Softening along with Oxygen and CO2 removal are the minimum boiler feedwater treatment activities to avoid problems associated with dissolved gases and scaling risk. If left unchecked, corrosion and scaling will cause considerable damage throughout a boiler system.
Boiler makeup water must be treated to reduce this dissolved salts to acceptable levels. Those levels will depend on the pressure of the boiler.
As generated steam leaves the boiler, the dissolved solids stay and increase the concentration level in the boiler water. There is a limit to the maximum concentration levels the boiler can tolerate.
Boiler systems are energy intensive so it’s everybody’s interest to maximize steam production and minimize fuel consumption. One of the firsts steps is to minimise blowdown, which represents a major energy loss, however that option is linked to the boiler feedwater quality.
Corrosive gases such as Oxygen (O2), carbon dioxide (CO2) and Hydrogen Sulfide (H2S) must be removed from boiler feedwater. Oxygen is a highly active element that, even in small amounts, can bring serious corrosion issues to feedwater lines, economizers, boiler internals and condensate return pipes.
Corrosion potential increases with the Oxygen content, being double for each 10°C increase in temperature.
When Oxygen and Carbon Dioxide are combined, the resulting corrosion rate can be up to 40% greater than the sum of the corrosion rates of the two gases acting individually. The combination of oxygen and ammonia is very corrosive to copper alloys too.
Carbon Dioxide (CO2) is most commonly associated with condensate corrosion. The main source of CO2 in the steam is the thermal breakdown of bicarbonate (HCO3–) and carbonate (CO3–2) alkalinity present in the feedwater:
2NaHCO3 + Heat –> Na2CO3 + CO2 + H2O
Na2CO3 + H2O + Heat –> 2NaOH + CO2
Carbon dioxide is not corrosive until it dissolves in the condensate and forms carbonic acid (H2CO3). Carbonic Acid reacts with iron to produce ferrous bicarbonate [Fe(HCO3)2], which is highly soluble and has no passivating effect.
Carbon Dioxide in the steam can be easily decreased by removing bicarbonate and carbonate alkalinity in the makeup water.
Oxygen can be reduced before entering the boiler by deaeration and chemical oxygen scavenging.
However, as the solubility of Oxygen in water decreases with temperature, a very cost effective way of reducing the content of dissolved oxygen (and other gases) is increasing the temperature of the feed water tank.
If a high proportion of make-up water is used, using the economiser as a heat source for heating the feed water, can substantially reduce the amount of oxygen scavenging chemicals required.
The most common Oxygen scavenger is Sodium Sulphite (Na2SO3), with an approximate dosing of 8 to 10 ppm
per 1 ppm of dissolved Oxygen.
Liquid catalysed Sodium Sulphite has a typical 45% concentration, so the economies of pre-heating the feed water are quite obvious.
The main reason why carbonate and silica scale take place is that solubility in water of these salts decreases as the temperature increases. As feedwater temperature rises to boiler water temperature, the concentration of the scale-forming salts exceeds solubility and precipitate, forming scales.
Calcium and Magnesium scales are common in boiler water systems when poor or no softening pre-treatment is used.
Calcium Sulfate (CaSO4) and Silica (SiO2) scales are common in heat transfer surfaces, where the boiler water next to the heating surface tends to become more concentrated as the steam bubbles form in low pressure boilers.
Silica, apart from forming scales in the boiler heat exchanging surfaces, can vaporize and be carried with the steam as silicic acid.
For all these reasons, hardness and silica must be measured in the boiler feedwater and removed in order to prevent boiler failures.
Iron oxides have always been present and caused problems in boiler water. These iron oxide precipitates are the main reason for many corrosion problems and overheat failures, as they tend to block heat transfer.
Iron enters the boiler water systems in two different ways:
Measuring and filtering Iron present in makeup water is obviously the first mechanism to minimise the Fe concentration in the boiler water system.
As ferrous ions (Fe+2) are the first product of corrosion, measuring the concentration of Fe+2 is a good method to identify active corrosion in boiler systems and evaluate the iron and corrosion prevention strategy.
REVERSE OSMOSIS DESALINATION
IRON AND MANGANESE FILTERS
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