Internal treatment and control of boiler feed water

Even if external water treatment is designed and executed, it is inevitable that problems such as corrosion and scale still occur in the long run of the boiler operation. Boiler water treatment chemicals are therefore used for protecting a boiler plant from the problems and are applied for the whole systems including the feed water line and the steam condensate line. In case of internal boiler water treatment, chemicals are added to the boiler to prevent the formation of scale by converting the scale forming compounds to free flowing sludges, which can be removed by-blow down. The chemicals are also added to prevent corrosion, priming and foaming.

Purpose of using boiler water treatment chemicals

  • Controlling pH and alkalinity to prevent scale formation and corrosion
  • Softening to prevent scale formation
  • Sludge dispersing for easy blow down to prevent scaling
  • Oxygen scavenging to prevent corrosion
  • Preventing foaming of boiler water
  • Neutralizing and film forming to prevent corrosion

Boiler water treatment chemicals and their functions

Phosphate base chemicals and alkali agent

Phosphate base boiler compounds for low-pressure boiler contain phosphates and alkali agents. Both phosphates and alkalies take part in preventing scale formation and corrosion on heating surfaces of a boiler by

  • reacting with hardness components
  • keeping silica substances water soluble
  • controlling of alkalinity of boiler water

In a medium pressure boiler caustic or coordinated phosphate treatment is applied. For high pressure boilers, in the light of high heat flux, a coordinated phosphate treatment is employed in order to prevent alkali corrosion caused by the concentrated free alkali. The chemicals generally used are : Na3PO4, NaHPO4, (NaPO3)6, NaOH etc.

Non-phosphate base chemicals

1) Polymer base compounds
Synthetic polyelectrolyte, Natural polyelectrolyte, etc. are used to prevent scale formation. The difference between polymer base and phosphate base boiler compounds is in the function that they prevent the scale formation of hardness components. The preventive mechanism is considered as follows:

  • Inhibit the crystal growth of scale components
  • Inhibit the crystallization of scale components
  • Keep the solids in suspension

2) Chelating agent
In chelate treatment, the hardness components contaminated in feed water are kept in a soluble state by the chelating agent. Typical chelating agents are ethylenediamine tetra acetate (EDTA) and nitrilo triacetate.

Sludge dispersant:
Hardness compounds and silica which enter into the boiler from the feed water become sludges of hydroxyapatite, magnesium hydroxide, magnesium silicate etc. by the effect of boiler compounds. Most of these are easily suspensible in water and are discharged by blow down from the boiler. In order to prevent the accumulation of sludge on the bottom of drum and to prevent scaling on the heating surface by hydroxyapatite, etc., sludge dispersants are frequently employed. Synthetic polymer e.g. acrylic acid polymer is generally used as Sludge dispersant.

Oxygen scavenging chemicals
Although oxygen can be reduced to less than seven parts per billion in modern deaerators, it is still necessary to reduce oxygen even further. This is accomplished by chemical scavenging in which either sodium sulfite (Na2SO3), hydrazine ( N2H4), hydroxylamine derivatives, hydroquinone/progallol-based derivatives is fed into the boiler feed water.

Sodium sulfite
Sodium sulfite is the most common scavenger in industry. The reaction involved is the following,

Na2SO3: +  O2 →  Na2SO4

Such that oxygen is removed to produce soluble sodium sulfite.  Sulfite is fed at about 40 parts to 1 part oxygen.

Hydrazine
The alternative to sulfite treatment is hydrazine, which has traditionally been used on higher pressure boilers. One reaction by which hydrazine removes oxygen is a two step process.

4Fe3O4 + O2 → 6Fe2O3

6Fe2O3 + N2H4 → 4Fe3O4 + 2H2O + N2

One interesting feature of these reactions is that in the first reaction the oxygen breaks down the protective oxide coating to form rust. In the second reaction, the hydrazine reduces the rust to the desired passive form. Only three parts 35 percent hydrazine solution is required per part of oxygen.
In using either hydrazine or sulfite, it is important to apply the following rules,

  • Feed the scavenger at earliest practical point
  • Feed continuously
  • Maintain residual sulfite or hydrazine according to levels recommended below

After the scavenging process of oxygen removal, the dissolved oxygen should be less than ten parts per billion.

Corrosion inhibitors
Volatile amines and filming amines are typically used as corrosion inhibitors to prevent the steam and condensate lines from corrosion. Volatile amines inhibit the corrosion by controlling the condensate pH. Filming amines are applied to form a water repellent film on metal surfaces which prevents direct contact with the corrosive substances such as oxygen and carbon di-oxide. Volatile amines eg., ammonia (NH3), cyclohexyl amine (C6H11NH2) etc., and filming amines eg., alkyl amines (RNH2) ( R=C10-C22) are used as corrosion inhibitors.

Antifoaming agents
Antifoaming agents are chemicals used to control foaming on the boiler water surface, caused by an increase of the dissolved or suspended solids or by contamination of oil and fat or organic matter in the boiler water. Chemicals of amide based, alcohol based or fatty acid ester based chemicals are used as antifoaming agents. These chemicals change the surface tension of the water.

Water quality control method for boiler feed water system

1) Water quality control target value
The boiler water quality control target value is set on the basis of standard criteria and considerations of the structure and operation conditions of a boiler. The following standard values for different parameters of boiler water should be maintained for smooth operation of boiler.

a) Control items for feed water

  • pH
  • Total hardness
  • Dissolved oxygen
  • Total iron and total copper
  • Conductivity
  • Silica
  • Hydrazine

b) Control items for boiler water

  • pH
  • Alkalinity
  • Total evaporation residue
  • Electrical conductivity
  • Chloride ion
  • Phosphate ion
  • Sulfite ion or hydrazine
  • Silica

2) Boiler water treatment methods

  • Caustic treatment
  • Coordinated phosphate treatment
  • volatile treatment

3) Concentration control of boiler water
Blow down is done to control the concentration of boiler water and to discharge sludge, impurities contained in the feed water and solids in the chemicals added.

4) Control of chemical injection
The purpose of controlling the chemical injection includes the adjustment of the water quality of the boiler system to the specified values by sufficient effect of the functions of chemicals chosen for an individual boiler. Some chemicals do not exert their total effect and in addition, may cause secondary problems depending on the excess or shortage of dosage and the point or method of injection. The control of chemical injection involves the following

  • Dissolution and dilution
  • Determination of dosage-Initial dosage, maintenance dosage
  • Injection point

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