Boiler Corrosion Problems

General problems in boilers are deposits, carryover, and corrosion, which are common to most systems. 

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Oxygen-influenced boiler corrosion problems (Cont.)... 

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. 

Condensate returns lines are often copper. Copper has good corrosion resistance to oxygen and carbon dioxide individually. When both gases are present in the condensate, copper is susceptible to corrosion. Copper picked up in the condensate system and returned to the boiler causes serious corrosion problems in the boiler and any steel feedwater and steam pipework. Boiler tubes should last for 25 years but can fail within one year in a mismanaged or ill-designed boiler system suffering from these faults. 

Boilers Plants always experience corrosion problems with boilers, and monitoring of corrosion has always presented a challenge relating to both access and simulation of exact conditions, e.g. condensation conditions. Applications in the monitoring of condenser tubes have been reported ... 

The periodic development and use of new steel alloys can improve ferrous corrosion resistance however, where economizer units are constructed of copper alloys, under certain conditions serious copper corrosion problems may result. This occurs when FW having a pH over 8.3 also contains small amounts of ammonia and dissolved oxygen (DO). The ammonia may be present, for example, as a result of the overuse or inappropriate application of certain amines. Further damage may occur from the plating-out of the copper-ammonia ion then created as a cathode on boiler tubes. This promotes anodic corrosion of the immediate surrounding anodic areas. 

It is generally acknowledged that no matter the size of boiler plant system or quality of FW provided, some form of chemical treatment is always necessary to counter deposition or corrosion problems associated with contaminants present in the incoming FW. 

Most waterside problems develop insidiously. Over time, scale and other types of deposit are gradually formed on internal heat transfer surfaces, which gradually raises the cost of providing heat energy. Some types of deposition can be very difficult and costly to remove. Corrosion wastes away the fabric of the plant (sometimes very quickly) and may produce an unexpected and untimely boiler plant shutdown, with a consequential loss of space heating, electricity, or process manufacturing capability. Likewise, fouling reduces the size of waterways and increases boiler operational problems. 

Where corrosion takes place, the origins of the metal oxides and salts formed from corroded boiler system metals should be traced in a systematic fashion to establish cause and effect and avoid misclassify-ing the fundamental waterside problem. Occasionally however, it is difficult to positively confirm the starting point of a corrosion problem because it is common for corrosion products to be transported from their point of origin and deposited elsewhere in the steam-water circuit, or alternatively to act as binders and contribute to fouling and contamination of the overall boiler plant system. 

Oxygenation treatment also reduces the risk of erosion-corrosion problems and limits iron transport to other parts of the boiler system where fouling could take place. 

Inadequate FW deaeration is a common cause of serious corrosion problems that affect many hundreds (if not thousands) of boiler plants of all sizes around the world. Despite the abundance of available literature advising the need to eliminate oxygen from boiler FW, inadequate deaeration continues to cause permanent waterside damage. 

In smaller boiler systems, the FW tank often acts as a common condensate receiver, MU water heater, and deaerating vessel. As such, the tank is subject to the same corrosion problem risks that befall deaerators, economizers, and FW lines. Smaller systems often are inadequately designed and constructed, with the result that they may suffer serious oxygen corrosion in a particularly short time. (It is not unknown for tank wall perforation to occur within 3 to 6 months of the installation of a new FW tank as a result of pitting corrosion.)... 

Selective Corrosion Problems Affecting Pre-Boiler Section Equipment... 

Almost all boiler section scale, fouling, deposit, or corrosion problems, however, tend to be associated with additional (and often very similar) problems both upstream and downstream of the boiler itself. Therefore, the presence of, for example, boiler deposits or localized corrosion issues should never be viewed in isolation. Rather, they should be investigated in the light of a likely chain of cause and effect, and the basic causes of the problems should be eliminated wherever possible, rather than simply the noticeable effects. 

For the limited number of smaller boiler plants where serious corrosion problems do develop, however, it typically is found upon investigation that corrosion does not take place in isolation. Rather, the... 

Table 7.6 Summary notes boiler section corrosion problems involving oxygen, concentration cells, and low pH.

Poor steam purity Contaminated steam leads to corrosion, erosion, sticky valves, and boiler operation problems. 

