Rewritten for Research Purposes by Nik Zafri bin Abdul Majid
Introduction
Water is the essential medium for steam generation. Conditioning it properly can increase the efficiency of the boiler as well as extend the boiler’s life. Treating boiler water also insures safe and reliable operation; without proper treatment, severe problems can develop, some so severe that the boiler itself can be destroyed.
This article will describe some of the more common and severe water-related problems that occur in industrial boilers and follows with a discussion on proper boiler water treatment. Each boiler and its water supply represents a unique situation. The information given here is a generalized discussion; it is important to enlist the help of experts, such as water service companies or consultants, to select the proper water treatment equipment and chemicals or to modify an existing program to increase boiler efficiency and reduce corrosion.
Boiler Water Problems
Boiler water problems generally fall into two classes: deposit-related and corrosion-related. Because the two often interact, it is very common to find a boiler experiencing both simultaneously. There are many instances where deposits cause corrosion and corrosion causes deposits. It is important to avoid both problems.
Deposit-Related Problems
Boiler Scale
One of the most common deposit problems is boiler scale. This happens when calcium, magnesium and silica, common in most water supplies, react with tube metal found in boilers to form a hard scale on the interior of the boiler tubes, reducing heat transfer and lowering the boiler’s efficiency. If allowed to accumulate, boiler scale can eventually cause the tubes to overheat and rupture. Scaling is one of the leading causes of boiler tube failures. Scale is equivalent to having a thin film of insulation between the furnace gases and boiler water.
It can drop a boiler’s efficiency by as much as 10-12%. Scale forms as the solubilities of the scaleforming salts in water decreases and the temperature and concentrations of the salts increases. When feedwater is elevated to boiler water temperature, the solubility of the scaleforming salts is decreased, and solid scale begins to form on the boiler systems.
Removing calcium and magnesium or other deposit-forming materials from the feedwater before they enter the boiler system is the best way to prevent scaling. Small amounts of hardness (calcium plus magnesium) can be effectively treated in the boiler and related system components by using boiler water treatment products such as chelates, polymers, and/or phosphates. Scale formation also occurs in economizers, feedwater pumps and related service lines. It also forms in low-pressure boilers where no pre-treatment or poorly maintained treatment chemicals, boiler water treatment products or pretreatment chemicals such as sodium zeolite are used. It is not normally found in boiler systems where demineralization is used or in high pressure, high purity systems.
Large amounts of hardness that cannot be successfully treated using boiler water treatment products must be treated by some other process.
Silica Scale
Silica scale is another kind of scale that affects boilers, much in the same manner as calcium and magnesium scale. Silica is found in most water supplies and it is not as easily removed as calcium and magnesium.
Silica can form several types of deposits, such as amorphous silica and magnesium silicate. Amorphous silica appears on boiler surfaces as a smooth, glass-like deposit that is very difficult to remove. A hydrofluoric acid-based cleaner is used to clean such affected surfaces. Magnesium silicate has a rough-textured tan to off-white appearance, and, while easier to remove than amorphous silica, is still difficult to remove.
Silica scale is found primarily in lower pressure systems where the pretreatment system uses sodium zeolite for softening and is not designed for silica removal. Silica-based deposits can also be found in high pressure systems where silica leakage through the anion unit(s) has occurred. Deposits form more readily as silica levels in-crease and hydrate alkalinities decrease. Silica deposits have high insulating properties which limit heat transfer and thus boiler efficiency and may also cause the failure. Silica can also distill from the boiler as silicic acid. Any silica carryover can promote deposits on steam turbine blades. Silica carryover at pressures about 600 psig (40 bar), becomes more serious as pressure increases.
Silica control can be done through pretreatment and proper boiler blowdown, and in low pressure boilers by maintaining at least a 3:1 ratio of hydrate alkalinity to silica in the boiler water.
Effects of Boiler Scale
The chemical structure of the scale, it’s porosity and the design and operational method of the boiler all influence the amount of heat lost. For example, 1/8-inch (3mm) of scale can cause a 2.0-3.0% loss in fire-tube boilers and water tube boilers.
A second but more serious effect from scale is the overheating of boiler tube metal, causing eventual tube failure. In modern boilers with high heat-transfer rates, even extremely thin layers of scale will cause a serious elevation in the temperature of tube material.
