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CONDENSATE TREATMENT - BOILERS Corrosion in steam and condensate return lines in boilers and associated equipment is a problem encountered by many heating plants. The problem may be quite severe and result in frequent replacement of lines, valves, traps, etc. Failures most frequently occur at threaded joints because the metal thickness has been reduced at these points. Horizontal lines are usually more severely attacked than vertical lines due to incomplete draining of the condensate in those systems that operate intermittently. The principal causes for steam and condensate corrosion are the dissolved gases - carbon dioxide and oxygen. These gases must be condensed with steam before they become aggressive. The chief source of carbon dioxide is the bicarbonate and carbonate alkalinity of the feed-water. Once subjected to boiler temperature, these alkalinities undergo thermal decomposition and liberate carbon dioxide, which becomes entrained with the steam. The decomposition is illustrated by the following equations;
2NaHCO + HEAT NaCO + H2O + CO2
NaCO + H2O + HEAT + NaOH + CO2 When carbon dioxide dissolves in water during the condensate state it creates carbonic acid. This acid decreases the condensate pH and makes it aggressive to metals. The heating plant utilises amines to neutralise the condensate pH. Feeding amines to the boiler or steam distribution line does this and because amines are volatile they become gaseous and are transported throughout the steam distribution system. The amines condense with the steam thus neutralising or raising the pH of the condensate making the condensate non-corrosive. Our operating parameters are to maintain condensate pH levels between 8.0 – 8.6.
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   The map above shows the rate of hardness in mg/l as calcium carbonate in England and Wales. |
