Need of Boiler Feed water Treatment | Boiler water treatment process

Boiler feed water specifications | Specifications of Boiler water

Boiler water is the term used specifically to refer to water which is heated inside the actual boiler . Boiler water is provided by the boiler feed water from the boiler as either heated water or steam.

Boiler feed water: The water  which directly enters the Boiler. Boiler feed water is made of Makeup  water  and  Return Condensate.

Makeup Water: Make water  is the original raw water derived from  well or  any other source and purified to the extent required.

Return condensate: Return Condensate is water which passes through boiler , gets converted to steam and after serving some purpose condensed back to liquid water and returned to feed water tank.

A Boiler plant is a critical part of an Industrial concern. In any process Industries, the steam generators are act as vital role.  The purity of boiler water and steam is one of the most important criteria for ensuring component availability an reliability in power generating systems.

Carry Over: Moisture , solids , silica and other associated solids passing from a boiler with steam is called carryover

Raw Water :

From the chemical analysis of raw water  we get all the information about quality of raw water. It may vary source to source.

Following are the different types of impurities which are generally present in any types of water. The types and percentage of impurities vary according to source of the water like river, lake, bore well etc.

Raw water Impurities and its impact on boiler:

1.Total Alkalinity:

In raw water, total alkalinity primarily covers bicarbonate. A small amount of carbonate might also be present. Hydroxide ions do not exist in raw water. Total alkalinity in raw water is important since it represents a potential source of scale.

2. Bicarbonate – Carbonate

The primary source of alkalinity in raw water is due to bicarbonate ions. On Heating bicarbonate ions decomposes to Carbonates and Carbon di oxide. If a soluble salt results, further heating will decompose the carbonate to caustic (OH) and carbon di oxide

2 HCO₃ → CO₃ + H₂O  + CO₂

CO₃ + H2O → 2 OH  + CO₂

Caustic in boiler water may or may not be desirable depending  upon the  operating pressure . Carbon di Oxide is a primary source of condensate line corrosion.

3. Calcium:

In majority of raw water calcium is the main source of hardness. In boiler and cooling water systems calcium salts are the main cause of scale.

4. Chloride:

Scale is not a problem where chloride salts are concerned, but they are very corrosive in oxidizing environment.  Chlorides are prominent in crevice corrosion and pitting. In all cases of designs or operation, steps should be taken to prevent the concentration of chlorides.

5. Iron:

Iron is normally found in soluble ferrous form.  On contact with air or oxidizing agents, iron is converted to ferric form and forms insoluble hydroxides or oxides. If iron is more than 0.3 ppm or greater, it should be reduced or controlled.

6. Magnesium:

Magnesium usually forms hydroxides or silicates in boiler water, which are desirable types of sludge. If boiler water alkalinity is low, undesirable magnesium phosphate often forms which is sticky and scale forming. In cooling water system, magnesium only precipitates when pH is over 10.0.

7. pH:

The raw water pH will be in the range of 5.5 to 8.0. Low pH can result in corrosion and high pH can result in scale formation.

8. Silica:

It is usually found in ground water and silica content  of 70-80 ppm is not unusual. In high  concentration, silica can cause scale problems in cooling water systems, as well as in  boilers.

Silica control is important especially in high-pressure boilers, since silica can vaporize and deposit in low-pressure areas of turbine, as well as, super-heaters.

9. Sulphate:

Most raw waters contain sulphates . Presence of sulphates aggravates  corrosion and  sulphates are found in scales  usually as calcium sulphate.

10. Total Dissolved Solids (TDS) :

It is a general indication of  corrosiveness of   raw  water . The conductivity test, though not accurate is a convenient method to check the TDS . Any change in raw water composition is reflected by the change in conductivity.

11. Hardness:

Hardness of water is expressed in parts of calcium carbonate per million parts of water. It also expressed in grains of CaCo3 per gallon of water 14.28 parts of CaCo3 is equals to one degree of hardness.

There are two types hardness in water.

  Temporary Hardness :

Temporary Hardness causes carbonate hardness which is soft and porous.

  Permanent Hardness :

Permanent Hardness in water cause non carbonate hardness which is hard in nature.

Why treatment required for boiler feed water:

Water  is the essential medium for steam generation . Conditioning it properly can increase the efficiency of boiler and as well as extend the boiler’s life. Treating boiler water also ensure safe and reliable operation. Without proper treatment boiler itself can be destroyed .

Boiler water  problem generally falls into two classes, deposit related and corrosion related . There are many instances  where deposit causes corrosion and corrosion causes deposit.

Boiler feed water has to be treated

a) To prevent corrosion in the boiler and steam system.

b) To prevent the formation of scales and deposits on heating surface.

Maintain a high level of steam purity.

The maintenance of steam purity is also important. Certain impurities can be carried over or dissolved in steam and can be deposited in the super heater or on the turbine blades. Gaseous impurities are objectionable where the steam is condensed after use or is used in heating system. Due to these conditions foaming and priming will happen.

