Inhibitors for Boiler Scale Deposits

Phosphate treatment programs precipitate calcium and magnesium hardness from boiler feedwater in the form of hydroxyapatite and serpentine. These compounds form sludge, which, when properly conditioned with polymers, is removed from the boiler via blowdown. When a significant excess of soluble orthophosphate and free hydroxide (caustic) alkalinity is maintained in the boiler water, calcium carbonate formation can be virtually eliminated. Polymeric dispersants or sludge conditioners are added with the phosphate to condition the sludge for improved removal from the boiler.

ChemTreat’s phosphate scale inhibitors always include polymers either blended in the solution or as supplements to prevent boiler scale deposits from forming on heat transfer surfaces. Phosphate treatment programs are recommended for scale deposits in the following conditions:

• Infrequent operator attention and testing
• Poor or nonexistent feedwater deaeration
• Water softeners are in use, but control is less than ideal
• Low treatment costs are desirable
• Boiler design, operating pressure, and steam production rates are compatible with precipitating chemistries

Care should be taken whenever phosphate scale inhibitors are used under the following set of operating conditions:

• High iron levels (>0.5 ppm) in the feedwater
• Feedwater hardness greater than ASME guidelines (>1.0 ppm)
• Feedwater economizers present
• Exceptional boiler internal cleanliness required

Polyelectrolytes, commonly referred to as polymers, are used in virtually all industrial boiler water treatment programs, either alone or in combination with other components such as phosphates or chelants. Over the past 30 years, the use of all-polymer internal boiler water chemistries has evolved for low- to intermediate-pressure boilers. This has been driven by the development of thermal hydrolytically-stable polymers, optimum molecular weights, functionality specific to hardness sequestration and iron oxide/hydroxide dispersion, and the ability to accurately test for active polymer residuals. Refinements in polymer chemistry, in conjunction with improved technologies for the production of high- and consistent-quality boiler water makeup, has allowed all-polymer treatment to be widely implemented. The all-polymer approach, while not applicable to all boiler systems, does have benefits over phosphate and chelant programs when applied correctly.

Boiler water polymers function as dispersants and weak sequestrants. Phosphate chemistry works by precipitating calcium as any of several insoluble calcium phosphate compounds. This material can deposit on tube and drum surfaces, and its buildup can impede heat transfer on generating tubes and cause tube failures. Polymers can be used in conjunction with phosphate programs to condition the precipitated compounds, prevent their accumulation, and aid in their removal from the boiler.

Unfortunately, it is not possible to purge all of the insoluble material from the boiler via a limited amount of blowdown. Thus, varying degrees of deposition are generally found in phosphate-treated boilers where hardness incursion has occurred. Many plants find the insoluble material generated with phosphate treatment programs undesirable. Chelants form soluble compounds with calcium and magnesium during hardness excursions provided there is sufficient free chelant present. Problems arise with chelating chemistries because of the poor thermal hydrolytic stability of free EDTA, the most commonly used boiler water chelant. The inability to maintain a free residual can result in hardness scale formation during upsets. Chelants can be very aggressive to boiler metal if fed in the wrong location or overfed.

All-polymer chemistry can overcome the drawbacks of both phosphate and chelant programs. Polymers sequester hardness, forming soluble compounds. Some commercially-available polymers become insoluble when exposed to high calcium hardness conditions. Polymers also disperse insoluble materials. (Weak sequestrants are not aggressive to boiler metallurgies, and free residuals can be maintained and tested for.)

The ChemTreat boiler water product line includes phosphate/polymer, chelant/polymer, phosphate-traced/polymer, and true all-polymer products. The polymers utilized in these products are optimized for molecular weight, thermal hydrolytic stability, and functionality specific to dispersion and sequestration. Most products contain a blend of two polymers, allowing us to incorporate the best polymer functionalities into a specific product. Polymer applications are limited to boiler operating pressures below 900 psig. Operating pressure/temperature, feedwater quality, and boiler cleanliness dictate dosage requirements.

Chelation depends on sequestration, whereby chemistries bind ions like calcium, magnesium, and iron and keep them in a soluble state. Cleaner boilers can more often be achieved with chelants than with any form of precipitating treatment. However, chelants are usually more expensive than phosphate programs since they react on a mole-to-mole basis. Furthermore, the control and subsequent testing requirements for chelation are more stringent and demanding than for phosphates. Chelant treatment programs typically have the following conditions:

• Dissolved oxygen levels below 25 ppb.
• Removal of calcium and magnesium to below 0.5 ppm as calcium carbonate.
• Removal of silica and/or alkalinity from the makeup water.
• Plant steam production and operating requirements preclude the use of a precipitating program.
• Boiler design prevents the use of phosphate treatment.

Because complete chelation is theoretically never attained, ChemTreat incorporates polymeric dispersants in all chelant-based programs to assist in preventing iron and mineral boiler scale deposits. Since the predominant chelation reactions take place in the boiler feedwater and not in the boiler water, considerable care must be given to selecting the proper chemical feed points. ChemTreat field representatives are experts at applying chelant technologies to all types of boiler systems.