Methods of Feedwater Treatment
The treatment may be internal, which involves the addition of chemicals to the water to make the salts causing scale and sludge less likely to bond to pipe work and heat exchanger surfaces. In the case of boilers and cooling towers, blowdown of the water on a continuous or regular basis is required. Care must be taken in the case of blowdown to ensure that the exiting water temperature does not damage the drains or that the chemicals pollute waterways. The consent of the water authorities is required in order to determine what levels can be discharged into waterways. In certain cases further treatment may be necessary before discharge.
In the case of boilers operating at low pressure, organic materials such as natural and modified tannins, starches, or alginates are added to aid blowdown. For boilers operating at high pressure, synthetic materials such as polyacrylates and polymethacrylates have been developed. The most commonly used chemicals for boiler feedwater treatment are phosphates and hydrazine.
External treatment involves the removal of impurities from the water by various methods before it enters the plant; this is the most effective method of water treatment. This category of treatment involves one or more of the following processes.
In sedimentation the water to be treated flows slowly through a tank, allowing the suspended material in the water to fall to the base of the tank. The use of coagulating compounds, such as aluminum and ferric sulfate, in creases the efficiency.
It is during oxidation that iron and manganese in suspension are removed from the water. Oxidizing agents (chlorine, ozone, hydrogen peroxide, potassium permanganate, etc.) or direct aeration is used; the metals in
Solution are converted to insoluble oxides, which are removed by filtration. The use of gaseous chlorine is not recommended if the water contains or — ganics, or else car — cinogenic by-products can be formed (trihalomethanes). Gaseous chlorine also presents health problems in the case of leaks and is corrosive.
If the solution is allowed to flow through a granular bed such as sand, the larger particulate matter remains on the surface, while the smaller material is collected in the thickness of the granular bed. Pressurization of the filter accelerates the process. Besides sand, other materials used as filtering media are anthracites, manganese dioxide, and activated carbon.
The sense of touch allows one to determine if water is hard or soft. For a domestic application in a hard-water area, more soap is required to produce lather than is required in a soft-water area.
The temporary hardness salts can be removed by boiling; these salts may be classified as alkaline or carbonate hardness salts. These salts in solution are calcium carbonate, CaC03, calcium hydrogen carbonate, Ca(HC03)2, and magnesium hydrogen carbonate, Mg(HC03 )2. Heating Ca(HC03)2 produces w’ater, carbon dioxide, and calcium carbonate, and this compound is deposited on heat exchangers.
The salts that cause permanent hardness are calcium sulfate, CaS04, calcium chloride, CaCl2, magnesium sulfate, MgS04, and magnesium chloride, MgCl2. These are known as nonalkaline or noncarbonate hardness salts and cannot be removed by boiling; they must be removed by chemical treatment.
The internal process complements the external process by taking care of any contamination that may enter the water from the process.
The following is a brief introduction to the various types of water softening plants encountered.
Lime Soda. If carbon dioxide is in solution in water and calcium hydroxide is added, the resulting precipitation product is CaC03; this can be removed by sedimentation.
If the water is temporarily hard due to the presence of Ca(HC03)2, and calcium hydroxide is added, the resulting products will be a precipitate.
If the water is permanently hard due to MgS04, and lime is added, the precipitates calcium sulfate, CaS04, and magnesium hydroxide, Mg(OH)4, result, which are removed by sedimentation.
Permanent hardness can also be due to the presence of CaS04, in which case the addition of soda (sodium carbonate), Na2C03, produces sodium sulfate, Na2S04, and calcium carbonate, CaC0 5; this precipitate once again is removed by sedimentation.
Ion Exchange. When water flows through a resin ion exchange material bed, some of the undesirable ions are adsorbed and replaced with less objectionable ones. The process may be either
• Base exchange
The base exchange process removes both the temporary and permanent hardness salts from the water by allowing the water to flow through resin beads containing sodium zeolite, Na2Z.
When the permanent hardness salt CaS04 passes through the bed, calcium zeolite (CaZ) and sodium sulfate (Na2S04 ) are formed, which are then flushed away.
