Feed water: Why water Treatment is Necessary for Boiler

 

 Feed water: Why water Treatment is Necessary for Boiler 

Contents

  1. Water for Boilers
  2. Impurities in Water
  3. Boiler feedwater
  4. Purity requirements of feedwater
  5. Boiler Deposits
  6. Corrosion
  7. Boiler water carryover

Water for boilers

All natural waters contain varying amounts of suspended and dissolved matters as well as dissolved gases. The type and amount of impurities in fresh water vary with the source (lake,river,well) and with the area of location. Impurities in water become an important consideration when water is to be used for steam generation. With the trend toward higher – pressure boilers,  pretreatment has become the key to successful operation of industrial power plants. Feedwater must be pretreated to remove impurities to control deposition, carryover, and corrosion in the boiler system. Poor quality water gives poor quality steam. The first step in any filters are a practical solution to most problems of water clean-up.

Impurities in Water

All natural waters contain  various types and amounts of impurities. These impurities cause boiler problems and as such consideration must be given to the quality and treatment required of the quality and treatment required of the water used for generating steam. For any type of treatment, sediment filtration (usually with cartridge filters) is the first step.

Natural Water

Natural waters contain suspended matter, dissolved gases. Water being a universal solvent dissolves minerals, rocks and soil that come into contact with it. It dissolves gases from air and gases that are given off from organics in the soil. It picks up suspended matter from the earth. Additionally it may also be contaminated with industrial wastes and process materials.

Dissolved Minerals

Dissolved minerals picked up by the water consist mainly of calcium carbonate (dolomite), magnesium sulfate (Epsom salts), silica (sand), sodium chloride (common salt), hydrated sodium sulfate ( Gluber salt ), and smaller quantities of iron, manganese, fluorides, aluminum, and other substances. The nitrates and phosphates found in water are usually due to sewage contamination.

Water hardness

water containing high amounts of calcium and magnesium minerals is hard water. The amount of hardness in natural water can vary from a few ppm to 500 ppm. Calcium and magnesium compounds are relatively insoluble in water and tend to precipitate out. This causes scale and deposit problems. such water must be treated to make it suitable for steam generation.

Dissolved gases in water

Water contains varying amounts of dissolved (21% oxygen , 78% nitrogen, 1% other gases including carbon dioxide).Water can contain up to 9 ppm oxygen at room temperature and atmospheric pressure. As the temperature increase, the solubility of oxygen decreases, but water under pressure can hold higher amounts of dissolved oxygen. Nitrogen, being inert, has little effect on water used in boilers. Water can contain 10 ppm of carbon dioxide, sometimes much more than that due to decaying vegetation and organics in soil. Hydrogen sulfide and methane may be dissolved in water but this is are. These gases can be troublesome when they are present in the feed water.

Other impurities in water

Natural waters contain varying levels of soil, sand, turbidity, colour, precipitated minerals, oil, industrial wastes and other suspended solid particles. Turbidity is due to very fine organic materials and microorganisms, as well as suspended clay and silt.  Colour is due to the decaying vegetable matter.

Boiler feedwater

Boiler feed water is the water supplied to the boiler. Often, steam is condensed and returned to the boiler as part of the feedwater. The water needed to supplement the returned condensate is termed make up water. Make-up water is usually filtered and treated before use. Feedwater composition therefore depends on the quality of the make-up water and the amount of condensate returned. Sometimes people think that there is a great deal of similarity between the requirements for potable (drinking) water and the requirements for boiler feedwater. The minerals in drinking water are considered desirable and are absorbed by the body. On the other hand, minerals in water cannot be handled as well by boilers. Although a boiler is a big mass of steel, it is more sensitive to water impurities than the human stomach. For this reason, a lot of care is needed in filtration and treatment of the boiler water supply.

Purity requirements of feedwater

Feedwater is a matter both of quantity of impurities and the nature of impurities. Hardness, iron, and silica, for example, are of more concern than sodium salts. The purity requirements of feedwater depend on how much feedwater is used as well as toleration of the particular boiler design (pressure, heat transfer rate etc.). In today high- pressure boilers practically all impurities must be removed. The feedwater (make-up water) from outside needs to be treated for the reduction or removal of impurities by first filtration, and followed by softening, evaporation, deariation, ion exchange etc. Internals treatment is also required for the conditioning of impurities within the boiler system, to control corrosion, as reaction occur in the boiler itself and the steam pipelines.

Boiler deposits  

Water evaporating in the boiler causes impurities to concentrate. Boiler scale results from suspended matter setting out on the metal or dissolved impmeurities precipitating out on heat transfer surfaces and becoming hard and adherent.

Impurities that from deposits

Bicarbonates of calcium and magnesium dissolved in water break down under heat and give off carbon  dioxide forming insoluble carbonates. These carbonates  precipitate directly on the boiler metal and or form sludge in the water that deposits on boiler surfaces. Sulfate and silica generally precipitate directly on the boiler metal and are much harder to condition. Silica (sand) if present in water can from exceedingly hard scale. Suspended or dissolved iron coming in the feedwater will also deposit. On the boiler metal oil and others process contamintants can from deposits as well and promote deposition of other impurities. Sodium. Compounds usually do not deposit unless the water is almost. Completely evaporated to dryness, as my occur in a starved tube. Deposits are seldom composed of one constituent Alone, but are generally a mixture of various types of solid sediments, dissolved minerals, corrosion products like rust, and other water contaminants.

