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It is a tasteless, odourless colourless liquid in its pure state. It is the only inorganic material existing in three forms a. ICE b. WATER c. STEAM (Vapour) These three things occurs within the earth's natural temperature range. Water can be converted into steam at a convenient temperature. Because of this nature, the water is ideal for process and for generating power. General : Generally speaking 100% pure water is not existing. All natural water contains impurities like dissolved solids, dissolved gases and suspended matters. Water is a universal solvent. It dissolves the rocks and soil it touches. It dissolves gases from atmosphere. It picks up suspended matters from earth. Above all, it gets contaminated by industrial wastes life dyes, oil and process materials. To be more precise the types of impurities the water contains on what it contacts and amount of impurities depends on time it contacts (retention time).
Type of Water : Fresh water supply can be classified into two major catergories by the source of water viz-a-viz
Source of Water
Generally speaking, ground water like borewell or well water will be a continous flow of water and contains more dissolved solids like irons, silica and other minerals. Whereas in surface water like river water and others the total dissolved solids will be less which results in total hardness. However continous addition of industrial wastes like dyes, processed chemicals Et.al in river water contaminate the same. In bore well continous consumption of water makes the situation still worse. Sea Water : The sea water is the another source of water, but it contains about 36gms of minerals (almost solids) in the litre. The minerals contents in fresh water is far les than that of sea water. some experiments is being carried out to convert the sea water into ordinary water (for industrial use) , however, the treatment cost per litre is so high that it cannot be used for any industrial purposes.
River Water : Usually the river water will be containing less hardness than that of ground water. for instance, the bore well water total hardness will be around 700 to 800 Mg/l, whereas the same can be around 300Mg/l in river water, However, the river water will contain more suspended solids which will affect the costly equipment if the water is used directly.
Minerals : The water picks up minerals from rocks consists chiefly of:
|Sl. No.||Chemical Name||Common Name|
|1.||Calcium Carbonate||CaCo3 Lime-stone|
|2.||Magnesium Carbonate||MgCo3 Dolomite|
|3.||Calcium Sulphate||CaSo4 Gypsum|
|4.||Magnesium Sulphate||MgSo4 Epsum Salt|
|6.||Sodium Chloride||Nacl Common Salt|
|7.||Hydrated Sodium Sulphate (Na2SO4 . 2H20)||Glauber Salt|
The above minerals are contributing to the major part of the total dissolved solids (TDS) over and above, small quantites of iron, manganese, chlorides and other minerals, wastes from industries like dyes, processed chemical contaminates the ground water.
Purity of Feed water : The feed water purity is a matter of both amount of impurities and nature of impurities. for instance the impurities like hardness, iron and silica are more concern than sodium salts. The purity requirement for any feed water depends on how much steam is produced. Hence the boiler design and the feed water parameters plays a vital role in selecting the treatment. The low pressure boilers can tolerate slightly higher feed water hardness. However, Proper internal treatement by chemical should be supplemented.
1. Removal of suspended matters
2. Removal of Dissolved solids for prevention of scales
3. Conditioning to prevent precipitation
4. Elimination of certain gasses to prevent corrosion
5. Prevention of foaming and carry over
6. Prevention of caustic embrittlement
7. Removal of oil and grease
The boiler feed water impurities can be eliminated by the mixture of both external and internal treatment. The treatments are classified as follows:
1. External Internal
2. Precipitation Chemical
5. Reverse Osmosis
Precipition Method: It is a functional way of treating water externally. Even though it is not being used now a days. Let us briefly discuss about this method.
Exchange Method: This is the method of softening water by zeolite. The name is generally applied to the process whereby sodium is substituted by another metal by exchange material not necessarily of zeolite class. In the process, calcium and salt are retained by the resin and equivalent amount of sodium salt passed out with water leaving the resin. Once the exchange capacity is exhausted regeneration is carried out with strong salt solution.
Cation Exchange: It is exchanged for one cation for another exchange of calcium magnesium, sodium, for hydrogen by means of exchange material (Hydrogen resin). the exhausted exchange material is regenerated by passing the solution of mineral acid through the resin (usually HCL).
Anion Exchange: In the anion exchange process, the sulphate chloride, nitrate ions Et.al. are exchanged by hydrogen ions. The exchange is carried out by passing the water through the bed of synthetic resin.