A temporary hardness salt such as calcium carbonate (CaCO 5) passing through a sodium zeolite bed will produce calcium zeolite (CaZ) and sodium carbonate (Na2C03). This solution is flushed away. But the temporary hardness salt calcium hydrogen carbonate, Ca(HC03)2, passing through a sodium zeolite bed, will produce calcium zeolite and sodium hydrogen carbonate, NaHC03. The latter increases the alkalinity of water, causing foaming of the boiler water due to the formation ot sodium hydroxide, NaOH. Similar reactions are possible involving magnesium chloride and sodium zeolite.
After a time, depending on the concentration of salts and the flow rate, the remaining sodium zeolite is converted to either calcium or magnesium zeolite. When the zeolite becomes saturated, the resin bed must be regenerated. The regeneration process is achieved by backwashing (flushing) the bed with fresh water to remove some of the remaining solids, followed by passing a solution of salt through the resin bed. This flushing removes the calcium chloride (CaCl) and the sodium zeolite; a final rinse removes any salt remaining, allowing the process to continue.
The dealkalization process removes the temporary hardness in water. This uses an acid resin bed for regeneration—in this case sulfuric acid (H2S04).
To remove the majority of the salts from water, a mixture of resins is used; the process in this case is called demineralization.
Basically, the hardness salts of calcium and magnesium ions are ex changed for sodium ions in the dealkization process; the carbonate and bicarbonate salts, which cause high levels of alkalinity, are replaced with chloride ions. Reverse osmosis can also be used to produce demineralized water.
Precipitation Softening. This process depends on sufficient holdup time within a vessel to allow sedimentation and clarification to occur. A coagulation chemical such as alum or iron salts added to the solution will improve the process efficiency.
Evaporation. The process of evaporation or distillation in the past was carried out in submerged-tube evaporators. These have been superseded by flash-type evaporators, which are more economical to run and reduce scale problems. The process is suitable for brackish water, where the cost of chemical methods is excessive. The resulting distilled water is not palatable and requires aeration to make it potable.
The operating cost is high due mainly to the energy used for the heat source, cooling water, and the necessary chemical treatment. To reduce the running costs, waste process heat or solar collectors are used.
Falling-film vertical evaporators, direct expansion systems, and vacuum freezing techniques may also be used.
Reverse Osmosis. The process of osmosis is used by plants to obtain food and moisture from the soil. The density of the sap in the roots of the plant is greater than that of the soil water surrounding it. The root wall provides a semipermeable membrane, and the difference in suction across it is the osmotic pressure.
In reverse osmosis, the osmotic pressure is increased manually to get the water to flow from a high-density area through a semipermeable membrane to the lower-density weaker solution. The water will pass through the membrane and leave the solids behind. A pressure of about 2.76 MPa will extract 90% or more of the dissolved absorbed solids; further refinement may be achieved through a base exchange process.
Magnetic Water Treatment. This method is used in marine engineering and district heating networks in Russia. The hard water to be treated, either hot or cold, flows first through a filter and then at high velocity through permanent magnets. The magnetic field influences the nature of the crystallization of the hardness salts. This results in numerous nuclei being formed in the solution, creating sludge instead of a hard scale, which is easily removed by blowdown.
The removal of all gases in the water by means of traps or chambers will improve the pumping characteristics and reduce corrosion and noise.
In the case of hot water, the oxygen in the water becomes about twice as corrosive for every 20 °С increase in temperature; hence, removal of the oxygen is of prime importance. Oxygen is extremely corrosive in hot-water systems containing demineralized water
In groundwater, gases such as carbon dioxide, hydrogen sulfide, and radon may be dissolved under pressure. For efficient removal, an intensive degassing process (GDT, or Gas-Degas Technology) has been developed by the GDT Corporation (USA). It consists of groundwater and air (or ozone) being intensively mixed in a venturi injector, followed by optimum residence time in a reactor vessel, and finally the efficient removal of unwanted stripped gases in a centrifugal separator.
By removing oxygen completely, corrosion by this gas is eliminated. It can be achieved by the addition of sodium sulfite or hydrazine, which reacts with oxygen. The reaction product will not normally cause any problems.
Chemicals such as disodium or the polyphosphates are used to precipitate scale-forming solids in the water. If alkalinity control is required, caustic soda
Or soda ash is used in controlled amounts. For some boiler water, rreatment — chelating agents are used to full advantage.
