The Pressure Vessel
2-14
750-96 (revised 2010)
Model CB Packaged Boiler Manual
2.10 — Preparation for Extended Layup
Many boilers used for heating or seasonal loads or for standby service may have extended periods of non-use.
Special attention must be given to these so that neither waterside nor fireside surfaces are allowed to deteriorate
from corrosion.
Too many conditions exist to lay down definite rules. There are two methods of storage: wet or dry. Your water
consultant or feedwater treating company can recommend the better method depending upon circumstances in
the particular installation.
Whichever method is used, common sense dictates a periodic recheck of fireside and waterside conditions during
the layup to allow variations from the above methods for special area or job site conditions.
Although pollution control regulations may continue to limit the permissible sulphur content of fuel oils, care must
be taken to avoid corrosion problems that sulphur can cause, especially in a boiler that is seasonally shutdown.
Dormant periods, and even frequent shutdowns, expose the fireside surfaces to condensation below the dew
point during cooling. This moisture and any sulphur residue can form an acid solution. Under certain conditions,
and especially in areas with high humidity, the corrosive effect of the acid will be serious enough to eat through or
severely damage boiler tubes or other metal heating surfaces during the time that a boiler is out of service.
This condition does not generally occur during normal firing operation since the high temperature of operation
vaporizes any condensation. However, proper boiler operation must be maintained, especially with a hot water
boiler, to prevent the flue gases from falling below the dew point.
At the start of layup, thoroughly clean the fireside by removing any soot or other products of combustion from the
tubes, tube sheets, and other fireside surfaces. Brushing will generally suffice. Sweep away or vacuum any accu-
mulation. The fireside surfaces may be flushed with water. However, all moisture must be eliminated after flushing
and the surface dried by blowing air or applying some for of heat. It is good practice to protect the cleaned sur-
faces by coating them with an anti-corrosive material to prevent rust.
To prevent condensation from forming in the control cabinet, keep the control circuit energized.
Dry storage is generally employed when the boiler will be out of service for some time or where freezing tempera-
tures may exist. In this method the boiler must be thoroughly dried because any moisture would cause corrosion.
Both fireside and waterside surfaces must be cleaned of scale, deposits, soot, etc. Steps must be taken to elimi-
nate moisture by placing moisture-absorbing materials such as quick lime (at 2 pounds for 3 cubic feet of volume)
or silica gel (at 5 pounds for 30 cubic feet of volume) on trays inside the vessel. Fireside surfaces may be coated
with an anti-corrosive material or grease or tar paint. Refractories should be brushed clean and wash coated. All
openings to the pressure vessel, such as manhole and hand holes, should be shut tightly. Feedwater and steam
valves should be closed. Damper and vents should be closed to prevent air from reaching fireside surfaces. Peri-
odic inspection should be made and absorption materials renewed.
Wet storage is generally used for a boiler held in standby condition or in cases where dry storage is not practical.
The possibility of freezing temperatures must be considered. Care must again be taken to protect metal surfaces.
Variables preclude definite recommendations. However, it is suggested that the pressure vessel be drained, thor-
oughly cleaned internally, and re-filled to overflowing with treated water. If deaerated water is not available, the unit
should be fired to boil the water for a short period of time. Additional chemicals may be suggested by the water
consultant to minimize corrosion. Internal water pressure should be maintained at greater than atmospheric pres-
sure. Nitrogen is often used to pressurize the vessel. Fireside surfaces must be thoroughly cleaned and the refrac-
tory should be wash coated.
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