CHAPTER2
In practice, the valve(s)
of
the bottom blowdown
are opened periodically in accordance with an
operating schedule and/or chemical control
tests.
From the standpoint
of
control, economy and
results, frequent short blows are preferred to
infrequent lengthy blows. This
is
particularly
true when suspended solids content
of
the water
is
high. With the use
of
frequent short blows a
more uniform concentration
of
the pressure ves-
sel water is maintained.
In
cases where the feedwater is exceptionally
pure, or where there
is
a high percentage
of
return
condensate, blowdown may
be
employed less
frequently since less sludge accumulates in the
pressure
vessel.
When
dissolved
and/or
suspended solids approach
or
exceed predeter-
mined limits, manual blowdown
to
lower these
concentrations
is
required.
Generally it
is
recommended that a steam boiler
be
blown down at least once in every eight·hour
period, but this may vary depending upon water
and
operating
conditions.
The
blowdown
amounts and schedule can be recommended by .
your
Cleaver~Brook:s
local representative.
A hot water boiler does not normally include
openings for surface blowdown and bottom
blowdown since blowdowns are seldom
prac-
ticed. The need remains to
be
alert to system
water losses and corresponding amount
of
raw
water make·up. A water meter
is
recommended
for water
make-up lines.
:Manual Blowdown Procedure
Slowdown
is
most effective at a time when
generation
of
steam
is
at the lowest rate since
feedwater input then
is
also low providing a
minimum dilution
of
the boiler water with low
concentration feedwater.
Make sure the blowdown piping and the tank, if
used, are in proper operating condition, the dis-
charge vents are clear
of
obstruction, and that the
waste
is
piped
to
a point
of
safe discharge.
Most blowdown lines are provided with two
valves, generally a quick opening valve nearest
the boiler and a slow opening globe type valve
downstream. Two slow opening valves or tan-
CHAPTER 2·12
THE
PRESSURE VESSEL
dem valves may be used. Valves will vary
depending upon pressure involved and the make
or manufacture.
If
seatless valves are installed,
follow the manufacturer's recommendations.
If
a quick opening valve and globe type
or
slow
opening valve are in combination, the former is
normally opened first and
closed last with blow-
ing down accomplished with the globe
or
slow
opening valve.
When opening the second
or
down stream valve,
crack it slightly
to allow the lines to warm up,
then continue opening slowly.
CAUTION!
Do
not
pump
the
lever
action
valve
open
and
closed,
as
water
hammer
is
apt
to
break
the
valve
bodies
or
pipe
fittings
or
damage
to
the
equipment
could
occur.
The length
of
each blow should be determined by
actual
water analysis. Lowering the water
in
the
gauge glass approximately
1/2"
is
often accept-
able as a guide
to
adequate blow. However, this
should not be
interpreted
as
a rule since water
analysis procedures should prevail.
If
the glass
cannot
be
viewed by the party operating the
valve, another operator should watch the glass
and direct the valve operator.
Close the downstream (slow opening) valve first
and
as
fast as possible. Then close the valve next
to the boiler. Slightly crack the downstream
valve and then close it tightly.
Under no circumstances should a blowdown
valve be left open and the operator should never
leave until the blowdown operation
is
completed
and valves
closed.
750·150
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