The condensate if recovered and reused can be useful for the plant but if it is not treated appropriately can be hazardous to the maintenance of the plant. Improper treatment of condensate leads to corrosion of the piping returning to the feed water tank and the ultimately to the boiler. The traces of two gases: CO2 and O2 are the causes to the problems associated with condensate corrosion.
EFFECT OF CARBON DIOXIDE
Alkalinity naturally occurs in water in the form of carbonates and bicarbonates and hence boiler feed water has pH between 8.5 to 9.5. This portion of alkalinity breaks down to form carbon di oxide under the pressures and temperatures in boilers
2HCO3 + Heat → CO3 + CO2 + H2O,
CO3 + H2O + Heat → 2OH + CO2
The first reaction usually achieves 100% completion. The second typically achieves 80% or more completion at pressures over 10.3 bar; between 3.4 and 10.3 bar, completion varies from 30-80%; between 1 and 3.4 bar, completion varies from 10-30%. Load factor also has an effect on these reactions. Higher load factors will result in lower conversion rates.
Other sources of CO2 are free carbon dioxide in raw water (typically well water), use of soda ash as an alkalinity builder for internal boiler treatment, process contamination and decomposition of some organic compounds. Carbon dioxide, which again is a gas, is carried out with the steam. After the steam has done its work and condenses, some if not all, of the carbon dioxide will dissolve in the condensate. This results in the formation of carbonic acid:
CO2 + H2O → H2CO3
Dissolved carbon dioxide in the condensate can accelerate corrosion in several ways. Carbonic acid lowers condensate pH and causes corrosion. This low pH leads to corrosion of ferrous metals and ultimately to the formation of ferrous oxide precipitates. Carbonic acid corrosion typically manifest itself as a general thinning of the metal, often causing initial failures in areas where system metals are already thin, i.e.; pipe threads, etc. The thinning of condensate piping typically leads to high iron content in boiler feedwater. The result is boiler tube failures as the iron precipitates out onto the tubes once it reaches the elevated temperatures and pH''s of the boiler water. This deposition can cause galvanic as well as under deposit corrosion, accelerating the corrosion rates to the point of failure.
The proper approach to reducing carbon dioxide attack is determined by a number of factors that include: complexity, size and age of the system, the quantity and source of condensate returned and the raw water characteristics. Various mechanical means of reducing carbon dioxide attack are available that include: hot or cold process softening, ion-exchange of various types and degasification, demineralization and deaeration of feedwater.
Other considerations to reducing carbon dioxide attach would include: severity and location of the corrosion in the systems, pressures and reducing stations, the feasibility of remote feeding, the nature of the process as well as any government restrictions and regulations.
EFFECT OF OXYGEN
Traces of oxygen can enter the condensate system in a number of ways: by improper operation of a deaerating heater, disruption of feeding an oxygen scavenger, process contamination, in-leakage during shut-down/cool-down times, vacuum pumps, vented condensate receivers, leaking pump glands, etc. Whatever the cause, oxygen corrosion in condensate is typically rapid and terminal to the system. The severity of oxygen attack depends of the concentration of dissolved oxygen, the pH and the temperature.
One of the most severe aspects of oxygen corrosion is that it occurs as pitting. A pit is a concentration of corrosion in a small area of metal surface. This type of corrosion can cause total failure of equipment even though the amount of metal loss is relatively small.
The influence of temperature in condensate as well as other areas of the boiler system is particularly important. Corrosion rates are accelerated as the temperature increases in the presence of oxygen. Since temperature is a driving force in oxygen corrosion even small quantities of dissolved oxygen in condensate can cause severe corrosion.
Oxygen removal in the condensate system is determined by the source. If oxygen is flashing with the steam from the boiler, use or repair of a de-aerating heater could help eliminate this. If oxygen is a result of in-leakage due to faulty seals, etc. certain repairs can be made to repair these. Often oxygen in-leakage into a condensate system is a situation that cannot totally be eliminated and therefore must be dealt with in the most economical manner.