Boiler Water Treatment Considerations

For prolonged boiler life, a water treatment program recommended by Thermidaire is a must. Steam boilers are sensitive to even minor deposits on tube walls. Therefore, most feedwater treatment programs are oriented toward scale control. However, internal corrosion control cannot be ignored. Scald with locally high caustic concentrations can develop on metal components, strip away the protective magnetite film, and attack the base metal. This attack is characterized by gouging of the metal and its severity is directly proportional to increasing operating pressures.

Oxygen is highly corrosive when dissolved in water, and can result in corrosion pitting of heat transfer surfaces. Mechanical deaeration of the feed water using a deaerator is the typical first step in eliminating dissolved oxygen and other gases such as ammonia and carbon dioxide. Mechanical deaeration will reduce oxygen to as low as 0.005 cc/liter. In boilers below 1,000 psig, the oxygen-scavenging chemical is fed continuously, preferably at the storage compartment of deaerator or at suction of boiler feed pump, to provide turbulence and time for mixing. Specification of chemicals and dosages should be in accordance with the recommendations of the Owner's water consultant.

Generally, a total water treatment program should pursue the following four-point goal:

1. Maintenance of free caustic corrosion potential, accomplished by eliminating free hydroxide (caustic) though the addition of various phosphate salts.

2. Minimize caustic deposit formation due to scale and suspended solids by

chelation and sludge conditioners so that blowdown will remove this potential problem.

3. Eliminate oxygen content through deaeration or other mechanical means and chemical scavenging.

4. Prevention of carryover and foaming.

The accomplishment of these objectives generally requires treatment before and after introduction of water into the boiler. The selection of pre-treatment processes depends upon water source, chemical characteristics, amount of make-up water needed, plant operating practices, etc. These treating methods include filtering, softening, demineralizing, deaerating, and preheating.

After-treatment involves the addition of chemicals into the boiler water. This after-treatment is required to compensate for any variations in the pre-treatment of the boiler feedwater system and to assure the ultimate protection of the boiler. Because of the variables involved, no one "boiler compound" cannot be considered a cure-all, nor is it advisable to experiment with homemade treating methods. Sound recommendations and their employment should be augmented by periodic analysis of the raw water, boiler water, and condensate.

Feedwater Quality Limits

The ABMA specifies limits, as listed in Figure 5, for boiler water composition with respect to operating pressure to assure good quality steam. While there are other considerations (such as corrosion and deposit formation) in establishing boiler water composition, the limits indicate that requirements for feedwater purity become more stringent as operating pressure increases.

Notes:

1. Actual values within the range reflect the TDS in the feedwater. Higher values are for high solids, lower values are for low solids in the feedwater.

2. Actual values within the range are directly proportional to the actual value of TDS of boiler water. Higher values are for high solids, lower values are for low solids in the boiler water.

3. Dictated by boiler water treatment.

4. These values are exclusive of silica.

5. Expressed as equivalent calcium carbonate in PPM.

ABMA Boiler Quality

Improper and nonexistent feedwater treatment is the major factor causing boiler failure, which ultimately results in boiler down time and costly repairs. All fresh water available from natural sources requires varying degrees of treatment prior to use in a boiler. The impurities in fresh water ware many and each required special attention. Solids in the form of minerals, chemicals, and organic material are all found in fresh water and all have a different effect on the internal surfaces of a boiler.

Boiler feedwater treatment by a competent water treating company will result in prevention of scale and deposits, removal of dissolved gases (free oxygen), protection against corrosion, elimination of carry-over of water with steam, maximum boiler efficiency, and maintenance savings. It is imperative that the Owner contacts a company specializing in boiler feedwater treatment and obtain their recommendation and service.

Failure to heed the above warning will cause excessive damage to the boiler. It will be very costly to repair this damage to the boiler and place it back in proper condition. The warranty does not cover this type of maintenance. This is the Owner's responsibility.