Sulfite (Sodium Sulfite Na2SO3) has unique properties which allow it to remove oxygen from water. The reaction of sulfite and oxygen creates sulfate.
The main reason why sulfite is so important for boiler maintenance is that it removes oxygen from water. Certain percentages of oxygen are dissolved in the water. The more oxygen there is, the more corrosion will occur. This is because oxygen reacts with the iron present in both cast iron or steel boilers. Corrosion will degrade the metal surfaces of boilers, create pits/leaks, and decrease the overall efficiency of the system.
Sulfite is a common type of oxygen scavenger. Oxygen scavengers describe chemicals that remove oxygen via an oxidation-reduction (redox) reaction.
Sulfite has many advantages such as:
To contact a water treatment expert about sulfite, catalyzed sulfite, corrosion, or any other issues click here.
The purpose of using catalyzed chemicals in boiler treatment is for a faster reaction and result to occur. Catalyzed sulfite can accelerate the reaction of the oxygen with sulfite from 10 to 100 times the normal rate. Un-catalyzed sulfite reacts just as quickly at high temperatures, but as the temperatures drop the reaction rate decreases exponentially. Catalyzed sulfite reacts at a very rapid rate at all temperatures.
A catalyst is a chemical that causes an increase in a reaction rate. Cobalt is the most common catalyst used in sulfite. Cobalt can be used in very low concentrations because the catalyst itself is not consumed.
The catalyst becomes inactive at a 9.3 pH or greater, therefore it is extremely important to use a separate mix tank just for the catalyzed sulfite. The cobalt catalyst precipitates as a brown floc that typically settles to the bottom of the batch tank. If you see this material collecting in the batch tank, your sulfite has become regular, un-catalyzed sulfite.
There is a catalyzed form of BJ-10 that can be used just like the normal BJ-10. The dosage rates for the catalyzed version and the normal BJ-10 are interchangeable, and can be determined by the normal boiler calculations and maintaining a residual of 30-60 ppm sulfite in the boiler, and 5-10 ppm sulfite in the deaerator, feedwater, or surge tank.
Analytical technology is always improving, so make sure you are using the most current sulfite test method.
Sodium bisulfite and sodium metabisulfite (SMBS) are other forms of sulfite that can be used for boiler treatment. Metabisulfite is usually in a powder form, while bisulfite is in a liquid form. Additionally, metabisulfite is 2-3 times cheaper than bisulfite. Bisulfite is more acidic than metabisulfite, an important consideration for balancing your boiler’s pH correctly.
Hydrazine, nitrite, and citric acid are alternatives to sulfite, catalyzed sulfite, bisulfite, and metabisulfite. Use increased caution with hydrazine (N2H4) as it can be dangerous for humans. Nitrite (sodium nitrite NaNO2) is a film-forming chemical, this layer protects from oxidation. Nitrite is used to passivate the surface of the steam boiler by adding an oxide layer to the metal. This layer prevents the metal from exposure to the factors that cause corrosion. Lastly, Citric acid (C₆H₈O₇) has become more popular in recent years for its unique properties of being safer and better for the environment. Additionally, there are shorter passivation processing times. Citric acid is similar to nitrite in its role of providing a passivation layer to prevent corrosion in a boiler system.
Whether you need sulfite for corrosion control, scale inhibitors, or any other boiler chemical treatment, Chardon Labs is here to help. Reach out here.
