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November 08, 2016 0 Comments
Fresh water can be surface water from rivers, streams, reservoirs or ground water from wells. Generally ground water supplies are more consistent in composition than surface water supplies. Surface water quality is affected by rainfall, soil erosion and industrial wastes, but ground water is usually harder than surface water. The composition of fresh water also varies with the location and type and strata of the earth formations. In limestone areas, for example, water contains large quantities of dissolved calcium. Apart from the geographic variations, the local conditions of a particular area may have a great influence in the composition of the water.
Boiler feedwater 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.
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’s 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 then followed by softening, evaporation, deariation, ion exchange etc. Internal treatment is also required for the conditioning of impurities within the boiler system, to control corrosion, as reactions occur in the boiler itself and the steam pipelines.
Water evaporating in the boiler causes impurities to concentrate. Boiler scale results from suspended matter settling out on the metal or dissolved impurities precipitating out on heat transfer surfaces and becoming hard and adherent.
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 form exceedingly hard scale. Suspended or dissolved iron coming in the feedwater will also deposit on the boiler metal. Oil and other process contaminants can form 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 may 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.
Phosphate deposits are usually soft brown or gray deposits that can be easily removed by normal cleaning methods. They are normally found in boilers employing a phosphate internal treatment. They are the preferred reaction product when using a residual phosphate treatment on high hardness feed water. Since they are easily conditioned with organic sludge conditioners, they are relatively nonadherent. Calcium phosphate is usually the predominant compound in the boiler 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. Carbonate deposit can be easily checked by putting it in an acid solution. Bubbles of carbon dioxide will effervesce from the scale.
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.
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.
Iron deposits are very dark colored 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.
The major problem that deposits 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 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 metal. 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 itself normally occurs when 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 concentrated at 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 or cracking is a more serious type of boiler metal failure showing up as continuous intergranular cracks. This type of cracking occurs when the metal 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 and 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.
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 recommendations of a water treatment consultant need to be followed to minimize chances of such corrosion from occurring.
Uniform corrosion of boiler 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 deep pitting that causes only a small amount of total iron loss causes penetration and leakage in boiler tubes. Corrosion beneath certain types of boiler deposits can weaken the metal and cause tube failure. Likewise corrosion in steam condensate system can damage pipelines and equipment.
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.
The common methods for prevention of corrosion include:
The selection and control of chemicals for preventing corrosion requires an understanding of the causes and corrective measures.
Boiler water carryover is the contamination of steam with boiler water solids. Common causes of boiler water carryover are:
Steam purity can be measured with the use of a sodium ion analyzer. It measures the sodium ion content in a cooled steam sample that will correspond to the amount of boiler water solids contaminating the steam. The sodium ion analyzer can detect carryover down to 1 ppb sodium in steam.
Oil contamination in boiler feedwater is usually from pumps and other lubricated equipment. Oil can cause serious foaming due to saponification of oil by boiler water alkalies.
Suspended solids tend to collect on the surface film surrounding a steam bubble, which therefore resists breaking and builds up foam. The finer suspended particles become the greater is their collection on the bubble. The type as well as the quantity of suspended solids can affect carryover. Depending on the type of suspended solids, some boilers having high suspended solids operate without carryover, while others have carryover with low suspended solids.
Silica can be present in the steam as the result of general boiler water carryover, or it can go into the steam in a volatile form. In the later case, silica acts much like gas and is considered to be selectively carried over. As the boiler pressures increase above 400 psi, there is an increase in the tendency for silica to be selectively carried into the steam in amounts proportionate to the amount of silica present in the boiler water.
Suspended and dissolved solids in the boiler water tend to deposit in the steam and condensate system. Impurities carried over with the steam cause contamination in the many processes for which steam is used, resulting in overheating, corrosion and reduction of efficiency of the boiler itself and other equipment.
The basic preventive measure is to maintain the concentration of solids in the boiler water at recommended levels. High water levels, excessive boiler loads and sudden load changes are to be avoided. Very often contaminated condensate returned to the boiler system causes carryover. The return condensate should be filtered to remove suspended solids before being fed back to the boiler. Efforts should be made to trace the source of any excessive contamination and the problem rectified. The use of chemical antifoams is effective in controlling carryover due to concentration of impurities in the boiler water. Steam-separating equipment must be inspected for proper installation.
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