Eliminating oxygen (O2) and carbon dioxide (CO2) from boiler feedwater is imperative in order to prevent damages caused by corrosion to the water side of boilers.
The presence of dissolved oxygen in water causes localized corrosive damage which is also known as “pitting.” The higher the pressure and temperature of the water, the more corrosive the dissolved oxygen becomes so each boiler has different maximum dissolved oxygen requirements depending on its pressure and working temperature.
A water tube boiler, for example, normally works at pressures greater than 300 PSI and at temperatures greater than the steam saturation point (overheated steam). For this type of boiler, dissolved oxygen requirements are quite strict.
On the other hand, a fire tube boiler normally works at pressures less than 300 PSI and at temperatures around the steam saturation point (saturated steam). The requirements for this type of boiler, therefore, are less strict.
In the case of carbon dioxide, its dissolution in water causes low pH levels and the production of corrosive carbonic acid.
There are three main options for eliminating O2 and CO2 from water – vacuum deaeration, thermal deaeration, and chemical deaeration, with thermal and chemical deaeration being by far the most utilized methods.
Vacuum deaeration involves using either steam injectors or a vacuum pump to eliminate unwanted elements from the boiler feedwater and is the least implemented of the available options.
In thermal deaeration, boiler feedwater is heated until its saturation point inside a tank called a deaerator to facilitate the release of dissolved gasses and their subsequent elimination through a vent.
Atmospheric deaerators are both easy to use and economical and eliminate 99% of dissolved oxygen from water. Pressurized deaerators, while more effective in eliminating dissolved oxygen (removing 99.99%), are extremely costly, complicated, and difficult to operate and maintain.
Chemical deaeration involves adding oxygen scavenging chemicals to the boiler feedwater system. The most commonly used oxygen scavenger is sodium sulfite (Na2SO3) because of its low cost, rapid and effective reaction with dissolved oxygen, and the ease of measuring its content in the water. The disadvantage of using sodium sulfite is the increase in the amount of total dissolved solids (sodium sulfites) that must be removed from the water via blowdowns.
The primary alternative to sodium sulfite is hydrazine, which upon evaporating does not contribute to the total amount of dissolved solids. It is, however, highly toxic and expensive.
Calderas Powermaster manufactures only fire tube boilers that produce saturated steam, for which an atmospheric deaerator is recommended, which will eliminate 99% of the dissolved oxygen from the water, along with a small chemical product regime, which will control the remaining 1% so that it does not have negative effects on the boiler. (Note: if the rate of condensate return is greater than 60%, then a condensate tank is sufficient.)
Using a pressurized deaerator for a fire tube boiler that produces saturated steam is acceptable, but will end up being extremely costly to acquire, operate, and maintain. For these reasons, this option is generally not justifiable and not an alternative that we recommend our clients pursue.