Boiler feed water specifications:

Boiler feed water properties depended upon boiler operating pressure. According to boiler operating pressure the specifications of feed water as follow as

Problems in boiler due to low quality of Boiler water :

a) Foaming  :  Foaming is responsible for the majority of carry over problems in boilers and is usually the result of too high a concentration of solids in the boiler water.

b) Corrosion: The most common example being the attack of steel by oxygen. This occurs in water supply systems, pre-boiler systems, boilers, condensate return lines. Oxygen attack is accelerated by high temperature and by low pH.        4H2O + 3Fe →      Fe3O4 + 4H2.

c) Due to scale formation in a boiler, heat transfer is retarded and plates and tubes get over heated , bulge and rupture.

d) Due to electro chemical action , presence of oxygen in boiler water pitting and corrosion is caused.

e) CO2 in boiler water forms ferrous carbonate and converts it to ferric hydroxide and later to ferric oxide called rust.

f) Due to hydrolysis , unstable salts of Ca and Mg , mineral acids are formed and tubes get overheated.

g) Due to untreated water pitting and thinning of tube walls or plates is caused from water side.

h) Dissolved oxygen causes corrosion of surfaces from water side excess concentration of hydrogen or hydroxyl ions corresponding to decrease or increase in pH respectively.

i) pH : Corrosive action of water is minimum which the pH is distinctly on the alkaline side. i.e. 8 and above at pH 9.0 protective film of ferrous hydroxide is formed. Some times condensates owe their alkalinity to dissolved NH3 release NH3 with drop in pH.

Boiler Water Treatment Process:

A treatment for boiler feed water may be divided into two broad classifications – external and internal treatment.

External Treatment:

a) Raw water  is treated before it is fed into the boiler. This is known as External treatment. The goal  is to improve the quality of water before it is used as boiler feed water.

b) Technically , treatment methods are aimed at reducing alkalinity , hardness , silica , iron, manganese, turbidity , bacteria , organic matter and others . In boiler feed water treatment helps in preventing  corrosion , scaling , overheating.

c) One of the critical parameter is Hardness , Which is one of the major factor for scaling. Hardness is removed by Ion exchange process  or other external treatment like lime soda or Reverse Osmosis (RO).

Internal Treatment or  Chemical Treatment:

a) Beside external treatment , several water treatment programs (internal treatment ) have been developed to deal with problems of scale formation and boiler corrosion . The factor that must be considered in choosing  the program

c) In case of internal treatment chemicals are introduced into the boiler water and the precipitated salts are removed in Blow down.

For pH  :

To increase the pH of water, Caustic soda flakes and Tri Sodium Phosphate are used in solution form.

Na3Po4 + H2O  →      3Na + OH^– + HPo4^—

Na2Hpo4 →     2Na⁺+HPo4^—

NaH2Po4   →   Na⁺ + H + HPo4^—

4NaOH + Fe3O4   →         NaFeO2 + 2H2O

Fe + 2NaOH  →       Na2FeO2 + H2

For Hardness : 

To minimize the hardness , Sodium Hexa Meta Phosphate is used.

For Phosphate :

To maintain the phosphate level , Sodium Hexa Meta Phosphate or Sodium Phosphate are used.

For Dissolves Oxygen:

1. To minimize the dissolved Oxygen level; Hydrazine, Sodium Sulphite are used.
2. By increasing the feed water temperature 100° to 110°C, Dissolved Oxygen can be minimized.

2Na2So3 + O2   →     2Na2So4

N2H2 + O2    →      N2 + 2H2O

Caustic Alkalinity :

To maintain the caustic alkalinity,  caustic soda and Tri sodium phosphate are used in solution form.

2HCO3  →   CO2 + CO3 + H2O

CO3 + H2O →   CO2 + 2OH

CO2 + H2O  →   H2CO3 ( Carbonic Acid )

Fe + H2CO3 →    Fe ( HCO3 )2 + H2 ( Ferrous Bicarbonate)

Control of corrosion :

a) Maintenance of pH of water in the desired range is required to control corrosion.

b) Oxygen in feed water should be controlled with in 0.007 ppm, priming by good aeration, supplemented by Hydrazine dosing.

c) D.M water storage tank should be properly sealed.

d) Condensers and Heaters should be air tight.

e) Proper wet lay up of units during shutdowns.

Blow Down:

The object of blowing down a boiler is to maintain the concentration of dissolved and suspended solids within the specified limits to avoid priming and carryover.

Continuous Blow Down.

Intermediate Blow Down.

Continuous Blow Down:

This is installed in order to regulate and maintain the boiler water concentration at a constant level. The percentage of blow down should be controlled to a minimum necessary to achieve this.

Intermediate Blow Down:

This is resorted to only when the water analysis shows it to be necessary or when an immediate change high boiler T.D.S is required. And then only on the instructions of responsible person.

Precautions During Blow Down:

a) Steam drum water level must be closely monitored during any blowing down operation and maintained as nearly as possible normal.

b) When opening blow down or drain valves to admit steam or hot water into relatively cold plot lines, open slowly so as to avoid sudden shocks or water hammering which may cause serious damage. 

Boiler feed water specifications

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