FREQUENTLY ASKED QUESTIONS Index
FAQ’s on Boiler Water Treatment
| Boiler Water Treatment Chemicals manufactured by Ion Exchange (India) Ltd. | |
| Deposite Controllers Scale Inhibitor - Indion 2106 | |
| Miscellaneous - Indion 1012 | |
| Return Line Treatment | |
| Oxygen Scavenger | |
| Why do I have to chemically treat my boiler? | Go to Top | |
| 1) To prevent boiler scale | ||
| 2) To minimize corrosion to the feed water and steam & condensate system | ||
| 3) Improve boiler efficiency | ||
| 4) Reduce fuel, operating and maintenance costs | ||
| 5) Minimize maintenance and downtime | ||
| What is boiler scale? | ||
| The most common scale is white in appearance and is from calcium carbonate that has precipitated from hard feed water. Hard water contains calcium and magnesium and minerals that are hard to wash. Silica scale is brittle and has a glassy appearance. The most common scale is white in appearance and is formed by calcium carbonate that has precipitated from hard feed water. | ||
| What happens to all the scale which would normally form in the boiler? | Go to Top | |
| It is dissolved in the boiler water or precipitates out as part of the mobile sludge which is formed. | ||
| Won’t my boiler leak with no scale present? | ||
| Initially maybe, but a scale-free boiler can be made leak-free by a good boiler smith. Minor leaks will be plugged by the tannin in the treatment. Also it should be remembered many leaks are actually caused by scale, which has led to localized overheating, or corrosions. Even a “traditional” boiler of riveted and expanded construction ought to be leak-free and PT will make sure no localized overheating or corrosion will ever occur to cause leaks. | ||
| Won’t the high alkalinities cause caustic embrittlement? | Go to Top | |
| Caustic embrittlement is a complex problem with multiple causes. The use of specific tannins have been shown to prevent it. Additionally leak-free boilers allow no opportunity for caustic embrittlement to occur. | ||
| Aren’t high alkalinities dangerous? | ||
| History does not show the risk to be serious if simple maintenance and operating policies are followed. | ||
| Is Line side treatment more effective? | Go to Top | |
| It is not. Direct chemical treatment of boiler water can totally solve fouling, corrosion, caustic embrittlement and steam contamination problems. Line side treatments can only ease the problems of fouling and caustic embrittlement but can not deal with corrosion or steam contamination. | ||
| What happens to all the mud and clay build up? Surely this would make it impossible to run for very long without a washout? | ||
| Any solids which would have formed mud's or clays will either go into solution (due to the alkalinity) or will go into suspension in the boiler water. However those particles in suspension, again due to the alkalinity, form a totally mobile sludge which moves with the slightest water current, even when cold. It is not in any way adherent. Under high steam demand, and thus rapid boiler water circulation, this sludge fully mixes with the boiler water and appears as a brown colour in the gauge glasses. | ||
| I use lead fusible plugs. Is this OK? | Go to Top | |
| No. Lead is rapidly eroded when alkalinities reach or exceed pH12. A number of alternatives are available: | ||
| a. Lead/tin alloy plugs which give a 90+ day life and longer with copper electroplating on the water side; | ||
| b. Copper electroplated (on the water side) pure lead plugs; | ||
| c. Drop (button) plugs. | ||
| Can I use lead mud hole door seals? | ||
| No. Lead is rapidly eroded when alkalinities reach or exceed pH12. An alternative must be used. | ||
| Doesn't high alkalinity dissolve brasses and bronzes? | Go to Top | |
| Only up to certain point. Experience shows it is important not to have leaking valves situated below the normal water level. Such valves seats and stems, when made of brass or bronze. It should ideally be substituted by stainless steel or other alkaline-resistant materials. Note ONLY the seat and stem need replacement. Erosion of the component body will be very slow indeed. | ||
| Doesn’t high alkalinity dissolve glass, i.e.: gauge glasses. | ||
| Silica is dissolved gradually by alkalinity. Correct maintenance policies based on inspection, on set days, in steam will prevent problems occurring with tubular glasses. It is best to use reflex glasses if possible. Failures of these flat glasses are virtually unheard. The ability to dissolve silica ensures silica scale does not form on boiler surfaces. | ||
| Why should I care about steam purity? | ||
| Because it leads to salts being deposited in the steam passages causing accelerated internal corrosion and hot spots (failed superheater elements, pipes etc). It leads to solids being carried to the valves and pistons acting as a grinding paste when mixed with liquids and it leads to contaminated and thus compromised lubrication. All these are equally true for any auxiliary item of equipment that use steam, e.g.: air pumps, generators etc. | ||
