Boiler Water Treatment is necessary to Producing quality steam on demand depends on properly managed water treatment to control steam purity, deposits and corrosion. There are two main types of boiler feed water treatment Internal boiler Water Treatment method and external boiler water treatment. A boiler is the sump of the boiler system. It ultimately receives all of the pre-boiler contaminants. Boiler performance, efficiency, and service life are direct products of selecting and controlling feedwater used in the boiler.
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External Treatment. Chapter 04 - Aeration; Chapter 05 - Clarification; Chapter 06 - Filtration; Chapter 07 - Precipitation Softening; Chapter 08 - Ion Exchange; Chapter 09 - Membrane Systems; Boiler Water Systems. Chapter 10 Boiler Feedwater Deaeration; Chapter 11 Preboiler And Boiler Corrosion Control; Chapter 12 Boiler Deposits: Occurence And. Army Technical Manual (TM) 5-650, Central Boiler Plants and Public Works Technical Bulletin (PWTB) 420-49-05 Industrial Water Treatment Procedures define phosphate and dispersants (polymers or tannin) as the accepted treatment for deposit control. Phosphate comes as. Chemical treatment of hot water heating boilers is discussed in this section and also in chapter 4 (cooling water). This chapter is organized by chemical treatment (oxygen scavengers, internal treatments, condensate treatment, etc.).
When BFW boiler feedwater enters the boiler, the elevated temperatures and pressures cause the components of water to behave differently. Most of the components in the feedwater are soluble. However, under heat and pressure, most of the soluble components come out of the solution as particulate solids, sometimes in crystallized forms and other times as amorphous particles. When the solubility of a specific component in water is exceeded, scale or deposits develop. The boiler water must be sufficiently free of deposit forming solids to allow rapid and efficient heat transfer and it must not be corrosive to the boiler metal.
Deposit Control in Boiler Water Treatment
Deposits in boilers may result from hardness contamination of feed water and corrosion products from the condensate and feedwater system. Hardness contamination of the feed water may arise due to a deficient softener system. Deposits and corrosion result in efficiency losses and may result in boiler tube failures and inability to produce steam. Deposits act as insulators and slow heat transfer. Large amounts of deposits throughout the boiler could reduce the heat transfer enough to reduce the boiler efficiency significantly. Different type of deposits affects boiler efficiency differently. Thus it may be useful to analyze the deposits for its characteristics. The insulating effect of deposits causes the boiler metal temperature to rise and may lead to tube-failure by overheating.
Impurities causing deposits
The most important chemicals contained in water that influences the formation of deposits in the boilers are the salts of calcium and magnesium, which are known as hardness salts. Calcium and magnesium bicarbonate dissolve in water to form an alkaline solution and these salts are known as alkaline hardness. They decompose upon heating, releasing carbon dioxide and forming a soft sludge, which settles out These are called temporary hardness that can be removed by boiling. Calcium and magnesium sulfates, chlorides and nitrates, etc. when dissolved in water are chemically neutral and are known as non-alkaline hardness. These are called permanent hardness and form bard scales on boiler surfaces, which are difficult to remove. Non-alkalinity hardness chemicals fall out of the solution due to reduction in solubility as the temperature rises, by concentration due to evaporation which takes place within the boiler, or by the chemical change to a less soluble compound.
The presence of silica in boiler water can rise to the formation of bard silicate scales. It can also associate with calcium and magnesium salts, forming calcium and magnesium silicates of very low thermal conductivity. Silica can give rise to deposits on steam turbine blades which can reduce plant efficiency after been carried over either in droplets of water in steam or in the volatile form in steam at higher pressures. Two major types of boiler water treatment are Internal water treatment and External water treatment.
Internal Water Treatment
Boiler Internal treatment is carried out by adding chemicals to the boiler to prevent the formation of scale by converting the scale-forming compounds to free-flowing sludge, which can be removed by taking blowdown. This method is limited to boilers small package low-pressure boilers, where feed water is low in hardness salts, to low pressures- high TDS content in boiler water is tolerated, and when an only a small quantity of water is required to be treated. If these conditions are not applied, then high rates of blowdown are required to dispose off the sludge. They become uneconomical from heat and water loss consideration. both internal and external water treatment are recommended. Only inter boiler water treatment method is not enough to get good results.
