Water Treatment, Storage And Blowdown For Steam Boilers
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Block 3 The Boiler House
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
Module 3.9 Water Treatment, Storage and Blowdown for Steam Boilers
The Steam and Condensate Loop
3.9.1
Block 3 The Boiler House
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
Water Treatment, Storage and Blowdown for Steam Boilers Before boiler blowdown can be discussed and understood it is necessary to establish a definition of water along with its impurities and associated terms such as hardness, pH etc. Water is the most important raw material on earth. It is essential to life, it is used for transportation, and it stores energy. It is also called the universal solvent. Pure water (H20) is tasteless, odourless, and colourless in its pure state; however, pure water is very uncommon. All natural waters contain various types and amounts of impurities. Good drinking water does not necessarily make good boiler feedwater. The minerals in drinking water are readily absorbed by the human body, and essential to our well being. Boilers, however, are less able to cope, and these same minerals will cause damage in a steam boiler if allowed to remain. Of the worlds water stock, 97% is found in the oceans, and a significant part of that is trapped in the polar glaciers - only 0.65% is available for domestic and industrial use. This small proportion would soon be consumed if it were not for the water cycle (see Figure 3.9.1). After evaporation, the water turns into clouds, which are partly condensed during their journey and then fall to earth as rain. However, it is wrong to assume that rainwater is pure; during its fall to earth it will pick up impurities such as carbonic acid, nitrogen and, in industrial areas, sulphur dioxide. Charged with these ingredients, the water percolates through the upper layers of the earth to the water table, or flows over the surface of the earth dissolving and collecting additional impurities. These impurities may form deposits on heat transfer surfaces that may: o
Cause metal corrosion.
o
Reduce heat transfer rates, leading to overheating and loss of mechanical strength.
Table 3.9.1 shows the technical and commonly used names of the impurities, their chemical symbols, and their effects. Atmospheric moisture Evaporation and transportation from surface water bodies, land surface and vegetation
Evaporation from oceans
Precipitation Consumptive use
Well Water table
Percolation
Streams flow to oceans
Total surface and ground water flow to oceans Ocean
Fresh ground water
3.9.2
Saline Interface ground water Fig. 3.9.1 Typical water cycle The Steam and Condensate Loop
Block 3 The Boiler House
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
Table 3.9.1 Impurities in water Name Calcium carbonate Calcium bicarbonate Calcium sulphate Calcium chloride Magnesium carbonate Magnesium sulphate Magnesium bicarbonate Sodium chloride Sodium carbonate Sodium bicarbonate Sodium hydroxide Sodium sulphate Silicon dioxide
Symbol CaCO3 Ca(HCO3)2 CaSO4 CaCI2 MgCO3 MgSO4 Mg(HCO3)2 NaCI Na2CO3 NaHCO3 NaOH Na2SO4 SiO2
Common name Chalk, limestone Gypsum, plaster of paris Magnesite Epsom salts Common salt Washing soda or soda Baking soda Caustic soda Glauber salts Silica
Effect Soft scale Soft scale + CO2 Hard scale Corrosion Soft scale Corrosion Scale, corrosion Electrolysis Alkalinity Priming, foaming Alkalinity, embrittlement Alkalinity Hard scale
Raw water quality and regional variations Water quality can vary tremendously from one region to another depending on the sources of water, local minerals (see Figure 3.9.2). Table 3.9.2 gives some typical figures for different areas in a relatively small country like the UK. Soft to moderately soft
Newcastle upon Tyne
Slightly hard to moderately hard Hard to very hard York Leeds Manchester Lincoln Norwich Birmingham
Cardiff Bristol
London Brighton Southampton Fig. 3.9.2 Regional variations in water quality
Table 3.9.2 Water variation within the UK - All impurities expressed in mg /l calcium carbonate equivalents Alkaline Non-alkaline Total Total Non-hardness Area hardness hardness dissolved hardness salts (temporary) (permanent) solids (TDS) Leeds 12 10 22 24 46 York 156 92 248 62 310 Birmingham 28 72 100 130 230 London 180 192 372 50 422
The Steam and Condensate Loop
3.9.3
Block 3 The Boiler House
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
The common impurities in raw water can be classified as follows: o
Dissolved solids - These are substances that will dissolve in water. The principal ones are the carbonates and sulphates of calcium and magnesium, which are scale -forming when heated. There are other dissolved solids, which are non-scale forming. In practice, any salts forming scale within the boiler should be chemically altered so that they produce suspended solids, or sludge rather than scale.
o
Suspended solids - These are substances that exist in water as suspended particles. They are usually mineral, or organic in origin. These substances are not generally a problem as they can be filtered out.
o
Dissolved gases - Oxygen and carbon dioxide can be readily dissolved by water. These gases are aggressive instigators of corrosion.
o
Scum forming substances - These are mineral impurities that foam or scum. One example is soda in the form of a carbonate, chloride, or sulphate.
