TM 5-810-15
to make sure any residual dissolved 02 is not
cates the steam drum internals showing the chem-
permitted to pit the tubes.
ical feed line and the continuous blow pipe in
(2) Water hardness, expressed as calcium
addition to the feedwater line. The diameter and
carbonate (CaCo3) in ppm should be as close to
length of the drum are determined by the capacity
zero as possible under all conditions. This can be
of the boiler in the number of primary or cyclone
accomplished by proper feedwater treatment and
separators needed. These devices in addition to
boiler water testing.
separating steam and water, aid water circulation
(3) Another item of importance is to
by a reactive (pumping) action that promotes water
maintain the proper acidity or alkalinity (pH value)
flow along the length of the steam drum.
reading of the boiler water. For boilers in the range
(1) The drying screen or secondary
of this manual, a pH reading of 9.0 to 10.5 should
scrubbers further separates the steam and water
be acceptable depending on the chemical
particles so that the steam leaving the drum meets
composition of the water source and the type of
the desired steam purity condition. Another feature
treatment used. In the latter category, some of the
of the steam drum is the reserve water holding
more common types of treatment are: sodium
capacity which permits load swings besides being
zeolite, hot lime zeolite, phosphate hydroxide and
the collecting and distribution point of the steam.
coordinated phosphate. In some cases the use of a
(2) The primary function of the lower (mud)
demineralizer or evaporation may be desirable.
drum is to complete the circuit for the tubes in the
These latter methods are more appropriate for
boiler section and generally to act as a water
higher pressures and temperatures in a steam cycle
reservoir and supply source for the lower furnace
that has more complex problems due to the source
wall headers and tubes connected thereto. Except
of water for the boiler or boilers. These water
in unusual cases, the lower drum has no internals.
treatment methods are addressed in more detail in
It should be sized so that maintenance people can
chapter 7.
roll tubes into the drum holes as well as inspect
c. Boiler internals. Figures 3-17 and 3-18 indi-
those tubes. Some designs may permit rolling of
3-26
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