The Pressure Vessel
2-12
750-96 (revised 2010)
Model CB Packaged Boiler Manual
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 pre-
ferred 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 vessel 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 predetermined limits, manual blowdown to lower these concentra-
tions 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 rec-
ommended by a water treating company or a water consultant.
A hot water boiler does not normally include openings for surface blowdown and bottom blowdown since blow-
downs are seldom practiced. The need remains to be alert to system water losses and corresponding amount of
raw water makeup. A water meter is recommended for water makeup lines.
2.8.4 — Manual Blowdown Procedure
Blowdown 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 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 involved and make or manufacture. If seatless valves are installed, follow the manufac-
turer’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 blowing down accomplished with the globe or slow opening valve.
When opening the second or downstream valve, crack it slightly to allow the lines to warm up, then continue open-
ing slowly.
The length of each blow should be determined by actual water analysis. Lowering the water in the gauge glass
approximately 1/2” is often acceptable 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.
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.
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