Boiler Systems

Hydrazine has the advantage over sodium sulfite of not contributing dissolved soHds to the boiler system, thereby reducing the need for purging. At... 

S CAW-OX Hydra ne Solutions to Control Ocygen Corrosion in Boiler Systems, Olin Corp., Stamford, Coim., 1988. 

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. 

Fig. 4. Two-dmm Stirling power boiler system for pulverized coal. The flue gas exits via back-end environmental control devices.

For once-through boilers, the treatment must be without soHd residues, so all-volatile treatment (AVT) is used. AVT, which is also used in some dmm boiler systems, rehes on the feedwater chemical additives, ammonia and hydrazine, to provide water appropriate to the boiler. Because the additives are volatile, they do not accumulate in the boiler and provide only minimal protection during contaminant ingress. Most plants using AVT have some form of condensate poHsher to remove impurities from the condensate. 

The quahty of feed water required depends on boiler operating pressure, design, heat transfer rates, and steam use. Most boiler systems have sodium zeohte softened or demineralized makeup water. Feed-water hardness usually ranges from 0.01 to 2.0 ppm, but even water of this purity does not provide deposit-free operation. Therefore, good internal boiler water treatment programs are necessary. 

The average AFUE of all installed furnaces is 65 to 75 percent, much lower than post-1992 efficiency standards due to the different vintages of furnaces and boilers. Systems that are 40 years old or older are even less efficient (55-65%), but these represent a veiy small fraction of furnaces operating in the United States today. 

Chemical consumption Chemical consumption will be associated with boiler feed make-up water-treatment plant, dosing systems for feedwater and boiler system, treatment of cooling water circuits and effluent treatment. Typical chemical requirements for a thermal power plant are given in Table 15.9. 

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. 

Condensate purification In some boiler systems the condensate returning is retreated using ion exchange to minimise corrosion and deposit accumulation. This particularly applies to once-through boilers (mainly nuclear) where there may be no water/steam separator and perhaps limited facilities for blow-down. Additionally, some high heat-flux boilers (oil-fired) have been fitted with condensate polishing plant (CPP). 

Safety is, of course, paramount with modern boilers, and, as with other types of pressure vessels, they are subject to construction codes and operational regulations in almost all countries of the world. These requirements have been developed over many years and are designed to ensure the continuously safe operation of the overall boiler system. 

With the incorporation of an economizer into a boiler system, typically a 1% gain in boiler efficiency can be achieved for every 40 to 50 °F reduction in stack temperature. 

Feedwater tank water temperatures on package boiler systems not fitted with deaerator heaters should be as high as practically possible, while consistent with the avoidance of FW pump cavitation. 

Traps that stick open may allow considerable volumes of steam to blow through, thus resulting in a reduction in overall boiler system efficiency. 

In common with a FT boiler plant, a WT boiler plant contains several steam-water condensate systems and subsystems, although those of industrial and waste-heat WT boiler systems are generally larger and more complex, and power WT boiler systems are even more so. 

Combined Cycle Boiler Systems These boiler types are used to harness the waste heat from one power source to provide the heat source for a second power source, usually an electricity generator via a steam turbine. They are a form of cogeneration boiler plant. 

In almost all countries today, safety codes and regulations exist for the construction, operation, and inspection of all boilers and associated pressure vessels and boiler systems. Both HW and steam-raising plants are provided with several vital boiler appurtenances (appliances or fittings) and various subsystems containing auxiliaries (accessories) that must be maintained, monitored, and controlled. However, for small HW and LP steam boiler plants the inspection process may be rather cursory with regard to the pressure vessel internals and tends to concentrate primarily on ensuring the proper operation of the various appurtenances that provide for boiler safety. 

Auxiliaries are additional boiler fittings that provide controls for ease of operation. They may include additional valves (such as check valves on feed lines, nonreturn valves on steam distribution lines, and the various boiler system drain valves), gauges, connections, and devices to regulate FW, air, and fuel and to provide for the efficient production, pressure, temperature, quality, and flow of HTHW or steam. 

Economizers are heat transfer tube bundles that preheat MU water or FW flowing within the tubes by extracting waste heat from the flue gas during its exit path to the stack. They typically account for approximately 10% of the total boiler heat transfer surfaces, while absorbing only 7% of the total heat generated in the boiler system. 