| Boiler tubes are consumable items so what does it matter? | Go to Top | |
| They need not be! PT offers a 20+ year life from boiler tubes where regulation so allows. The lack of corrosions and thermal fatigue failures also, of course, extend to all other parts of the boiler. Corrosion prevention is just one part of the overall benefits available from using PT. | ||
| Why would I use something experimental on my loco? | ||
| Pretreatment (PT) is no experiment. It is a fully developed water treatment based on highly successful previous treatments. It takes these further due to improvements in the understanding of various phenomena and the availability of more effective and relevant chemicals. It should also be remembered PT was developed for locomotives. | ||
| Why the water source is of no concern at all? | Go to Top | |
| The high alkalinity created acts as a buffer to incoming water variation. | ||
| Is it complicated to administer? | ||
| It is not. A pre-prepared chemical mix is added to the locomotive's tank to treat the water on a regular basis. Long periods of standing without operation of any injectors will reduce the active antifoam concentration below safe levels. In such cases extra antifoam may be required depending on operating circumstances. | ||
| It sounds complicated to monitor? | Go to Top | |
| It is not. Only two boiler water parameters are to be measured and only a few remedies to incorrect conditions exist; these being blowdown and alkalinity regulation. | ||
| How often should a loco be blown down? | ||
| This depends on the results obtained when monitoring the boiler water. Few times per annum can be said to be normal. | ||
| How often should a loco be washed out? | Go to Top | |
| This varies and is results-driven. Once a year is fully possible when the treatment is operating as a preventative one rather than a corrective one. | ||
| What is the difference between a corrective and preventative application of the treatment? | ||
| In application terms basically nothing. A treatment period will be seen as corrective if scale and corrosion are present in the boiler from previous use. Treatment in a boiler using PT from new or containing no scale or corrosion will be preventative. In these circumstances there is nothing from past incomplete treatments to correct. | ||
| What happens in feedwater heaters? | Go to Top | |
| These should be as clean as possible to start with. In time they will be descaled and corrosions will be prevented but it will be a slower process than in the boiler. New items will remain scale and corrosion free. | ||
| Does PT stop corrosion and scale in water tanks, pipes and injectors? | ||
| Yes. Old scale and corrosion will be treated over time but this can be a slow process due to the much lower levels of chemical concentration in these parts. | ||
| Our locomotive stands idle for several months every year. How does this effect an application? | Go to Top | |
| No problem! PT treated water should, where frost is not going to be a problem, be left in water tanks and boilers. It continues to work when a locomotive is not in use. Thus it is much better to leave tanks and boilers filled rather than in empty and so-called “dry” conditions. | ||
| Would PT treated water descale and treat corrosion if left standing in a boiler or tanks when not in use? | ||
| Yes, absolutely. The period of time to fully correct such problems is hard to predict as each case is different, but the effect of such a treatment period will only be positive. | ||
| Is the regime identical for boilers with copper fireboxes and/or tubes? | Go to Top | |
| Not quite. There are couple of alterations with a lower TDS and pH required but that is about it. Dosing and monitoring remain the same whilst results will be little different. | ||
| Is Porta Treatment OK with brass tubes? | ||
| No, Porta Treatment can not be used with brass tubes. | ||
| Why did my feedwater pump fail due to excessive corrosion? | Go to Top | |
| If your feedwater pump has experienced excessive corrosion and/or failure it is most likely from inadequate amount of sulfite being fed to the feedwater tank. Make sure your sulfite is fed to the feedwater tank and that it is being feed though an injection quill that reaches near the center of the tank. | ||
| What tests should I perform on my condensate system? | ||
| Monitor: | ||
| 1) Insoluble and soluble iron | ||
| 2) Condensate pH (7.5-8.5 for most systems) at the furthest point from your boiler and possibly at multiple locations if your system is large | ||
| 3) Condensate corrosion coupons | ||
| Why are condensate pipes leaking or excessively corroding? | Go to Top | |
| Corrosion occurs from carbonic acid and oxygen pitting. Carbonic acid occurs from CO2 which is a breakdown molecule of the carbonate alkalinity condensing from water to form H2CO3. Oxygen pitting occurs as steam condenses and the vacuum created pulls air into the condensate system. | ||
