CHAPTER2
Solids become less soluble in the high tempera-
ture
of
the boiler water and tend to accumulate
on
heating surfaces.
Blowdown
and internal
chemical treatment is, therefore, required to
prevent the solids from forming harmful scale
and sludge.
Scale has a low heat transfer value and acts as an
insulation barrier.
This
retards heat transfer,
which not only results
in
lower operating ef-.
ficiency and consequently higher fuel consump-
tion
but,
more
importantly,
can
cause
overheating
of
boiler metal. This can result in
tube failures
or
other
pressure vessel metal
damage causing boiler down-time and costly
repairs.
Scale is
ca~
primarily by calcium and
mag-
nesium salts, silica and oil.
Any
calcium and
magnesium salts in the boiler water generally are
precipitated by the use
of
sodium phosphate,
along with organic materials, to maintain these
precipitates
or
"sludge"
in a fluid form. The
solids such as sodium salts and suspended dirt do
not readily form scale but, as the boiler water
boils
off
as relatively pure steam, the remaining
water is thicker with the solids.
If
this concentra-
tion
is
permitted to accumulate, foaming and
priming will occur and the sludge can cause
harmful deposits that bring about overheating
of
the metal.
The
lowering
or
removal
of
this concentration
requires the use
of
boiler water blowdown.
1. Types of
Blow
down
There are two principal types
of
blowdown: in-
termittent manual
blowdown
and continuous
blowdown.
:Manual Blowdown
Manual
or
sludge blowdown is necessary for the
operation
of
the boiler regardless
of
whether
or
not continuous blowdown
is
employed.
The blowdown tappings are located at the bottom
or
lowest part
of
the boiler so that
in
addition to
lowering the concentration
of
dissolved solids
in
750-150
THE
PRESSURE
VESSEL
the pressure vessel water, it also removes a por-
tion
of
the sludge which accumulates in the lower
part
of
the vessel.
Equipment generally consists
of
a quick opening
valve and a shut-off valve. These, along with the
necessary piping, are
~ot
normally furnished
with the boiler, but supplied by others. All piping
must
be to a safe point
of
discharge. Piping must
be
properly supported and free to expand.
Continuous Blowdown
Continuous blowdown
is
used in
<?Onjunction
with a surface blow-off tapping (furnished on
units
60" diameter and larger) and
is
the con-
tinuous removal
of
concentrated water.
The surface blow-off opening,
when
furnished,
is
on
the top center line
of
the pressure vessel and
is
provided with an internal collecting pipe ter-
minating slightly below the working water level
for the purpose
of
skimming surface sediment,
oil
or
other impurities from the surface
of
the
pressure vessel water.
A controlled orifice valve
is
used to allow a
continual-yet
controlled-flow
of
con-
centrated water .
Periodic adjustments are made to the valve set-
ting to increase
or
decrease the amount
of
blow-
down
in
accordance with water analysis results.
The
flow control valve and piping are generally
provided by others. All piping must
be
to a safe
point
of
discharge.
Frequency
or
Manual
Blowdown
When continuous blowdown
is
utilized, manual
blowdown
is
primarily
used
to
remove
suspended
solids
or
sludge.
The
continuous
blowdown removes sediment and oil from the
surface
of
the water along with a prescribed
amount
of
dissolved solids.
When surface
or
continuous blowdown
is
not
utilized, manual blowdown is used
to control the
dissolved
or
suspended solids in addition
to
the
sludge.
CHAPTER 2·11
To Next Page
Loading...