Boiler corrosion water-tube

It is not the intention here to consider in detail the subject of boiler feed-water conditioning and treatment for nuclear plant, but the general principles may be noted. Essentially, the same objectives apply as in fossil-fuelled plant, embodied in the three aims to minimise corrosion, deposition and steam-carryover. Requirements are more stringent in nuclear plant because there is no possibility of repairing tubes which have failed, let alone those which have suffered either deposition or corrosion. Again, certain tubes in nuclear plant have very modest design corrosion allowances so that only minimal loss of thickness from any cause can be tolerated. 

Oxygen corrosion, occurring as General etch corrosionAmiform rate corrosion (less common form) Localized corrosion (takes several forms and common where waterside conditions are less than ideal) Rare in correctly treated boilers but can affect drum waterline and tubes. More common in idle and low-load boilers, where water chemistry is unbalanced, under high MU conditions and after poor chemical cleaning. Also in peak-load boilers, especially where deposition can occur. Localized corrosion can be very serious, causing metal failure. 

Corrosion and mineral debris can form in condensate lines from a variety of means, and it is not uncommon for the resultant debris accumulated over many years to slough off and return to the boiler when operating conditions change. The result is often a thick boiler-bottom sludge that settles out in the water space or baked-on sludge, which mars efficient combustion and water tube heat transfer. 

Heat recovery units (water tube boilers) have experienced problems with corrosion and buildup of soot on the heat transfer surfaces. 

The main objective of the test program is to burn processed MSW in a fluidized-bed boiler environment to investigate combustion characteristics, heat transfer, corrosion and erosion, and control parameters. The tests, are being conducted in a CPC-owned 0.7 sq m fluidized-bed combustor which has been reworked to incorporate water tubes for heat extraction in both the bed region and exhaust. Additional tubes will be air cooled to simulate boiler temperatures and to observe erosion and corrosion phenomena, detailographic examination will be made of these materials. 

The boiler has a water-circulating tube wall (waterwall) construction. A layer of smelt freezes on the fireside of the wah due to coohng from the water in the tubes, and this frozen smelt provides a barrier between the tube and the reducing and/or oxidizing gases. Gas composition has been implicated as a major determinant of the corrosion rate on the fireside surfaces. The lower waterwaU is the most critical component. Unlike conventional boilers, a waterwall tube leak cannot be tolerated in a recovery boiler, since a smelt-water reaction has the potential for a catastrophic explosion. 

Caustic corrosion of unalloyed and low-alloy steel is encountered in some unusual situations. For example, in boilers traces of sodium hydroxide can become concentrated and cause local corrosion and caustic embrittlement. This occurs usually in boiler tubes that alternate between wet and dry conditions or in which deposits form. Boiler feed water permeates the deposits and evaporates. This causes concentration of the caustic material, to up to several percent, which is enough to destroy the protective magnetite and/or to initiate caustic embrittlement (Effertz et al., 1982 Hersleb, 1982). 

This is a problem in process plant steam heaters. There are always some residual carbonates in boiler feed water. When the water is turned into steam, some of these carbonates decompose into CO Thus, all steam is contaminated with CO. The CO being far more volatile than water gets trapped and accumulates in the high points of steam heaters. With time, the CO condenses in the water to form carbonic acid. This causes corrosion and tube leaks. To avoid CO accumulation, the exchanger high points can be vented. 

Power generation Cooling-water heat exchangers, flue gas desulfurization systems, fossil fuel boilers, steam generator tubes (nuclear), air heaters, steam turbine systems, vaults, atmospheric corrosion, gasification systems, mothballing... 

Phosphorus is also important in the production of steels, phosphor bronze, and many other products. Trisodium phosphate is important as a cleaning agent, as a water softener, and for preventing boiler scale and corrosion of pipes and boiler tubes. 

Water Treatment. Water and steam chemistry must be rigorously controlled to prevent deposition of impurities and corrosion of the steam cycle. Deposition on boiler tubing walls reduces heat transfer and can lead to overheating, creep, and eventual failure. Additionally, corrosion can develop under the deposits and lead to failure. If steam is used for chemical processes or as a heat-transfer medium for food and pharmaceutical preparation there are limitations on the additives that may be used. Steam purity requirements set the allowable impurity concentrations for the rest of most cycles. Once contaminants enter the steam, there is no practical way to remove them. Thus all purification must be carried out in the boiler or preboiler part of the cycle. The principal exception is in the case of nuclear steam generators, which require very pure water. These tend to provide steam that is considerably lower in most impurities than the turbine requires. A variety of water treatments are summarized in Table 5. Although the subtieties of water treatment in steam systems are beyond the scope of this article, uses of various additives maybe summarized as follows ... 