A third serious effect of scale formation is localized corrosion. Boilers with high heat transfer rates above 75,000 Btu/sqft/hr Effective Projected Radiant Surface (EPRS) are subject to localized corrosion, a situation where the deposits are actually causing the corrosion. This is a good ex-ample of the interaction between deposit-related and corrosion-related boiler water problems. Secondary corrosion is particularly present in systems with iron oxide deposits. The net effect is that the stack gas temperature may increase as the boiler absorbs less heat from the furnace gases, leading to increased pollution and more fuel consumption through inefficient operation.
Iron Deposits
Iron oxide is another compound which will accumulate on boiler surfaces. Iron enters the boiler in the feed-water or it can form in the boiler from corrosion. Iron oxides can be present in both soft and hard-scale deposits. Both types are frequently found at the same location, with the hard deposit existing as a layer next to the boiler tube and the soft layer on top of it.
Iron oxides are porous deposits, which will allow boiler water to seep through and “flash” to steam, leaving behind the dissolved solids. These dissolved solids in the boiler water, such as caustic and chelates, can concentrate in these localized areas to thousand of parts per million even though the water contains the normally recommended levels of these compounds. These excessive concentrations can result in rapid and severe metal dissolution and tube failure.
Minimizing Iron Deposit-Related Problems
The most obvious and effective way to minimize iron-related problems is to keep as muchiron out of the boiler as possible. The supply water should be subject to pretreatment techniquessuch as filtration, clarification, etc. Likewise, if steam condensate is returned to the boiler, actionshould be taken to minimize the corrosivity ofthe condensate through proper chemical treatment. chelates, polymers and phosphates (residual withand without polymer) can minimize iron deposits.Other areas that should receive attention include hot and cold lime softeners, filters, sodiumzeolite glands on feedwater pumps because they cancontribute iron to the system. These system components must be operating properly beforechemical treatment can be effectively applied.
Corrosion-Related Problems
Oxygen Attack
Dissolved oxygen interacts with boiler component surfaces, forming “pits” on the metal surface. These pits may eventually grow large enough to penetrate the metal, forcing a boiler shutdown.Oxygen present in boiler feedwater becomes very aggressive when heated, causing corrosive damage to preheaters and economizers. Oxygenwhich enters the boiler itself can also cause further damage to steam drums, mud drums, boiler tubes and headers. Damage can also occur to condensers and condensate piping from oxygenstill present in the steam.Controlling the oxygen content in the feedwater is done through deaeration and chemicaltreatment. Deaerators in steam generating systems use steam to strip oxygen from the feedwater. deaerator can effectively remove almost all the oxygen from the feedwater typically to less than 15ug/lppb (parts per billion) without the need to add an additional oxygen scavenger.
The most common scavenger is sodium sulfite,although other organic materials also work well.Some of these materials also form a protectiveoxide on large preheater and economizer surfaces. Scavengers cannot effectively substitutefor the function of the deaerator; if the oxygen content of the feedwater is greater than 50ug/1(ppb) then oxygen corrosion can occur evenwhen oxygen scavengers are used.Two of the most common causes of corrosionare the presence of carbon dioxide and oxygenin the condensate.
Carbon dioxide will form carbolic acid and reduce the pH of the condensateand cause acid attack while oxygen can directlyattack metal. The source of carbon dioxide incondensate is usually carbonate found in boilerwater carried over in the steam. Boilers using softened water are more prone to this than those using demineralized water. The presence of oxygenin condensate can be caused by poorly operating deaerators, leakage of air into vacuum condensers, leakage of cooling water and other factors.Treatment of condensate is done with neutralizing amines. Carbon dioxide reacts with water to form carbonic acid, a highly corrosive material that can attack equipment.
It cannot be emphasized strongly enough that the deaerator is the one piece of equipment in the water treatment process that should receive careful maintenance attention.
Caustic Attack
Caustic attack on boilers can take two forms: caustic gouging or caustic cracking, also called caustic embrittlement.
