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Introduction
Boiler Water Flow Schematic
Softener & Demineralization
Chemicals
Clarification
Filtration
Water Quality Monitoring
pH Treatment
Reverse Osmosis
Introduction
Untreated water contains dissolved minerals, gases and particulates. The removal or otherwise 'treatment' of each of these is critical to efficient boiler operation for different reasons. Minerals lead to scaling that acts as in insulator reducing boiler efficiency; gases can be corrosive and particulates can contribute to both problems. Water treatment is dynamic and varies from boiler to boiler and can vary month to month with the same boiler.
Water quality is primarily an issue with steam boilers that use a lot of make-up water. Closed-system hot water boilers are the least affected by water quality because they use the least amount of make-up water and operates at lower temperatures.
The common minerals in water that lead to scaling problems are iron, calcium, magnesium and silica. When water containing these dissolved minerals is heated, it looses its ability to hold the minerals in solution. When they come in contact with metal boiler parts, scale forms. In addition to reduced efficiency, scale can lead to boiler tube failure if the tubes are over-heated.
Oxygen and certain other gases in water are corrosive. Deaerators and chemicals that remove oxygen can reduce the corrosiveness of the water.
Primary indicators of boiler water treatment are pH, TDS (Total Dissolved Solids), TSS (Total Suspended Solids) and hardness.
Water pH
Water pH is a measure of its relative acidic or alkalinity. A neutral level is pH = 7. A number lower than 7 is acidic and higher than 7 is alkalinic (caustic). Both extremes are corrosive to boiler metal.
More on pH Treatment
Methods to Remove Water Impurities
The best way to remove impurities is before they enter the boiler. Small amounts of impurities can be effectively treated inside the boiler to keep them in solution or allow them to be discharged via blowdown.
External Treatment
External treatment refers to the chemical and mechanical treatment of the water source. The goal is to improve the quality of this source prior to its use as boiler feed water, external to the operating boiler itself. Such external treatment may include:
1. Clarification (removes solids, very large boiler systems)
2. Filtration (removes solids)
3. Softening and Demineralization (removes dissolved minerals)
4. Dealkalization
5. Deaeration and Heating (removes oxygen and other corrosive gases)
Internal Treatment
Even after the best and most appropriate external treatment of the water source, boiler feed water (including return condensate) still contains impurities that could adversely affect boiler operation. Internal boiler water treatment is then applied to minimize the potential problems and to avoid any catastrophic failure, regardless of external treatment malfunction.
1. Addition of chemicals (pH Control, Oxygen Removal, other)
2. Blowdown (removes accumulated solids from boiler water)
Monitoring Water Quality
Water quality monitoring varies from weekly litmus test strips to continuous electronic instrumentation and automated chemical treatment. The size of the boiler, the importance of water quality and the skills of the boiler operators are all factors in deciding how best to monitor boiler water quality.
More on Water Quality Monitoring
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Water Treatment Schematic
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Water Softener and Demineralization |
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Introduction
 The removal of impurities, such as calcium, magnesium, iron and silica which can cause scale, is known as water softening or demineralization. Common treatment methods to remove these impurities include lime softening, sodium cycle cation exchange (often called sodium zeolite softening), reverse osmosis, electro dialysis, and ion ex-change demineralization. Which treatment is most appropriate depends on the water supply quality, the purity requirements of the boiler, and to some extent - the budget.
Water Hardness is measured in grains per gallon or ppm. The conversion is 17.1 ppm = 1 grain
One cubic foot of softener resin is typically good for 30,000 grains in exchange. Softeners are typically set to regenerate once the resin is 90% exhausted. Regeneration is accomplished with a variety of chemicals for various purposes, but is commonly simple table salt brine, NaCl. The last part of the regeneration cycle is fresh water flush to prevent salt from entering the boiler.
