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CHELATING PROGRAMME      

  • In this program chelating agents viz., EDTA, NTA, etc., are added to feed water.
  • These salts form water soluble complexes with calcium & magnesium and prevent them from precipitating. In case they form scales, these salts de-stabilizes the crystalline growth.
  • This system of treatment is discouraged as the salts may carry over with steam and break into acidic components which may cause corrosion in post boiler sections.  

CARBONATE TREATMENT PROGRAM      

  • This method of treatment is implemented in boilers having operating pressure within 20Kgs/cm2 where soft & raw water are used as boiler feed water.
  • The above treatment should be used in conjunction with polymeric sludge conditioners for better results.
  • In this method of treatment precipitation of calcium & magnesium, either partially or completely, are induced by maintaining higher pH & higher alkalinity.
  • The residual hardness, if present, should be controlled with the addition of polymeric sludge conditioners.

PHOSPHATE TREATMENT PROGRAM

  • This method of treatment is implemented in boilers where DM or RO water is used as boiler feed water make up. Also in boilers having operating pressure above 20Kgs/cm2 which should be having DM or RO water make up.
  • In this method calcium is precipitated as calcium ortho phosphate sludge with the addition of blended phosphates.
  • This treatment is recommended in boilers where the feed water contains low sodium, low silica & have more than 90% condensate return.

OTHER ASPECTS OF BOILER WATER TREATMENT

      OXYGEN SCAVENGING
      SLUDGE CONDITIONING
      ALKALINITY BUILDER
      PREVENTION OF CARRY OVER

OXYGEN SCAVENGING   

  • Boiler feed water contains dissolved gasses viz., Oxygen, Carbon dioxide and hydrozen sulphide.
  • These gasses are removed through degasser. Oxygen in particular is removed through de-aerator.
  • The presence of Oxygen causes pitting corrosion.
  • The presence of CO2 causes grooving corrosion.
  • The presence of CO2 generally occur in post boiler section where it is neutralized with the help of amines.
  • Oxygen is removed with the addition of Hydrazine or Sulphites.
  • Sulphites are used in boilers having operating pressure upto 600psi. Above 600psi sulphites will decompose and loose their properties.
  • Above 600psi boilers hydrazine is used for removing oxygen.
  • Apart from removing oxygen, hydrazine forms a magnatite film over the metal surfaces in boiler system.
  • Also it contributes to the increase of pH.
  • Demerits of hydrazine are: 1. It is carcinogenic. 2. It reacts very slow.
  • To enhance the reaction rate it is supplemented with a catalyst. 

SLUDGE CONDITIONING

  • Boiler water contains suspended particles like precipitation of calcium, magnesium, silica, etc. These salts flock together and settle down at the bottom of boiler drum, low velocity areas, may adhere to boiler tubes to form a scale.
  • To prevent the above problem, a medium molecular weight polymeric dispersant is added. This dispersant gives same ionic charges to the precipitated salts, due to which repulsion occurs among the particles. Hence all these suspended matter is kept in kinetic motion and can be removed freely through blow down.

ALKALINITY BUILDER

  • Total alkalinity should be maintained at 15 to 20% of TDS in the boiler water.
  • Certain amount of caustic alkalinity is required to be maintained for following reasons:
  • To keep silica in soluble state.
  • To keep iron in soluble state.
  • In Phosphate program, in presence of caustic alkalinity magnesium preferentially reacts with hydroxyl ions and precipitate as mg(OH)2 which is a loose sludge and can be removed through blow down freely.
  • Otherwise magnesium reacts with phosphate and form magnesium phosphate which is a thick slimy sludge and can not be removed through blow down.

CARRY OVER

  • Some times suspended particles go along with steam. This is called as carry over. This occurs due to the following reasons:
  • Excess alkalinity causes foaming. When steam separates from water surface, the bubbles at the interface breaks and the suspended particles enveloped in the bubbles gets carried along with steam.
    Operating the boiler more than the designed capacity also causes carry over.
  • Presence of oil, organic matter & suspended matter in excess causes carry over.   The carry over causes scaling in the post boiler sections.