Caustic gouging causes deep elliptical depressions in metal boiler surfaces, which occur in areas of high heat flux or under heavy porous deposits, such as iron oxide deposits. This is another clear case of an interrelated deposit and corrosion problem. Underneath these deposits, boiler water can concentrate to the point where high caustic concentration accumulates, causing a localized corrosion. This very rapid action can take place and even cause a failure within a few days or even a few hours. Careful control of boiler water chemistry can prevent caustic gouging; if the “free hydroxide alkalinity” is set too high or uncontrolled, then caustic gouging may result. Prevention of porous deposit formation (such as iron oxide) eliminates a place for caustic gouging to occur.
Caustic cracking is a form of stress corrosion cracking that happens when a high concentration of caustic is present at a heated and stressed steel surface. These cracks can occur quickly and cannot be readily seen, sometimes causing a violent failure. All parts of the boiler are subject to this type of corrosion, including boiler tubes, headers, steam drums, mud drums, bolts, etc. Avoiding heated, stressed surfaces in boilers is not feasible, so care should be taken to prevent high concentrations of caustic from forming. However, maintaining an excessive “free hydroxide alkalinity” while using caustic to regenerate anion exchange resins and control the pH of the boiler water can cause high caustic concentrations.
Acid Attack
A third corrosion-related problem is caused when the boiler water pH drops below about 8.5. Known as acid attack, the effect exhibits rough pitted surfaces, with some of the pits being quite deep. Again, the presence of iron oxide deposits on boiler surfaces can encourage this kind of corrosion. A low boilerwater pH is usually caused by contamination of the boiler feedwater, from sources such as hydrochloric or sulfuric acid from leaks in demineralizers and condenser leaks of cooling tower water. Contamination can also occur from process leaks of acid or acid-forming materials into the return condensate system. Boiler feedwater pH should be continuously monitored.
Boiler Water Treatment
Boiler water treatment is grouped into three main areas:
- External treatment
- Internal treatment
- Condensate treatment
Quality requirements for boiler feed water and boiler water vary from system to system. Generally speaking, the higher the steam pressure, the higher the quality of water that is required. The table below shows generally good parameters for boiler operations but should not be applied to all situations. As stated before, a specific water treatment should be recommended by someone who has knowledge of both boiler water treatment practice and the conditions of the boiler to be treated.
External Treatment
This type of treatment involves the removal of impurities which from the boiler feedwater. Treatment falls into three categories, depending on what needs to be removed:
- Removal of suspended solids
- Removal of hardness and other soluble impurities
- Oxygen removal
- Removal of Suspended Solids
Untreated boiler feedwater frequently contains suspended matter such as mud, silt and bacteria. Left in the water, this material can cause problems, such as foaming or deposits in the boiler.
The process of clarification or filtration removes most suspended matter. One common method involves both processes; the water is first passed through a clarifier which removes most of the suspended matter, then a filter, which removes the rest.
To perform the clarification function, a flocculent aid is mixed with the raw water in either the raw water feed line or in the “rapid mix zone” of the clarifier. Primary clarification occurs in the “rapid mix zone”, where small solids are formed. The solids grow in size in the “slow mixing zone” and settle in the “settling zone”. The resulting sludge is removed from the bottom of the unit while the clarified water is drawn from the top by overflowing into a launder ( a device that functions much like a pool skimmer). A rake at the bottom of the clarifier moves slowly through the settled sludge to keep it from “setting up”, or solidifying.
Filtration can be done several different ways. The most common filters are granular media filters, made from sand, anthracite (hard coal) and garnet. Other types of filters, such as cartridge filters, sock filters and strainers are used in some installations. Filter media choice, filter bed depth and other design parameters are determined by the quality of the water and boiler requirements.
Removal of Hardness and Other Soluble Impurities
The second type of boiler water treatment involves the removal of impurities, such as calcium, magnesium and silica which, as discussed earlier, can cause scale. Common treatment methods to remove these impurities include lime softening, sodium cycle cation exchange (often called sodium zeolite softening), reverse osmosis, electrodialysis, and ion ex-change demineralization. Which treatment is most appropriate again depends on the water supply quality and the purity requirements of the boiler.
Quick or slaked lime added to hard water, reacts with the calcium, magnesium and, to some extent, the silica in the water to form a solid precipitate. The process typically takes place in a clarifier. The lime is added to the “rapid mix zone”, where it reacts with some of the calcium, magnesium and silica. The combined precipitate is removed from the bottom of the clarifier and the treated water is now softer than the untreated inlet water but still unsuitable for the boiler.