Operations
Quick Lime and Clarifiers
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 ex-change demineralization. Cation exchange is usually picked for lower pressure boilers (450 psig) and demineralization for higher pressure boilers (above 600 psig).
Ion Exchange
 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.
Cation = positively charged Ions
Anion = negatively charged 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 ex-changed for hydroxide ions. The hydrogen and hydroxide ions react to form water, now suitable for use in the boiler. A third ion exchange could be used to control alkalinity.
For higher purity water, more elaborate systems are employed, 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 salt brine, cation exchange beds are regenerated with hydrochloric or sulfuric acid and the anion exchange beds become regenerated with caustic soda. Salt brine regeneration is followed by a fresh water rinse to assure that no salt enters the boiler.
Dealkalizers
Dealkalizers reduce the alkalinity of softened water through a chloride anion exchange process. Softened water is passed through the anion exchange resin where bicarbonate, carbonate, sulfate and nitrate ions are exchanged for chloride ions. The anion exchange resin is regenerated by salt (NaCl) and softened water. Some dealkalizers add a small amount of caustic during the regeneration cycle to increase capacity and provide a slightly elevated pH level.
The primary benefit of using dealkalized water is the prevention of CO2 generation inside of the boiler. CO2 leaves the boiler with the steam and can form carbonic acid in the condensate, leading to the primary cause of condensate system corrosion.
Other Technologies
Other technology is sometimes employed to remove undesirable impurities from the water supply, including reverse osmosis, electro dialysis, and electro dialysis with current reversal. These are all known as membrane processes. Reverse osmosis uses semi permeable membranes that let water through but block the passage of salts. In the case of electro dialysis, the salts dissolved in the water are forced to move through cation-selective and anion-selective membranes, removing the ion concentration.
More Information
More on Reverse Osmosis
Water Treatment
Water Treatment Schematic
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Oxygen Scavengers
Sulfites - typically for boilers up to 800 psi; sulfites react with oxygen to form sulfates that are removed from the boiler via blowdown. There are two forms of sulfite: Catalyzed - uses a catalyst to improve reaction time; Non-catalyzed - slower reaction time and must be used in hot water
Hydrazine - typically for boilers over 800 psi. At pressure higher than 800 psi, sulfite begins to break down into acidic gases of sulfur. Sulfite also creates additional Total Dissolved Solids (TDS) which is a problem for high pressure applications. Hydrazine removes O2 without producing acidic gases or TDS, but is considered a possible carcinogen.
Hydroxides
Sodium Hydroxide - NaOH or Caustic Soda, or Soda Ash - used to maintain boiler water pH in the 10.0 - 11.5 range. Hydroxide increase boiler alkalinity to prevent acidic corrosion. If heavy scale is present, caustic soda can accumulate to cause Caustic Attack. See pH Treatment
Calcium-Hydroxide - reacts with calcium and magnesium bicarbonates to form sludge that is removed via blowdown.
Phosphates
Phosphate treatment causes calcium and magnesium to precipitate into sludge where it can be removed via blowdown.
Polymers
Polymers are long, complex molecules that attach to impurities and prevent them from sticking to boiler metal to form scale. This creates TDS that are removed via blowdown.
Chelants
Chelants can prevent scale from forming and over time, remove existing scale. Chelants in contact with 02 is corrosive. It must therefore be used in a 02- free environment.
Neutralizing Amines
Neutralizing amines hydrolyze in water to generate the necessary hydroxide ions required for neutralization of the carbon dioxide. The normal approach to treating systems with these amines is to feed sufficient quantity to neutralize the carbon dioxide and then provide small additional amounts to buffer the pH to 8.5 or 9.0. At this pH, continued preservation of the magnetite film (boiler metal) is also achieved. It is also implied that corrosion will not exist at a pH>8.0-8.5.