The amount of impurities present is extremely small and they are usually expressed in any water analysis in the form of parts per million (ppm), by weight or alternatively in milligrams per litre (mg /l). The following sections within this Module describe the characteristics of water.
Hardness Water is referred to as being either hard or soft. Hard water contains scale-forming impurities while soft water contains little or none. The difference can easily be recognised by the effect of water on soap. Much more soap is required to make a lather with hard water than with soft water. Hardness is caused by the presence of the mineral salts of calcium and magnesium and it is these same minerals that encourage the formation of scale. There are two common classifications of hardness: o
Alkaline hardness (also known as temporary hardness) - Calcium and magnesium bicarbonates are responsible for alkaline hardness. The salts dissolve in water to form an alkaline solution. When heat is applied, they decompose to release carbon dioxide and soft scale or sludge. The term temporary hardness is sometimes used, because the hardness is removed by boiling. This effect can often be seen as scale on the inside of an electric kettle. See Figures 3.9.3 and 3.9.4 - the latter representing the situation within the boiler. Carbon dioxide combines with water to form carbonic acid: CO2 Carbon dioxide
H20 Water
H2C03 Carbonic acid
Limestone (calcium carbonate) is dissolved by carbonic acid to form calcium bicarbonate: H2C03 Carbonic acid
CaCO3 Calcium carbonate
Ca(HCO3)2 Calcium bicarbonate
Fig. 3.9.3 Alkaline or temporary hardness
3.9.4
The Steam and Condensate Loop
Block 3 The Boiler House
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
Carbon dioxide combines with steam to form carbonic acid: Ca(HCO3)2 Calcium bicarbonate
Heat
CaCO3 Calcium carbonate
CO2 Carbon dioxide
H20 water
Similarly, magnesite (magnesium carbonate) is dissolved by carbonic acid to form magnesium bicarbonate: Mg(HCO3)2 Magnesium bicarbonate
Heat
MgCO3 Magnesium carbonate
CO2 Carbon dioxide
H20 water
Fig. 3.9.4 Non-alkaline or permanent hardness (scale + carbonic acid) o
Non-alkaline hardness and carbonates (also known as permanent hardness) - This is also due to the presence of the salts of calcium and magnesium but in the form of sulphates and chlorides. These precipitate out of solution, due to their reduced solubility as the temperature rises, and form hard scale, which is difficult to remove. In addition, the presence of silica in boiler water can also lead to hard scale, which can react with calcium and magnesium salts to form silicates which can severely inhibit heat transfer across the fire tubes and cause them to overheat.
Total hardness Total hardness is not to be classified as a type of hardness, but as the sum of concentrations of calcium and magnesium ions present when these are both expressed as CaC03. If the water is alkaline, a proportion of this hardness, equal in magnitude to the total alkalinity and also expressed as CaC03, is considered as alkaline hardness, and the remainder as non-alkaline hardness. (See Figure 3.9.5) Non-alkaline hardness (permanent)
Alkaline hardness (temporary)
Total hardness
Fig. 3.9.5 Total hardness
Non-scale forming salts Non-hardness salts, such as sodium salts are also present, and are far more soluble than the salts of calcium or magnesium and will not generally form scale on the surfaces of a boiler, as shown in Figure 3.9.6. 2NaHCO3 Sodium bicarbonate
Heat
Na2CO3 Sodium carbonate
Na2CO3 Sodium carbonate H20 water
Heat
CO2 Carbon dioxide
H20 water
2NaOH Sodium hydroxide
C02 Carbon dioxie
Adding the total hardness + non-hardness salts gives: Total hardness
Non hardness salts
Total dissolved solids (TDS)
Fig. 3.9.6 The effects of heat
The Steam and Condensate Loop
3.9.5
Block 3 The Boiler House
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
Comparative units When salts dissolve in water they form electrically charged particles called ions. The metallic parts (calcium, sodium, magnesium) can be identified as cations because they are attracted to the cathode and carry positive electrical charges. Anions are non-metallic and carry negative charges - bicarbonates, carbonate, chloride, sulphate, are attracted to the anode. Each impurity is generally expressed as a chemically equivalent amount of calcium carbonate, which has a molecular weight of 100.
