Page 271 - Boiler_Operators_Handbook,_Second_Edition

256 Boiler Operator’s Handbook

carryover or scale formation and blowdown is it. By re- load because they can operate with higher solids content
moving some of the boiler water from where it contains at lower loads. Usually carryover is the limiting factor
the highest concentration of solids we provide space for but scale formation can be so I also recommend raising
some makeup water that contains very little solids to the level at 50 ppm intervals doing so each year one
enter the boiler and reduce the overall concentration of month before the annual internal inspection while keep-
solids. ing a close watch on relative stack temperatures. Back
In a steam boiler that means removing the water off on any increase in stack temperature because it could
right after it has separated from the steam in the steam indicate scale formation. Since blowing down wastes
drum. That’s why the continuous blowdown piping is energy and water minimizing it is a wise operation; it’s
in the steam drum and the piping has the holes located worthwhile to minimize blowdown.
where they are. That was a hint for those of you who We used to adjust the blowdown manually but
didn’t put the piping back the last time you removed modern technology has produced instruments and
internals for inspection because you figured it was just a equipment that provide a reasonable degree of auto-
waste of your time. matic blowdown control. There are systems that provide
In hydronic systems the blowdown is usually continuous blowdown as intended, with continuous
drawn from the boiler at the same place as for steam measurement of TDS and modulating of a control valve
boilers but you may want to check the system for places to vary the rate of blowdown to maintain a maximum
where the solids are more concentrated. Usually the re- level but the more common systems are intermittent in
turn water will be more concentrated because the water operation.
shrunk as it cooled but contains the same weight of sol- The typical system incorporates a timer that opens
ids so blowing down return water will waste less of it. the continuous blowdown control valve at fixed inter-
I’ve also had some unusual encounters with mul- vals. The valve then remains open until the TDS, mea-
tiple drum boilers, older sterling designs, where the sured at a probe in the blowdown piping, drops below
solids managed to concentrate in one section of the the preset value. One potential problem with that type
boiler, not the one where the continuous blowdown con- of blowdown control is introduction of a surge of high
nection was, with scale forming despite maintenance of solids water fed to the boiler by opening up a previously
low TDS at the point of blowdown. Regardless of the shutdown system. The solids will be high in the boiler
system, its operating pressure and temperature, and the until the valve opens again. I would prefer a method
quality of the makeup water you should be aware that where the automatic control has a high and low set-
someone could have made a careless decision regarding ting and blowdown is continuous through a manually
the location of a blowdown connection. Any time you set valve with the automatic valve opening to dump
experience scale formation or problems with carryover additional blowdown when the high point is reached
that isn’t related to pressure fluctuations you should and close when the low point is reached. It doesn’t cost
re-evaluate the location of your testing and continuous any more than the system with valve timing, constantly
blowdown connections. monitors solids, provides a continuous flow of water
We determine how much to blow down by the to any blowdown heat recovery system, and will react
TDS reading (described above in testing). The ABMA immediately when additional solids are introduced to
has set standards for proper levels of solids concentra- the boiler. The only thing that’s better is a modulating
tion for boilers according to operating pressure and control valve but they’re also rather expensive for small
your boiler’s instruction manual may contain that table boilers.
or specific recommendations for what levels of solids Blowoff is designed to remove solids that settled
concentration to run at. Note, that’s a recommendation, out of the boiler water. The sources include solids from
not an absolute value. You may find that your boilers makeup water, rust and other solid particles returned
can operate with a considerably higher level of solids with condensate, and the intentional production of
without forming scale or carryover. It depends on many sludge by chemical water treatment. It contributes to
factors including boiler load. I always recommend a the reduction of dissolved solids but at a considerable
customer raise their settings for TDS levels gradually expense in water and energy because bottom blowoff is
until some problem is detected or they get as high as not recovered in any way. Use continuous blowdown to
4,000 ppm either stopping at that value or backing down remove dissolved solids concentration and limit bottom
below the value where problems occurred. blowoff to its purpose of removing sediment which will
They’re also told to establish values for each boiler vary depending on the quality of the makeup water, de-