| What is boiler carryover? | ||
| If your boiler water suspended solids are excessive, some solids may collect on the surface of a steam bubble and exit the boiler with the steam. | ||
| Why do I need to feed a polymer or phosphate to my boiler? | Go to Top | |
| Phosphate and polymers are required in boilers to prevent the calcium and magnesium from precipitating to the boiler tubes. Theoretically, some calcium and magnesium will leak though the softener. The calcium and magnesium minerals attach themselves to the polymer or phosphate and are discharged either through surface blowdown or during bottom blow downs. | ||
| What is a single drum program? | ||
| A single drum program is referred to as a chemical mixture that contains all the boiler chemical components (sulfite, amine, phosphate/polymer, and alkalinity) in one container. | ||
| Why do I have to feed sulfite to my boiler? | Go to Top | |
| Sulfite is referred to as an oxygen scavenger. Sulfite is the most commonly used oxygen scavenger. It is used to eliminate or replace dissolved oxygen. | ||
| How do I calculate how much condensate is returning back as a boiler feed? | ||
| To calculate your return condensate percentage multiply 100 by 1 - (feedwater silica/makeup water silica) | ||
| What is boiler blowdown? | Go to Top | |
| Blowdown is the removal of the concentrated dissolved and suspended solids. By blowing down the water from the system, lower concentrated water dilutes the existing water in the boiler. | ||
| How much do I need to blowdown my boiler? | ||
| It depends on how many impurities you have in your feedwater. The goal is to determine what the limiting factor is (dissolved solids, alkalinity, silica, or iron) in your boiler water and set your blowdown setting at that limit. To determine the amount of blowdown percentage divide 1 by your concentration ratio and multiply by 100. | ||
| Why is my boiler water red? | Go to Top | |
| If your boiler water is red in appearance, it may be from a number of possible reasons. Some of the most common ones are: | ||
| 1) Inadequate levels of sulfite | ||
| 2) Over feeding alkalinity | ||
| 3) Condensate contamination | ||
| 4) Overfeeding an acidic sulfite product that depresses the boiler water pH | ||
| 5) Low alkalinity | ||
| Why is my boiler scale only on my bottom tubes? | ||
| If you have poor performing softeners, if your softeners are being by-passed, or if you do not have softeners or hard water is entering the boiler. Remember phosphates and polymers are only used as polishers to remove the minimal amounts of Calcium and magnesium that enter the boiler. | ||
| Your pre-treatment is designed to remove 95% of the calcium and magnesium. If you are using a straight phosphate program with no polymer, make sure to perform bottom blowdown twice a day. A phosphate chemical is designed to sink to the boiler bottom after the calcium and magnesium is attached. | ||
| What is a Deaerator? | Go to Top | |
| A deaerator is mechanical way of removing dissolved oxygen from the water. There are different types of deaerators and multiple manufactures. Remember you still need to feed an oxygen scavenger to remove the dissolved oxygen that the deaerator does not remove. | ||
| What boiler tests should I perform? | ||
| If you do not have man power or time to perform a bunch of tests. We recommend testing at minimum the boiler water sulfite residual, boiler water conductivity, and feedwater/softener hardness. Also, we recommend monitoring your chemical inventory daily as a double check to ensure sufficient chemical is being added to the chemical. | ||
| What is the boiler chemical amine? | Go to Top | |
| There are two main categories of amines, neutralizing amines and filming amines. Neutralizing amines are the most common. Amines are designed to increase the condensate pH to minimize condensate corrosion. Make sure to check how your steam is being used. In some applications, there may be restrictions on using amines. | ||
| What items are common boiler failures? | ||
| 1) Oxygen Pitting | ||
| 2) Short-Term Overheating | ||
| 3) Long-Term Overheating | ||
| 4) Caustic Gouging | ||
| What should be the boiler temperature of water? | Go to Top | |
| The boiling point of water depends on pressure. At atmospheric pressure, water boils at 100°C. As pressure increases, the boiling point increases. At 22,000 kPa, where water is converted to steam, the boiler point is lowered. | ||
| What is boiler feedwater? | ||
| Boiler feedwater is referred to as the water entering the boiler. It is a mixture of returned condensate and fresh make-up water. | ||
| Why do I have to return my condensate? | Go to Top | |
| Condensate is hotter than make-up water and it contains valuable BTUs. The warmer the feedwater in the tank, the less energy you need to heat the water to make steam. So it is important to return as much condensate as possible. | ||