Characteristics of phosphate deposit

Phosphate deposits are usually soft brown or gray deposits that can be easily removed by normal cleaning methods. They are Normally found in boiler employing a phosphate internal treatment. They are the reaction product when using a residual phosphate treatment on high hardness feed water. Since they are easily condition with organic sludge conditioners, they are relatively nonadherent. Calcium phosphate is usually the predominant compound in the boiler deposit.

Characteristics of Carbonate deposit

Carbonate deposits are usually granular and sometimes porous. The crystals are relatively large and often matted together with finely divided particles of other materials making the scale look dense and uniform. Carbonated deposit can be easily checked by putting it in an acid solution. Bubbles of carbon dioxide will effervesce from the scale.

Characteristics of sulfate deposit

Sulfate deposit is brittle, does not pulverize easily, and will not effervesce when put in an acid solution. It is much harder and denser than a carbonate deposit due to its smaller crystal structure.

Characteristics of silica deposit

Silica deposits are very hard and resemble porcelain. Their crystals are very small, forming a dense, impervious scale. This scale is extremely brittle, very difficult to pulverize, and not soluble in hydrochloric acid.

Characteristics of iron deposit

Iron deposits are very dark coloured and are due to corrosion products or iron contamination in feedwater. Iron deposits are usually  magnetic in nature. They are soluble in hot  acid, giving a dark- brown solution.

Problems caused by deposits.

The major problem that deposit cause is tube failure from overheating. This is due to the fact that the deposits act as an insulator and excessive deposits prevent efficient heat transfer through the tubes to the water. This causes the metal to become overheated and over time the metal fails. These deposits can also cause plugging or partial obstruction of boiler tubes, leading to starvation and subsequent overheating of the tubes. Underneath the deposit layer corrosion may also occur. Deposits cause unscheduled outages, increased cleaning time and expenses. Boiler deposits reduce overall operating efficiency resulting in higher fuel consumption.

Corrosion

Corrosion is basically the reversion of a metal to its ore form. Iron, for example, reverts to iron oxide as a result of corrosion. The process of corrosion is actually not so simple, it is a complex electro-mechanical reaction. Corrosion may generally be over a large metal surface but sometimes it results in pinpoint penetration of mental. Though basic corrosion is usually due to reaction of the metal with oxygen, other factors including stresses produce different forms of attack. Corrosion may occur in the feedwater system as a result of low pH water and the presence of dissolved oxygen and carbon dioxide. Corrosion in the boiler water alkalinity is too low or too high or when the metal is exposed to oxygen-bearing water during either operation or idle periods. High temperatures and stresses tend to accelerate the corrosion. In the steam & condensate System and pipelines corrosion is generally the result of contamination with carbon dioxide and oxygen

Cracking in boiler metal may occur due to cyclic  stresses created by rapid heating and cooling. These stresses are points where corrosion has roughened or pitted the metal surface. This is usually because of improper corrosion prevention. sometimes even with properly treated water corrosion fatigue cracking occurs. These cracks often originate where a dense protective oxide film covers the metal surfaces, and cracking occurs from the action of applied cyclic stresses. Corrosion fatigue cracks are often thick, blunt, and across the metal grains. They start at internal tube surfaces and are most often circumferential on the tube.

Caustic embrittlement

Caustic embrittlement or cracking is a more serious type of boiler metal failure showing  up as continuous intergranular cracks. This type of cracking occurs when the metals is stressed, water contains caustic with a trace of silica, and some mechanism, such as a slight leak, is present allowing the boiler water to concentrate on the stressed metal. Caustic embrittlement is more of a problem in older boilers with riveted drums as they cause stresses an crevices in the areas of rivets and seams. In the newer welded drum boilers this type of cracking is less frequent but the rolled tube ends are still vulnerable to attack. The possibility of caustic cracking should be a consideration in water treatment.

Other causes of boiler corrosion  

Chelate residuals in excess of 20 ppm as CaCO3 or improperly applied chelate treatment may produce boiler system corrosion. Concentration of boiler solids at high heat input areas might also produce corrosion. The Recommendation of a water consultant  need to be followed to minimize chances of such corrosion from occurring.

Corrosion problems

Uniform corrosion of boilers metal surfaces is bound to occur and is not of much concern as all boilers experience a small amount of general corrosion. Corrosion, however, takes  many forms and pitting that causes only a small amount of total iron loss causes penetration and leakage in boiler tubes. corrosion beneath certain types of boilers deposits can weaken the metal and causes tube failure. Likewise corrosion in steam condensate system can damage pipelines and equipment.

Corrosion measurement

Hydrogen gas sampling of the boiler steam is done to measure the corrosion potential of the boiler water. This test for corrosion is based on the release of hydrogen gas when iron corrodes. Measuring the amount of hydrogen gas released detects boiler water conditions and indicates if corrosion conditions exist in an operating boiler.

Basic corrosion prevention methods 

The common methods for prevention of corrosion include:

  1. Filtration of solid suspended impurities & particles from water
  2. Removing dissolved oxygen from the boiler feedwater
  3. Maintaining alkaline conditions in the boiler water
  4. Keeping the boiler internal surfaces clean
  5. Protecting boilers during out of services periods
  6. Using a chemical treatment programme to counteract corrosive gases in steam and
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