Boiler steam, corrosion

For higher-pressure boilers demineralization is necessary to minimize total dissolved solids in the boiler. This water is normally carried in steel pipework, but if condensate is returned and the condensate has become contaminated (for example, with carbon dioxide or copper ions) more corrosion-resistant materials such as copper are required. Downstream of the boiler, steam pipework is usually steel with steel or stainless steel expansion bellows. 

The explicit aims of boiler and feed-water treatment are to minimise corrosion, deposit formation, and carryover of boiler water solutes in steam. Corrosion control is sought primarily by adjustment of the pH and dissolved oxygen concentrations. Thus, the cathodic half-cell reactions of the two common corrosion processes are hindered. The pH is brought to a compromise value, usually just above 9 (at 25°C), so that the tendency for metal dissolution is at a practical minimum for both steel and copper alloys. Similarly, by the removal of dissolved oxygen, by a combination of mechanical and chemical means, the scope for the reduction of oxygen to hydroxyl is severely constrained. 

Apart from the oxygen corrosion that results in HW and LP steam heating systems where water losses occur as a result of leaking pump mechanical seals, excess BD, faulty steam traps, and other sources, a subsequent effect is the development of fouling. This effect stems from the production of corrosion debris and (high iron content) sludge that eventually settle out in the boiler. This corrosion debris, sludge, and other foulants must be periodically removed from the boiler by BD, which merely adds to the water loss, and the cycle perpetuates. 

Process contamination. Sticky films (LP boilers) Varnish (HP boilers) Steam discoloration FW pH fluctuations Acid corrosion Stable foams leading to carryover... 

The FW oxygen scavengers mentioned earlier are volatile and can therefore provide additional protection against post-boiler section corrosion induced or enhanced by oxygen in-leakage. These particular scavengers also break down under pressure to produce some level of volatile ammonia, which can neutralize any carbon dioxide found in the steam-condensate system. 

The /main modifications took place in the burners. The boiler itself has not been modified. In the tests it achieved 60% of nominal output of steam. With better monitoring of superheater temperatures, it is expected to achieve an output of 90%. The test program will be developed this year (1980) and will include emission analysis, boiler efficiency, corrosion, and other aspects. CESP also plans to analyse smaller boilers which will operate this same year. 

Completely soluble in water and brine. Cationic behavior makes it useful for the formulation of corrosion Inhibitors. Film-forming amine. Steam volatile for boiler-condensate corrosion control. Low combining weight makes it an efficient neutralizing amine. 

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... 

Boiler Water Treatment. Alkanolamines, in general, provide excellent corrosion protection to steel in many appHcations. When used in boiler water treatment, AMP provides excellent protection to steel and copper in steam lines through efficient absorption of CO2, effectual distribution ratio for... 

Monobutylamines are easily soluble in water and hydrocarbons and can generally be steam distilled. These properties lead to uses in soaps for water and oil emulsions, and as corrosion inhibitors in steam boiler appHcations (see Corrosion and corrosion inhibitors Emulsions). Morpholine is also extensively used as a corrosion inhibitor in steam boiler systems. In addition, it is widely used as an intermediate in the production of delayed-action mbber accelerators. 

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 ... 

A good summary of the behavior of steels in high temperature steam is available (45). Calculated scale thickness for 10 years of exposure of ferritic steels in 593°C and 13.8 MPa (2000 psi) superheated steam is about 0.64 mm for 5 Cr—0.5 Mo steels, and 1 mm for 2.25 Cr—1 Mo steels. Steam pressure does not seem to have much influence. The steels form duplex layer scales of a uniform thickness. Scales on austenitic steels in the same test also form two layers but were irregular. Generally, the higher the alloy content, the thinner the oxide scale. Excessively thick oxide scale can exfoHate and be prone to under-the-scale concentration of corrodents and corrosion. ExfoHated scale can cause soHd particle erosion of the downstream equipment and clogging. Thick scale on boiler tubes impairs heat transfer and causes an increase in metal temperature. 

The term steam quaUty refers to the amount of dry steam present relative to Hquid water in the form of droplets. The steam deUvered from the boiler usually contains some water. Excessive amounts can result in air entrapment, drying problems following exposure, and unacceptable steam levels (>3% water or <97% quaUty steam). Excessive amounts of water deposits dissolve boiler chemicals onto the load to be sterilized. Boiler chemicals are used to prevent corrosion in the lines. Inappropriate boiler chemicals, also called boiler amines, may introduce toxicity problems (see CORROSION AND CORROSION control). 

Condensate Polishing. Ion exchange can be used to purify or poHsh returned condensate, removing corrosion products that could cause harmful deposits in boilers. Typically, the contaminants in the condensate system are particulate iron and copper. Low levels of other contaminants may enter the system through condenser and pump seal leaks or carryover of boiler water into the steam. Condensate poHshers filter out the particulates and remove soluble contaminants by ion exchange. 

Ca.ustic Corrosion. Concentration of caustic (NaOH) can occur either as a result of steam blanketing (which allows salts to concentrate on boiler metal surfaces) or by localized boiling beneath porous deposits on tube surfaces. 

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. 

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. 

Steam Purity. Boiler water soHds carried over with steam form deposits in nonreturn valves, superheaters, and turbine stop and control valves. Carryover can contaminate process streams and affect product quaHty. Deposition in superheaters can lead to failure due to overheating and corrosion, as shown in Figure 6. 

Metal Cleaning. Citric acid, partially neutralized to - pH 3.5 with ammonia or triethanolamine, is used to clean metal oxides from the water side of steam boilers and nuclear reactors with a two-step single fill operation (104—122). The resulting surface is clean and passivated. This process has a low corrosion rate and is used for both pre-operational mill scale removal and operational cleaning to restore heat-transfer efficiency. 

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. 

Leaky valves are also a cause of erosion. Most turbine erosion-corrosion problems come from damage that takes place when the unit is not running. A shght steam leak into the turbine will let the steam condense inside the turbine, and salt from the boiler water will settle on the inside surfaces and cause pitting, even of the stainless blading. There must be two valves with a drain between them, i.e., a block valve on the header and an open drain in the line before it reaches the closed trip-throttle valve. 

In a turbine that is running, erosion-corrosion is pretty much confined to units that are operating on saturated steam with inadequate boiler-water treatment. This type of erosion takes place behind the nozzle ring and around the diaphragms where they fit in the casing. 

To protect steam boilers and their tubes from corrosion, E. Cumberland used cathodic impressed current in America in 1905. Figure 1-10 has been taken from the corresponding German patent [35]. In 1924 several locomotives of the Chicago Railroad Company were provided with cathodic protection to prevent boiler corrosion. Where previously the heating tubes of steam boilers had to be renewed every 9 months, the costs fell sharply after the introduction of the electrolytic... 

Aikaiinity Bicarbonate (HCOs" ), carbonate (COs , and hydroxyl (OH ), expressed as CaCOs Foaming and carryover of solids with steam embrittlement of boiler steel bicarbonate and carbonate produce CO2 in steam, a source of corrosion Lime and lime-soda softening, acid treatment, hydrogen zeolite softening, demineralization, dealkalization by anion exchange, distillation, degasifying... 

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