Power plant: Water sources & impurities present in it.

Water is basically the combination of 2 parts of Hydrogen & 1 part of Oxygen. It is very impossible to get pure water in the nature . Pure water does not exist in nature due to its characteristic as solvent. Number of matters like gases, minerals and organic materials dissolve in the water easily.

Water mixes with  fine particles wherever it flows such as silt, sand, iron,mud & organics etc. Biological growths like algae & bacteria take place in the water. Thus, water is usually contaminated with dissolved & un-dissolved solids along with living matters!

Raw Water sources in nature:

  • Rain water:
  • Surface water:Rivers, Streams, Ponds, Lakes, and Reservoirs
  • Ground water :Springs, Shallow Wells and Deep Wells

For Boilers, heat exchangers ,domestic & other industrial purpose, we cannot use these above water directly from their sources.Before using it, need to carryout some treatment to get required quality for particular application. water being universal solvent has lot of impurities int it.

Rain water is said to be very purest form of water, but after falling on ground it mixes with river, lake,seal water & soil, which makes it impure & unsuitable for direct use.

Effects of impure or raw water :

  • Scales in Boilers and Heat exchangers.
  • Poor quality boiler steam.
  • Corrosion of boilers, heaters and piping.
  • Stains, discoloration, spots.
Impurities present in water:

  • Dissolved impurities also called as Ionic impurities
  • Undissolved impurities also called Non-ionic impurities
  • And Gaseous impurities

1-Dissolved or Ionic impurities:

These are again divided into Cationic & Anionic

Cat ionic dissolved impurities

  • Magnesium
  • Calcium
  • Sodium
  • Potassium
  • Iron
  • Mangnese

Calcium and magnesium are the most common dissolved impurities found in water. When calcium and magnesium salts are present in considerable amount it imparts hardness to water i.e. it does not allow lather to form with soap. 

The salts of calcium and magnesium causes temporary (Carbonate) & permanent hardness (Non carbonate).

Sodium & Potassium are always present in any kind of water, but they will not harm much if the percentage of concentration of these salts are less.

Iron present in the water can cause corrosion & clogging. Iron can be in dissolved & precipitated form.

An ionic dissolved impurities

  • Chlorides
  • Sulphates
  • Nitrates
  • Phosphates
  • Silica.
  • Bi carbonates
  • Carbonates
  • Hydroxide
  • Fluorides

Chlorides: can be found  in all kind of water. The chloride  may be in combination with one or more cations, calcium, magnesium, iron and sodium. Chlorides of these salts are present in water because of their high solubility in water. 

Excessive chloride in water causes corrosion. It also impart taste to water. Chlorides are prominent in crevice corrosion and pitting.

Sulphates: Most raw water contains sulphates due to leaching and erosion of sulphate minerals and oxidation of sulphides. They can also be due to industrial waste discharge and farm drainage.

 Sulphate also occur due to aerobic oxidation of organic matter. Sulphate in water causes corrosion and scaling in boiler. Sulphate are normally found as calcium, magnesium and sodium salt. Sulphate are also aggressive to concretes.

Silica:Most natural water contain silica up to 100  ppm. Silica is an oxide of silicon which is a major constituents of igneous and meta morphic rocks, of clay minerals.Silica can exist in various form as simple silicates or as a complex polymeric material. Colloidal silica is rarely present in Bore well water but is commonly present in surface water. 

Silica can cause deposits on boiler, cooling tower, turbines ,pipe lines etc

2-Undissolved non ionic impurities:

These include
  • Colour
  • Taste and odour
  • Turbidity or suspended solids.
  • Oil.
  • Organic matter
  • Colloidal silica

Colour: is normally expressed in Hazen units and is caused by the presence of colloidal suspension and aquatic growth. It is also caused by dissolved organic substance due to decomposition of vegetation. Some water may have colour due to presence of iron.

Turbidity is caused by the presence of suspended solids in water. It is a measure of total weight of dry solids present whereas turbidity is an optical effect. The suspended impurities include clay, sand, algae and precipitated iron.

Taste and odor: Organics and some inorganic chemicals present in the water contribute taste and odour. These chemicals can originate from municipal or industrial waste or from natural resources such as decomposition of vegetable matter.

Organic impurities: Organic impurities can be due to vegetable decomposition. It is also due to organic matter which comes from animal and human fecal matter or it’s degradation products, industrial waste and agriculture pesticides and herbicides.

3-Gaseous impurities:
 Gaseous impurities mainly include
  • Carbon di-oxide,
  •  Oxygen,
  • Hydrogen sulphide,
  • Ammonia
  • Chlorine.
Carbon di-oxide: Surface water contains small amount of free CO2 but well water may contain more than 100 ppm as Carbon-di-oxide is found in waters coming in contact with decaying organic matter or carbonaceous material.

Depending on the pH of water carbon di- oxide may be present either as free carbonic acid (dissolved CO2 gas) or in a semi carbide form (as bicarbonates) or in a combined form (as carbonates), free CO2 depresses the pH and thus accelerates corrosion.

Oxygen: is practically absent in natural water, but is usually present in surface waters rather in high concentration. Oxygen is corrosive to metals but its  absence in natural waters could produce other obnoxious gases such as methane H2S etc.Dissolved oxygen is highly corrosive and should be removed either chemically or mechanically especially in Boiler feed water.

Hydrogen sulphide: is found in some well waters in areas where the soil contains certain types of organic matter, decomposing under anaerobic conditions .H2S creates unpleasant odour to water, promoters metal corrosion and  causes  clogging of pipes. 

Ammonia: is  due to industrial and agricultural pollution. It is corrosive to copper and brass at pH 9.0. Ammonia can be removed by various methods-Deaeration, chlorination or by hydrogen cation exchange if in ionic form.

In order to safeguard the Boiler, heat exchangers & pipe lines, it is utmost important to carryout the water treatment process to get required quality of water as feed water,cooling water etc.

Water treatment is divided into
Internal treatment: This treatment is done in Boiler drums, cooling water etc

External water treatment : It is carried out out side the unit (Boiler, Cooling water)

External water treatment process includes following sequential  sub processes & systems

  • Sedimentation: Normally happens in water reservoirs
  • Clarification: Removes turbidity , kills microorganisms
  • Filtration: Involves multi grade filters to remove turbidity
  • Ultra-filtration: Process of bringing down the water turbidity to <0.2 NTU
  • Reverse osmosis process: Removes reactive silica & conductivity
  • De-mineralisation: Removes Cation & Anion impurities present in water

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