| What is boiler make-up water? | ||
| Make-up water is referred to as the fresh water that is added to the feedwater tank. | ||
| What is cycles of concentration? | Go to Top | |
| Cycles of Concentration is refers to how many times you reuse your water. The purpose of a boiler is to reuse water. To calculate your cycles of concentration divide your boiler water silica residual by your feedwater silica. | ||
| How do I remove boiler scale? | ||
| One solution is to hire an outside company that specializes in acid cleaning. If scale is light, do not remove the scale, just make sure your softeners are functioning properly and use a polymer designed for gradual scale removal. | ||
| How do I prevent boiler scale? | Go to Top | |
| 1) Have a good operating softener | ||
| 2) Make sure the brine tanks is half filled with salt at all times | ||
| 3) Perform softener hardness and feedwater checks daily | ||
| 4) If using a phosphate chemical program, blowdown the boiler two times a day | ||
| Where do I feed my chemicals? | ||
| Sulfite and alkalinity to the feedwater tank or drop leg of a Deaerator Phosphate/Polymer to the feedwater line or drop leg of a Deaerator or steam drum. Amine preferred to the steam header, but you can feed it to the feedwater line or drop leg of a Deaerator. One drum program or day tank to the feedwater line or drop leg of a Deaerator. | ||
| How do I feed my chemicals? | Go to Top | |
| Preferably neat (straight from the drum). It provides a more consistent chemical dilution. | ||
| How do I wet lay up a boiler? | ||
| If you plan on keeping your boiler idle for more than a month, dry lay-up is the preferred method. If the boiler needs to be readily available to service. Add additional sulfite and alkalinity (if you are using an acidic sulfite) to the boiler. Maintain at least 100ppm of sulfite and check sulfite residuals weekly. Also, it is important to ensure the boiler tubes stay fully in the water to prevent tube corrosion. | ||
| How do I dry lay up a boiler? | Go to Top | |
| Drain, clean, allow time to dry, insert desiccant or hydrated lime to absorb the oxygen, and carefully seal the boiler to prevent air leaks. Inspect the desiccant or hydrated lime periodically. | ||
| How to improve poor boiler steam-fuel ratio and increase boiler efficiency? | ||
| For occasional low efficiency-clean the burner tips and fuel oil pumps filters, check for viscosity of fuel oil, burner tip holes and atomizing steam pressure. | ||
| For continuously low efficiency-check flame colour, if the colour of the flame is not bright golden yellow, combustion is poor. Take remedial measures mentioned as above and additionally check the following. | ||
| If stack temperature is high, there is soot deposition in the boiler. Stop the boiler and carry out cleaning of the boiler. Check water side deposition/scale formation. If scale formation is observed, plan for cleaning the boiler with appropriate method. Evaluate for installation of economizer and soot blowing frequency. | ||
| Soot deposition in my boiler is heavy. How to reduce soot formation and deposition? | Go to Top | |
| The reasons for heavy soot deposition are: | ||
| i) Poor quality of fuel with higher ash content metals, high insolubility. Ensure the quality of fuel | ||
| ii) Poor combustion - improve the combustion by checking Atomizing Steam pressure by cleaning burner tip and fuel oil system filters and checking viscosity near burner tip | ||
| Blow down losses are heavy, how to reduce blow down rate? | ||
| i) Monitor boiler water treatment. All volatile treatment is better than conventional treatment if there is techno-economical feasibility | ||
| ii) Check water treatment process for seepage of Chloride, Silica, etc. Check TSP quality for Chloride content | ||
| iii) Fine control of CBD with increased frequency of Blow down water will help in saving of energy & boiler water | ||
| There is corrosion problem in the pressure parts on F.W. circuit, how to overcome the problem? | Go to Top | |
| i) Check proper deaeration in the Deaerator. D.O. should be less than 7PPB | ||
| ii) Maintain N2H4 at desired level in the BFW | ||
| iii) Maintain pH more than 9 in the CBD water | ||
| iv) Closely monitor TSP level in the CBD water | ||
| v) Check feasibility of change over of BFW treatment to AVT | ||
| vi) Verify for proper selection of oxygen scavenger if used | ||
| What are the symptoms of caustic attack? | ||
| Localized wall loss on the inside diameter (ID) surface of the tube, resulting in increased stress and strain in the tube wall. | ||
| What are the causes of caustic attack? | Go to Top | |
| Caustic attack occurs when there is excessive deposition on ID tube surfaces. This leads to diminished cooling water flow in contact with the tube, which in turn causes local under-deposit boiling and concentration of boiler water chemicals. If combined with boiler water chemistry up sets of high pH, it results in a caustic condition which corrosively attacks and breaks down protective magnetite. | ||
