Steam Boiler Chemical Treatments
Water treatment chemicals for steam boiler systems generally address two areas of concern, oxygen pitting corrosion and scale or deposit formation. Today’s equipment has made great strides in helping to reduce these effects; however chemical treatment is needed to finish the job.
The most used chemical to prevent oxygen pitting is Sodium Sulfite. Sulfite reacts with dissolved oxygen to form sulfate, thus virtually eliminating oxygen. We continually keep excess sulfite levels in the boiler to react with any incoming dissolved oxygen. In some steam boiler systems, Sodium Nitrite is used. There are other chemicals that can be used for this purpose.
Scale Deposit Control
Wherever moderate to high hardness make up water exists, steam boilers typically have water softeners to treat the incoming water. Softeners exchange the scale forming Calcium with non-scaling Sodium. Water treatment chemicals like Phosphate, Phosphonate and Polymers are used to protect against the small amount of hardness that may get past the softeners. These chemicals also address problems associated with other contaminants that may enter the boiler due to system upsets. Additional products are used, such as Alkalinity Boosters, to help these chemicals perform efficiently.
Steam & Condensate Treatment
These chemicals, specifically Morpholine, Cyclohexylamine and Diethylaminoethanol are added to the boiler and travel out with the steam. They are sometimes injected directly into the steam line. These chemicals eventually condense down the steam line to neutralize the acidic pH of the steam condensate that normally forms when the steam condenses. These are the only chemicals added to the boiler that leave with the steam under normal operating conditions.
Cooling Tower Chemical Treatments
Water treatment chemicals for cooling towers vary based on the type of make-up water quality and cooling system type. Basically, these chemicals are comprised of Corrosion Inhibitors, Scale/Deposit Inhibitors, Dispersants and Biocides. Cooling Tower chemical treatments are designed to prevent corrosion, scaling, fouling and microbiological growth on the heat transfer surfaces and internal piping. It is important to note that cooling tower sumps are designed to collect dirt and debris preventing the accumulation in the condenser/absorber/exchanger equipment. Periodically, the tower sumps should be physically cleaned to remove this material.
Typically, Water Treatment Products contain a combination of Inhibitors and Dispersants to make them easier to use.
Biocides are always fed separately.
Sodium Molybdate, Phosphonate and Phosphates are primarily used for steel corrosion inhibition. Typically higher levels are maintained in systems where the make-up water is relatively “soft”, containing low calcium and magnesium hardness. Dispersants should be used especially when phosphates are applied to prevent the precipitation of calcium phosphate on the heat exchange surfaces. Tolyltriazole and Benzotriazole are used for copper corrosion protection.
Polymers and Phosphonate are primarily used to “tie-up” scale forming minerals such as calcium and iron. By combining with these minerals, the inhibitors prevent “hard” scale from forming. If soft deposits are found, dispersant levels should be increased.
Most water treatment programs incorporate a multi-chemical dispersant product. These contain Polyacrylate, Polymaleics, and Copolymers. Many polymer products are used for a specific mineral or chemical application. There are polymers for Calcium, Iron, Phosphate, etc. Polymers have become increasingly important in recent years.
Many various types of Biocides are available today. Many companies use a few types to keep their costs down. They are usually all effective for control. However, some are better for algae and slime formation, while others are better for bacteria control. A proper water treatment biocide program should be able to prevent the formation of algae and slime as well as maintain low bacteria colony counts. Bromine-based biocides are the primary biocide used today. They are effective against algae, slime and bacteria. Bromine is effective over a wide pH range and this is the reason that Bromine is preferred over the less costly Chlorine compounds. Some biocide programs use chlorine because of its low cost; however a large amount of chlorine is required in most cooling towers because the pH levels are higher than normal.
Other types of Biocides include Carbamates, Quats, and Isothiazolin compounds, all of which can provide adequate control.
Closed Loop Chemical Treatments
Water Treatment chemicals for Closed Loop Systems are primarily designed to prevent corrosion and microbiological growth. Because these systems normally experience very little make-up, chemical addition should be minimal.
Closed Loop system types are chilled and heating hot water and secondary heating and cooling systems.
Nitrite-based products are often used for closed loops systems. Typical control ranges for Nitrite levels are 750 – 1500 ppm. Typically, higher control levels are maintained for hot water systems where corrosion potential is greater.
Nitrite treated systems must be checked routinely for bacteria levels, as they can be a source of nutrient for bacteria.
Also, it is very important that microbiological laboratory analysis be performed periodically for Sulfate and Iron Reducing Bacteria. These bacteria can be very harmful to steel and copper if allowed to grow. Regular aerobic dip slides will not monitor for this bacteria.
Molybdate – based products have grown in popularity over recent years. They are more expensive to use, but are able to maintain consistent protection and will not encourage microbiological growth. Typical control ranges are 150 – 300 ppm as Molybdate.
Biocides – At times it may be necessary to add a biocide to closed loop systems. Be sure that the biocide chosen is effective for anaerobic bacteria as well as Sulfate and Iron Reducing Bacteria.