Strong Base Anion Exchange: Strong base resin are totally ionised through the complete pH range. Removed silica and carbonic acid in sodium slit form the cation resin is converted to sodium- bydroxide in the resin. The process is given below:
NaCl + ROH -----> NaOH + ROI
Distallation Method: Raw water is distilled in evaporator to procude distillate. It is almost completely free from dissolved solids. This method is not being used for low and medium pressure boilers. There are other methods like Deaeration, reverse osmosis and activated carbon filters. Deaeration: It is used to remove the dissolved oxygen and carbondioxide.In this method the steam deaerator breaks up water into spray and then pass the steam across and throw it to force out distilled gases like oxygen and carbondioxide. The oxygen content can be reduced to 0.5 Mg/L. Again this process not required for low and medium pressure boilers.
Reverse Osmosis: It is the latest and sophisticated method of treating the watter. In this process a synthetic semi-permeable membrance is used to remove the minerals to the maximum extent. Water is treated in this process the outlet will be of very good quality and total dissolved solids (TDS) of around 100-110 Mg/L. This method is so costly that is not being used for boilers and the another major problem in this is, disposal of effluent water which comes out of this process.
Activated Carbon Filter: This filter is being used ro remove chlorine, colour and odour. when the raw water directly fed into any costly equipments like softener, D.M. Plant and R.O. Plant suspended matters in the water which enters along with the water into the system may affect the costly material like resin and R.O. Membrane. To avoid this problem A.C. Filter with pressure Sand Filter (PSF) is being used before the water fed into the system.
Internal treatment of water is the method in which chemical is being used to treat the feed water inside the boiler and is essential where or not the water is pre-treated by external treatment, regardless of the quantity of feed water. In some cases, external treatment of water is not necessary and the water can be treated by internal method alone. Internal treatment can constitute slow treatment when the boiler operates at low pressure. Large amount of condensate return and the raw water is of good quality, then the internal treatment alone will be sufficient. However, medium and high pressure boilers requires external treatment of make-up water along with the internal treatment as even a small amount of oxides can cause very high fuel consumption or may reduce the life of the equipment.
Internal Treatment Programme: Purpose of internal treatment programme can be classified as follows: React with incoming feed water hardness to prevent it from precipitation on the boiler metal as a scale.
Conditioning of any matter such as hardness, sludge in the boiler and make it non-sticky to the boiler metal.
Provide anti-foam protection to the reasonable concentration of cycle and to avoid carry-over.
Schavenge the oxygen from feed water and to provide enough alkalinity to prevent boiler from corrosion.
In addition to these, the programme is designed in such a way that it also protects the steam condensate system from corrosion.
Chemical used in Internal Treatment: Today all th eboilers use varieties of internal treatment chemicals. Phosphate has been the main chemical until the polymers came into being. But for the low and medium pressure boilers still we continue to use phosphate for maintaining scale from metal surfaces.In internal treatment where polymer condition the calcium and magnesium in the feed water. Polymer forms soluble complexes with the hardness, where as phosphate precipate the hardness. Sludge conditioners are also used to aod in conditioning the precipated hardness, while selecting these process care should be taken so that they are both effective and stable at the operating pressures. Certain synthetic and organic material are used as anti-foam agent. In some cases to schavenge the oxygen in the feed water separate chemicals are used as Hydrazine, DEWBORN-102-A Et.al. Condenste system protection can be accomplished by using filming amines such as DEWBORN 181.
Internal Treatment for Scale Prevention
Compounds in Raw Water Treating Chemicals Sludge Formed
Calcium Bicarbonate DEWBORN - 101 A ( Carbonate & Phosphate )
- - Tricalcium Phosphate
Calcium Sulphate - Calcium Carbonate
- - Tricalcium Phosphate
Magnesium Sulphate - Magnesium Hydroxide
Internal Treatment with Sulphate: When calcium and magnesium sulphates enter through the boiler feed water into the boiler it becomes insoluble due to boiler temp. When with the phosphate and alkalinity producing hydroxipatite which is less sticky, more readily conditioned product than Calcium Phosphates. Magnecium also gets removed similar to calcium sulphate and both these sludges are getting removed through blow down.