| What are symptoms of Oxygen pitting? | ||
| Aggressive localized corrosion and loss of tube wall, most prevalent near economizer feedwater inlet on operating boilers. Flooded or non-drainable surfaces are most susceptible during outage periods. | ||
| What are the causes of Oxygen pitting? | Go to Top | |
| Oxygen pitting occurs with the presence of excessive oxygen in boiler water. It can occur during operation as a result of in-leakage of air at pumps, or failure in operation of preboiler water treatment equipment. This also may occur during extended out-of-service periods, such as outage sand storage, if proper procedures are not followed in lay-up. Non-rainableocations of boiler circuits, such as super heater loops, sagging horizontal super heater and re heater tubes, and supply lines, are especially susceptible. More generalized oxidation of tubes during idle periods is sometimes referred to as out of- service corrosion. Wetted surfaces are subject to oxidation as the water reacts with the iron to form iron oxide. When corrosive ash is present, moisture on tube surfaces from condensation or water washing can react with elements in the ash to form acids that lead to a much more aggressive attack on metal surfaces. | ||
| What are the symptoms of hydrogen damage? | ||
| Inter granular micro-cracking. Loss of ductility or embrittlement of the tube material leads to brittle catastrophic rupture. | ||
| What are the causes of hydrogen damage? | Go to Top | |
| Hydrogen damage is most commonly associated with excessive deposition on ID tube surfaces, coupled with a boiler water low pH excursion. Water chemistry is upset, such as what can occur from condenser leaks, particularly with salt water cooling medium, and leads to acidic (low pH) contaminants that can concentrate in the deposit. Under-deposit corrosion releases atomic hydrogen which migrates into the tube wall metal, reacts with carbon in the steel (decarburization) and causes inter granular separation. | ||
| What are the symptoms of acid attack? | ||
| Corrosive attack of the internal tube metal surfaces, results in an irregular pitted or, in extreme cases, a" Swiss cheese” appearance of the tube ID. | ||
| What are the causes of acid attack? | Go to Top | |
| Acid attack most commonly is associated with poor control of process during boiler chemical cleanings and/or inadequate post-cleaning passivation of residual acid. | ||
| What are the symptoms of Stress corrosion cracking? | ||
| Failures from SCC are characterized by a thick wall, brittle-type crack. May be found at locations of higher external stresses, such as near attachments. | ||
| What are the causes of stress corrosion cracking? | Go to Top | |
| SCC most commonly is associated with austenitic (stainless steel) superheater materials and can lead to either trans granular or inter granular rack propagation in the tube wall. It occurs where a combination of high tensile stresses and a corrosive fluid are present. The damage results from cracks that propagate from the ID. The source of corrosive fluid may be carryover into the super heater from the steam drum or from contamination during boiler acid cleaning if the super heater is not properly protected. | ||
| What are the symptoms of water side corrosion fatigue? | ||
| ID initiated, wide trans granular cracks which typically occur adjacent to external attachments. | ||
| What are causes of water side corrosion fatigue? | Go to Top | |
| Tube damage occurs due to the combination of thermal fatigue and corrosion. Corrosion fatigue is influenced by boiler design, water chemistry, boiler water oxygen content and boiler operation. A combination of these effects leads to the breakdown of the protective magnetite on the ID surface of the boiler tube. The loss of this protective scale exposes tube to corrosion. The locations of attachments and external weldments, such as buck stay attachments, seal plates and scallop bars, are most susceptible. The most likely to progress during boiler start-up cycles. | ||
| What are the symptoms of Super heater Fireside Ash Corrosion? | ||
| External tube wall loss and increasing tube strain. Tubes commonly have a pock-marked appearance when scale and corrosion products are removed. | ||
| What are the causes of Super heater Fireside Ash Corrosion? | Go to Top | |
| Fireside ash corrosion is a function of the ash characteristics of the fuel and boiler design. It usually is associated with coal firing, but also can occur for certain types of oil firing. Ash characteristics are considering the boiler design when establishing the size, geometry and materials used in the boiler. Combustion gas and metal temperatures in the convection passes are important considerations. Damage occurs when certain coal ash constituents remain in a molten state on the super heater tube surfaces. This molten ash can be highly corrosive. | ||
