Page 254 - Boiler_Operators_Handbook,_Second_Edition

Consumables 239

or dump it otherwise. Part of that effort is to avoid the on the value of the chemicals you purchase. Especially
heavy cost of treating the plant’s discharge of waste wa- after a switch to chelant or polymer treatment from
ter which is highly concentrated with solids and chemi- 100 % phosphate (maintaining some phosphate as an
cals compared to water that’s simply wasted to a drain. indicator of the operation of the others isn’t a bad idea)
It’s an action I am glad to see. We should all understand your chemical treatment representative should produce
that especially blowdown contains considerably more a blowoff schedule that is less frequent. If not, check it
solids and chemicals than normal waste water so mini- yourself. I suggest making certain that you perform a
mizing blowdown is important. bottom blowoff before shutting down a boiler for in-
Another consideration is the draining of a boiler spection at a time before shutdown to match the sched-
and refilling it with fresh water during every annual uled interval for blowoff plus a shift or day. Use a shift
outage. As the cost of treating sewage continues to rise if you’re blowing off more frequently than every two or
and concern for the treatment of very caustic waters more days. Then, slowly drain the boiler after cooling
grows there may come a time when dumping a boiler is and look at the bottom of the mud drum and headers
restricted. If your plant doesn’t have a connection to a right after you open the boiler to see if there’s any ac-
sanitary sewer, and many don’t, I strongly recommend cumulated sludge or mud. If none is there and the boiler
you rent a tank trailer to store your boiler water while had been operating normally (not in standby or other
performing your annual inspections. That way you low load modes) before the shutdown, increase the in-
aren’t discharging caustic water into the environment terval between blowoffs so that it matches the interval
unnecessarily and you save on the cost of the chemicals you just established. After a period of time you’ll come
it contained (although loss of sulfite is expected) and the up with a small accumulation in the mud drum which
cost of treating fresh makeup water. means you just exceeded your limit and stick with the
I’ve also seen a fair number of operators use boiler interval that you know works.
bottom blowoff as a means of water level control. In the On steam systems blowdown heat recovery sys-
chapters on control I mention one probable reason that tems capture much of the heat and a little bit of the
an operator feels compelled to do this but even if the water that’s dumped by continuous blowdown. The
controls malfunction there’s no reason to consistently blowdown is dumped into a flash tank which operates
waste water and boiler chemicals in order to maintain at a pressure slightly above the deaerator pressure. Since
boiler water level. If the level tends to rise it can be pre- the water is much hotter than the saturation temperature
vented by restricting feedwater flow. at that pressure some of the water flashes into steam.
It’s also not sensible to use bottom blowoff as a The steam is separated by some internals then flows to
means to reduce the dissolved solids content (TDS) of the deaerator where it replaces some of the boiler output
the boiler water instead of using continuous blowdown, that would need to be used to heat the feedwater. The
what we sometimes call surface blowdown. Removal remaining water then flows to the heat exchanger. Low
of the boiler water to limit the TDS is best done with pressure plants and small high pressure plants may not
the continuous blowdown because it removes the most be able to justify the flash tanks so all the blowdown
concentrated water in the boiler, the water that’s left water flows to the heat exchanger. The heat exchanger
right after the steam is separated. Bottom blowoff tends transfers heat from the blowdown to the makeup water.
to be a blend of the boiler water and feedwater that just In low pressure plants the heat exchanger can be
dropped to the bottom drum so it contains a much lower as simple as a barrel set above the boiler feed tank and
concentration of solids. None of the water or heat is re- arranged so the makeup water is fed into the barrel then
covered from blowoff; water and heat is recovered by a overflows into the boiler feed tank. The blowdown is
good blowdown heat recovery system. passed through a coil of tubing in the bottom of the tank
What about that bottom blowoff? It’s used to then to the blowdown control valve (which can be man-
remove sediment, mud and sludge that either enter ually set) or an automatic one. In a plant with multiple
the boiler with the feedwater or created by the water low pressure boilers each one could have its own coil.
treatment. With chelants and polymers the scale form- If the flow isn’t throttled after the heat exchanger the
ing salts are sequestered or trapped in solution and boiler water will flash in the coil, making a lot of noise
removed in the continuous blowdown so blowoff isn’t and eventually damaging the coil.
needed anywhere near as much as it used to be. The A heat exchanger in high pressure plants should
chemical salesman may give you a schedule for blow- be of high pressure construction and heat the makeup
off that wastes chemicals and his commission is based water before it goes to the deaerator. The control valve