For boiler water treatment in case of smaller boiler capacities, a softening plant is often used for treatment of the make-up water. Starting with a hydrogencarbonate content of ∼0.36 mmol/l or a carbon hardness of ∼2 °dH in the feed water, this kind of treatment is no longer sufficient.
By softening, the carbon hardness in the water reacts to sodium bicarbonate. For example according to the following formula:
Ca(HCO3)2 + 2Na [resin] → 2NaHCO3
Inside the steam boiler, pressure, temperature and time cause the sodium bicarbonate to react to sodium carbonate (soda), water, and free carbonic acid. For example according to the following formula:
2NaHCO3 → Na2CO3 + H2O + CO2
Under the same conditions, the soda and water again react to caustic soda and even more free carbonic acid. For example according to the following formula:
Na2CO3 + H2O → 2NaOH + CO2
The free carbonic acid in the boiler water becomes volatile, and follows the steam into the condensate. The unbuffered condensate reacts acidic, which causes strong corrosion to unprotected steel parts. This becomes visible by a brownish colour of the condensate or boiler water.
In order to minimize corrosion in the boiler- and condensate system, the respective guidelines limit the concentration of carbonic acid in the boiler feed water to 25 ppm CO2. This equals a carbon hardness of about 0.56 mmol/l or 3 °dH.
Especially for the foodstuffs industry, this is still too much. 1 ppm CO2 already cause a pH-value of about 5.5, and thus strong corrosion throughout the condensate system. Unless the condensate system is completely made of stainless steel, a decarbonisation plant or demineralisation (reverse osmosis) is required.
The resulting caustic soda increases the alkalinity of the boiler water. Starting with an alkalinity (KS 8.2) of about 12 ... 15 mmol/l, the boiler water tends to foam. The permitted values can only be maintained by blow-down of boiler water. The required boiler blow down rate is directly proportional to the carbon hardness in the make-up water. For example, at a carbon hardness of 5 °dH, the blow down rate lies at about 14% of the steam capacity. With a carbon hardness of 10 °dH, it raises to 23%.
A thinning of the make up water water with condensate lessens the required boiler blow down rate, since condensate is almost salt-free water. However, in case of low condensate return or high carbon hardness, the boiler feed water accordingly can no longer be sufficiently treated with a softening plant alone. Decarbonisation or demineralisation of the make-up water is required in these cases.
If, due to operating or heat economical reasons, the condensate is not deaerated fully, the feed water needs to be demineralised with a reverse osmosis or an ion exchanger. Only that way, the carbonic acid can be removed from the water-steam system.
Often, the carbonic acid in the condensate is neutralised by dosing of ammonia (NH4OH) to the feed water. This causes the carbonic acid to react to ammonium carbonate (NH4HCO3). In order to neutralise 1 ppm carbonic acid, about 0.6 ppm ammonia are required.