Steam blanketing is a condition that occurs when a steam layer forms between the boiler water and the tube wall. Under this condition, insufficient water reaches the tube surface for efficient heat transfer. The water that does reach the overheated boiler wall is rapidly vaporized, leaving behind a concentrated caustic solution, which is corrosive. 

Porous metal oxide deposits also permit the development of high boiler water concentrations. Water flows into the deposit and heat appHed to the tube causes the water to evaporate, leaving a concentrated solution. Again, corrosion may occur. Caustic attack creates irregular patterns, often referred to as gouges. Deposition may or may not be found in the affected area. 

Stress Corrosion Crocking. Stress corrosion cracking occurs from the combined action of corrosion and stress. The corrosion may be initiated by improper chemical cleaning, high dissolved oxygen levels, pH excursions in the boiler water, the presence of free hydroxide, and high levels of chlorides. Stresses are either residual in the metal or caused by thermal excursions. Rapid startup or shutdown can cause or further aggravate stresses. Tube failures occur near stressed areas such as welds, supports, or cold worked areas. 

Boiler Deposits. Deposition is a principal problem in the operation of steam generating equipment. The accumulation of material on boiler surfaces can cause overheating and/or corrosion. Both of these conditions frequentiy result in unscheduled downtime. Common feed-water contaminants that can form boiler deposits include calcium, magnesium, iron, copper, aluminum, siUca, and (to a lesser extent) silt and oil. Most deposits can be classified as one of two types scale that crystallized directiy onto tube surfaces or sludge deposits that precipitated elsewhere and were transported to the metal surface by the flowing water. 

In water-wall incinerators. The internal walls of the combustion chamber are lined with boiler tubes that are arranged vertically and welded together in continuous sections. When water walls are employed in place of refrac toiy materials, they are not only useful for the recovery of steam but also extremely effective in controlling furnace temperature without introducing excess air however, they are subject to corrosion by the hydrochloric acid produced from the burning of some plastic compounds and the molten ash containing salts (chlorides and sulfates) that attach to the tubes. 

In applying electrolytic protection, galvanized tubes can be installed downstream from copper components in water boilers without danger of Cu " -induced pitting corrosion. The protection process extends the application range for galvanized tubes with respect to water parameters, temperature and material quality beyond that in the technical regulations [16, 17]. 

The water supply for boilers is usually treated. Treatment depends on the quality of the water supply, the pressure of the boiler, the heat flux through the tube walls and the steam quality required. Most waters require de-alkalization. The water produced in this process is nonscaling and potentially corrosive (see above). 

Fresh waters are, in general, less corrosive towards copper than is sea-water, and copper is widely and satisfactorily used for distributing cold and hot waters in domestic and industrial installations . Copper and copper alloys are used for pipes, hot-water cylinders, fire-back boilers, ball floats, ball valves, taps, fittings, heater sheaths, etc. In condensers and heat exchangers using fresh water for cooling, tubes of 70/30 brass or Admiralty brass are usually used, and corrosion is rarely a problem. 

The three principal concentration mechanisms postulated as being responsible for on-load corrosion processes by Mann are dry-out, concentration in crevices, and concentration in porous deposits. (Clean boiler tube surfaces on which high-pressure water is boiled under forced convection do not develop concentration factors of more than about two.)... 

Infrared Methods Commercial instrumentation for recording infrared radiation has been available for some years and has been explored by the electrical power industry in the UK for assessing corrosion in boiler tubes at power-station shut-down. An external heal source is played onto the outside of boiler tubes at the same time as cold water is circulated inside the tubes. Hot spots due to poor heat conductivity caused by excessive corrosion product indicated areas of high corrosion. 

Water treatment monitoring and control is often a knife-edge operation and must be tailored to the overall operation of the boiler because waterside and gas-side problems usually are interlinked. Consequently (and as with other types of WT boiler), not only should the utility boiler FW be essentially free of dissolved oxygen to prevent waterside pitting corrosion of the economizer and other boiler components, but also the temperature must be high enough to prevent dewpoint condensation and subsequent acid attack on the gas side of the economizer tubes. 

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