Although condensate polishers mitigate corrosion problems in the condensate, FW, and boiler, it should be remembered that they do noth-... 

Some basic BW treatment objectives include keeping boiler surfaces clean and corrosion-free to minimize fuel bills and managing variable quality FW smoothly and efficiently to limit upsets and other downstream problems. But the nature of potential boiler deposition problems changes with increases in pressure and, simply put, is primarily concerned with a reduction in simple, hardness-related deposits and an increase in complex, iron oxide deposits. The effect of dirty boilers on fuel costs can be seen in Figure 10.4. 

The introduction of modem, very high pressure or temperature boilers has led to water chemistry control problems and cause-and-effect corrosion problems due to phosphate hideout. Under... 

Erythorbates are safe products and there are no harmful breakdown products, although when early formulations utilized ammonia as a PH buffer (and neutralizer for part of the carbon dioxide), copper corrosion problems resulted. However, erythorbates are not steam-volatile,and consequently there is no post-boiler oxygen scavenging potential available. Thus, in the event of complete breakdown of the product at high pressure, oxygen-induced, ammonia corrosion of copper may continue unchecked. 

Corrosion of steel by carbonic acid is probably the most common problem in the post-boiler section, producing pipe grooving and general metal wastage, especially in threaded joints. This form of corrosion is not self-regulating and the reaction products can produce more carbon dioxide, thus perpetuating the corrosion problem. Typically, the condensate pH level is depressed to around 5.0 to 5.5. 

Morpholine is still the standard by which other amines are compared for pH control, and AMP has commonly been employed to control carbon steel boiler tube erosion-corrosion problems in European gas-cooled reactor stations. 

Where corrosion problems exist, rates may vary considerably throughout the system, but total metal loss may typically exceed 0.25 ppm M (where M = Fe/Cu/Ni) in the condensate without the use of amine. This compares badly to recommended maximum limits of 0.05 ppm M for up to 580 psig/40 bar boilers, 0.03 ppm M for up to 870 psig/60 bar, and 0.02 ppm M beyond that. 

The primary boiler plant problem here is cold-end corrosion, caused by the destructive effects of sulfuric acid produced within the convection area. Further problems include acid rain, which occurs when sulfur gases are emitted and widely dispersed to eventually produce sulfuric acid in the upper atmosphere, which precipitates as rain. 

The ultimate answer to cold-end corrosion problems is to totally eliminate sulfur from all fuels used, although from a political or economic standpoint, this is seldom a viable option. At the boiler plant facility itself, more practical options include raising the exit gas temperatures to prevent the dew point from being reached and using selective fuel treatments. 

Corrosion problems affecting pre-boiler section equipment 209... 

The Carnot cycle is not a practical model for vapor power cycles because of cavitation and corrosion problems. The modified Carnot model for vapor power cycles is the basic Rankine cycle, which consists of two isobaric and two isentropic processes. The basic elements of the basic Rankine cycle are pump, boiler, turbine, and condenser. The Rankine cycle is the most popular heat engine to produce commercial power. The thermal cycle efficiency of the basic Rankine cycle can be improved by adding a superheater, regenerating, and reheater, among other means. 

The hardness deposits coat the inside of the boiler s tubes, interfere with heat transfer, and overheat the tubes. The carbon dioxide, which is also generated from the dissolved solids, creates more serious corrosion problems in downstream heat exchangers. When the steam condenses, the carbon dioxide may remain trapped in the reboiler or preheater as a noncondensable gas. Actually, there is no such thing as a noncondensable gas. Even C02 is somewhat soluble in water. As the C02 dissolves in the condensed steam, it forms carbonic acid, a relatively weak acid (pH typically between 5 and 6). Strong acids will have pH values of 1 to 2. Pure water has a pH of seven. Carbonic acid is particularly corrosive to carbon steel heat-exchanger tubes. 

The release of alkaline compounds is also an important factor in coal combustion. Most high-temperature corrosion problems in fossil fuel boilers are the result of salt melts (54,55). Alkali metal compounds volatilize from coal at the high temperatures of conventional combustion and subsequently condense on heat transfer surfaces. The lower temperatures of FBC are expected to reduce salt volatility, a fact that has been confirmed by Vogel et al. (16). 

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