pH value
Another term to be considered is the pH value; this is not an impurity or constituent but merely a numerical value representing the potential hydrogen content of water - which is a measure of the acidic or alkaline nature of the water. Water, H2O, has two types of ions - hydrogen ions (H+) and hydroxyl ions (OH-). If the hydrogen ions are predominant, the solution will be acidic with a pH value between 0 and 6. If the hydroxyl ions are predominant, the solution will be alkaline, with a pH value between 8 and 14. If there are an equal number of both hydroxyl and hydrogen ions, then the solution will be neutral, with a pH value of 7. Acids and alkalis have the effect of increasing the conductivity of water above that of a neutral sample. For example, a sample of water with a pH value of 12 will have a higher conductivity than a sample that has a pH value of 7. Table 3.9.3 shows the pH chart and Figure 3.9.7 illustrates the pH values already mentioned both numerically and in relation to everyday substances. Table 3.9.3 The pH scale pH Hydrogen ion concentration value H+ 0 100 7 10-7 14 10-14
3.9.6
Hydroxyl ion concentration H10-14 10-7 100
Nature Acid Neutral Alkaline
The Steam and Condensate Loop
Block 3 The Boiler House
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
pH value 0
Lemon juice 2.3 Wine 2.8 to 3.8 Vinegar 3.1
1
1.1 Hydrochloric acid (0.36% HCI) 1.2 Sulphuric acid (0.49% H2SO4)
2
2.0 Hydrochloric acid (0.036% HCI) 2.1 Sulphuric acid (0.049% H2SO4) 2.4 Acetic acid (6% CH3COOH)
3
Marshy water 4.0
4
Beer 4.0 to 5.0
5
Water, chemically pure 7.0
Sea water 8.3
2.9 Acetic acid (0.6% CH3COOH) 3.4 Acetic acid (0.06% CH3COOH)
Fruit juice 3.5 to 4.0
Milk 6.3 to 6.6
0.1 Hydrochloric acid (3.6% HCI) 0.3 Sulphuric acid (4.9% H2SO4)
5.2 Boric acid (0.2% H3BO3)
6 7
8 8.4 Sodium bi. carb. solution (0.42% NaHCO3) 9
9.2 Borax solution (1.9% Na2B407)
10 10.6 Ammonia solution (0.017% NH3) 11
11.1 Ammonia solution (0.17% NH3) 11.6 Ammonia solution (1.7% NH3)
Lime-water, saturated 12.3
Fig. 3.9.7 pH chart
The Steam and Condensate Loop
12
12.0 Potassium hydroxide solution (0.056% KOH)
13
13.0 Potassium hydroxide solution (0.56% KOH) 13.0 Sodium hydroxide solution (0.4% NaOH)
14
14.0 Potassium hydroxide solution (5.6% KOH) 14.0 Sodium hydroxide solution (4% NaOH)
3.9.7
Block 3 The Boiler House
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
Questions 1. Temporary hardness salts are reduced by: a| Raising the water temperature
¨
b| Lowering the water temperature
¨
c| Raising the pH value
¨
d| Letting the water settle
¨
2.
What is the effect of CO2 in a steam system?
a| The formation of scale
¨
b| The formation of sludge
¨
c| Corrosion
¨
d| Acidity
¨
3.
Which of the following forms soft scale or sludge?
a| Magnesium sulphate
¨
b| Sodium carbonate
¨
c| Sodium bicarbonate
¨
d| Calcium bicarbonate
¨
4.
Which of the following are principal dissolved solids that are scale forming?
a| Carbonates and sulphates of sodium
¨
b| Calcium bicarbonate
¨
c| Carbonates and sulphates of magnesium
¨
d| Bicarbonate of sodium and magnesium
¨
5.
What is the effect of temperature on calcium and magnesium sulphates?
a| They separate out as soft scale and sludge
¨
b| They precipitate out of solution and form hard scale
¨
c| Foaming and carryover occurs
¨
d| The TDS is increased
¨
6.