Sludge Conditioning in Internal Treatment: The suspended solids carried along with feed water into the boiler affect both the boiler cleanliness and steam purity. The suspended solids for varying tendencies to deposit hot metals. The suldge conditioners play a vital role in preventing these sludges from depositing and form insulating boiler scale. The role of sludge conditioners is to keep boiler water suspended solids from depositing and form scale from the boiler metal. DEWPHOS 102, DEWTREAT 101 chemicals have been incorporated with sludge conditioners to keep the sludges in the conditioning stage so as to remove through blow-down easily.
1) Scale or deposits form in improperly treated boiler water reducing the effeciency of the boiler and increasing fuel consumption considerably.
2) CORROSION: It is due to dissolved oxygen and will cause tube failure and unplanned boiler shut-down.
3) CARRY OVER: It is a sludge or colour along with the steam which can contaminate the process so for example, the carry over of colour may spoil the knitted cloth in the vinches. The treatment of boiler feed requires the planned programme that involves proper selection of treatment method of application and control limits. It also requires continuous maintainance of best feedwater quality, chemical control and boiler water concentrations. The feed water blow-down parameters must be consistently monitored.
Foaming: Foaming in boiler caused by small stable bubbles and the steam entering in the boiler water surfaces. The water fall around each bubble is more stable by increase in suspended or dissolved solids in boiler water may also be caused by oil detergent or organic matter.
Caustic Embrittlement: Caustic embrittlement is cracking on boiler steel resulting from stresses metal by concentrate of caustic . As it is, there is no simple test for detecting embrittlement. A special device has to be used for detecting casutic embrittlement, but usually by detecting and maintaining caustic alkalinity and boiler blow down water, caustic embrittlement can be avoided. Basically, caustic embrittlement in boiler designing have reduced the caustic embrittlement problem considerably.
Internal Treatment for Caustic Embrittlement: Practical experience has shown that sludge conditioner such as used in DEWBORN - 101 A or DEWPHOS 102 applied to the boiler water are effective in preenting caustic cracking of boiler steel. This method os called co-ordinated phosphate - ph method in which all of the boiler alkalinity is analytically maintained in the form of phosphate rather than caustic soda.
Advantage of Internal Treatment: The prime advantage in the internal treatment is that it can eliminate the need for expensive external treatment equipment, this give a definite economic advantage. In addition, to this internal treatment reduces the requirement for man-power, as the external treatment equipment requires additional man-power. However, internal treatment with moderate external treatment can eliminate the problem to the maximum extent. A qualified technical personnel specialised in water treatment may be consulted for selecting the appropriate method of treatment according to the system.
Treatment Monitoring: Monitoring of chemical can be classified as follows :
Chemical Feeding: Chemical is being added into the boilers either nu chemical feed pumps or directly into the feed water line. In some cases, chemical is being added into the feed water tank as a sludge dosage.
Controlling of Chemical Dosage: Chemical dosage purely depends upon the amount of feed water and the quality of feed water. while selecting the dosage proper care has to be taken. for example, DEWBORN 101A & DEWPHOS 102 being suggested on the basis of feed-water total hardness whereas DEWBORN 182 catalysed hydrazine hydrate is being dosed on the basis of feed water has to be maintained by way of slightly extra dosage during te initial period, so that any change in the feed water parameters will be taken care bythe reserve of this chemical. Boiler Water Analysis : Routine control test of boiler feed water and blow-down water has to be carried out. So that, any change in the parameters can be adjusted by way of reducing or increasing the chemical dosage.
Parameters to be Tested: Water parameters testing can be varying according to the operating pressure of boilers usually for the low pressure boilers and medium pressure the total hardness, p' alkalinity, M' Alkalinity, Caustic alkalinity(2P-m) pH, phosphate, silica, chlorides, iron, total dissolved solids (TDS) has to be checked, in both the feed water and boiler blow-down water with regualr intervals so as to control the above said parameters as per ISI standards.
Chemical Requirement of Feed Water and Boiler Water for Low Pressure Boilers.
Serial No. Characteristic F.W. B.B.W.
1. Total Hardness as CaCo3 10 NIL
2. pH 8.5 to 9.5 11 to 12
3. Dissolved Oxygen 0.1 -
4. Silica as Sio2 upto 5 -
5. Total Alkalinity as CaCo3 - 700
6. Caustic alkalinity as CaCo3 - 350
7. Sodium sulphate as Na2SO3 - 30 - 50
8. Sulphate / Caustic Alkalinity - Ratio - 2.5 & Above
Units of Water Analysis: The most common unit is parts per million(Ppm) this can be better explained with the following example. 1ppm of salt means one milligram of salt in 1 litre of water for the high pressure boiler, still parts per billion is being used. But all parameters expressed as CaCo3 excepting few like Iron, silica, et.al. The units are expressed in CaCo3 because molecular weigh of calcium carbonate is 100.
