Treatment of Boiler Feed Water

 

Treatment of Boiler Feed Water

Additionally, an internal treatment should be used as a complement to avoid corrosion and scaling in the feed-water system, as well as protect against corrosion in the steam condensate systems.

Specific doses of conditioning materials are added to the water during the conditioning process, which is an important addition to the water treatment programme. The following are some of the most regularly used products:

Phosphates-dispersants, polyphosphates-dispersants (softening chemicals): these products react with the alkalinity of boiler water to neutralise the hardness of the water by forming tricalcium phosphate, an insoluble compound that can be disposed of and blown down on a continuous or periodic basis through the bottom of the boiler.

Natural and synthetic dispersants (anti-scaling ingredients) improve the conditioning products’ dispersive qualities. They might be anything from:

Lignosulphonates and tannins are examples of natural polymers.

Polyacrilates, maleic acrylate copolymer, maleic styrene copolymer, polystyrene sulphonates, and other synthetic polymers

Sequestering agents are inhibitors that have a threshold effect, such as inorganic phosphates.

Sodium sulphite, tannis, hydrazine, hydroquinone/progallol-based derivatives, hydroxylamine derivatives, ascorbic acid derivatives, and other oxygen scavengers The oxides and dissolved oxygen are reduced by these scavengers, whether they are catalysed or not. Metal surfaces are usually passivated as well. Whether or not a deaerating heater is utilised will determine the product to use and the dose required.

Anti-foaming or anti-priming chemicals are a combination of surface-active compounds that change the surface tension of a liquid, eliminate foam, and prevent tiny water particles from being carried over into the steam.

Soda ash, caustic, and different sodium phosphates are among the softening agents utilised. The calcium and magnesium compounds in the feed water react with these substances. To react selectively with magnesium hardness, sodium silicate is utilised. Calcium bicarbonate in the feed water is broken down or interacts with caustic soda to generate calcium carbonate at boiler temperatures. 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

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Coagulation or dispersion are the two most common methods for conditioning sludge inside a boiler. It is preferable to coagulate the sludge to create big flocculent particles when the overall amount of sludge is substantial (as a result of high feed-water hardness). Blow-down can be used to get rid of it. Based on the fee-water analysis, coagulation can be achieved by carefully adjusting the amounts of alkalis, phosphates, and organics utilised for treatment. It is desirable to utilise a higher percentage of phosphates in the treatment when the amount of sludge is modest (low feed water hardness). Phosphates separate sludge particles and generate separated sludge particles. To keep the sludge particles spread throughout the boiler water, a higher percentage of organic sludge dispersants is utilised in the treatment.

Sludge conditioning compounds include organic materials from the tannin, lignin, and alginate classes. These organics must be carefully chosen and treated so that they are both effective and stable at the boiler’s working pressure. Anti-foam agents are made from synthetic organic components. Sodium sulphite and hydrazine are two compounds used to scavenge oxygen. In feed-water systems, various mixtures of polyphosphates and organics are employed to prevent scale and corrosion. Condensate corrosion is prevented with volatile neutralising amines and filming inhibitors.

Chemical solution tanks and proportioning pumps, as well as specific ball briquette chemical feeders, are common internal chemical feeding systems. Softening chemicals (phosphates, soda ash, caustic, etc.) are usually introduced directly to the fee-water at the boiler drum’s entrance. They can also be fed through a separate pipe that empties into the boiler’s feed-water drum. Chemicals should be discharged in the boiler’s fee-water section so that reactions can take place in the water before it reaches the steam