Drawing courtesy of National Coil Coaters Association, A coil of metal is unwound and then cleaned or pretreated, usually by a dip process. But the AVT chemistry, without any buffering effect, was unable to neutralize the acidity or alkalinity of cooling water ingresses or other pollutions, when they concentrate in the SGs. No definitive, universally-agreed-upon range for excess caustic (OH) alkalinity exists. Varying types of treatment are used at different locations to avoid scale, corrosion, or foaming. Impurities in water will leave solid deposits as steam evaporates. Magnesium and magnesium alloys have attracted much attention as biodegradable biomedical materials due to their biodegradability, close elastic modulus to bone tissue, and good bone tissue response, especially in bone implant applications. PO4 readily reacts with calcium hardness and alkalinity to form an insoluble sludge of hydroxyapatite. An excess residual of 1 to 3 ppm is typically carried in the boiler to protect against oxygen intrusion and maintain the protective magnetite film. However, they offer the benefit of promoting a black iron magnetite surface. In fact both environments were able to coexist in different local parts of the SG, it became obvious that it was impossible to avoid corrosion of Alloy 600MA tubes, whatever which Na/PO43 molar ratio was met (2.02.6). In some applications, such as in food and dairy plants, the use of steam condensate treatment is restricted or prohibited. Barry Dooley, in Heat Recovery Steam Generator Technology, 2017. Figure 11. This is very common for fluoropolymer finishes for coating items such as bread pans, cake pans, and cookie sheets. Categories include the following: no change in chemistry since commissioning; using incorrect or outdated guidelines; continuing to use reducing agents in combined cycle/HRSGs and thus risking or experiencing single-phase FAC; continuing to use the wrong phosphate treatment (usually not using only TSP); not having a chemistry manual for the unit, plant or organization; incorrect addition point for chemicals (most often reducing agent with AVT(R)); not questioning use of proprietary chemical additions (phosphate blends, amines, FFP) and therefore not knowing the composition of chemicals added to the unit/plant; not determining through monitoring the optimum feedwater pH to prevent/control FAC. Historically most of the pressurized water reactor (PWR) units were using the Phosphate treatment based on fossil fired unit experience. By continuing you agree to the use of cookies. The remaining oxygen is removed by chemical scavengers fed to the feedwater storage section of the deaerator. SEM surface view of a crystalline phosphate treatment. The main objective of secondary side water chemistry is to build and maintain protective layers on CS. All PWR plants started phosphate chemistry with concentrations of 26mg/kg for high-purity plants and 4050mg/kg for plants with frequent condenser leaks. The phosphate solution shall be prepared using desalinated (demineralized) or condensate water. Internal oxidation (IO) model hypothesizes faster oxidation at the stressed crack tip where preferential oxidation of Cr leaves Ni-rich substrate underneath the crack tip filled with voluminous corrosion products. Operating low and medium pressure boiler plants at maximum efficiency requires careful control over the boiler water chemistry. Using neutralizing amines to maintain the pH above 6.0 provides additional protection from pH excursions. Oxidizing conditions are favored by air ingress or presence of copper ions on SG tube deposits. Generally, pH does not adversely affect the corrosion rate of steel above a pH of 6.0. Galvanneal and galvanize were treated with pre-phosphate plus oil versus controls: GA with a dry-film lubricant (DFL) and bare (oiled-only) GA, GI (courtesy of D. E. Bullard). In the case of difficult-to-etch CRES steels, to ensure removal of the existing surface layer, use 50g KBr per liter of 1:9:10 concentrated sulfuric acid, concentrated phosphoric acid, and water. More commonly, it is obtained as a 50% active liquid or in more dilute liquid versions depending on the source of supply. The definition of low and medium pressure is somewhat discretionary. The largest commercial examples of fluoropolymer products coated in this fashion include home ovenware or bakeware such as pie tins, bread pans, and cookie sheets. These chemicals react much slower with dissolved oxygen as compared to sodium sulfite. Knife coating has also been done with fluorinated coatings. Sodium erythorbate is a non-volatile oxygen scavenger that can be used as an alternative to sodium sulfite and hydrazine. The coating is continuously pumped into the pan, while the overflow recycles back to the supply reservoir, where it is remixed and filtered. The term may also be applied to raw water intended for use in boilers, treated boiler feedwater, steam condensate being returned to a boiler, or boiler blowdown being removed from a boiler. Alternatively, sodium carbonate (Na2CO3, aka soda ash) may be used. For this reason, most low and medium pressure boiler applications favor the use of liquid sodium (or potassium) hydroxide as the alkalinity builder. With this phosphate treatment, wastage corrosion was experienced, resulting in SG leaks. Phosphate treatment is based on the formation of insoluble metal phosphates, such as Pb5(PO4)3Cl, Cd3(PO4)2. Several mill-annealed Alloy 600 tubes and few thermally treated Alloy 600 tubes suffered from stress corrosion cracking. Reproduced with the permission of Elsevier. Figure10.11. Severe corrosion of carbon steel support plates led to the accumulation of corrosion products in the crevice between the support plates and tubes leading to tube denting. [8], Boiler sludge concentrations created by coagulation treatment may be avoided by sodium phosphate treatment when water hardness is less than 60mg/L. It also has the advantage of converting red iron oxide (hematite) into black iron oxide (magnetite). Although dealkalization of boiler makeup is a common practice in high pressure boiler installations, many low and medium pressure boilers operate on soft water makeup. The treatment processes involved are: treatment with chelating agent, phosphate treatment, ferrite treatment. In this chapter, a phosphating treatment of magnesium alloys has been introduced to biomedical application, focusing on the formation process of a phosphate coating, surface microstructure, and the phosphating system. However, since it appears on the Generally Recognized as Safe (GRAS) list of food additives, it does not pose the same health and safety concerns as hydrazine and the other alternatives. In addition to reacting with oxygen in the pre-boiler and boiler, volatile scavengers carry with the steam into the condensate system where they further react with dissolved oxygen. Denting stopped with high AVT chemistry. Alkaline-clean in a solution of 3% trisodium phosphate and 3% sodium carbonate for 5 minutes at 82C. The main advantages of this application technique are its very high throughput and greater than 95% application efficiency. These phosphate coatings are often of a new no-rinse (dried in place) microcrystalline type applied by roll coating, and are primarily used on hot-dip galvanneal coatings, although there have been limited uses on hot-dip galvanize and electrodeposited ironzinc coatings (Bittner and Schubach 1997). This article will clear away some of the clutter by presenting a basic approach to providing effective chemical treatment for low and medium pressure boilers. The class of volatile oxygen scavengers include carbohydrazide, methylethylketoxime (MEKO), hydroquinone and diethylhydroxylamine (DEHA). In the time of phosphate treatment, utilities were used to operate with rather high impurity levels, much higher than now, particularly for seawater-cooled plants. One part of hydrazine is required to react with 1 part of dissolved oxygen. In various western countries different measures, like boric acid treatment or molar ratio control, were implemented to improve secondary side chemistry. A. Drexler, in Steam Generators for Nuclear Power Plants, 2017. [12] Acceptable phosphate concentrations decrease from 140mg/L in low pressure boilers to less than 40mg/L at pressures above 1,500 pounds per square inch (10,000kPa). Trication ZnPO4NiMn is often used for galvanized sheet surfaces (Fig. This treatment is a modification of the iodo-phosphate treatment described above for carbon steel. [16] Steam locomotives and boilers generating steam for use in direct contact with contaminating materials may not recycle condensed steam. The reason for the necessity of such a buffering effect was that the condenser tubes, historically made of copper alloys due to their high thermal conductivity, were not always tight. These solid deposits thermally insulate heat exchange surfaces initially decreasing the rate of steam generation, and potentially causing boiler metals to reach failure temperatures. Feedwater of the receiver and evaporator shall be ammoniated. Robots are used to assemble the brakes and problems develop when the parts stick to each other. This is available as a 98% flake form having a 76% Na2O content. This product is available as a 99% active powder having a 58% Na2O content. Water Treatment Requirements for Low and Medium PressureBoilers, Closed Loop Systems: Chemical Treatment Alternatives, Microbiological Control in Cooling Water Systems, Corrosion Control in Cooling Tower Systems, Scale and Fouling Control in Cooling Tower Systems, Chemical Treatment Requirements for Condensate Systems, Chemical Treatment Requirements for Steam and Hot Water Systems, Natural Pathogen Control Chemistry in Cooling Tower Operation, Essential Instrumentation for Reverse Osmosis Systems, Reverse Osmosis Improves Boiler Efficiency, Reducing Wastewater Discharge by Conservation, Reuse and Recycling, Corrosion Control in Water Distribution Systems, Reducing Fuel Costs Via Improved Boiler Feedwater Quality, Water Treatment Requirements