Boiler Feed-Water Treatment

Boiler Feed-Water Treatment

The importance of correct feed-water treatment for economic operation and for extending life of boiler and equipment cannot be over emphasized. Feed-water treatment is essential in boilers, feed-systems, etc., more particularly in modern boilers of a high evaporative rate. (The faster a steam boiler or generator will convert water to steam, the more rapidly will the solids in the water concentrate up.) So, large and small water-tube boilers, the typical fire-tube packaged boiler, and steam generators are all examples of this in varying degrees. As all untreated waters carry natural salts, they have to be treated to prevent scale forming.
The three main reasons for water treatment are:

  • Prevention of Corrosion in feed boiler, steam and condensate systems.
  • Elimination of Scale.
  • Economic boiler operation without carryover.


Corrosion will reduce metal thickness of tubes or shell. Result: pressure must be reduced and finally boiler condemned.

Scale reduces the heat flow from fire side to water. Result: high fire temperatures are needed to maintain down is insufficient.

Table General Boiler feed water limits.

Boiler Feed Water
Drum
Pressure
(psi)
Iron
(ppm Fe)
Copper
(ppm Cu)
Total
Hardness
(ppm CaCO3)
0-3000.1000.0500.300
301-4500.0500.0250.300
451-6000.0300.0200.200
601-7500.0250.0200.200
751-9000.0200.0150.100
901-10000.0200.0150.050
1001-15000.0100.0100.0
1501-20000.0100.0100.0

Basic Chemistry of the Effect of Impurities in the Boiler

The different constituents present in the feed water have different effects on the boiler, table below shows each constituent along with its effect on the boiler.

Table The effect of different impurities on the boiler.

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 embitterment.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, (CI)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) and Manganese (Mn)Deposits in boiler, in large amounts can inhibit heat transfer.Aeration, filtration, ion exchange.Most common form is ferrous bicarbonate.
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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