External Water Treatment
External treatment is used to remove suspended solids, dissolved solids (particularly the calcium and magnesium ions which are major causes of scale formation), and dissolved gases (oxygen and carbon dioxide). The external treatment processes available are ion-exchange or demineralization process, RO reverse osmosis and Deaeration. Before any of these are used, it is necessary to remove suspended solids and turbidity from the raw water, because these may foul the resins used in the subsequent treatment sections.
Methods of pre-treatment sedimentation, clarification, coagulation, and aeration of water will be discussed later.
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For industrial companies using a boiler for its facility, some type of boiler feed water treatment system is usually necessary to ensure an efficient process and quality steam generation. The most appropriate boiler feed water treatment system will help the facility avoid costly plant downtime, expensive maintenance fees, and boiler failure as a result of scaling, corrosion, and fouling of the boiler and downstream equipment.
But what is a boiler feed water treatment system and how does it work?
The complex answer to this question (which largely depends on the quality and quantity of makeup water needed for the boiler on an individual basis) is simplified and broken down for you below:
What is a boiler feed water treatment system?
A boiler feed water treatment system is a system made up of several individual technologies that address your specific boiler feed water treatment needs.
Treating boiler feed water is essential for both high- and low-pressure boilers. Ensuring the correct treatment is implemented before problems such as fouling, scaling, and corrosion occur, will go a long way in avoiding costly replacements/upgrades down the line.
An efficient and well-designed boiler feed water treatment system should be able to:
- Efficiently treat boiler feed water and remove harmful impurities prior to entering the boiler
- Promote internal boiler chemistry control
- Control return-line corrosion
- Avoid plant downtime and boiler failure
- Prolong equipment service life
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What’s included in a basic boiler feed water treatment system?
As mentioned above, the exact components of a boiler feed water treatment system depend on the quality of water being drawn from in relation to the quality of water makeup needed for the specific boiler (according to the manufacturer’s recommendations), but in general, a basic boiler feed water treatment system typically includes some type of:
- Filtration and ultrafiltration
- Ion exchange/softening
- Membrane processes such as reverse osmosis and nanofiltration
- Coagulation/chemical precipitation
Depending on the impurities present in your water, any combination of these treatments might best suit your facility and make up your treatment system, and depending on the needs of your plant and process, these standard components are usually adequate. However, if your plant requires a system that provides a bit more customization, there might be some features or technologies you will need to add on.
What does a boiler feed water treatment system typically remove?
A boiler feed water treatment system might be made up of the technologies necessary to remove problematic dissolved solids, suspended solids, and organic material, including any number of the following:
- Iron: either soluble or insoluble, iron can deposit on boiler parts and tubes, damage downstream equipment, and affect the quality of certain manufacturing processes
- Copper: can cause deposits to settle in high-pressure turbines, decreasing their efficiency and requiring costly cleaning or equipment change-outs
- Silica: if not removed to low levels, especially in high-pressure boilers, silica can cause extremely hard scaling
- Calcium: can cause scaling in several forms depending on the chemistry of the boiler feed water (e.g. calcium silicate, calcium phosphate, etc.)
- Magnesium: if combined with phosphate, magnesium can stick to the interior of the boiler and coat tubes, attracting more solids and contributing to scale
- Aluminum: deposits as scale on the boiler interior and can react with silica to increase the likelihood of scaling
- Hardness: also causes deposits and scale on boiler parts and piping
- Dissolved gasses: chemical reactions due to the presence of dissolved gases such as oxygen and carbon dioxide can cause severe corrosion on boiler pipes and parts
How does a boiler feed water treatment system work?