What is the treatment for scale forming salts in boiler feedwater?
a| They are chemically treated to modify the pH
¨
b| The feedwater tank is raised to at least 85°C
¨
c| They are chemically treated to produce suspended solids
¨
d| They are removed by filtration means
¨
Answers
1: a, 2: c, 3: d, 4: c, 5: b, 6: c
3.9.8
The Steam and Condensate Loop
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
Module 3.9 Water Treatment, Storage and Blowdown for Steam Boilers
The Steam and Condensate Loop
3.9.1
Block 3 The Boiler House
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
Water Treatment, Storage and Blowdown for Steam Boilers Before boiler blowdown can be discussed and understood it is necessary to establish a definition of water along with its impurities and associated terms such as hardness, pH etc. Water is the most important raw material on earth. It is essential to life, it is used for transportation, and it stores energy. It is also called the universal solvent. Pure water (H20) is tasteless, odourless, and colourless in its pure state; however, pure water is very uncommon. All natural waters contain various types and amounts of impurities. Good drinking water does not necessarily make good boiler feedwater. The minerals in drinking water are readily absorbed by the human body, and essential to our well being. Boilers, however, are less able to cope, and these same minerals will cause damage in a steam boiler if allowed to remain. Of the worlds water stock, 97% is found in the oceans, and a significant part of that is trapped in the polar glaciers - only 0.65% is available for domestic and industrial use. This small proportion would soon be consumed if it were not for the water cycle (see Figure 3.9.1). After evaporation, the water turns into clouds, which are partly condensed during their journey and then fall to earth as rain. However, it is wrong to assume that rainwater is pure; during its fall to earth it will pick up impurities such as carbonic acid, nitrogen and, in industrial areas, sulphur dioxide. Charged with these ingredients, the water percolates through the upper layers of the earth to the water table, or flows over the surface of the earth dissolving and collecting additional impurities. These impurities may form deposits on heat transfer surfaces that may: o
Cause metal corrosion.
o
Reduce heat transfer rates, leading to overheating and loss of mechanical strength.
Table 3.9.1 shows the technical and commonly used names of the impurities, their chemical symbols, and their effects. Atmospheric moisture Evaporation and transportation from surface water bodies, land surface and vegetation
Evaporation from oceans
Precipitation Consumptive use
Well Water table
Percolation
Streams flow to oceans
Total surface and ground water flow to oceans Ocean
Fresh ground water
3.9.2
Saline Interface ground water Fig. 3.9.1 Typical water cycle The Steam and Condensate Loop
Block 3 The Boiler House
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
Table 3.9.1 Impurities in water Name Calcium carbonate Calcium bicarbonate Calcium sulphate Calcium chloride Magnesium carbonate Magnesium sulphate Magnesium bicarbonate Sodium chloride Sodium carbonate Sodium bicarbonate Sodium hydroxide Sodium sulphate Silicon dioxide
Symbol CaCO3 Ca(HCO3)2 CaSO4 CaCI2 MgCO3 MgSO4 Mg(HCO3)2 NaCI Na2CO3 NaHCO3 NaOH Na2SO4 SiO2
Common name Chalk, limestone Gypsum, plaster of paris Magnesite Epsom salts Common salt Washing soda or soda Baking soda Caustic soda Glauber salts Silica
Effect Soft scale Soft scale + CO2 Hard scale Corrosion Soft scale Corrosion Scale, corrosion Electrolysis Alkalinity Priming, foaming Alkalinity, embrittlement Alkalinity Hard scale
Raw water quality and regional variations Water quality can vary tremendously from one region to another depending on the sources of water, local minerals (see Figure 3.9.2). Table 3.9.2 gives some typical figures for different areas in a relatively small country like the UK. Soft to moderately soft
Newcastle upon Tyne
Slightly hard to moderately hard Hard to very hard York Leeds Manchester Lincoln Norwich Birmingham
Cardiff Bristol
London Brighton Southampton Fig. 3.9.2 Regional variations in water quality
Table 3.9.2 Water variation within the UK - All impurities expressed in mg /l calcium carbonate equivalents Alkaline Non-alkaline Total Total Non-hardness Area hardness hardness dissolved hardness salts (temporary) (permanent) solids (TDS) Leeds 12 10 22 24 46 York 156 92 248 62 310 Birmingham 28 72 100 130 230 London 180 192 372 50 422
The Steam and Condensate Loop
3.9.3
Block 3 The Boiler House
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
The common impurities in raw water can be classified as follows: o
Dissolved solids - These are substances that will dissolve in water. The principal ones are the carbonates and sulphates of calcium and magnesium, which are scale -forming when heated. There are other dissolved solids, which are non-scale forming. In practice, any salts forming scale within the boiler should be chemically altered so that they produce suspended solids, or sludge rather than scale.
o
Suspended solids - These are substances that exist in water as suspended particles. They are usually mineral, or organic in origin. These substances are not generally a problem as they can be filtered out.
o
Dissolved gases - Oxygen and carbon dioxide can be readily dissolved by water. These gases are aggressive instigators of corrosion.
o
Scum forming substances - These are mineral impurities that foam or scum. One example is soda in the form of a carbonate, chloride, or sulphate.
