BOILER WATER TREATMENT

The treatment and conditioning of boiler feed water must satisfy three main objectives:

• Continuous heat exchange
• Corrosion protection
• Production of high quality steam

External treatment is the reduction or removal of impurities from water outside the boiler. In general, external treatment is used when the amount of one or more of the feed water impurities is too high to be tolerated by the boiler system in question. There are many types of external treatment (softening, evaporation, deaeration, membrane contractorsetc.) which can be used to tailor make feed-water for a particular system. Internal treatment is the conditioning of impurities within the boiler system. The reactions occur either in the feed lines or in the boiler proper. Internal treatment may be used alone or in conjunction with external treatment. Its purpose is to properly react with feed water hardness, condition sludge, scavenge oxygen and prevent boiler water foaming.

External treatment

The water treatment facilities purify and deaerate make-up water or feed water. Water is sometimes pretreated by evaporation to produce relatively pure vapor, which is then condensed and used for boiler feed purposes. Evaporators are of several different types, the simplest being a tank of water through which steam coils are passed to heat the water to the boiling point. Sometimes to increase the efficiency the vapor from the first tank is passed through coils in a second tank of water to produce additional heating and evaporation. Evaporators are suitable where steam as a source of heat is readily available. They have particular advantages over demineralization, for example, when the dissolved solids in the raw water are very high.

Certain natural and synthetic materials have the ability to remove mineral ions from water in exchange for others. For example, in passing water through a simple cation exchange softener all of calcium and magnesium ions are removed and replaced with sodium ions. Since simple cation exchange does not reduce the total solids of the water supply, it is sometimes used in conjunction with precipitation type softening. One of the most common and efficient combination treatments is the hot lime-zeolite process. This involves pretreatment of the water with lime to reduce hardness, alkalinity and in some cases silica, and subsequent treatment with a cation exchange softener. This system of treatment accomplishes several functions: softening, alkalinity and silica reduction, some oxygen reduction, and removal of suspended matter and turbidity.
Chemical treatment of water inside the boiler is usually essential and complements external treatment by taking care of any impurities entering the boiler with the feed water (hardness, oxygen, silica, etc.). In many cases external treatment of the water supply is not necessary and the water can be treated only by internal methods.

Internal treatment

Internal treatment can constitute the unique treatment when boilers operate at low or moderate pressure, when large amounts of condensed steam are used for feed water, or when good quality raw water is available. The purpose of an internal treatment is to

1) react with any feed-water hardness and prevent it from precipitating on the boiler metal as scale;

2) condition any suspended matter such as hardness sludge or iron oxide in the boiler and make it non-adherent to the boiler metal;

3) provide anti-foam protection to allow a reasonable concentration of dissolved and suspended solids in the boiler water without foam carry-over;

4) eliminate oxygen from the water and provide enough alkalinity to prevent boiler corrosion.

In addition, as supplementary measures an internal treatment should prevent corrosion and scaling of the feed-water system and protect against corrosion in the steam condensate systems.

During the conditioning process, which is an essential complement to the water treatment program, specific doses of conditioning products are added to the water. The commonly used products include:

• Phosphates-dispersants, polyphosphates-dispersants (softening chemicals): reacting with the alkalinity of boiler water, these products neutralize the hardness of water by forming tricalcium phosphate, and insoluble compound that can be disposed and blow down on a continuous basis or periodically through the bottom of the boiler.
• Natural and synthetic dispersants (Anti-scaling agents): increase the dispersive properties of the conditioning products. They can be:
  • Natural polymers: lignosulphonates, tannins
  • Synthetic polymers: polyacrilates, maleic acrylate copolymer, maleic styrene copolymer, polystyrene sulphonates etc.
• Sequestering agents: such as inorganic phosphates, which act as inhibitors and implement a threshold effect.
• Oxygen scavengers: sodium sulphite, tannis, hydrazine, hydroquinone/progallol-based derivatives, hydroxylamine derivatives, hydroxylamine derivatives, ascorbic acid derivatives, etc. These scavengers, catalyzed or not, reduce the oxides and dissolved oxygen. Most also passivate metal surfaces. The choice of product and the dose required will depend on whether a deaerating heater is used.
• Anti-foaming or anti-priming agents: mixture of surface-active agents that modify the surface tension of a liquid, remove foam and prevent the carry over of fine water particles in the steam.

The softening chemicals used include soda ash, caustic and various types of sodium phosphates. These chemicals react with calcium and magnesium compounds in the feed water. Sodium silicate is used to react selectively with magnesium hardness. Calcium bicarbonate entering with the feed water is broken down at boiler temperatures or reacts with caustic soda to form calcium carbonate. Since calcium carbonate is relatively insoluble it tends to come out of solution. Sodium carbonate partially breaks down at high temperature to sodium hydroxide (caustic) and carbon dioxide. High temperatures in the boiler water reduce the solubility of calcium sulphate and tend to make it precipitate out directly on the boiler metal as scale. Consequently calcium sulphate must be reacted upon chemically to cause a precipitate to form in the water where it can be conditioned and removed by blow-down. Calcium sulphate is reacted on either by sodium carbonate, sodium phosphate or sodium silicate to form insoluble calcium carbonate, phosphate or silicate. Magnesium sulphate is reacted upon by caustic soda to form a precipitate of magnesium hydroxide. Some magnesium may react with silica to form magnesium silicate. Sodium sulphate is highly soluble and remains in solution unless the water is evaporated almost to dryness.

There are two general approaches to conditioning sludge inside a boiler: by coagulation or dispersion. When the total amount of sludge is high (as the result of high feed-water hardness) it is better to coagulate the sludge to form large flocculent particles. This can be removed by blow-down. The coagulation can be obtained by careful adjustment of the amounts of alkalis, phosphates and organics used for treatment, based on the fee-water analysis. When the amount of sludge is not high (low feed water hardness) it is preferable to use a higher percentage of phosphates in the treatment. Phosphates form separated sludge particles. A higher percentage of organic sludge dispersants is used in the treatment to keep the sludge particles dispersed throughout the boiler water.
The materials used for conditioning sludge include various organic materials of the tannin, lignin or alginate classes. It is important that these organics are selected and processed, so that they are both effective and stand stable at the boiler operating pressure. Certain synthetic organic materials are used as anti-foam agents. The chemicals used to scavenge oxygen include sodium sulphite and hydrazine. Various combinations of polyphosphates and organics are used for preventing scale and corrosion in feed-water systems. Volatile neutralizing amines and filming inhibitors are used for preventing condensate corrosion.

Common internal chemical feeding methods include the use of chemical solution tanks and proportioning pumps or special ball briquette chemical feeders. In general, softening chemicals (phosphates, soda ash, caustic, etc.) are added directly to the fee-water at a point near the entrance to the boiler drum. They may also be fed through a separate line discharging in the feed-water drum of the boiler. The chemicals should discharge in the fee-water section of the boiler so that reactions occur in the water before it enters the steam generating area. Softening chemicals may be added continuously or intermittently depending on feed-water hardiness and other factors. Chemicals added to react with dissolved oxygen (sulphate, hydrazine, etc.) and chemicals used to prevent scale and corrosion in the feed-water system (polyphosphates, organics, etc.) should be fed in the feed-water system as continuously as possible. Chemicals used to prevent condensate system corrosion may be fed directly to the steam or into the feed-water system, depending on the specific chemical used. Continuous feeding is preferred but intermittent application will suffice in some cases.