ASME Pressure Vessels and Water Storage Tanks - page 46

Boiler Blowdown Vessels
46 •
Boiler Blowdown Tanks (BBT) are used
to control blowdown of a boiler so that
the level of dissolved solids is minimized
to reduce scaling in the boiler. BBT
also enable high-pressure water to
safely flash to steam.
Required boiler maintenance includes
periodic removal of water to control
the buildup of suspended solids and
particulates as well as the concentration
of treatment chemicals.
Surface water blowdown is often done
continuously to reduce the level of
dissolved solids and bottom blowdown
is performed periodically to remove
sludge from the bottom of the boiler.
The amount of blowdown necessary
depends on boiler operating pressure,
amount of makeup water, impurity
levels in the makeup water and the
dissolved solids concentrations that a
given boiler can tolerate.
A properly designed BBT has a
capacity of not less than twice the
volume of the boiler water so that the
boiler can be blown down completely
without any restriction.
The process of removing this water is
referred to as boiler blowdown. Boiler
blowdown can also refer to the
discharged water itself.
Blowdown water has the same
temperature and pressure as the boiler
water. Local laws usually do not permit
this high temperature and pressure
water to be discharged into the sewer.
An acceptably designed blowdown
tank system reduces the temperature
by mixing cold water with the effluent.
It is a general rule that the temperature
of the water leaving the BBT shall not
exceed 150
º
F and 5 psig.
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