Filming Amines
Filming amines function by forming a protective barrier against both oxygen and carbon dioxide attack. These amines form films directly with the condensate line metal and develop a barrier to prevent contact of the corrosive condensate with the return piping. By design, film formers have been developed to function best at a pH of 5.5-7.5. In addition, these amines are highly surface-active and will slough loosely adherent iron oxide and other corrosion products back to receiving points or to the boiler. Care must be exercised with the feed of filming amines.
Combination Amines
 Over the past several years, combinations of filming and neutralizing amines have been shown to be extremely effective, particularly in complex systems. While the combination amine is still functionally a filmer, the neutralizing amine portions provide for reduction in fouling potential and more uniform coverage of the filmer.
Filming amines and combination amines are generally fed to steam headers. Dosages are based on steam production.
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Water Quality Monitoring
Water Treatment
pH Treatment
Clarification
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Introduction
Water clarification is the process of removing suspended solids from water. Most of the suspended matter in water would settle, given enough time, but in most cases the amount of time required would not be practical. The time required for settling is dependent on many factors, including: weight of the particle, shape of the particle, size of the particle, and viscosity and/or frictional resistance of the water, which is a function of temperature.
The process of clarification generally involves the addition of coagulation chemicals to a water supply, mixing under heavy agitation to cause the particles to cling together and grow larger, and mechanically removing them from the water.
Clarification is commonly used for large boilers that consume large volumes of water from surface water sources.
Clarification is commonly used in combination with filtration for total removal of solids.
Operation
Chemical Treatment
Coagulation
Coagulation, the first step in complete clarification, is the neutralization of the electrostatic charges on colloidal particles. Because most of the smaller suspended solids in surface waters carry a negative electrostatic charge, the natural repulsion of these similar charges causes the particles to remain dispersed almost indefinitely. To allow these small suspended solids to agglomerate, the negative electrostatic charges must be neutralized. This is accomplished by using inorganic coagulants (water soluble inorganic compounds), organic cationic polymers or polyelectrolytes.
The most common inorganic coagulants are:
1. Alum-aluminum sulfate - Al2(SO4)3
2. Ferric sulfate - Fe2(SO4)3
3. Ferric chloride - FeCl3
4. Sodium aluminate - Na2AI204
Organic coagulants serve the same function as the inorganic metal salts, but the process is simpler because the charge neutralization reaction is the only concern. Polymer addition has no effect on pH or alkalinity, so no supplemental chemical feed is required to control either.
Polymers can be envisioned as long chains with molecular weights of 1000 or less to 5,000,000 or more. Along the chain are numerous charged sites. In primary coagulants, these sites are positively charged. The sites are available for adsorption onto the negatively charged particles in the water. To accomplish optimum polymer dispersion and polymer/particle contact, initial mixing intensity is critical. The mixing must be rapid and thorough. Polymers used for charge neutralization cannot be over-diluted or over-mixed. The farther upstream in the system these polymers can be added, the better their performance.
Because most polymers are viscous, they must be properly diluted before they are added to the influent water. Special mixers such as static mixers, mixing tees and specially designed chemical dilution and feed systems are all aids in polymer dilution.
Flocculation - The Second Step of the Coagulation Process
Once the negative charges of the suspended solids are neutralized, flocculation begins. Charge reduction increases the occurrence of particle-particle collisions, promoting particle agglomeration. Portions of the polymer molecules not absorbed protrude for some distance into the solution and are available to react with adjacent particles, promoting flocculation. Bridging of neutralized particles can also occur when two or more turbidity particles with a polymer chain attached come together. It is important to remember that during this step, when particles are colliding and forming larger aggregates, mixing energy should be great enough to cause particle collisions but not so great as to break up these aggregates as they are formed.
In some cases flocculation aids are employed to promote faster and better flocculation. These flocculation aids are normally high molecular weight anionic polymers. Flocculation aids are normally necessary for primary coagulants and water sources that form very small particles upon coagulation. A good example of this is water that is low in turbidity but high in color (colloidal suspension).
