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 contractors etc.) 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.

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.

IMPURITYRESULTING INGOT RID OF BYCOMMENTS

Soluble Gasses

Hydrogen Sulphide (H2S)Water smells like rotten eggs: Tastes bad, and is corrosive to most metals.Aeration, Filtration, and Chlorination.Found mainly in groundwater, and polluted streams.

Carbon Dioxide (CO2)Corrosive, forms carbonic acid in condensate.Deaeration, neutralization with alkalis.Filming, neutralizing amines used to prevent condensate line corrosion.

Oxygen (O2)Corrosion and pitting of boiler tubes.Deaeration & chemical treatment with (Sodium Sulphite or Hydrazine)Pitting of boiler tubes, and turbine blades, failure of steam lines, and fittings etc.

Suspended Solids

Sediment & TurbiditySludge and scale carryover.Clarification and filtration.Tolerance of approx. 5ppm max. for most applications, 10ppm for potable water.

Organic MatterCarryover, foaming, deposits can clog piping, and cause corrosion.Clarification; filtration, and chemical treatmentFound mostly in surface waters, caused by rotting vegetation, and farm run offs. Organics break down to form organic acids. Results in low of boiler feed-water pH, which then attacks boiler tubes. Includes diatoms, molds, bacterial slimes, iron/manganese bacteria. Suspended particles collect on the surface of the water in the boiler and render difficult the liberation of steam bubbles rising to that surface.. Foaming can also be attributed to waters containing carbonates in solution in which a light flocculent precipitate will be formed on the surface of the water. It is usually traced to an excess of sodium carbonate used in treatment for some other difficulty where animal or vegetable oil finds its way into the boiler.

Dissolved Colloidal Solids

Oil & GreaseFoaming, deposits in boilerCoagulation & filtrationEnters boiler with condensate

Hardness, Calcium (Ca), and Magnesium (Mg)Scale deposits in boiler, inhibits heat transfer, and thermal efficiency. In severe cases can lead to boiler tube burn thru, and failure.Softening, plus internal treatment in boiler.Forms are bicarbonates, sulphates, chlorides, and nitrates, in that order. Some calcium salts are reversibly soluble. Magnesium reacts with carbonates to form compounds of low solubility.

Sodium, alkalinity, NaOH, NaHCO3, Na2CO3Foaming, carbonates form carbonic acid in steam, causes condensate return line, and steam trap corrosion, can cause embrittlement.Deaeration of make-up water and condensate return. Ion exchange; deionization, acid treatment of make-up water.Sodium salts are found in most waters. They are very soluble, and cannot be removed by chemical precipitation.

Sulphates (SO4)Hard scale if calcium is presentDeionizationTolerance limits are about 100-300ppm as CaCO3

Chlorides, (Cl)Priming, i.e. uneven delivery of steam from the boiler (belching), carryover of water in steam lowering steam efficiency, can deposit as salts on superheaters and turbine blades. Foaming if present in large amounts.DeionizationPriming, or the passage of steam from a boiler in "belches", is caused by the concentration sodium carbonate, sodium sulphate, or sodium chloride in solution. Sodium sulphate is found in many waters in the USA, and in waters where calcium or magnesium is precipitated with soda ash.

Iron (Fe) andDeposits in boiler, in large amounts can inhibit heat transfer.Aeration, filtration, ion exchange.Most common form is ferrous bicarbonate.

Manganese (Mn)

Silica (Si)Hard scale in boilers and cooling systems: turbine blade deposits.Deionization; lime soda process, hot-lime-zeolite treatment.Silica combines with many elements to produce silicates. Silicates form very tenacious deposits in boiler tubing. Very difficult to remove, often only by flourodic acids. Most critical consideration is volatile carryover to turbine components.

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