Lime softening treatment is followed by either sodium cycle cation exchange or ion exchange demineralization. Cation exchange is usually picked for lower pressure boilers (450 psig) and demineralization for higher pressure boilers (above 600 psig).
Ion exchange is just what it implies: a process that exchanges one type of ion (charged particle) for another. Many troublesome impurities in supply water are ions, making this process extremely important in boiler water treatment. Ion exchange takes place in a closed vessel which is partially filled with an ion exchange resin. The resin is an insoluble, plastic-like material capable of exchanging one ion for another. There are two types: cation and anion resins. Each is capable of exchanging one or the other types of ions.
Another method of ion exchange involves a sodium exchange softener, where hard water enters the unit and the calcium and magnesium are exchanged for sodium. The treated water will normally have most of the hardness removed, but will still contain other impurities. This method is suitable only for low pressure boilers.
If very pure water is required, for high pressure boilers for example, then demineralization is required. A demineralizer contains one or more cation exchange beds, followed by one or more anion exchange beds.
In the demineralizer, water is treated in two steps. First, it is passed through the cation exchange bed, where the cations (calcium, magnesium and sodium) are exchanged for hydrogen ions. The treated water is now free of cations but is too acidic and cannot yet be used in the boiler.
In the second step the water passes through the anion exchange bed where the anions (sulfate, chloride, carbonate and silica) are exchanged for hydroxide ions. The hydrogen and hydroxide ions react to form water, now suitable for use in the boiler.
For higher purity water, more elaborate systems are employed than the one shown here, but the basic principle remains the same.
Ion exchange resins have a limited capacity and will eventually become exhausted. They can be regenerated however; sodium cycle cation exchange beds are regenerated with brine, cation exchange beds are regenerated with hydrochloric or sulfuric acid and the anion exchange beds become regenerated with caustic soda.
Other technology is sometimes employed to remove undesirable impurities from the water supply, including reverse osmosis, electrodialysis, and electrodialysis with current reversal. These are all known as membrane processes. Reverse osmosis uses semipermeable membranes that let water through but block the passage of salts. In the case of electrodialysis, the salts dissolved in the water are forced to move through cation-selective and anion-selective membranes, removing the ion concentration.
Oxygen Removal with a Deaerator
The third type of boiler water treatment involves the removal of dissolved oxygen in the water.
A deaerator (sometimes called a dearating heater) takes advantage of the fact that the solubility of oxygen in water decreases as the water gets hotter. The oxygen is removed by spraying the untreated boiler water onto trays in the deaerator, where it makes inti-mate contact with steam rising through the tray. The steam heats the water while stripping the oxygen. Proper functioning of the deaerator requires that the two non-condensable gases, oxygen and nitrogen be vented away from the water being treated. Deaerated water should have an oxygen concentration of less than 15ppb (ug/l).
Maintenance Suggestions with a Deaerator
Here are some things to remember to keep the deaerator operating properly:
1. Steam should be vented from the deaerator. The deaerator vent should always be open to remove scrubbed gases. Steam is carried along with the gases. The plume should form about six inches from the top of the vent and be visibly steam for only two feet. This is enough to remove gases; more than two feet is a waste of steam.
2. Check the oxygen concentration of the deaerated feedwater to be sure it is functioning properly. This can be done using simple calorimetric tests. Be sure to turn the oxygen scavenger off before taking any measurements.
Internal Treatment
Internal boiler water treatment continues the process of purifying the water begun using external treatment methods. All treatment additives discussed here are designed to assist with managing corrosion or deposits. A good internal treatment program can protect boilers which use a proper quality of feedwater but it cannot protect boilers with grossly contaminated boiler feedwater. It is essential to have both external and internal treatment procedures that are effective, well-maintained and closely monitored.
Boilers are typically protected from corrosive attack by a thin film of magnetite (a black magnetic iron oxide) which forms on the surface of the boiler metal. Water treatment programs should be designed that encourage and maintain this protective film by maintaining the proper pH, assuring the absence of oxygen through a deaerator, the use of an oxygen scavenger, and the employment of other chemical additives.