There are various WT and FT boiler economizer designs, classified as either steaming economizer and nonsteaming economizer types according to thermal performance. These economizers are constructed in either bare tube or finned tube (extended surface) patterns. They may be positioned horizontally or vertically within the boiler system, in either cross-flow or counterflow arrangements. 

For any particular FSHR design, capital payback periods are clearly reduced by half if the equipment is installed in a boiler system that... 

It is therefore not surprising to find that where boiler systems operate under these constraints, they suffer rapid and extensive oxygen-initiated corrosion to the internal surfaces of the FW system, the feed lines, the boiler shell, and all internal heating surfaces. With poor oxygen scavenging, heavy pitting corrosion and tuberculation is found, especially on the tubes and at the waterline of the FW tank and boiler shell. 

There are several fundamental waterside problems that may occur, each involving a combination of physicochemical reactions. Some of these problems are specific for particular boiler system types and designs, while others are more general in nature and have the potential to cause trouble to all manner of HW and steam-raising plants. 

In any boiler system, there are several fundamentally different problems that may develop. Some problems may specifically affect only the waterside boiler surfaces, economizers, or condensate system, but all will ultimately adversely affect the overall HW or steam-system cycle and raise the cost of doing business. 

In practice, both fouling and deposition actions are likely to occur simultaneously to some degree or other in a boiler system. Furthermore, corrosion processes and the entry of contaminants into the steam-water system usually results in some form of deposition occurring elsewhere in the system. 

Foulants and contaminants may originate virtually anywhere in the overall steam-water circuit. Some may be derived from pre-boiler systems (such as an economizer or deaerator) or post-boiler systems (such as a steam trap or a condensate line), but they inevitably find their way into the boiler and cause significant damage and expense. 

The potential for corrosion as a result of the reactions of noncondensable gases present in steam-water circuits is a major area of risk. The dissolved oxygen (DO) content of MU water is recognized as a primary source of gas entering a boiler system, and effective deaeration of MU and FW is therefore critical. 

Ammonia (NH3) also may be present in some boiler systems, either by design or by the breakdown of amines, hydrazine, and other organic compounds. It attacks components and equipment constructed of copper and brass. 

Economizers are not usually designed to generate steam, and any deposits found in them therefore are not likely to be a result of carbonic acid corrosion or contamination from steam. Rather, the transport and buildup of corrosion debris within an economizer tends to originate from corrosion processes occurring either in the economizer itself or in some upstream part of the pre-boiler system. Economizer deposits typically develop in the presence of oxygen and possess a high iron content. 

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. 

The corrosion of steel and other metals in a boiler system takes place when an electrochemical cell is established, although the rates of corrosion and the types of corrosion mechanisms involved are highly dependent on the particular circumstances that develop during the operation of individual boiler plants. A failure to adequately control corrosion ultimately leads to the failure of boiler surface components or other components and items of equipment in the system. 

Oxygen is almost always a contributing factor to corrosion mechanisms therefore, the effective removal of dissolved oxygen (DO) is of paramount importance in controlling the rate of boiler system corrosion, irrespective of the size, design, or pressure of the plant. 

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. 

Where feed lines have short pipe runs, where hot wells or FW tanks are of small volume, or when FW is too cold, there often is insufficient time for full DO scavenging to take place, even when using catalyzed scavengers. The inevitable result of this lack of contact time is the formation of oxygen-induced corrosion products, which by various secondary mechanisms may settle out to form permanent deposits within the boiler system. These deposits may develop in several forms (e.g., where DO removal is particularly poor, they often appear as reddish tubercles of hematite covering sites where pitting corrosion is active). Active pitting corrosion combined with the presence of waterside deposits ultimately may lead to tube failure in a boiler or other item of system equipment and result in a system shutdown. 

All boiler system waterside surfaces need the protection given by the smooth, hard, tenaciously adherent magnetite layer. The magnetite film sometimes may sparkle because of the precipitation of fine magnetite crystals onto the metal-oxide surface. Magnetite film formation is best achieved under stable, low-oxygen content operating conditions at a pH level of 10.5 to 11.5 (possibly up to 12.0). 

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