Color Removal
By far the most difficult impurity to remove from most surface waters is color (from dissolved or colloidal suspensions of decayed vegetation) and other colloidal suspensions.
Color in surface water normally is a result of its contact with decayed vegetation and is composed of tannins and lignins, the components that hold together the cellulose cells in vegetation. In addition to their undesirable appearance in drinking water, these organics can cause serious problems in downstream water purification processes. For example:
1. Expensive demineralizer resins can be irreversibly fouled by these materials
2. Some of these organics have chelated trace metals, such as iron and manganese within their structure, which can cause serious deposition problems in a cooling system.
There are many ways of optimizing color removal in a clarifier:
1. Prechlorination (before the clarifier) significantly improves the removal of organics as well as reducing the coagulant demand.
2. The proper selection of polymers for coagulation has a significant impact on organic removal.
3. Color removal is affected by pH. Generally, organics are less soluble at low pH.
Conventional Mechanical Clarification
The simplest form of clarification uses a large tank or horizontal basin for sedimentation of flocculated solids. The basin may contain separate chambers for rapid mix, slow mix and settling. The first two steps are important for good clarification. An initial period of turbulent mixing is necessary for contact between the coagulant and the suspended matter, followed by a period of gentle stirring to increase collisions between particles and increase floc size. Typical retention times are 3-5 minutes for rapid mix, 15 to 30 minutes for flocculation, and 4-6 hours for settling.
The coagulant is added to the waste water in the rapid mix chamber or just upstream, and the water passes through the mix chambers into the settling basin. As the water passes along the length of the basin (or out to the circumference in the case of a circular clarifier), the flocculated particles settle to the bottom and are scraped into a sludge collection basin for removal and disposal. Clear water flows over a weir and is usually held in a tank called a clearwell.
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Water Filtration
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 Introduction
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 re-moves the rest.
Operation
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.
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Clarification
Water Treatment
Water Treatment Schematic
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Introduction
Water quality monitoring varies from weekly litmus test strips to continuous electronic instrumentation and automated chemical treatment. The size of the boiler, the importance of water quality and the skills of the boiler operators are all factors in deciding how best to monitor boiler water quality.
Common water monitoring is for oxygen, Total Dissolved Solids (TDS), and pH. A different type of instrument is required for each.
Sample Meters and Instruments
DLR Mechanical Services
The DLS10000 series Blowdown Heat Recovery System adjust automatically to changing system demands, and recover up to 90% of the heat normally lost during boiler surface blowdown operation.
Blowdown/Heat Recovery systems will usually result in a payback in a few short months from fuel savings alone.
The DLS 10000 series Packaged Blowdown Heat Recovery System provides several features not found in other units:
1. It automatically controls the surface blowdown to maintain the desired level of total dissolved solids (TDS) in the boiler, reducing the amount of blowdown to a minimum.
2. It recovers the heat from the high temperature blowdown, and transfers it to the incoming cold make-up water, maximizing boiler efficiency.
3. The conductivity controller controls the actual boiler conductivity ( TDS ) level, keeping blowdown to the required minimum and reduces chemical costs
4. The BTU system records the actual energy saved and the amount of make-up water used. Invaluable information for the boiler operator and plant engineer
5. The Stainless Steel blowdown heat exchangers are uniquely designed to handle the blowdown and make up water. The unique spiral plate design provides U Factors as high as 1000 BTU/SqFt/Degree, and maintains high fluid velocities preventing scaling and fouling.
Â
Significant Fuel Savings for Any Size Boiler: Transfers the blowdown heat to the make-up, thereby decreasing fuel costs. Â
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Introduction
What is pH?
Acidic and basic (alkaline) are two extremes that describe chemicals, just like hot and cold are two extremes that describe temperature. Mixing acids and bases (alkaline) can cancel out their extreme effects; much like mixing hot and cold water can even out the water temperature. A substance that is neither acidic nor basic (alkaline) is neutral.