Oxygen Scavengers
Most of the oxygen in boiler feedwater is removed by the deaerator but trace amounts are still present which can, over time, cause boiler corrosion. To prevent this, oxygen scavengers are added to the boiler water, preferably in the storage tank of the deaerator so the scavenger will have the maximum time to react with the residual oxygen. Under certain conditions, such as when boiler feedwater is used for attemperation to lower steam temperature, other locations are preferable.
The most commonly used oxygen scavenger is sodium sulfite. It is inexpensive, very effective and rapidly reacts with the trace amounts of oxygen. It is also easily measured in boiler water.
In most cases it i the oxygen scavenger of choice. There are instances in some higher pressure boilers (generally above 900 psig), that some of the sulfite may decompose and enter the steam, causing problems in the condensate systems and condensing steam turbines. In these cases, substitute (usually organic-based) oxygen scavengers can be used.
New oxygen scavengers have been introduced in recent years. The decision to use them or rely on sodium sulfite should only be made by those qualified to make boiler water treatment decisions. In all cases the new product should be carefully added and its effectiveness evaluated in accordance with operating procedures.
Other Chemical Additives
Phosphate
Used almost as often as oxygen scavengers, phosphate plays several important roles in boiler water treatment:
It buffers the boiler water pH to minimizethe potential for boiler corrosion.It precipitates small amounts of calcium ormagnesium into a soft deposit which canthen accumulate in mud drums or steamdrums rather than as hard scale.It helps to promote the protective oxide filmon boiler metal surfaces.
Common phosphate compounds added to treat boiler water include sodium phosphate (monosodium phosphate, disodium phosphate or trisodium phosphate) or sodium polyphosphate. They all function approximately the same; the choice of which to use depends on the quality of the boiler water and the handling requirements of the user.
As phosphate functions as a precipitating boiler water treatment, creating a sludge as it reacts with hardness, a procedure should be established to remove the sludge during a routine boiler shutdown. The rate of sludge accumulation varies according to the hardness of the water entering the boiler and the operating conditions of the boiler. Although boilers treated using phosphates (without chelates or polymers) tend to require more frequent cleaning, they also tend to show very low corrosion rates.
Chelates and Polymers
Rather than precipitate with hardness compounds, chelates and polymers “solubilize” or combine with hardness (calcium and magnesium and to some extent iron) to form a stable chemical compound. Sometimes they are used in conjunction with phosphate.
The resulting compounds can be eliminated by blowdown.
The two most commonly used chelates are nitrilotriacetic acid (NTA) and ethylenediamine tetraacetic acid (EDTA). Either product can be used in low pressure (up to 150psig), while EDTA is preferred for higher pressure boilers. They have been successfully used for many years; how-ever, since many water treaters base the chelate dosage on the amount of hardness encountered in the boiler feedwater, other water treatment programs are often chosen when large swings in feedwater hardness occur on a frequent basis. A well-operating deaerator is important when using chelates.
Most polymers used in boiler water treatment are synthetic in composition. Some act like chelates but none are as strong as EDTA. Polymers disperse suspended solids; thus they are referred to as “dispersents”. A wide variety of different polymers are available; some are effective in controlling hardness deposits, while others in controlling iron deposits.
In some cases the most effective treatment program uses a combination of chelates and polymers. Again, the decision to use one or the other or a combination of both should be made by those who understand the functions of various polymers and the needs of the boiler. The uses of steam might also be considered. Steam used in food processing has specific chemical addition restrictions. Lastly, these treatment measures are only effective when boiler feedwater pretreatment is effective.
Blowdown
Blowdown is a very important part of any water treatment program. Its purpose is to limit the concentration of impurities in the boiler water. The right amount of blowdown is critical: too much results in energy loss and excessive chemical treatment cost; too little and excessive concentrations of impurities build up. There are no hard and fast rules as to the amount of blowdown because of the variation in water quality varies from place to place. It can range from 1% (based on feedwater flow) to as much as 25%.
Location varies; it can be from beneath the water surface in the steam drum, from the mud drum or bottom header, or it can be from the bottom of the boiler. Blowdown can be continuous or intermittent. Here are some principles to help establish an effective blowdown program:
1. In drum-type boilers, the concentration of the water should be controlled by blowdown from the steam drum. Continuous blowdown is preferred.
