Handbook Of Industrial Water Treatment 5jy42

http://www.idoub.com/doc/21506235/Boiler-Handbook-Guide http://www.gewater.com/handbook/boiler_water_systems/ch_21_Fireside.jsp#SOURCE AND NATURE OF FIRESIDE DEPOSITION Corrosion is one of the main causes of reduced reliability in steam generating systems. It is estimated that problems due to boiler system corrosion cost industry billions of dollars per year. Deposition is a major problem in the operation of steam generating equipment. The accumulation of material on boiler surfaces can cause overheating and/or corrosion. The quality of feedwater required is dependent on boiler operating pressure, design, heat transfer rates, and steam use. Most boiler systems have sodium zeolite softened or demineralized makeup water. Feedwater hardness usually ranges from 0.01 to 2.0 ppm, but even water of this purity does not provide deposit-free operation. Therefore, good internal boiler water treatment programs are necessary. Impurities in fuels can cause deposit formation and fireside metal surface corrosion. Compounds of aluminum, barium, copper, iron, magnesium, manganese, and silica have all been used to control combustion fouling and corrosion. The most severe problems are generally found in combustion equipment firing fuels that significantly deviate in composition from the fuel on which the equipment design was based. Combustion of solid fuels such as coal and bark (also referred to as "hog fuel") can also lead to fireside slagging. Sodium, calcium, silica, iron, and sulfur content are of primary concern in the burning of solid fuels. Other metal oxides, such as alumina, titania, and potassium oxide, can also aggravate slagging. High-temperature corrosion can proceed only if the corroding deposit is in the liquid phase and the liquid is in direct with the metal. Deposits also promote the transport of oxygen to the metal surface. Corrosion is caused by the combination of oxide layer fluxing and continuous oxidation by transported oxygen.

What is Scale Formation ? Scale formations in boilers are responsible for lost efficiency, increased maintenance and operating costs not to mention lost revenue due to outages and downtime. Most scale formations in boilers can be traced to the presence of hardness in the make-up water. This hardness reacts in the high temperatures environment within the boiler to form and insoluble scale. This insoluble scale coats the heat transfer surfaces, acting as an insulator to impede heat transfer. Hardness isn't the only cause if scale formation in boilers, other impurities such as iron, silica, copper, oil, etc. are often found in samples of boiler scale. In fact, it is rare to find scale which isn't the result of several of these impurities. Normally pre-softening the water before feeding it to the boiler is the first step in eliminating scale formations. Even when the make-up is soft, there is still a need for chemical scale inhibitors inside the boiler. With proper treatment the problems of lost efficiency, tube damage and lost production can be avoided or greatly reduced. Proper treatment requires the right balance of chemical treatment and control.

How do I stop Scale Formation from happening ? The first and foremost aspect of stopping scale formation is to have a good idea of the make-up water that is feeding your system. If you aren't sure, have a certified laboratory complete a fully analysis on this water so you can make an informed decision on what exactly the potential problems you may encounter. After determining these specific aspects of your make-up water then your water treatment expert can guide you through a program that fits your situation. Just a few items that may be of concern when putting together a good water treatment program for your boiler. A complete program will include sludge build-up, pH levels, oxygen removal, condensate treatment, and alkalinity levels. Back to Top

What is corrosion ? Corrosion in boilers can almost always be traced to one or both of two problems. The most common cause is dissolved oxygen entering the system via the feed-water. The oxygen causes very localized corrosion to occur in the form of pitting. The pits are small but deep pinpoint holes which eventually can penetrate tube walls and cause their failure. Another common cause of corrosion in boiler systems is low pH within the boiler. This reduced pH may result from carbon dioxide infiltration or form contamination by other chemicals. Oxygen corrosion is normally controlled by driving the oxygen from the feed-water in a deaerating heater or by chemically removing it with an oxygen scavenger such as sodium sulfite. There are many contaminates which can infiltrate a boiler system and cause low pH levels to develop. Manufacturing wastes such as sugar or acids from plating operations which can be returned to the boiler with condensate can be a source of problems because they concentrate in the boiler. Oxygen can infiltrate the boiler system at virtually any point. When dissolved, oxygen is present in boiler feed water attach on feed lines, pumps and economizers can be expected. The severity of the attach depends upon the concentration of the oxygen and the temperature of the water. Back to Top How do I stop Corrosion ? You can use a deaerator which is defined as a piece of equipment which heats water with steam to insure essentially complete removal of dissolved gases. There are several types of deaerator available, each having its own advantages and disadvantages. Internal treatment for dissolved oxygen corrosion is normally accomplished by the addition of sodium sulfite. Most oxygen scavengers contain a catalyst which speeds the reaction of the sulfite with the oxygen. In systems equipped with a deaerator the sulfite should be fed to the storage tank of the deaerator or to either the suction or pressure side of the feed water pump. In systems which do not have a deaerator, the sulfite can be fed at almost any point in the feed water system, including the condensate tank. Internal treatment for carbon dioxide is normally accomplished by the use of a volatile amine. "Amine" refers to any of a number of chemicals derived from ammonia. There are two major groups of amines in practice as water treatment chemicals today. There are normally referred to as "neutralizing amines" or "filming amines" depending upon whether they neutralize the acid formed by carbon dioxide or form a protective film on the metal. Filming amines do not neutralize the carbonic acid which forms in condensate systems. Instead, they form a film on the metal which is non-wettable, or impervious to water. this protective film prevents the corrosive impurities from ing the metal. Neutralizing amines function by increasing the pH of the condensate. Normally they are fed at such a rate that the pH of the condensate is maintained slightly above 7.0. Satisfactory reduction of carbon dioxide corrosion is possible with the use of a neutralizing amine. it is necessary to supplement this type of condensate protection with an oxygen scavenger to remove dissolved oxygen. Whether condensate corrosion is controlled by chemical treatment or a combination of mechanical and chemical methods, it is important that careful checks and testing be incorporated as a part of the treatment program. No treatment can be better than the way in which it is applied. Consult a water treatment expert to get you started on the right foot.

