Industrial Water Engineering, Inc.

CHEMICAL WATER TREATMENT

toll free 1-800-543-6519

GSA # GS-10F-7884A (1/31/2007)

DOD Cage # 0MV90

 

 

Mud drum of boiler treated with BW-5302.

Boilers are used to heat water into steam. As water is heated, the temperature and pressure at which water first forms steam is called the saturation temperature. As heat is added to the steam, the steam temperature goes higher than the saturation temperature and the steam is called superheated. Steam is widely used in industrial processes. A boiler is never a stand alone process but is part of a system.

Boilers are generally divided into two types-fire tube boilers and water tube boilers.   A fire tube boiler has fire or heat applied to the inside of the tube and water is heated to steam on the outside of the tube. A common fire tube design is the Scotch Marine boiler.  In a water tube boiler, water is inside the tube and fire is on the outside of the tube. Water tube boilers often have both a steam drum and a mud drum. The drums are connected by water tubes.

The choice for boiler water treatment is based on two factors: 1) feedwater chemistry (hardness) and boiler operating pressure.

Boiler treatment is used to prevent scale, corrosion, carry-over, and foaming. Scale is the direct result of poor control of calcium, magnesium, and silica in the feedwater. Corrosion is usually due to oxygen in the system. Oxygen also greatly accelerates the corrosion due to carbon dioxide. Carry-over is the direct result of too many dissolved solids (TDS) in the boiler.

Scale control is accomplished by internal treatment or external treatment or a combination of both. The higher the pressure the greater the external treatment requirements and the greater the need for maintaining return condensate quality. External treatment includes equipment such as water softeners, demineralizers, dealkalizers, and reverse osmosis systems. Internal treatment is either solubilizing or precipitation.

Precipitation is the reaction with carbonate to form calcium carbonate or with phosphate to form calcium phosphate. The resulting mineral is then conditioned and removed with blowdown. Carbonate is formed when the soluble bicarbonates react with hydroxide that is added as part of the treatment. Phosphate programs use soluble sodium phosphates that are added to react with the calcium hardness. The conditioning chemicals are terpolymers that prevent the precipitate from sticking to boiler surfaces. BW-5687 is a precipitation type of treatment.

Our solubilizing programs tie up or condition the calcium hardness so that it does not precipitate. Other programs may use chelants such as EDTA and NTA. Excessive feed of chelants will destroy boiler metals. Organic phosphates and terpolymers used in conjunction may be used in boilers with feedwater less than 1 mg/L total hardness. Organic phosphates and polymers are also able to remove metallic iron oxides so that heat transfer systems remain very clean. Our solubilizing programs programs will not attack parent metal surfaces like chelants. 

On boilers under 300 BHP, the need to feed scale control chemicals and oxygen scavengers separately does not make sense. The need for multiple test kits, feed systems, and manpower to maintain the program often results in failure. For these package boilers, a single-package program (BW-560 and BW-570) will do an excellent job. These packages are monitored with an easy to use sulfite test and a conductivity meter.

Magnesium hardness is precipitated as magnesium silicate or magnesium hydroxide. The hydroxide required in both these reactions may be from the conversion of bicarbonate ion with heat or by the addition of hydroxide with treatment. The quantity of hydroxide allowed is directly related to boiler pressure. Permissible levels of hydroxide ion decreases as pressure increases.

In the past 5 years, polymer technology has improved at strides similar to those found in home computers. Polymers are designed with two or more monomers put together in a predetermined molecular structure.  These polymers are very effective at conditioning boiler sludge and maintaining clean boiler surfaces.

Three gases may be present in feedwater or generated in the boiler. These gases are:

•         OXYGEN-Low levels will cause severe pitting
•         CARBON DIOXIDE-Reacts with water to form corrosive carbonic acid.
•         AMMONIA-In the presence of oxygen, ammonia is very corrosive to copper.

Oxygen is a major source of corrosion and many other forms of corrosion are dependent on oxygen. Oxygen also allows metal oxides (hematite and magnetite-iron oxides) to enter the boiler and form deposits subject to caustic hideout and other under deposit corrosion mechanisms.

Corrosion of metal roughly doubles for each 30^o F increase in temperature. Oxygen corrosion should be controlled as far downstream of the boiler as practical. Two methods of oxygen control exist, mechanical and chemical.

Mechanically, oxygen is removed as water is heated to near the flashpoint in a special heater called a dearator heater or DA heater.  The oxygen content will be lowered to 0.02 to 0.007 ppm depending on type and operational characteristics.

After mechanical removal, oxygen is chemically controlled with either sulfite, hydrazine, or other compounds. The reactions are:

    Sulfite + Oxygen-----> Sodium sulfate

   Hydrazine + Oxygen-------------> Nitrogen and Water

Additionally, hydrazine converts hematite to magnetite. Hydrazine will start to revert to ammonia at temperatures above 500^o F. Sulfite forms Hydrogen sulfide gas that is very corrosive and toxic. Sulfite is limited to low pressure boilers less than 300 PSIG.

Hydrazine is a carcinogen and requires special handling. Some companies have patent positions on in-situ hydrazine replacements that are easier to handle. High costs has forced most users of hydrazine to invested in proper chemical feed equipment to eliminate personnel from handling hydrazine.

Both sulfite and hydrazine should be used in the catalyzed form. Catalyzed oxygen scavengers allow oxygen removal faster at lower temperatures compared to uncatalyzed material. BW-590 and BW-5910 are catalyzed sulfites. BW560 and BW-570 single package treatments contain catalyzed sulfite.