The amount of impurities present is extremely small and they are usually expressed in any water analysis in the form of parts per million (ppm), by weight or alternatively in milligrams per litre (mg /l). The following sections within this Module describe the characteristics of water.
Hardness Water is referred to as being either hard or soft. Hard water contains scale-forming impurities while soft water contains little or none. The difference can easily be recognised by the effect of water on soap. Much more soap is required to make a lather with hard water than with soft water. Hardness is caused by the presence of the mineral salts of calcium and magnesium and it is these same minerals that encourage the formation of scale. There are two common classifications of hardness: o
Alkaline hardness (also known as temporary hardness) - Calcium and magnesium bicarbonates are responsible for alkaline hardness. The salts dissolve in water to form an alkaline solution. When heat is applied, they decompose to release carbon dioxide and soft scale or sludge. The term temporary hardness is sometimes used, because the hardness is removed by boiling. This effect can often be seen as scale on the inside of an electric kettle. See Figures 3.9.3 and 3.9.4 - the latter representing the situation within the boiler. Carbon dioxide combines with water to form carbonic acid: CO2 Carbon dioxide
H20 Water
H2C03 Carbonic acid
Limestone (calcium carbonate) is dissolved by carbonic acid to form calcium bicarbonate: H2C03 Carbonic acid
CaCO3 Calcium carbonate
Ca(HCO3)2 Calcium bicarbonate
Fig. 3.9.3 Alkaline or temporary hardness
3.9.4
The Steam and Condensate Loop
Block 3 The Boiler House
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
Carbon dioxide combines with steam to form carbonic acid: Ca(HCO3)2 Calcium bicarbonate
Heat
CaCO3 Calcium carbonate
CO2 Carbon dioxide
H20 water
Similarly, magnesite (magnesium carbonate) is dissolved by carbonic acid to form magnesium bicarbonate: Mg(HCO3)2 Magnesium bicarbonate
Heat
MgCO3 Magnesium carbonate
CO2 Carbon dioxide
H20 water
Fig. 3.9.4 Non-alkaline or permanent hardness (scale + carbonic acid) o
Non-alkaline hardness and carbonates (also known as permanent hardness) - This is also due to the presence of the salts of calcium and magnesium but in the form of sulphates and chlorides. These precipitate out of solution, due to their reduced solubility as the temperature rises, and form hard scale, which is difficult to remove. In addition, the presence of silica in boiler water can also lead to hard scale, which can react with calcium and magnesium salts to form silicates which can severely inhibit heat transfer across the fire tubes and cause them to overheat.
Total hardness Total hardness is not to be classified as a type of hardness, but as the sum of concentrations of calcium and magnesium ions present when these are both expressed as CaC03. If the water is alkaline, a proportion of this hardness, equal in magnitude to the total alkalinity and also expressed as CaC03, is considered as alkaline hardness, and the remainder as non-alkaline hardness. (See Figure 3.9.5) Non-alkaline hardness (permanent)
Alkaline hardness (temporary)
Total hardness
Fig. 3.9.5 Total hardness
Non-scale forming salts Non-hardness salts, such as sodium salts are also present, and are far more soluble than the salts of calcium or magnesium and will not generally form scale on the surfaces of a boiler, as shown in Figure 3.9.6. 2NaHCO3 Sodium bicarbonate
Heat
Na2CO3 Sodium carbonate
Na2CO3 Sodium carbonate H20 water
Heat
CO2 Carbon dioxide
H20 water
2NaOH Sodium hydroxide
C02 Carbon dioxie
Adding the total hardness + non-hardness salts gives: Total hardness
Non hardness salts
Total dissolved solids (TDS)
Fig. 3.9.6 The effects of heat
The Steam and Condensate Loop
3.9.5
Block 3 The Boiler House
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
Comparative units When salts dissolve in water they form electrically charged particles called ions. The metallic parts (calcium, sodium, magnesium) can be identified as cations because they are attracted to the cathode and carry positive electrical charges. Anions are non-metallic and carry negative charges - bicarbonates, carbonate, chloride, sulphate, are attracted to the anode. Each impurity is generally expressed as a chemically equivalent amount of calcium carbonate, which has a molecular weight of 100.
