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Water Treatment for Steam Boiler Applications

Water Treatment for Steam Boiler Applications

A properly exercised water treatment program for steam boiler applications is essential to satisfy three main objectives: continuous heat exchange, production of high quality steam, and corrosion/deposition protection. A water treatment program consists of both external and internal treatment where preventative measures are achieved through mechanical and chemical means.

Water is known as the universal solvent because it dissolves more substances than any other liquid due to the polar nature of the H2O molecule. Solids, minerals, and nutrients that are carried in water depend on its originating source. Influent water used for drinking, boiler make-up, cooling and process applications is derived from one of three sources: underground (wells), surface (ground), or reuse (water treatment plants).

External boiler treatment consists of the mechanical and chemical removal of suspended solids, dissolved solids, and dissolved gasses. The main objective of external treatment is to significantly improve the quality of feedwater before entering the boiler. Ion exchange/softening, demineralization, and reverse osmosis are mechanical methods used to remove dissolved solids. Dissolved gasses such as oxygen and carbon dioxide are removed through deaeration and dealkalization.

The primary step in boiler water pre-treatment is ion exchange to remove hardness. A sodium zeolite softener removes soluble hardness in the form of calcium and magnesium through ionic exchange with sodium. The removal of hardness prevents scale and helps to maintain continuous heat exchange and efficiency. 1/8” of scale will result in a heat loss of 20% and an increase in fuel consumption of 4%. Reverse osmosis yields a high quality makeup which allows for the boiler to run at higher cycles of concentration, reducing boiler blow-down and the demand for further makeup and decreasing fuel consumption.

A deaerator uses pressure and temperature to drive off dissolved gasses and heats feedwater to the desired temperature before entering the boiler. An efficient deaerator is capable of reducing dissolved oxygen levels to 7 PPB. The remaining dissolved oxygen is removed by using an oxygen scavenger. The most common oxygen scavenger used is catalyzed sodium sulfite which should be fed upstream of the deaerator. Dissolved oxygen in the feedwater and boiler system will result in corrosion in the form of oxygen pitting. Oxygen pitting can produce catastrophic failures even if only a small amount of metal has been lost. Dissolved CO2 is formed from the breakdown of calcium and sodium carbonates in the boiler and carried over to the condensate return. A dealkalizer will remove dissolved CO2 from the condensate return.

Once the feedwater is treated, chemical products are added to the boiler to decrease the potential for deposition, scale, and corrosion. Phosphates, chealants, and polymers are used as primary inhibitors to prevent scale. Phosphate reacts with calcium to form hydroxyapatite, and chealants react with silica and iron forming a harmless sludge which can be removed with daily bottom blowdown. Phosphate/chealant programs only work well with softened makeup. A polymer program can be implemented to keep calcium and magnesium ions in solution prior to removal through blowdown. A polymer program works well with or without softened makeup.

Sodium hydroxide, or caustic, is used to increase alkalinity. Maintaining an alkalinity range of 300-600 ppm measured as p-alkalinity helps to minimize the corrosive effects of heated water on the boiler metallurgy. Alkalinity is consistent to pH, maintaining proper boiler alkalinity will ensure that boiler pH is also in range.

Neutralizing amines are volatile compounds that leave the boiler with the steam and prevent carbonic acid corrosion in condensate receivers and return piping by adjusting pH to neutral range. Neutralizing amines use in certain processes are regulated by the FDA. In food and beverage plants only morpholine, DEAE or cyclohexylamine can be used. The FDA limits the allowable level of each in treated steam to 10 ppm of morpholine, 15 ppm of DEAE, and 10 ppm of cyclohexylamine. The FDA maximum limit is 25 ppm total amine when any or all are used in combination, where individual limits are not exceeded. Filming amines are another option to protect the condensate return system.

Overall, internal boiler treatment is proportional to water use and controlled through blowdown and chemical feed rates. Typically, maximum boiler cycles are determined by the measure of chlorides. Conductivity is also used as a measure to determine boiler cycles and blowdown settings. ASME guidelines and LSI calculations of the makeup source will aid in determining the solubility limits of impurities at high temperature and pressure. Typically one impurity will reach its solubility limit before others, thus becoming the determining factor in blow down settings. A well understood external and internal water treatment program will decrease overall fuel consumption, decrease overall water consumption, decrease maintenance, prevent failures, and loss of production.

With the addition of our water treatment division, our engineers can design a complete boiler system based around your application that includes external and internal boiler treatment options. A complete package of our extensive services will include the initial survey and analysis, design and implementation, boiler start-up with established control parameters, and water treatment chemicals and monthly testing services. An excellent option for our New York City based customers is the Fulton line of skid mounted boiler packages. A typical plant room package will include: boiler blowdown separator, feed water and condensate return tank, water treatment and control system with interconnecting mechanical and electrical services. These systems are especially beneficial for projects where on-site work is difficult or limited by time, where access is remote or restricted, or where security is an important consideration.

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