Cleaver-Brooks CBH 40 - Page 28

Chapter 2-The Pressure Vessel
TYPES OF SlOWDOWN
There are two principal types of blowdown: inter-
mittent manual blowdown and continuous blow-
down.
14. Manual Slowdown
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 dissolved solids in the pressure vessel
water, it also removes a portion 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 not 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.
B. Continuous Slowdown
Continuous blowdown is used in conjunction with
a surface blow-off tapping (furnished on units 60"
diameter and larger) and is the continuous
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 con-
tinual - yet controlled - flow of concentrated water.
Periodic adjustments are made to the valve setting
to increase or decrease the amount of blowdown in
accordance with the test analysis.
The flow control valve and piping are generally
provided by others. All piping must be to a safe
.point of discharge.
C. frr®quenc:y of Manual Slowdown
When continuous blowdown is utilized, manual
blowdown is primarily used to remove suspended
solids or sludge. The continuous blow'down
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.
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 in-
frequent lengthy blows. This is particularly true
2-8
when suspended solids content of the water is high.
With the use of frequent short blows a more
uniform concentration of the pressure vessel water
is maintained.
In cases where the feedwater is exceptionally pure,
or where there is a high percentage of return con-
densate, blowdown may be employed less frequen-
tly since less sludge accumulates in the pressure
vessel. When dissolved and/or suspended solids ap-
proach or exceed pre-determined limits, manual
blowdown to lower these concentrations is
required.
It is generally 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 a schedule should be recommended by a water
treating company or a water consultant.
A hot water boiler does not normally include
openings for surface blowdown and bottom blow-
down since blowdowns are seldom practiced. 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.
D. Manual Slowdown Procedur®
Blowdown is most effective at a time when
generation of steam is at the lowest rate since feed-
water input then is also low providing a minimum
dilution of the boiler water with a low concentra-
tion feedwater.
Make sure blow-off piping, and tank if used, are in
proper operating condition and discharge vents
clear of obstruction, and that waste is piped to a
point of safe discharge.
Most blow-off 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 tandem valves may be
used. Valves will vary depending upon pressure in-
volved and make or manufacture. If seatless
valves are installed follow the manufacturers'
recommendations.
If a quick opening valve and a globe type or slow
opening valve are in combination, the former is
normally opened first and closed last with blowing
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 open the slow opening valve
first and pump the lever action valve open and
closed as water hammer is apt to break the valve
bodies or pipe fittings.
The length of each blow should be determined by