1) Scaling and fouling
3) Microbiological growth
Scaling and Fouling Scaling is the precipitation of hard and adherent salts of relativity low soluble water constituents, like calcium and magnesium, on the metal surface. These scales have very poor thermal conductivity and their control is therefore absolutely essential for proper heat transfer efficiency. Some of the common scales are: 1. Calcium carbonate 2. Calcium sulphate 3. Silicate scales 4. Calcium orthophosphate 5. Magnesium salts 6. Iron salts
Calcium Carbonate This is the most commonly encountered scale in cooling water systems and it forms an extremely hard and adherent deposit.
Ca(HCO3)2 -----> CaCO3 + CO2 + H2O
Calcium bicarbonate is present in almost all cooling waters. Increasing temperature and pH decomposes the bicarbonate and CO2. Although calcium bicarbonate is moderately soluble in water, calcium carbonate is not. It precipitates at the hot spots of the heat exchangers and a dense adherent scale is often formed on the heat exchanger surfaces. Suspended matter may get entrapped into the CaCO3. Scale layer forming a deposit larger in volume, but lesser in density.
Calcium sulphate has much higher solubility than carbonate and this fact is used as the basis of scale control by acid free. The sulphate ions replace alkalinity enabling operation of the cooling system at higher cycles without exceeding carbonate solubility limits. Silicate Scales Silicate scales deposition must be prevented as they are extremely difficult to remove once formed. Silicate scaling can be prevented by: 1. Limiting the silica in the water to 160 mg/1 max. 2. (Mg mg/l as CaCO3) X (SiO2 mg/l as SiO2) should be kept below 35,000 Calcium orthophosphate Orthophosphate, in the cooling water, is formed by the reversion of polyphosphate, commonly used as an inhibitor. The orthophosphate readily combines with the calcium ions present in the water, forming the highly insoluble and trouble-some calcium orthophosphate sludge. This sludge has very poor thermal conductivity and can seriously affect heat transfer. The sludge also leads to under deposit corrosion and therefore its control and conditioning, if and when formed, is extremely important.
Magnesium salts have less scaling potential because a. They are generally more soluble than the calcium slats with the result that the late are precipitated first. b. Their concentration is usually considerably lower than that of calcium salts.
Factors that effect scaling
1. Temperature: The common scalants found in cooling water, CaCO3 and CaSO4, exhibit inverse solubility i.e. their solubility decreases with increasing temperature. Generally the amount of scale increases with increase in teperature.
2. pH or alkalinity: The solubility of CaCO3 decreases with increasing pH. Alkaline pH usually increases the scaling potential. However, some materials like silicate are more soluble in the alkaline range.
3. Solubility: For water borne deposits to form, the potential scaling material should be carried as a soluble constituent of the cooling water to some degree. Under the conditions, each potential scalant exhibits a definite solubility limit. Once this limit is exceeded, the solution gets supersaturated and precipitated forms leading to scaling. In addition, other dissolved solids also influence scale forming tendencies. In general, higher the level of scale forming dissolved solids, greater the chances of scale formation.
Factors that affect fouling
1. Water characteristics: Fouling will depend on the suspended impurities carried by the water. Surface waters have greater fouling tendency, as the amount of suspended matter picked up by them is greater.
2. Temperature: Fouling tendency increases with increasing temperature. Heat transfer surfaces which are hotter than the cooling water, accelerate fouling.
3. Velocity: Fouling is greater in areas of low velocity while it is less severe in areas of high velocity. Normal velocity is 1 to 2 metres/second.
4. Microbial growth: Micro-organisms can deposit on any surface. Certain bacteria like iron bacteria utilize corrosion products leading to voluminous deposits. Also, slim secreted by bacteria, acts as binders and entrap material which normally would not have deposited.
