In our post last week, oxygen corrosion in boilers was discussed. The control of this corrosion is the topic of today's blog. Mechanical deaeration and chemical deaeration are the methods for controlling this corrosion.
Dearators work by heating the water and venting the gases. This operation reduces oxygen in the makeup by more than 99% leaving behind oxygen concentrations measured in the parts-per-billion range. At feedwater and boiler water temperatures, even this level can create severe corrosion. Therefore, the last traces of oxygen must be chemically scavenged from the feedwater. Proper mechanical deaeration is always necessary for a well run and cost effective oxygen scavenging water treatment program. We have alot of information about deaerators in previous blog posts including:
The selection of the proper oxygen scavenger for any specific application will depend on many factors - your water treatment provider should be consulted before a program is instituted. However, there are a few general considerations that should always be taken into account. They are:
- Sulfite is the most commonly used oxygen scavenger because of its relatively low cost, speed or reaction, ease of handling and effectiveness. However it is rarely used at boiler pressures above 900 psig.
- Numerous non-sulfite based oxygen scavengers have been developed over the years that offer specific benefits over sulfite. Oxygen scavengers that add minimal dissolved solids to the water are usually the product of choice for high pressure boiler systems. Many of these low solids oxygen scavengers also act as metal passivators that protect metal surfaces from general corrosion. Others are volatile and carryover with the boiler steam to protect steam and condensate systems from corrosion due to oxygen in-leakage.
- If treated feedwater is to be used for desuperheating, the use of sulfite could result in deposit formation throughout the superheater. In these situations the low solids non-sulfite based oxygen scavengers are the products of choice.
The selection of the proper oxygen-control program can be made only after a thorough investigation of the specific boiler operating conditions. Consideration should also be given to boilers that are taken out of service and stored in a wet or dry state, since they are still susceptible to corrosion on unprotected internal surfaces.