method is to place a desiccant as silica gel
in the boiler and the boiler is then closed up
for drying. In either case, water leakage
over, or sweating of, the boiler metal
surfaces must be protected against. Since
this type of moisture is saturated with
oxygen, its contact with metal surfaces will
cause excellerated rates of corrosion. So
long as the boiler metal surfaces remain
free of moisture, no appreciable corrosion
will occur.
If the boiler’s past history indicates that
external sweating of the boiler internals is a
problem, then additional heater should be
installed in the furnace at strategic locations
in order to insure that the boiler pressure
vessel is maintained above the dew point.
This concept must also include the
superheater section if the boiler has a
superheater. If you allow dew to form, then
the purpose of lay up is defeated, as the
boiler tube metal will rapidly corrode both
internally as well as externally.
For extended dry standby, blanketing with
nitrogen should be considered. This
method will dry out the metal and if
maintained properly, it will help to maintain
a moisture free atmosphere on the internal
areas of the boiler.
Boiler Filled With Water
In this method, protection can be
obtained for the boiler metal if:
1. Correct chemical conditions are
maintained in the boiler water.
2. The boiler water is mixed adequately to
maintain uniform conditions throughout
the boiler.
3. The boilers are completely filled with
treated water as not to allow any boiler
metal surfaces to come in contact with
the air.
Sufficient caustic soda (or equivalent
alkalinity builder) should be added to the
boiler water in order to produce a Hydroxyl
(OH-) Alkalinity of 350 to 600 ppm. In
addition, sufficient sodium sulfite must be
added to the boiler water to establish a
sodium sulfite residual of 30 – 50 ppm as a
minimum.
The mixing of the water in the boiler must
be thorough so that correct chemical
conditions can exist in every section of the
boiler. Mixing of the water can be
accomplished by circulating water from one
section of the boiler to another with the use
of a pump, as in prolonged standby, the
boiler is not fired off, and as such, can not
mix the boiler water by means of circulation
through heating up the boiler. If the pump
method is practical, additional chemicals
can be trickled into the boiler as they are
needed during the circulation procedure.
Some boiler water sludge will form during
standby conditions and this must be treated
for. The cleaner the boiler surface metal,
the lower the potential for corrosion. Either
a Polymaleic, Sulfonated Copolymer,
Maintenance
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