WATER AND WASTE WATER TREATMENT PLANTS
INTRODUCTION
The natural water contains solid, liquid and gaseous impurities and therefore, this water cannot be used for the generation of steam in the boilers. The impurities present in the boilers. The impurities present in the water should removed before it’s in steam generation. The necessity for reducing the corrosive nature & quantity of dissolved and suspended solids in feed water has become increasingly important with the advent of high pressure, critical & supercritical boilers.
IMPURITIES IN WATER:
The impurities present in feed water are classified as give below:
- Undissolved and suspended solid materials.
- Dissolved salt & Minerals.
- Dissolved gases.
- Other materials (as oils acid) either in mixed or unmixed forms.
1.Undissolved and suspended solids materials
- Turbidity and sediment
Turbidity in the water is suspended insoluble matter including coarse particles (mud, sediment sand etc.) that settle rapidly on standing. Amounts range from almost zero in most ground waters and 25,000 ppm in muddy and turbulent river water. The turbidity of feed water should not exceed 5 ppm. These materials filtration. Their presence is undesirable because heating or evaporation produces hard stony scale deposits on the heating surface & clog fluid system. Both are objectionable as they cause damage to the boiler system.
2. Sodium and potassium Salts
These are extremely soluble in water and do not deposit unless highly concentrated. Their presence is trouble some as they are alkaline in nature and accelerate the corrosion.
3.Chlorides
Majority of the chlorides cause increased corrosive action of water.
4.Iron
Most common soluble Iron in water is Ferrous Bicarbonate. The water containing Ferrous Bicarbonate deposits becomes yellowish and reddish sediment of Ferric Hydroxide if exposed to air. Majority of ground surface water contains less than 5 ppm but even 0.3 ppm can create trouble in the feed water system by soft scale formation and accelerating the corrosion.
2. Dissolved Salts and Minerals
Calcium and Magnesium Salts present in the water in the form of Carbonates, Bicarbonates, and Sulphates and Chlorides, the presence of these Salts is recognized by the hardness of the water (hardness of water is tested by soap test). The hardness of water is classified as temporary and permanent hardness. The temporary hardness is caused by the Bicarbonates of Calcium and Magnesium and can be removed by simple boiling. The boiling coverts the soluble Bicarbonates into less soluble Carbonates which can be removed by simple blow down method. The permanent hardness of the water is caused by the presence of Chlorides, Sulphates and Nitrates of Calcium an Magnesium and they cannot be on heating surfaces
3. Dissolved Gasses
- Oxygen
It presents in surface water in dissolved form with veriable percentage depending upon the water temperature and other solid contents in water. Its presence is highly objectionable as it is corrosive to Iron, Zinc, Brass and other Metals. It causes corrosion and pitting of Waters Lines, Boiler Exchangers. Its effect is further accelerated at high temperatures.
2.Carbon Dioxide
The river water contains 50 ppm and well water contain 2 to 50 ppm of CO2. It also causes the corrosion of stream, water and condensate lines. It also helps to accelerate the corrosive action of Oxygen. The gases are H2S, CH4, N2 and many others but their percentage are negligible, therefore, their effects are not discussed here.
4. Other Materials
- Free Mineral Acid
Usually present as sulphuric or Hydrochloric Acid and causes corrosion. The presence is reduced by neutralization with Alkalies.
2.Oil
Generally, the Lubricating Oil is carried with steam into the condenser and through the feed system to the Boiler. It causes sludge, scale and foaming in Boilers. It is generally removed by strainers an baffle separators. The effects of all the impurities present in the water are the scale formation on the different part of the Boiler system corrosion. The scale formation reduces the heat transfer rates and clog the flow passage and endanger the life of the equipment’s by increasing the temperature above safe limit. The corrosion phenomenon reduces the life of the plant rapidly, therefore, it is absolutely necessary to reduces the impurities below a safe limit for the proper working of the power plant.
BASIC WATER CHEMISTRY
RAW WATER & IMPURITIES
Water as it occurs in nature “pure'”, and whatever be the source, always contains impurities either in solution or in suspension. The determination of these impurities makes analysis of water necessary and removal and control of these impurities make water treatment essential.
Some of the most common Cations in water are: Calcium, Magnesium, Sodium and Iron rarely Ammonium Magnesia. These cations are associated with Anions like bicarbonates, carbonates, Hydroxides (The sum of which is termed as Alkalinity), sulphates and chlorides presence of Nitrates and Phosphates is normally not very common. In the water treatment field, the preferred method of expression of these dissolved impurities is in terms of equivalent calcium carbonate is a good “as CaCO3”. This is because calcium carbonate is a good common denominator as it a molecular weight of 100, which facilitates calculations. Moreover, in this form of analysis, the sum of cations or total cation always equals the total Anions. Quantitatively, these are expressed in parts per million or milligrams/litters. One part per million equals one thousand of one percent (0.0001%). One part per million means one part in a million parts, one 1b. in a million 1 bs. of water. Of all the dissolved impurities, hardness is perhaps the most troublesome. Hardness as due to compounds of calcium and magnesium. One heating water containing these salts, Carbon Dioxide is released from solution and the Bicarbonates are converted into carbonates which are insoluble and from scales and deposits. Other salts of Calcium & Magnesium like Sulphates and Chlorides have lower solubility than Sodium salts Bicarbonates or temporary hardness and Chlorides, Sulphates, Nitrates etc., of calcium and Magnesium are known as neutral or permanent hardness.
If you have any questions let us know
Do you have any questions? Write and our specialists will answer you.