http://www.hamadaboiler.com/en/water/qa.htm What Causes Boiler Deposists?

Boiler scale is caused by impurities being precipitated out of the water directly on heat transfer surfaces or by suspended matter in water settling out on the metal and becoming hard and adherent. Evaporation in a boiler causes impurities to concentrate. The high temperatures break down some minerals, cause others to become less soluble. In general, water in with hot metal will tend to deposit out impurities as it evaporates.

Answer to :back Which are some Common Types of Boiler Deposits?

In untreated boiler-water, the formation of deposits is like a back-to-nature movement. That is as minerals are deposited out from water they form many types of crystalline and rock-like structures such as are encountered in the earth's strata. Deposits are seldom composed of one constituent alone but are generally a mixture of various types of minerals, corrosion products and other water contaminants. The most common types of boiler deposits may contain : Calcium cabonate, sulphate or silicate; magnesium hydroxide or silicate, iron oxide, and silica, sludge deposits form boiler water which has been treated may also contain calcium and magnesium phosphates.

Answer to :back What are the Characteristics of a carbonate deposi?

A carbonate deposit is usually granular and some-times of a very porous nature. The crystals of calcium carbonate are large but usually are matted together with finely divided particles of other materials so that the scale looks dense and uniform. A carbonate deposit can be easily identified by dropping it in a solution of acid. Bubbles of carbon dioxide will effervsce from the scale.

Answer to :back What are the Characteristics of a Sulphate Deposit?

A sulphate deposit is much harder and more dense than a carbonate deposit because the crystals are smaller and cement together tighter. A sulphate deposit is brittle, does not pulverize easily, and does not effervesce when dropped into acid.

Answer to :back What are the Characteristics of a Silica Deposit?

A high silica deposit is very hard, resembling percelain. The crystal of silica are extremely small, forming a very dense and impervious scale. This scale is extremely brittle and very difficult to pulverize. It is not soluble in hydrochloric acid and is usually very light coloured.

Answer to :back What are the Characteristics of an Iron Deposit?

Iron deposits, due either to corrosion or iron contamination in the water, are very dark coloured. Iron deposits in boilers are most often magnetic. They are soluble in hot acid giving a dark brown coloured solution.

Answer to :back What Problems do Deposits Cause?

The biggest problem that deposits cause is over-heating and failure of boiler tubes. A deposit acts as an insulator and excessive deposits prevent an efficient transfer of heat through the tubes to the circulating water. This causes the metal itself to become over heated. When the overheating is severe enough the metal fails. Boiler deposits can also cause plugging or partial obstruction of corrosive attack underneath the deposits may occur. In general, boiler deposits can cut operating efficiency, produce boiler damage, cause unscheduled boiler outages, and increase cleaning expense.

What is Carrosion?

Stated simply, general corrosion is the reversion of a metal to itsore form. Iron, for example, reverts to iron oxide as the result of corrosion. The process of corrosion, however is a complex electro chemical reaction and it takes many forms. Corrosion may produce general attach over a large metal surface or it may result in pinpoint penetration of metal. While basic corrosion in boilers may be primarily due to reaction of the metal with oxygen, other factors such as stresses, acid conditions, and specific chemical corrodents may have an important influence and produce different forms of attack.

Answer to :back Where is Corrosion Usually Experienced?

