Treatment of Boiler Feed Water

Three key goals must be met while treating and conditioning boiler feed water:

Heat exchange is ongoing.

Corrosion resistance

High-quality steam production

The  reduction or removal of pollutants from water outside the boiler is  known as external treatment. When the amount of one or more of the feed  water pollutants is too high for the boiler system to tolerate, external  treatment is usually used. External treatment (softening, evaporation, deaeration,  membrane contractors, and so on) can be used to tailor manufacture  feed-water for a certain system. The conditioning of pollutants within  the boiler system is known as internal treatment. The reactions take  place in either the feed lines or the boiler itself. Internal treatment  can be used in conjunction with or instead of external treatment. Its  job is to react properly with feed water hardness, condition sludge,  scavenge oxygen, and keep boiler water foaming at bay.

Treatment from the outside

Make-up  water or feed water is purified and deaerated at the water treatment  facilities. Evaporation of water is occasionally used to produce  relatively pure vapour, which is subsequently condensed and used as  boiler feed. Evaporators come in a variety of shapes and sizes, the most  basic of which is a water tank through which steam coils are passed to  heat the water to the boiling point. To improve efficiency, vapour from  the first tank is sometimes fed through coils in a second water tank to  provide extra heating and evaporation. Where steam as a source of heat  is readily available, evaporators are ideal. When the dissolved solids  in the raw water are quite high, they have a distinct advantage over  demineralization.

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Mineral  ions are removed from water by certain natural and manufactured  materials in exchange for others. A simple cation exchange softener, for  example, removes all calcium and magnesium ions from water and replaces  them with sodium ions. Simple cation exchange is occasionally employed  in conjunction with precipitation type softening because it does not  lower total solids in the water supply. The hot lime-zeolite method is  one of the most common and effective combo treatments. This entails  treating the water with lime to reduce hardness, alkalinity, and, in  certain circumstances, silica, followed by a cation exchange softener.  Softening, alkalinity and silica reduction, some oxygen reduction, and  removal of suspended debris and turbidity are all features of this  treatment system.

Chemical  treatment of water within the boiler is usually required, and it  complements the exterior treatment by removing any contaminants that may  have entered the boiler with the input water (hardness, oxygen, silica,  etc.). In many circumstances, external water treatment is not required,  and the water can be treated entirely internally.

Internal therapy

When  boilers operate at low or moderate pressure, substantial amounts of  condensed steam are used for feed water, or acceptable quality raw water  is available, internal treatment can be the only option. An internal  treatment’s goal is to help you feel better.

1) react with any hardness in the feed water to prevent scale from forming on the boiler metal;

2) make any suspended substance in the boiler, such as hardness sludge or iron oxide, non-adherent to the boiler metal;

3)  provide anti-foam protection so that a reasonable concentration of  dissolved and suspended particles in the boiler water can be maintained  without foam carry-over;

4) Remove oxygen from the water and add enough alkalinity to keep the boiler from corroding.

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