Why Is Sulfite Used In Boilers?
Sulfite is a very important chemical used to scavenge oxygen in steam boilers. Sulfite reacts with oxygen and produces sulfate.
As you know, dissolved oxygen in boilers can cause a rapid corrosion known as pitting. An oxygen scavenger is a chemical that consumes oxygen through an oxidation-reduction (redox) reaction. Hydrazine and nitrite are also oxygen scavengers, but the most widely used is sulfite.
Sulfite is inexpensive, safe, easily fed and monitored, reacts quickly with oxygen, and is compatible with most other boiler treatment chemicals.
What Does Catalyzed Mean?
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.
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.
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.
How Do I Monitor Catalyzed Sulfite In A Boiler?
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.