5. Macrofouling: Certain macro-organisms like corbicula clam and mussels cause severe fouling problems due to their ability to proliferate on tube sheet faces and within tubing. As small as 0.2 mm in size at the onset corbicula are transported in to the cooling water system from water source as larvae or small juveniles. They have the ability to self-fertilize, grow and reproduce even at temperatures above 65 Degree F. They not only cause condenser tube blockages, but the decomposition of this trapped biogrowth can also lead to corrosion of stainless steel and copper alloy tubing during plant outages. Macroscopic biofouling can cause as much as 3-4% loss of production in a 600 mw power station, (according to a recent survey of the Electric Power Research Institute, U.S.A)
6. Corrosion products: Insoluble corrosion product mixes with other foulants like debris, micro-organisms, etc. and aggrevates fouling. It also serves as a nutrient for iron bacteria, promoting their growth.
7. Oil: Oil often adheres to the metal surface and has the ability to bind deposits. Oil has very poor thermal conductivity and can seriously affect heat transfer. Oil serves as a nutrient for micro -organisms reflecting in increased microbial growth. Oil also, prevents film forming inhibitors from reaching and passivating the metal surfaces. The percent reduction in overall hest transfer coefficient due to different scalants at varying scale thickness is given in Table 1 to Table 4. the overall efficiency of a power plant depends upon the vacuum is the exhaust steam condenser. Condensers operating at high vacuum permit expansion of the steam to the lowest possible temperature. The overall clean heat transfer coefficients for the condensers in thermal power plants are usually high and are therefore very sensitive to the scale thickness. A deposit thickness of even a fraction of a millimeter would result in drastic reduction in the heat transfer efficiency and in turn in condensers vaccum. The increased heat rejection will ultimately lead to drop in power generation. In the auxiliary system where water is used for bearing cooling, deposit formation would result in excessive heating of the metal leading to its eventual failure. It is always prudent to prevent deposit formation by suitable treatment on condensers/heat exchangers rather than letting it form and then cleaning it mechanically or chemically. Deposit formation will not only result in revenue loss due to lowered generation rate but will also lead to shortened equipment life due to periodic cleaning. Cleaning can result in irreplaceable metal loss besides the fact that it can never return the equipment to its original state of cleanliness. This also means that the period between successive cleanings is progressively reduced. Maintaining deposit free surfaces therefore assumes great importance and suitable conditioning of the circulating water is an absolute necessity. The constant generation rate derived with proper conditioning more than pays for then additional cost of treatment besides keeping the equipment healthy.
1. pH: The corrosion potential increases as the pH is lowered.
2. Oxygen and other dissolved gases: Some of the other gases besides oxygen mentioned earlier that influence corrosion are CO2, NH3, H2S and Chlorine. The presence of these gases increases the corrosion potential of the circulating of water.
3. Dissolved and suspended solids. Normally higher solids content would mean increased corrosion potential due to the higher conductivity. However it is just the reverse because the hardness salts presents in the water forms a passive film on the metal, which inhibits corrosion. This is the reason why soft water is more corrosive that hard water. Suspended solids influence corrosion by erosive or abrasive action. They can also settle on metal surface producing localized corrosion cells.
4. Microbial growth: Microbial growth promotes the formation of corrosion cells and also the by products of some organisms are corrosive.
5. Velocity: In high velocity and turbulent waters, oxygen is rapidly distributed and passivation layer of corrosion inhibitors are often removed resulting in increased corrosion. High velocity can also lead to erosion of metal surface, protective films and oxides. At the same time low velocity can lead to deposition giving rise to localized corrosion cells.
6. Temperature: As the temperature increases the diffusion of oxygen to the metal surface also increases, promoting corrosion. Above 70 deg.cel. the loss of dissolved oxygen exceeds the amount made available in diffusion, and a decreased in corrosion rate occurs.
Algae: Air, water, and sunlight are the three basic requirement for algae growth. The distribution decks and side walls of a cooling tower fulfill all these requirements and there fore represents an excellent growth environment for algae.
Fungi: Fungi lack chlorophyll and are therefore non-photo synthetic, resulting in dependence on nutrients provided by organic matter. In cooling water systems fungi can used wood as a source of nutrients an can destroy the wood.
Bacteria: There are many kinds of bacteria found in cooling water systems. 1. Pseudomonas. 2. Sulphate reducing bacteria. 3. Iron bacteria. 4. Nitrifying Bacteria. Identifying these bacteria in cooling water there are many types of procedures available which include both micro biological testing and inorganic testing for further details contact our technical personnel.