Specific treatment processes vary depending on the requirements of the boiler and quality/chemistry of the feed and makeup water, but a typical boiler feed water treatment system will usually include the following steps:
Makeup water intake
Makeup water, or the water replacing evaporated or leaked water from the boiler, is first drawn from its source, whether raw water, city water, city-treated effluent, in-plant wastewater recycle (cooling tower blowdown recycle), well water, or any other surface water source.
Coagulation and chemical precipitation
After all the large objects are removed from the original water source, various chemicals are added to a reaction tank to remove the bulk suspended solids and other various contaminants. This process starts off with an assortment of mixing reactors, typically one or two reactors that add specific chemicals to take out all the finer particles in the water by combining them into heavier particles that settle out. The most widely used coagulates are aluminum-based such as alum and polyaluminum chloride.
Sometimes a slight pH adjustment will help coagulate the particles, as well.
Filtration and ultrafiltration
The next step is generally running through some type of filtration to remove any suspended particles such as sediment, turbidity, and certain types of organic matter. It is often useful to do this early on in the process, as the removal of suspended solids upstream can help protect membranes and ion exchange resins from fouling later on in the pretreatment process. Depending on the type of filtration used, suspended particles can be removed down to under one micron.
Ion exchange softening
When pretreating boiler feed water, if there’s high hardness complexed with bicarbonates, sulphates, chlorides, or nitrates, a softening resin can be used. This procedure uses a strong acid cation exchange process, whereby resin is charged with a sodium ion, and as the hardness comes through, it has a higher affinity for calcium, magnesium, and iron so it will grab that molecule and release the sodium molecule into the water.
After the softening process, some boiler feed water treatment systems will utilize dealkalization to reduce alkalinity/pH, an impurity in boiler feed water that can cause foaming, corrosion, and embrittlement. Sodium chloride dealkalization uses a strong anion exchange resin to replace bicarbonate, sulfate, and nitrate for chloride anions. Although it doesn’t remove alkalinity 100%, it does remove the majority of it with what can be an easy-to-implement and economical process. Weak acid dealkalization only removes cations bound to bicarbonate, converting it to carbon dioxide (and therefore requiring degasification). It is a partial softening process that is also economical for adjusting the boiler feed water pH.
Reverse osmosis (RO) and nanofiltration (NF)
Reverse osmosis (RO) and nanofiltration (NF) are often used down the line in the boiler feed water treatment system process so most of the harmful impurities that can foul and clog the RO/NF membranes have been removed. Similar processes of separation, they both force pressurized water through semipermeable membranes, trapping contaminants such as bacteria, salts, organics, silica, and hardness, while allowing concentrated, purified water through. Not always required in boiler feed water treatment, these filtration units are used mostly with high-pressure boilers where concentration of suspended and dissolved solids needs to be extremely low.
Deaeration or degasification
Boiler Water Treatment 101
At this point in the boiler feed water treatment process, any condensate being returned to the system will mix with the treated makeup water and enter the deaeration or degasification process. Any amount of gasses such as oxygen and carbon dioxide can be extremely corrosive to boiler equipment and piping when they attach to them, forming oxides and causing rust. Therefore, removing these gases to acceptable levels (nearly 100%) can be imperative to the service life and safety of the boiler system. There are several types of deaeration devices that come in a range of configurations depending on the manufacturer, but generally, you might use a tray- or spray-type deaerator for degasification or oxygen scavengers.
Boiler Water Treatment Basics
After the boiler feed water has been sufficiently purified according to the boiler manufacturer’s recommendation and other industry-wide regulations, the water is fed to the boiler where it is heated and used to generate steam. Pure steam is used in the facility, steam and condensate are lost, and condensate return is pumped back into the process to meet up with the pretreated makeup water to cycle through pretreatment again.
Boiler Water Treatment Manual Instructions
SAMCO has over 40 years of experience helping our customers design and custom-engineer boiler feed water treatment systems. If you have any questions, be sure to visit our website for more information about boiler feed water treatment here. We also have an article you might be interested in about how much a boiler feed water treatment system might cost you and who we recommend as qualified boiler feed water treatment companies for you to consider as you search for all the options available for your plant.
Boiler Water Treatment Training Manual
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