Boiler corrosion design

This last design (chloride anion exchange) is very often specified when some form of dealkalization plant is under consideration. Selection usually is made on the grounds of operator safety and reduced risk of boiler corrosion. Although it may have the attraction of not creating any potential for acid handling or acid introduction into the FW line or not requiring a deaerator, these are mere diversions. 

A volatile, amine-based chemical treatment usually added to the boiler FW, designed to neutralize the corrosive effects of carbonic acid in steam/condensate and raise condensate pH. 

Feedwater treatment is designed to protect the feedwater system and, to some extent, the boiler. Most systems contain carbon steel piping. Carbon steel corrosion (Fig. 23a) is considerably slower at a pH between 9.0 and 11.0. In aH-ferrous feedwater systems, the preferred pH range is therefore 9.2 to 9.6, although some systems are operated at a pH as high as 10. In systems where copper alloys are present, high concentrations of ammonia accelerate corrosion of the copper alloys. In those systems the preferred pH is 8.8—9.2. 

Internal cathodic protection of water tanks and boilers is most economical if it is taken care of at the design stage. It can, however, be installed at a later stage as a rehabilitation measure to halt the progress of corrosion. Tanks and boilers in ships were described in Section 17.4. Further applications of internal protection are dealt with in Chapter 21. 

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. 

Lees and Whitehead" have shown that differences in boiler design lead to differences in furnace atmospheres, which are subsequently reflected in differences in scale morphology and corrosion performance. Hence they report that there is no unique scale morphology which is characteristic of furnace wall corrosion. They also warn that the scale that is examined during an investigation may not be an exact reflection of the scale on the tube surfaces during operation due to the possible hydrolysis of the scale on cooling (when hot flue gas is replaced by moist air) and the redistribution of phases in the scale due to the loss of the incident heat flux. 

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. 

Variable-load operation may result in higher heat fluxes than on baseload, as well as unusual chemical conditions. If, however, after allowing for these, and remedying them as far as is practicable, a boiler persistently suffers from on-load corrosion, then the cure may lie in a design change. 

Today boiler vessels are usually fabricated from special boiler plate and firebox steels of varying thickness, while their auxiliaries (supplementary equipment) and appurtenances (boiler accessories and instruments, especially those employed for safety reasons) may be produced from any of several different constructional metals, alloys, and other materials, including cast iron, copper alloys, stainless steels, and so forth. Tubes and tube plates may be variously constructed of carbon steel, low-alloy steels, or special alloy steels, with each design providing for particular required levels of thermal and mechanical stress and corrosion resistance. The overall boiler plant system may have a life expectancy in excess of 50 to 60 years, although individual components may need to be replaced periodically during this period. 

Stay bolts are also used in many FT boiler designs. Typically, these bolts are provided with a drilled weep hole that leaks if deterioration of the stay bolt occurs due to waterside corrosion, thus providing an early warning to the operator. 

Hot water generators and LP steam-raising plants of below 15 psig are designed to operate with minimal blowdown and to suffer negligible circulatory losses. As a result, clean, sediment-free water of almost any characteristic nature (e.g., soft and corrosive or hard and scale-forming) is likely to be suitable as a source of boiler water makeup. 

When the impeller or casing is made of cast iron (common in many designs of FW pumps and other pre-boiler items of equipment), under certain operating circumstances graphitic corrosion can occur. 

Today many designers favor the use of particular grades of stainless steel and titanium alloys, but older condensers are often constructed of copper alloys, which may provide a source for copper corrosion, the products of which can be transported back to the boiler. 

In typical cause-and-effect mode, where chlorides penetrate the deposit or where a localized overconcentration of hydroxyl ions occurs, the magnetite film is disrupted and particular forms of very damaging corrosion occurs. In addition, where localized heat flux exceeds design limits within a boiler and may be accompanied by departure from nucleate boiling (DNB) conditions, overheating and metal failure may also occur. 

Thus, the proper control of deposition, corrosion, and fouling and boiler structural integrity are interdependent functions, and all these phenomena are directly related to boiler design and real-time operating conditions. 

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. 

Chemical treatment programs are often individually designed for particular boiler plant systems but usually contain oxygen scavengers, pH boosters, and corrosion inhibitors. In addition, the formulations employ materials specifically designed to limit the degree of deposition and control the mechanisms of deposition. 

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. 

Of course, this argument is perfectly true where it can be positively demonstrated that MU water requirements really are very low. Once again however, if this is not the case, then—most treatment programs are designed primarily for corrosion control and do not compensate for undue hardness entering the boiler—calcium carbonate scale can and does develop over time. This process takes place even where the MU water is relatively soft, and results in the formation of insulating boiler tube deposits or boiler vessel sludge. 

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.)... 

Thus, a failure to properly balance and control various external and internal water chemistry parameters may lead to one or more corrosion mechanisms occurring, including several forms involving oxygen. Many types of corrosion are of themselves unfortunately relatively common phenomena, and the mechanisms also are common to all sizes and designs of boiler. 

The precise protocols necessary to achieve effective corrosion control will vary dependent on individual boiler design and operation. For example, control of alkalinity is fundamental in controlling corrosion mechanisms. In small to midsize, general-purpose and industrial boilers, it is common practice to obtain adequate BW alkalinity as part of any water treatment program that operates under a free-caustic regimen. This approach generally is perfectly acceptable, and such programs normally can be relied on to ensure a clean, scale- and corrosion-free boiler. 

It will be noted from the preceding discussion that impurities may induce more than one form of corrosion. The particular types of corrosion and the influence of these impurities work in a developing chain of cause and effect, depending in large part on the specific operating circumstances in question. Such factors include the boiler system basic configurational design, the localized areas of stress, and the temperature of various metal surfaces. 

For all types of boilers, from the most simple HW heaters to immense power generating plants, one of the most fundamental objectives of any water treatment program is (as stressed several times) to minimize boiler section waterside corrosion, especially those common types of corrosion involving oxygen. The actions taken to achieve this objective are generally the same, irrespective of boiler size or design. These actions include ... 

The potential for electrochemical corrosion in a boiler results from an inherent thermodynamic instability, with the most common corrosion processes occurring at the boiler metal surface and the metal-BW interface (Helmholtz double layer). These processes may be controlled relatively easily in smaller and simpler design boilers (such as dual-temperature, LPHW heating, and LP steam boiler systems) by the use of various anodic inhibitors. 

However, some of the most widely used phosphonates tend to be poor calcium sulfate inhibitors, and they may adversely affect the corrosion of copper in the boiler, so care in formulation design and product application is very important. 

For the smallest low-pressure heating boilers, simple one-drum programs that are designed for greater than 99% condensate return are used. Where condensate is only 70 to 80%, boiler waterside surfaces get dirty quickly, under-deposit and oxygen pitting corrosion may develop, and tubes may fail. 

Many types of WT boiler are of a one-drum design, but where mud drums are fitted, these should also be inspected. As its name suggests, much of the mud, sludge, dislodged scale particles, and other general debris in the boiler ends up in the bottom or mud drum. This material should be removed and the drum inspected for underdeposit corrosion, wall thinning, erosion, and other problems. 

Cathodic protection is a useful supplement to other forms of water treatment, as a general corrosion inhibiting device in HW boilers, or where specific design configurations can lead to inadequately protected localized metal in steam boilers. Where BW makeup demands are minimal and boiler output is fairly constant, cathodic protection devices can also provide some measure of protection against hardness scales. Calcium carbonate salt is formed as a floc-culant or soft sludge rather than a hard scale, due to the peptizing effects of a zinc hydroxide complex formed from zinc ions in alkaline BW. 

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