pH value
Another term to be considered is the pH value; this is not an impurity or constituent but merely a numerical value representing the potential hydrogen content of water - which is a measure of the acidic or alkaline nature of the water. Water, H2O, has two types of ions - hydrogen ions (H+) and hydroxyl ions (OH-). If the hydrogen ions are predominant, the solution will be acidic with a pH value between 0 and 6. If the hydroxyl ions are predominant, the solution will be alkaline, with a pH value between 8 and 14. If there are an equal number of both hydroxyl and hydrogen ions, then the solution will be neutral, with a pH value of 7. Acids and alkalis have the effect of increasing the conductivity of water above that of a neutral sample. For example, a sample of water with a pH value of 12 will have a higher conductivity than a sample that has a pH value of 7. Table 3.9.3 shows the pH chart and Figure 3.9.7 illustrates the pH values already mentioned both numerically and in relation to everyday substances. Table 3.9.3 The pH scale pH Hydrogen ion concentration value H+ 0 100 7 10-7 14 10-14
3.9.6
Hydroxyl ion concentration H10-14 10-7 100
Nature Acid Neutral Alkaline
The Steam and Condensate Loop
Block 3 The Boiler House
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
pH value 0
Lemon juice 2.3 Wine 2.8 to 3.8 Vinegar 3.1
1
1.1 Hydrochloric acid (0.36% HCI) 1.2 Sulphuric acid (0.49% H2SO4)
2
2.0 Hydrochloric acid (0.036% HCI) 2.1 Sulphuric acid (0.049% H2SO4) 2.4 Acetic acid (6% CH3COOH)
3
Marshy water 4.0
4
Beer 4.0 to 5.0
5
Water, chemically pure 7.0
Sea water 8.3
2.9 Acetic acid (0.6% CH3COOH) 3.4 Acetic acid (0.06% CH3COOH)
Fruit juice 3.5 to 4.0
Milk 6.3 to 6.6
0.1 Hydrochloric acid (3.6% HCI) 0.3 Sulphuric acid (4.9% H2SO4)
5.2 Boric acid (0.2% H3BO3)
6 7
8 8.4 Sodium bi. carb. solution (0.42% NaHCO3) 9
9.2 Borax solution (1.9% Na2B407)
10 10.6 Ammonia solution (0.017% NH3) 11
11.1 Ammonia solution (0.17% NH3) 11.6 Ammonia solution (1.7% NH3)
Lime-water, saturated 12.3
Fig. 3.9.7 pH chart
The Steam and Condensate Loop
12
12.0 Potassium hydroxide solution (0.056% KOH)
13
13.0 Potassium hydroxide solution (0.56% KOH) 13.0 Sodium hydroxide solution (0.4% NaOH)
14
14.0 Potassium hydroxide solution (5.6% KOH) 14.0 Sodium hydroxide solution (4% NaOH)
3.9.7
Block 3 The Boiler House
Water Treatment, Storage and Blowdown for Steam Boilers Module 3.9
Questions 1. Temporary hardness salts are reduced by: a| Raising the water temperature
¨
b| Lowering the water temperature
¨
c| Raising the pH value
¨
d| Letting the water settle
¨
2.
What is the effect of CO2 in a steam system?
a| The formation of scale
¨
b| The formation of sludge
¨
c| Corrosion
¨
d| Acidity
¨
3.
Which of the following forms soft scale or sludge?
a| Magnesium sulphate
¨
b| Sodium carbonate
¨
c| Sodium bicarbonate
¨
d| Calcium bicarbonate
¨
4.
Which of the following are principal dissolved solids that are scale forming?
a| Carbonates and sulphates of sodium
¨
b| Calcium bicarbonate
¨
c| Carbonates and sulphates of magnesium
¨
d| Bicarbonate of sodium and magnesium
¨
5.
What is the effect of temperature on calcium and magnesium sulphates?
a| They separate out as soft scale and sludge
¨
b| They precipitate out of solution and form hard scale
¨
c| Foaming and carryover occurs
¨
d| The TDS is increased
¨
6.
What is the treatment for scale forming salts in boiler feedwater?
a| They are chemically treated to modify the pH
¨
b| The feedwater tank is raised to at least 85°C
¨
c| They are chemically treated to produce suspended solids
¨
d| They are removed by filtration means
¨
Answers
1: a, 2: c, 3: d, 4: c, 5: b, 6: c
3.9.8
The Steam and Condensate Loop
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