Page 255 - Boiler_Operators_Handbook,_Second_Edition

240 Boiler Operator’s Handbook

on the heat exchanger outlet is usually controlled by the purposes, anything from makeup for cooling towers (I
level in the bottom of the flash tank. That way the heat know it’s hot, but it’s also distilled water) to use as sani-
exchanger is always flooded and the blowdown is not tary water (where it has to be cooled). In chemical and
flashing into steam which can leave deposits that plug petroleum facilities there’s considerable water used in
up the heat exchanger. scrubbers and condensate makes a great replacement for
Blowdown heat recovery does save some energy scrubber makeup. In other words, if it has to be wasted
and, with a flash tank, a little bit of water. The real sav- try wasting it in another system instead of fresh water.
ings, however, is in the water that would be used to cool Appropriate recovery of condensate is another
the blowdown if you didn’t have the heat recovery sys- matter. I’ve found that several plants allow considerable
tem. Blowdown dumped through the blowoff flash tank waste of high temperature condensate by collecting it in
will dump some heat in the form of steam up the vent open systems where as much as 15% of the water and
but the 212°F water has to be cooled to less than 140°F over 50% of the energy in that condensate is lost in flash
before it’s dumped in the sewer and the typical practice is steam. High pressure condensate should be recovered
to use good city water for it. It will take a volume of cold in a way that prevents flashing. The best way to recover
water about equal to the blowdown to cool it before it’s high pressure condensate is to return it to the deaerator.
dumped in the sewer. You’re wasting all that cold water if Some of the condensate may flash off in the deaera-
you aren’t using the blowdown heat recovery system. tor but it simply displaces some of the steam required
The best way to reduce water waste in a steam for deaeration. There’s no reason to be concerned for
plant is to recover condensate and use it as boiler feed- oxygen in high pressure condensate, but it’s typically
water. There are many reasons for this in addition to returned in a manner that allows some scrubbing of it to
saving water. Recovery of condensate recovers heat, remove any oxygen that may be in it from start-up and
eliminating the need to heat cold makeup water before other operations.
it’s fed to the boiler. Condensate is basically distilled wa- Having explained that condensate that would
ter, converted to steam in the boiler and then condensed flash off should be recovered in a manner that uses that
so it doesn’t require all the pretreatment and chemical steam it only makes sense that any signs of steam escap-
treatment needed for fresh makeup water. Recovery of ing from a condensate tank vent line is a problem that
condensate saves money that would be spent on ad- requires an operator’s attention. The normal reason we
ditional fuel, boiler water treatment chemicals, and the have steam leaking is leaking traps. Trap maintenance is
additional water required for blowdown to remove the very important in reducing water waste.
solids brought in by fresh city water.
All too frequently the only consideration for recov-
ering condensate is the value of the heat. After evaluat- TREATMENT CHEMICALS
ing several condensate recovery projects I can assure
you the cost of heating the water is minimal. The cost of I’m always listening to plant chiefs complain about
the water itself is more valuable than the heat and the the price of water treatment chemicals. They aren’t
cost of chemicals adds even more to it. Treat condensate cheap and they sure aren’t anything that you want to
as a valuable resource. treat casually. In the normal plant they’re about two
Recovery of condensate is the best way to minimize percent of the total cost. The amount of chemicals we
water waste from a boiler plant but there are times when use are a function of the amount of makeup water enter-
recovery for use as boiler feedwater is undesirable. Wast- ing the plant so preserving water is the first important
ing of condensate is not unusual in a chemical or petro- step in minimizing the cost of water treatment chemi-
leum facility because the potential for contamination of cals. The following section deals with water treatment
that condensate is so high. In some cases we recovered because it’s definitely one of the most important things
some of the energy from it using a heat exchanger but that boiler operators have to do. Considerations of the
that doesn’t preserve the water. Capability to monitor the chemicals as consumables are addressed here.
water and filter it with carbon filters and other measures, The concentrated treatment chemicals are defi-
including reverse osmosis, make it possible to recover nitely hazardous waste if they escape their containers
and use condensate in those plants today. or treatment equipment. They are hazardous to handle
In instances where the capital cost expenditure to and can cause severe burns. I know the attitude about
recover condensate is so high that recovery can’t be justi- how we can be perfectly competent in handling the
fied it’s possible that the condensate can be used for other material and shouldn’t need the protective gear because