for Low and Medium Pressure Boilers, Practical Methods for Achieving Cooling Tower Water Savings, Enhanced Cooling Tower Maintenance Saves Water, Reclaiming Wastewater for Use as Cooling Tower Makeup, A Green Approach to Cooling Tower Operation, Using Soft Water for Cooling Tower Makeup, We Dont Have a Water Shortage ProblemWe Have a Water Distribution Problem. As an alternative to precipitating treatment programs, chelating agents are often used to keep calcium and magnesium soluble thereby avoiding the formation of an insoluble sludge. Scale deposits are also a fundamental cause of overheating and stress boiler tube failures. In the early 1970s (under phosphate chemistry treatment) Alloy 600MA started to suffer from corrosion. In general, experience suggests that a range of 85 to 300 ppm OH alkalinity is typical for boilers operating in the low and medium pressure ranges. Amines are best applied by injection with a quill into the main steam header, but they can also be applied directly to the boiler where they distill off into the steam phase. This method can be adapted to spraying techniques. 2-acrylamido-2-methyl propane sulfonic acid (AMPS), and various co-polymers and ter-polymers of AA/AMPS. [8], Boilers operating at pressures less than 200 pounds per square inch (1,400kPa)[10] may use unsoftened feedwater with the addition of sodium carbonate or sodium hydroxide to maintain alkaline conditions to precipitate calcium carbonate, magnesium hydroxide and magnesium silicate. Black iron magnetite is a more passive (corrosion resistant) form of iron as compared to red iron hematite. A schematic of a coating line is shown in Figure10.11. Phosphate treatment, as a popular pretreatment of magnesium alloys in industry, has been proven to be an effective method for improving the surface biocompatibility of titanium alloys and stainless steel, such as osteoconductivity or osseointegration. Most coatings are solvent based so VOC content (volatile organic compounds) can be high, but volatile gases are incinerated. Finally, a short commentary is given on likely future trends. It also offers very precise control of film build. S. Mizutani, S. Sakai, in Studies in Environmental Science, 1997. final pH contacting with 10 times volume distilled water. Magnesium and magnesium alloys have attracted much attention as biodegradable biomedical materials due to their biodegradability, close elastic modulus to bone tissue, and good bone tissue response, especially in bone implant applications. Eight (8) parts of sodium sulfite are required to react with one (1) part of dissolved oxygen. Ion exchange softeners essentially remove all hardness and iron from the boiler makeup. Figure10.12. if a non-precipitating chemical program is desired, for reaction with magnesium hardness and to adjust boiler alkalinity, such as polyacrylate, polymethacrylate and various co- and ter- polymers. A continuous substrate is called a web. However, with the development of more chemically refined synthetic polymers, the use of natural organic dispersants continues to decline. Replacement water is required to continue steam production. [8], Coordinated control of pH and phosphates attempts to limit caustic corrosion occurring from concentrations of hydroxyl ions under porous scale on steam generating surfaces within the boiler. Medium pressure boilers fall in the range of 150 to 650 psig. Early on, boiler operators applied natural substances like potato starch and tannin/lignin extracts from tree bark. Many large boilers including those used in thermal power stations recycle condensed steam for re-use within the boiler. Sorbitol anhydride esters have been recently approved by the FDA for use in treating steam condensate in food plants up to a dosage of 15 ppm in the steam. One unusual application is automotive brake dampers. [13], Chelants like ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetic acid (NTA) form complex ions with calcium and magnesium. Soda ash reacts under boiler temperature and pressure to produce sodium hydroxide and free carbon dioxide. Recommended alkalinity similarly decreases from 700mg/L to 200mg/L over the same pressure range. However, the fast degradation rate leads to early loss of mechanical property, high increase in pH value, and hemolysis, which limit their clinical application. Available as a 90% active powder or less-concentrated liquid, sodium sulfite reacts rapidly with residual dissolved oxygen to form harmless sodium sulfate. If left unchecked, the propagation of the pit ultimately leads to tube failure. With adequate alkalinity, addition of sodium phosphate produces an insoluble precipitate of hydroxyapatite with magnesium hydroxide and magnesium and calcium silicates. In general, boiler blowdown is controlled such that the suspended solids produced by these precipitation reactions do not exceed 500 ppm. However, in the specialty chemical market, the various brand names and proprietary formulations create some confusion as to the best practice for boiler water treatment.