Corrosion may occur in the feed-water system as a result of low pH water and the presence of dissolved oxygen and carbon dioxide. Corrosion in the boiler proper generally occurs when the boiler water alkalinity is low or when the metal is exposed to wxygen bearing water either during operation or idle periods. High temperatures and stresses in the boiler metal tend to accelerate the corrosive mechanisms. In the boiler metal tend to accelerate the corrosive mechanisms. In the steam and condensate system corrosion is generally the result of c ontamination with carbon dioxide and oxygen. Specific contaminants such as ammonia or sulphur bearing gases may increase attack on copper alloys in the system.

Answer to :back What is Corrosion Fatigue?

This type of cracking in boiler metal may occur by two different mechanisms. In the first mechanism, cyclic stresses such as are created by rapid heating and cooling are concentrated at points where corrosion has roughened or pitted the metal surface. This is usually associated with improper corrosion prevention. The second type of corrosion fatigue cracking occures in boilers with properly treated water.

In these cases corrosion fatigue is probably a misnomer. These cracks often originate where the metal surfaces are covered by a dense protective oxide film and cracking occurs from the action of applied cyclic stresses. Corrosion fatigue cracks are usually thick, blunt and cross the metal grains. They usually start at internal tube surfaces and are most often circumferential on the tube.

Answer to :back What is Caustic Cracking?

Caustic cracking (caustic embrittlement) is a serious type of boiler metal failure characterized by continous, mostly intergranular cracks. The following conditions appear to be necessary for this type of cracking to occure : 1. 2. 3. 4.

The metal mus be stressed, the boiler-water must contain caustic, at least a trace of silica must be present in the boiler-water, and some mechanisms, such as a slight leak, must be present to allow the boiler water to concentrate on the stressed metal.

Answer to :back What Problems does Corrosion Cause?

Corrosion, in general, causes difficulty from two standpoints. The first is deterioration of the metal itself, and the second is deposition of the corrosion products to form deposits. Generally, uniform corrosion of boiler surfaces is seldom of real concern. Corrosion, however, takes many insidious forms and deep pits resulting in only a minimum of iron loss may cause pentration and leaking of boiler tubes. Corrosion underneath certain types of boiler deposits can so weaken the metal that failure of tubes occurs. In steam condensate system, replacement of lines and equipment due to corrosion can be a costly problem.

Answer to :back What Measures are taken to Prevent Boiler System Corrosion?

Corrosion, in general, causes difficulty from two stand-points. The first is deterioration of the metal itself, and the second is deposition of the corrosion products to form deposits. Generally, uniform corrosion of boiler surfaces is seldom of real concern. Corrosion, however, takes many insidious forms and deep pits resulting in only a minimum of iron loss may cause penetration and leaking of boiler tubes. Corrosion underneath certain types of boiler deposits can so weaken the metal that failure of tubes occurs. In steam condensate system, replacement of lines and equipment due to corrosion can be a costly problem. Why Water Treatment is Needed : As feed-water enters a boiler the heat causes hardness (cal-cium and magnesium salts) to come out of solution. Untreated the hardness deposits on the hot boiler metal to from scale. As water evaporates in the boiler the feed-water impurities concentrate. Even small amounts ot iron, copper, and silica can accumulate in the boiler-water and cause serious deposit problems in higher pressure boilers. Since scale can cause overheating and failure of boiler metal, preventive water treatment is needed. The corrosion of boiler system metal is a complex process and takes many forms: general attack, localized pitting, and various types of cracking in stressed metal. In general, the main factors causing corrosion are dissolved gases in the water (primarily oxygen) and acid conditions. High temper-atures speed up the

corrosion process.Corrosion is damaging from several standpoints: it causes weakening and failure of metal and produces corrosion products which can cause boiler deposits. High concentrations of dissolved and suspended matter in boiler-water can cause foaming of the water at the steam release surface. This produces carry-over of the water and its impurities into the steam. Carry-over results in deposits and other problems in turbines, engines and other processes using steam. While mechanical and operational factors also cause carry-over, proper control of water conditions is important in producing pure steam.

Iron deposits, due either to corrosion or iron contamination in the water, are very dark coloured. Iron deposits in boilers are most often magnetic. They are soluble in hot acid giving a dark brown coloured solution.

Answer to :back What Problems do Deposits Cause?

The biggest problem that deposits cause is over-heating and failure of boiler tubes. A deposit acts as an insulator and excessive deposits prevent an efficient transfer of heat through the tubes to the circulating water. This causes the metal itself to become over heated. When the overheating is severe enough the metal fails. Boiler deposits can also cause plugging or partial obstruction of corrosive attack underneath the deposits may occur. In general, boiler deposits can cut operating efficiency, produce boiler damage, cause unscheduled boiler outages, and increase cleaning expense.