The pH scale measures how acidic or basic (alkaline) a substance is. It ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic, and a pH greater than 7 is basic (alkaline). Each whole pH value below 7 is ten times more acidic than the next higher value. For example, a pH of 4 is ten times more acidic than a pH of 5 and 100 times (10 times 10) more acidic than a pH of 6. The same holds true for pH values above 7, each of which is ten times more alkaline (another way to say basic) than the next lower whole value. For example, a pH of 10 is ten times more alkaline than a pH of 9.
Pure water is neutral, with a pH of 7.0. When chemicals are mixed with water, the mixture can become either acidic or basic (alkaline). Vinegar and lemon juice are acidic substances, while laundry detergents and ammonia are basic (alkaline).
Chemicals that are very basic or very acidic are called "reactive." These chemicals can cause severe burns. Automobile battery acid is an acidic chemical that is reactive. Automobile batteries contain a stronger form of some of the same acid that is in acid rain. Household drain cleaners often contain lye, a very alkaline chemical that is reactive.
An Acid is a substance that produces H3O+ (H+) when it is dissolved in water. It is a proton donor and an electron pair acceptor or a species that donates protons. For example: HCl, NH4, AlCl3.
A Base is a substance that produces an OH- when it is dissolved in water (Arrhenius). A proton acceptor (Bronsted), or an electron donor. For example: NaOH, KOH, CH3NH2.
H2O can act as an acid or a base because it auto-ionizes itself, meaning it gives protons back and forth within itself, thus acting as both an acid and a base.
Boiler pH
Natural water is usually between 6.5 and 7.5 pH. A common recommendation is to maintain boiler water at 8.5 pH.
Acidic water is corrosive. Alkalinic water is more prone to scaling.
Alkalinity is a measure of the bicarbonate (HCO3), carbonate (CO3) and hydroxyl (OH) ions in the water. pH and alkalinity ratings are NOT the same and are NOT proportional. pH is rated on the Scale and alkalinity is measured in parts per million (ppm). A typically recommended alkalinity rating is 140 - 700 ppm for boilers operating below 300 psi.
Controlling pH
pH is controlled by either removing water impurities or adding other chemicals to neutralize the condition. For example, Caustic Soda, an alkaline, is added to neutralize CO3, carbonic acid.
pH Related Corrosion
Acid Attack
When the boiler water pH drops below about 8.5, a corrosion called acid attack can occur. The effect exhibits rough pitted surfaces. 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.
Caustic Attack
Caustic attack on boilers is a localized attack due to extremely high pH (12.9 +). It can take two forms: caustic gouging or caustic cracking, also called caustic embrittlement. Caustic attack or caustic corrosion is often encountered in phosphate treated boilers in which deposits of phosphates or other 'scale' occur in high heat transfer areas. Boiler water can permeate the porous deposit resulting in localize corrosion. When it is coupled with significant heat flux, concentration of the boiler water occurs rapidly speeding the corrosion.
The corrosion action is a result of the formation of caustic-ferritic compounds through the dissolving of the protective magnetite film. Once the process begins, the iron in contact with the boiler water will attempt to restore the protective magnetite film. Caustic corrosion (typically in the form of gouging) continues until the deposit is removed or the caustic concentration is reduced to normal.
Caustic soda (NaOH) is the only normal boiler water constituent that has high solubility and does not crystallize under typical boiler conditions. Its caustic concentration can be as high as 10,000-100,000 ppm.
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.
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 Introduction
Reverse Osmosis systems, RO, use pressure to force water through a semi-permeable membrane. Over 95% of all impurities can be removed. RO systems are one of the most expensive systems to install, but can substantially reduce additional chemical treatment requirements, leading to an acceptable payback.
RO systems are typically used ahead of DI (de-ionization) systems. The DI system can further treat the water for impurities and control pH.
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Water Treatment
Water Softener
Water Treatment Schematic
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