After its energy content is exhausted, steam returns to the liquid phase. Although at lower temperatures, relative to steam, this condensate still contains a reasonable amount of energy, almost 20% of saturated steam at same pressure. It is actually distilled water at a temperature higher than ambient, almost free of dissolved solids. Returning it to the boiler, relieves a sequence of water treatment duties, while reducing boiler water blowdown and loss of energy. Lost of condensate implies that cold treated water has to be fed to the boiler to accomplish material balance. This means more make- up water with additional treatment costs and additional fuel to heat it up to boiler water temperature. And this colder feed water tends to reduce boiler steam output and energy efficiency. Collecting and returning condensate to the boiler is another fundamental efficiency measure and is the basis of steam usage option of energy and water reuse. It may seem that condensate might be the ideal boiler water, but not quite.
Condensate may be contaminated due to steam application and physical conditions. If in steam usage, condensate contamination is expected and probable, continuous treatment must be considered and a reliable quality control is essential to indicate dangerous contamination levels as fast as possible. Parameters like conductivity or turbidity might be sufficient. For example, the presence of oil and greases from heat exchanger leaks in a refinery, demands mechanical filtration in paper cartridge filters, and oil surges that cannot be handled by existingt filters, require immediate discarding. Any oil injection to boiler water may cause immediate fouling due to carbonization of these materials on heat- transferring water- side surfaces and this can provoke immediate overheating and material failure.
Another issue is related to the physical difference between steam and water. When steam condenses, the flow volume shrinks and this causes a depression that may allow ambient gases to enter the pipe and dissolve in condensate. The same potential corrosion problems, related to specific dissolved gases, appear. Carbon dioxide reduces the pH of the condensate causing acid attack, while oxygen attacks metal directly. Before the condensate is returned to the boiler, it is fed to a deaera-tion device but in the collecting pipes some treatment must be provided to reduce corrosion. Corrosion of condensers, steam traps and condensate piping is common. Adding chemicals, such as neutralizing amines, helps to keep condensate pH high, but can only protect the system against acid attack from COi i Most commonly used are morpholine and cyclohexylamine.
Film amines can protect from oxygen attack. These compounds form a very thin organic material film over metal surfaces, blocking oxygen, but they have little effect on condensate pH. Heat transfer is minimally affected because the film is thin. Commonly used amines are dodecylamine and octadecylamine.
Since condensate systems tend to be quite large, using a mixture of amines, film amines with faster and slower condensing ones, improves equipment protection throughout the system. Chemical and physical properties of amines should be chosen depending on system length, to select the best mix. Nevertheless, attack remains and condensate lines and accessories usually present undissolved corrosion products that can be removed by an activated carbon or coke filter.
Just to emphasize that condensate with high contamination risk must not return to the boiler, especially if it contains significant amounts of hydrocarbons or any organic products, acids or caustics, seawater etc. Injection of any of these products into the boiler may cause serious accidents, boiler damage and eventual production disruption. But, depending on the contaminant, condensate may be reused as reasonably clean water, for a number of services.
Condensate is a valuable resource and even the recovery of small quantities is often economically justifiable. But how much condensate return is expected to be a good amount? Huge industrial plants, with long distance between systems and a wide variety of steam usages, tend to have lower recovery, while utility companies must have a high recovery rate as business standard and design parameter. In an average industry, like a refinery or petrochemical complex, 70% of steam demand being recovered as condensate is a very good amount. But even in these places, 30% or less, whatever the reason, is pretty awful. Condensate collection is rarely metered, and this rate is measured by water and steam balance and by itself can be a good overall measurement of energy efficiency performance for the industry.
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