| What are the causes of High-temperature Oxidation? | ||
| High-temperature oxidation can occur locally in areas that have the highest outside surface temperature relative to the oxidation limit of the tube material. Determining the actual root cause between the mechanisms of ash corrosion or high-temperature oxidation is best done by tube analysis and evaluation of both ID and OD scale and deposits. | ||
| What are the symptoms of Waterwall Fireside Corrosion? | Go to Top | |
| External tube metal loss (wastage) leading to thinning and increasing tube strain. | ||
| What are the causes of Waterwall Fireside Corrosion? | ||
| Corrosion occurs on external surfaces of water wall tubes when the combustion process produces a reducing atmosphere (sub stoichiometric). This is common in the lower furnace of process recovery boilers in the pulp and paper industry. For conventional fossil fuel boilers, corrosion in the burner zone usually is associated with coal firing. Boilers having maladjusted burners or operating with staged air zones to control combustion can be more susceptible to larger local regions possessing a reducing atmosphere, resulting in increased corrosion rates. | ||
| What are the symptoms of Fireside Corrosion Fatigue? | Go to Top | |
| Tubes develop a series of cracks that initiate on the outside diameter (OD) surface and propagate into the tube wall. Since the damage develops over longer periods, tube surfaces tend to develop appearances described as “elephant hide,” “alligator hide” or craze cracking. Most commonly seen as a series of circumferential cracks. Usually found on furnace wall tubes of coal-fired once-through boiler designs, but also has occurred on tubes in drum-type boilers. | ||
| What are the causes of Fireside Corrosion Fatigue? | ||
| Damage initiation and propagation result from corrosion in combination with thermal fatigue. Tube OD surfaces experience thermal fatigue stress cycles which can occur from normal shedding of slag, soot blowing or from cyclic operation of the boiler. Thermal cycling, in addition to subjecting the material to cyclic stress, can initiate cracking of the less elastic external tube scales and expose the tube base material to repeated corrosion. | ||
| What are the symptoms of short term over heat? | Go to Top | |
| Failure results in a ductile rupture of the tube metal and is normally characterized by the classic “fish mouth” opening in the tube where the fracture surface is a thin edge. | ||
| What are the causes of short term over heat? | ||
| Short-term overheat failures are most common during boiler start up. Failures result when the tube metal temperature is extremely elevated from a lack of cooling steam or water flow. A typical example is when superheater tubes have not cleared of condensation during boiler start-up, obstructing steam flow. Tube metal temperatures reach combustion gas temperatures of 1600°F or greater which lead to tube failure. | ||
| What are the symptoms of Long term over heat? | Go to Top | |
| The failed tube has minimal swelling and a longitudinal split that is narrow when compared to short-term overheat. Tube metal often has heavy external scale build-up and secondary cracking. | ||
| What are the causes of Long term over heat? | ||
| Long-term overheat occurs over a period of months or years. Super heater and reheat superheater tubes commonly fail after many years of service, as a result of creep. During normal operation, alloy superheater tubes will experience increasing temperature and strain over the life of the tube until the creep life is expended. Furnace water wall tubes also can fail from long-term overheat. In the case of water wall tubes, the tube temperature increases abnormally, most commonly from waterside problems such as deposits, scale or restricted flow. In the case of either superheater or water wall tubes, eventual failure is by creep rupture. | ||
| What are the causes of graphitization? | Go to Top | |
| Long-term operation at relatively high metal temperatures can result in damage in carbon steels of higher carbon content, or carbon-molybdenum steel, and result in a unique degradation of the material in a manner referred to as graphitization. These materials, if exposed to excessive temperature, will experience dissolution of the iron carbide in the steel and formation of graphite nodules, resulting in a loss of strength and eventual failure. | ||
| What are the symptoms of erosion? | ||
| Tube experiences metal loss from the OD of the tube. Damage will be oriented on the impact side of the tube. Ultimate failure results from rupture due to increasing strain as tube material erodes away. | ||
| What are the causes of erosion? | Go to Top | |
| Erosion of tube surfaces occurs from impingement on the external surfaces. The erosion medium can be any abrasive in the combustion gas flow stream, but most commonly is associated with impingement of fly ash or soot blowing steam. In cases where soot blower steam is the primary cause, the erosion may be accompanied by thermal fatigue. | ||
