Several methods are available for storing idle boilers to prevent
corrosion.
The choice will depend on whether or not long term storage is required or
if, for short time storage, the boiler need to be available at short notice.
LONG TERM STORAGE
For long term storage, the following methods are available:
1 Using a corrosion inhibitor such as recommended for diesel
cooling water treatment. In this case the boiler is filled with water correctly
treated with the appropriate amount of corrosion inhibitor, a suitable
corrosion inhibitor is a mixture of sodium nitrite and borate at a
concentration of 1000 to 3000 ppm of each chemical; the boiler must be
drained and washed out to remove these inhibitors before it is prepared for
steaming.
2 Dry storage - completely emptying the boiler, drying out
by gentle heat and then blow in a moisture absorbing and vaporizing powder like
Shell VPI, and sealing the boiler gastight.
3 Using cyciohexylamine carbonate, or cyclohexylammonium
nitrite.
When using any of the above methods, it is important to consider the external
boiler surfaces. If the external surfaces are not effectively cleaned to remove
all the vanadium and sulphate deposits then condensation will produce strong
acids and rapid external corrosion of boiler tubes etc.
Thus the first step in protecting the extemal surfaces is to clean them
thoroughly using the appropnate cleaning techniques. If heat is available to
prevent moisture condensing on the metal surfaces then further action may be
unnecessary. However, additional insurance is possible by spraying the tubes
with a protective film of a film-forming corrosion inhibitor.
All valves shall be checked for leakage in spindle glands to prevent corrosion
of boiler outside shell.
SHORT TERM STORAGE
Even for short term storage, neglect of boilers during standing periods
frequently leads to the onset of corrosion in the steam and water spaces from
which serious attack may develop during subsequent working periods.
Corrosion can start as soon as a boiler is emptied, and the attack will be
especially severe if pools of water are left in the boiler. In all cases the
boiler should be dried out as soon as practicable.
During short term idle periods, one of the following methods of protection,
whichever is most convenient, should be used.
1. Completely filling the boiler with alkaline water
containing sufficient catalysed sodium sulphite to combine with the dissolved
oxygen and leave an excess of 200 ppm Na2SO3 before sealing the boiler, the water should be raised to the
boiling point to induce mixing by circulation and to complete the chemical
reaction.
2. Completely filling the boiler with alkaline water
containing sufficient hydrazine to combine with the dissolved oxygen and leave
an excess of hydrazine, for short periods of storage, up to three days, the
hydrazine excess should be at least 25 ppmN2H4 before sealing the boiler, the water should be raised to the
boiling point to induce mixing by circulation and to complete the chemical
reaction.
When using the above methods, the alkalinity should be raised to the
recommended value by the addition of the appropriate chemical normally used. It
is also necessary to check periodically the excess of sulphite or hydrazine in
the boiler and to restore any that has been oxidized.
If the boiler is not being emptied then the sodium sulphate or hydrazine should
be added a short time before steaming ceases. The treatment, in solution,
should be directly injected into the boiler or added as a single shot at the
feed tank, via a tundish fitted with a pipe leading into the feed pump suction.
When using these methods, precautions should be taken to prevent
atmospheric condensation upon the fire-side surfaces of the boiler, which would
lead to severe external corrosion.
Wet storage methods are not advised for idle boilers in
circumstances in which the water in the boiler or pipe work could freeze during
idle periods.
All wet storage methods could be completed with connection of a small nitrogen
bottle, thus putting the boiler in a very low pressure preventing any oxygen
from entering from the open air in case of small leakages.
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