CleaverBrooks CB Ohio Special 100 HP, Operation, Service And Parts Manual

Page: 56 / 200

Chapter 3 — Waterside Care and Requirements
3-14 Part No. 750-184
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 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.
Frequency of 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.
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. The length and frequency of the blwdown is particularly
important when the 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 the
concentrations is required.
It is generally recommended that a steam boiler be blown down at
least once in every eight-hour period, but frequency may vary
depending upon water and operating conditions. The blowdown
amounts and schedule should be recommended by your local
Cleaver-Brooks authorized representative.
A hot water boiler does not normally include openings for surface
blowdown and bottom blowdown 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.
Manual Blowdown Procedure
Blowdown is most effective at a point in time when the generation
of steam is at the lowest rate and feedwater input is also low, thus
providing a minimum dilution of the boiler water with low
concentration feedwater.