25 May 2005
COMMONLY ASKED QUESTIONS AND ANSWERS ON BOILER
What is meant by neutralized conductivity?
Boiler water most often contains hydroxyl (OH-) alkalinity. The OH- contribution to
conductivity is disproportionately large compared to other ion species.
Furthermore, the OH- contribution towards conductivity is not used in calculating
TDS. The OH- conductivity is therefore neutralized using an organic acid (i.e.,
gallic acid). Organic acids are used because they do not contribute to
What is the relationship between conductivity and TDS?
Conductivity is a measurement of electrical conductance of ionized species
dissolved in water and is reported in units of micromhos or microsiemens per
centimeter. TDS is the actual amount of mineral in solution measured in ppm. In
boiler water, neutralized conductivity times 0.7 yields a good approximation to
TDS. When tannin is used as a dispersant, the factor increases proportionally to
the amount of tannin present.
Why are there different types of alkalinity to be concerned with?
Alkalinity can exist as bicarbonate (HCO3-), a carbonate
(CO32-), or as hydroxyl (OH-). Natural waters usually contain bicarbonate or
carbonate alkalinity. Hydroxyl alkalinity is required in steam boilers below
6205 kilopascals (900 pounds per square inch gauge) and is either provided by
addition of caustic or by the breakdown of bicarbonate and carbonate alkalinity in
the boiler. The breakdown of bicarbonate and carbonate alkalinity also produces
carbon dioxide, which forms carbonic acid in the condensate system.
How often should bottom blowdown be done?
There is no absolute rule for frequency of bottom blowdown. It can vary between
once per shift to once or twice a week. The required frequency depends on the
boiler, the feedwater quality and the type of chemical treatment program. A
precipitating treatment program reacts with hardness in the feedwater to form a
sludge that must be removed through bottom blowdown. A solubilizing treatment
program keeps hardness in solution and creates little in the way of sludge.
Where is the best location to feed sulfite?
Sulfite, like any oxygen scavenger, should be fed into the storage section of a
deaerator. This ensures the removal of oxygen before the feedwater enters the
What is the difference between makeup water and feedwater?
Makeup water is water that comes from a source outside of the boiler system.
Feedwater is a combination of makeup water and condensate return.