For a high-pressure boiler plant with a high evaporation rate, demineralized feedwater is classified as having an electrical conductivity of less than
0. 2 fiS cm’1; for less critical plant conditions an electrical conductivity greater than 0.2 jxS cm -1 may be acceptable.
The water chemistry relating to power plants operating at high temperatures and pressures is a complex issue. To determine if water is corrosive or scale forming, use is made of the Langelier or Ryznar index. For further information, refer to the VGB guidelines for plants operating at pressures above 68 bar (VGB-450L) and the Scandinavian recommendations (DENA).
Either straightforward drainage or blowdown can readily remove sludge from the plant. It is, however, necessary in some cases to ensure that the residual solids are free-flowing; this is achieved by the use of tannin, lignin, seaweed derivatives, and starch organics.
Water level control and the use of organic antifoam chemicals are essential in steam plants in order to break down the bubbles at the water surface in steam systems, which cause foaming.
Condensate formed in steam systems may require treatment to remove the carbon dioxide in suspension or free-flowing in the condensate. Due to the nature of the water source, carbon dioxide and oxygen as dissolved gases are always present to some degree in water supplies, and in some instances hydrogen sulfide (H2S) and ammonia (NH3) may be present, producing a weak carbonic acid gas and causing elevated-temperature corrosion of metals. Treatment in this case is achieved by the addition of chemicals, pH control, oxygen removal, etc.
When dealing with a large steam plant, the chemistry and the methods of water treatment required are complex.
The efficiency of water separation varies considerably from boiler to boiler. The purity of the steam supplied to a steam turbine should be checked. On the basis of the results, the maximum allowable salt concentration in the boiler water can be determined. This concentration may be much lower than the values given in the table.
When the heat load even locally exceeds 230 kW m-2 the target values for drum pressure, 160 bar (except for Si02), should be used for all boiler pressures. For feedwater, the recommended values for >67 bar should be used.
1. The maximum p-value is independent of feedwater treatment.
2. The gauge pressure when using phosphates to reduce the residual hardness and when using a coordinated phosphate method for pH
Control fall in the pressure range of 35-90 bar P04 between 10 and 20 mg kg~ and in the range 67-125 bar between 7 and 15 mg-1.
3. 725 = Conductivity of boiler water from a neutralized sample at 25 °C.
Although the table is outside the scope of most industrial ventilating engineering requirements, it does indicate the many problems to be considered in the measurement techniques.
To finish this section, a typical flow diagram has been included (Fig. 4.42 ).
Ft is essential that the engineer not lose sight of the numerous potential problems related to microbiological concentration. These include
• Microbiological fouling in heat exchanger pipelines, cooling towers, etc.
• Microbiological corrosion in pipe work
• The effects of contaminated water on human health
In the case of a closed water system, once the correct water treatment is provided, the incidence of microbiological fouling or corrosion is virtually eliminated, provided that the addition of fresh water is not a frequent occurrence. It is, however, essential to have water tests carried out at regular intervals by a water laboratory.
In the case of an open water system, the problem is compounded due to the addition of microorganisms from the atmosphere. Water temperature control is critical to stop the water from becoming a breeding soup culture for the microorganisms.
The aerosols formed in an open system, if inhaled, can cause various forms of Legionella. No one biocide is adequate to control these, as there are some 30 known groups, the most virulent being Legionella pneumophila.
It is essential to practice good design of all open systems by adhering to set guidelines. A well-planned and effective maintenance program is of prime importance.
The use of ozone for water treatment is now well established and has the following advantages:
• An efficient biocide
• Low owning and operating costs compared with other methods
• No chemical handling, storage, or discharge problems
• Simple methods of automatic control
Ozone is more effective than chlorine in deactivating poliovirus, Cryptosporidium parvum, Giardia lamblia, and other protozoa. It also improves the color, taste, and odor of water dramatically. However, since no residual amount remains, it is always necessary to add a small amount of a more stable disinfectant as well (sodium hypochlorite, chlorine dioxide, etc.).
The disadvantages are
TM.13, Minimising the Risk of Legionnaire’s Disease (CIBSE),
Approved Code of Practice on the Prevention or Control of Legionellosis (Health and Safety Commission, UK).
The Control of Legionellosis Including Legionnaires’ Disease (Health and Safety Executive, UK).
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