| What are the causes of mechanical fatigue? | ||
| Fatigue is the result of cyclical stresses in the component. Distinct from thermal fatigue effects, mechanical fatigue damage is associated with externally applied stresses. Stresses may be associated with vibration due to flue gas flow or soot blowers (high-frequency low-amplitude stresses), or they may be associated with boiler cycling (low-frequency high-amplitude stress mechanism). Fatigue failure most often occurs at areas of constraint, such as tube penetrations, welds, attachments or supports. | ||
| What is the application of filming amine? | Go to Top | |
| Filming amines are various chemicals that form a protective layer on the condensate piping to protect it from both oxygen and acid attack. The filming amines should be continuously fed into the steam header with an injection quill based on steam flow. The two most common filming amines are octadecylamine (ODA) and ethoxylated soya amine (ESA). Combining neutralizing and filming amine is a successful alternative to protect against both acid and oxygen attack. | ||
| What is the application of polymers in boiler water treatment? | ||
| Most polymers used in feed water treatment are synthetic. They act like chelates but are not as effective. Some polymers are effective in controlling hardness deposits, while others are helpful in controlling iron deposits. Polymers are often combined with chelates for the most effective treatment. | ||
| What is the application of Neutralizing amine? | Go to Top | |
| Neutralizing amines are high pH chemicals that can be fed directly to the feedwater or the steam header to neutralize the carbonic acid formed in the condensate (acid attack). The three most commonly used neutralizing amines are morpholine, diethyleminoethanal (DEAE) and cyclohexylamine. Neutralizing amines cannot protect against oxygen attack; however, it helps keep oxygen less reactive by maintaining an alkaline pH. | ||
| Explain the common metallurgical problems occur in the boiler? | ||
| Common metallurgical problems are: | ||
| Waterwall Tubes | ||
| Most failures observed have been caused by overheating, both long and short-term. As the metal temperature rises above 900 F, the normal structure of the carbon and low alloy steels begins to break down, and carbide spheroidization occurs. Typical contributors to failure include: | ||
| Waterside deposits | ||
| Corrosion fatigue | ||
| Hydrogen damage | ||
| Caustic/acid phosphate gouging | ||
| Graphitization | ||
| Pitting | ||
| Ash Corrosion | ||
| Ash Corrosion | ||
| Flame Impingement | Go to Top | |
| Floor Tubes | ||
| Floor tube failures occur when a thermal differential exists, with the top half of the tube becoming hotter than the bottom half. Deposits plate out, leading to under deposit corrosion mechanisms. The tube often fails by overheating. | ||
| Roof Tubes | ||
| Most roof tube failures can be attributed to corrosion fatigue or internal corrosion, such as hydrogen damage. | ||
| Economizer Tubes | ||
| The majority of all economizer failures occur by oxygen corrosion, erosion, or thermal fatigue. | ||
| Super heater Tubes | ||
| Like water wall tube failures, most problems arise from overheating. The most susceptible areas are lead tubes and tube bends. Common causes of failure include: | ||
| High temperature, liquid-phase corrosion | ||
| Vanadium attack (oil-fired units) | ||
| High temperature oxidation | ||
| Creep rupture failures | ||
| Dissimilar metal weld failures | Go to Top | |
| Headers | ||
| Most failures occur at the circumferential weld connections or at the tube stub weld. Circumferential cracking can be observed at or near the weld, and is attributable to thermal expansion fatigue and/or creep damage. | ||
| Steam/Mud Drum | ||
| Generating tubes at the rolled-in end of the drum may have small circumferential cracks on the inside of the tube. These cracks are attributable to corrosion fatigue brought on by thermal or mechanical stresses. | ||
| Dissimilar Metal Welds | ||
| Tube tie welds or dissimilar metal welds are often made from a stainless steel or nickel base alloy. Problems arise from the difference in thermal expansion coefficients between the austenitic and ferritic weld, causing cracking. | ||
| Piping | ||
| High energy piping systems, including main steam, hot reheat steam and cold reheat steam lines, often fail by creep damage as a result of long-term service at elevated temperatures under stress. Fatigue and fabrication defects may also play a role. Girth welds tend to leak-before-break, while longitudinal seams may have more catastrophic and unpredictable ruptures. Feed water piping is susceptible to flow-accelerated corrosion (FAC), caused by the dissolution of the protective oxide layer. | ||
| Go to Top | ||