2. Also in drum boilers, blowing from the mud drum or bottom headers removes suspended solids from the boiler. Trying to control the concentration of impurities by blowdown from this location can cause a severe disruption of circulation in the boiler, causing damage to the boiler. Bottom blowdown should be of short duration, on a regular basis. These are determined by boiler design, operating conditions and the accumulation rate of suspended solids.
3. Fire tube boiler blowdown can be either continuous or intermittent. It can be blown down from below the surface or from the bottom. Type, frequency and duration depend on boiler design, operating conditions and the type of water treatment program.
A way to reduce the energy loss is to install a continuous blowdown heat recovery device. These are now economical for blowdowns as low as 500 lb/hr.
Other Internal Treatment Materials
Caustic, in the form of sodium hydroxide or potassium hydroxide or a combination of the two, can be used to control the pH of boiler water. Sometimes caustic is used in conjunction with polyphosphate.
Other chemicals used in boiler water treatment are ammonia and hydrazine. Since both materials are nonprecipitating and will volatize into the steam, they are commonly referred to as “all volatile treatment” (AVT). Used frequently in large electrical power generating plants, they are less common in other industries because hydrazine must be treated as an extremely hazardous substance and AVT treatment is ineffective for harder water.
Condensate Treatment
Corrosion of condensers, steam traps and condensate piping is common. Adding a basic material, such as amines to the steam will neutralize the acid as the steam condenses, keeping the pH of the condensate high. Neutralizing amines can only protect the system from acid attack from carbon dioxide.
The most commonly used amines are morpholine, cyclohexylamine and diethylaminoethanol (DEAE). A mixture of amines is usually required, since most steam/condensate systems are quite large and contain numerous condensers. Using a mixture assures protection through-out the system because some amines condense faster and the slower condensing ones will be able to protect equipment that is farther down the system. Someone who is very familiar with the steam/condensate system and knows the chemical and physical properties of amines should be employed to assist in the selection of the best mix of amines.
Protection from oxygen attack can be achieved using film amines. These compounds form a very thin film of organic material on metal surfaces which acts as a barrier to oxygen but has little effect on the pH of the condensate. There is minimal effect on heat transfer because the film is thin. Like neutralizing amines, they are added to the boiler water or they can be added directly to the steam. Two commonly used amines are dodecylalamine and octadeccylamine.
Benefits From a Proper Water Treatment Program
Emphasis has been made in several instances of the importance of using knowledgeable people to ensure proper evaluation of water treatment needs. It is always best to use someone familiar with the boiler system operation as well.
As an example, an Arizona manufacturer saved almost $100,000 a year after contracting with an outside water treatment firm who had run a computerized analyses on the efficiency of the firm’s six chillers. The company was spending over $50,000 on water treatment chemicals, an amount they considered excessive.
The consultants found one of the six units operating at only 56% of its maximum efficiency. The problems identified included scale, which was treated with an inhibited acid. A second analysis was performed and the efficiency of the unit rose dramatically to 99.5%. Based on this improvement, the manufacturer ordered a complete cleanup program on the entire system.
Another problem that was discovered was that the facility’s five cooling towers had accumulated a significant amount of dirt an biological growth. This was removed by scraping and washing with pressure hoses.
As a result of this cleaning and new water treatment program, this manufacturer will save about $26,000 on water treatment chemicals and a projected $70,000 in energy costs because of cleaner heat transfer surfaces in the condenser tubes. Not included in the savings figure is the 2.5 million gallons of water that is also saved, resulting from increased cycles of concentration within the cooling towers.
Operator’s Checklist for Water Treatment Systems Maintenance
As has been stressed, water treatment is a highly specialized, highly individual process and, as such, should only be undertaken with the advice and help of outside water service companies or consultants. Such organizations are in a position to analyze the water used in a boiler at several different stages, make recommendations for additives or treatment and, in many cases, provide a monitoring service to be sure the proper chemical balance is maintained.
Reference :
CIBO - ENERGY EFFICIENCY HANDBOOK (Chapter 2)
COUNCIL OF INDUSTRIAL BOILER OWNERS (CIBO),
6035 BURKE CENTRE PARKWAY, SUITE 360BURKE, VA 22015,
Edited BY RONALD A. ZEITZ
