Superheater tubes

After the waterwaH tubes deHver the saturated steam back into the top of the boHer dmm, moisture is separated out by a series ofbaffl.es, steam separators, and cormgated screens. The water removed drops down into the hot water contained in the steam dmm. The steam travels out through either a dry pipe, which leads to a superheater header, or a series of superheater tubes that connect directiy into the top of the steam dmm. The superheater tubes wind back into the top of the furnace and/or a hot flue-gas backpass section, next to the economizer, where heat from the combustion gases exiting the furnace superheats the steam traveling through the tubes. 

Coal deposits from east of the Mississippi River generally have acidic mineral constituents, ie, they are richer in siUca and alumina and tend to produce higher melting ash mixtures. These materials do not soften until above 1000°C and have limited problems with deposition on the inside walls of the boiler (slagging) or on the superheater tubes inside the boiler (fouling). 

Corrosion by essentially pure steam arises principally in connection with power generating plant. Temperatures up to about 600°C in association with pressures up to about 15 MN/m are involved, although in the most advanced super-critical installations being planned for the future, temperatures of 650°C and pressures of 40MN/m are under consideration. The highest temperatures occur in the superheater tubes, but it is probable that the severest corrosion conditions on these components will arise from the presence of fuel ashes on the outside of the tubes rather than from the steam on the inside. Perhaps the most critical components in such installations will... 

Table 7. It Comparison of internal and external scaling of superheater tubes after 6 950h in steam at 13 -8 MN/m and 500-670°C ...

Exiting steam from the roof and convection-pass cooling sections is collected in headers and typically passes through a primary superheater tube bundle, where a controlled amount of superheat is provided. In the superheater the steam discharges through an outlet header and across a spray attemperator (which provides the steam temperature control) and is then delivered to the control valves for distribution and subsequent use in a turbine or other items of process equipment. 

Superheater tube bundles are designed for different arrangements within the boiler ... 

The formation of superheater deposits in and around the superheater tube outlets and receiving header is a relatively common but serious superheater problem that may occur. These deposits are caused by contamination from BW carryover and also by gross contamination of attemperation water used to control the degree of superheat. 

Finally, if a tube steam leak occurs in a boiler, then depending on the internal arrangements steam may cut into or cause a damaging groove in part of the superheater tube bundle. 

Depending on the particular component alloy employed (such as a superheater tube alloy), metal burning may take place at temperatures of 1,000 to 2,000 °F (538-1,093 °C), resulting in a hole developing in the tube. 

Hydrogen sulfide also is formed, and this instantly reacts with iron and steel to form thin deposits of black ferrous sulfide in the superheater tubes ... 

Baffles and combustion area tile, brickwork, and other refractory surfaces are subject to risks of thermal breakdown perhaps as a result of severe local overheating caused by poor flame control. Where baffles are missing or broken, this adds to the risk of local overheating (especially of superheater tubes) due to excessive gas combustion temperatures. As a result, intact baffles and fireside boiler-metal surfaces may suffer from spalling. Check for missing or broken tube hangers and unsupported tubes. 

The materials of construction vary considerably with different boiler designs. Conventional drum boilers generally use carbon steel boiler tubes and stainless steel superheater tubes. 

The damage continued to the top of the furnace, but screen and superheater tubes were not damaged. One soot blower was blown onto the roof of an adjoining building and another knocked loose. 

Moderate Superheater tube Superheater tube parted from vibration fatigue, causing carryover of boiler water from steam drum and into furnace through superheater leak... 

Heavy (90 days) Superheater tube Carryover from sheared S/H tube... 

COMMENTS The advantage of using reheat is to reduce the moisture content at the exit of the low-pressure turbine and increase the net power of the Rankine power plant. The one reheat Rankine basic cycle shown in Fig. 2.13 can be expanded into more than one reheat if desired. In this fashion it is possible to use higher boiler pressure without having to increase the maximum superheater temperature above the limit of the superheater tubes. 

Clearly the oil is a much cleaner fuel than the original coal, from the point of view either of the environmentalist or of the plant engineer concerned with fouling of steam and superheater tubes. The sulfur contents of the oils are 0.1-0.2%, which is acceptable, but the nitrogen contents are about 0.6%, which may cause undesired NOx emissions. Some of the more toxic elements, (mercury, selenium, fluorine, and cadmium) have not yet been determined in oil. It is not clear what will be done with the solid residue whether it will be disposed of as waste or whether its small carbon content, typically 20-50% depending on the... 

Figure 15.2 is not a complete picture of a larger industrial-type boiler. Mainly, it does not show the superheat and economizer tubes. Figure 15.3 gives a better idea of the relative arrangement of the steam-generating tubes, superheat tubes, and economizer tubes. 

Of course, if the steam from the kettle will be used to reboil some nearby column, entrainment is not a major problem. If the steam is going into superheater tubes, however, serious entrainment will lead to solid deposits inside the superheater tubes, and this may cause these tubes to overheat and fail. 

Spray water for use in a spray attemperator should be of the highest quality. Solids entrained in die spray water enter the steam and can cause troublesome deposits on superheater tubes and turbine blades. 

Blast furnace gases, brick kilns, annealing furnaces, boilers, acid production, reactors, superheater tubes, nuclear pile instrumentation, soaking pits, glass tank flues. 

Therefore, negative pressure was decreased to reduce tramp air. Also, the operational reciprocating compressor was replaced by a centrifugal rotary screw type compressor. Further, ash build up on the boiler superheater tubes was a problem, impeding heat transfer to cool down the flue gas. This problem was resolved by using acoustics to cause the ash to fall off the superheater and economizer tubes. This allowed lower fuel consumption, resulting in decreased NOx emissions.1... 

Fouling is the accumulation of mineral-derived ash on the superheater and reheater tubes in the convective (heat exchanger) section downstream from the furnace. Fouling restricts the flow of exhaust gases and impedes heat transfer through superheater tube walls and thereby reduces the amount of steam generated. 

The presence of sodium and vanadium complexes in the fuel oil ash can, under certain plant operating conditions, result in considerable harm to the equipment. Spalling and fluxing of refractory linings is associated with the presence of sodium in the fuel. Above a certain threshold temperature, which will vary from fuel to fuel, the oil ash will adhere to boiler superheater tubes and gas turbine blades, thus reducing the thermal efficiency of the plant. At higher temperatures, molten complexes of vanadium, sodium, and sulfur are produced that will corrode all currently available metals used in the construction of these parts of the plant. TTie presence of trace amounts (ASTM D-1318) of vanadium (ASTM D-1548, IP 285, IP 286) in fuel oil used in glass manufacture can affect the indicator of the finished product. 

Magnesium oxide particles in the 2- to 7-p.m median particle size range deposit in the convection pass preferentially, while finer sized particles are not deposited and tend to get carried into the back end of the boiler. When an MgO of 2-p.m median particle size is fed into the fuel at treatment rates of 0.5-1.0 parts by weight Mg to 1.0 part V in the oil, the quantity of MgO deposited on the superheater tubes is 2-3 times that of the V and Na. This treatment rate results in a high Mg V ratio, which is needed to raise the ash-melting point and reduce tube deposits. 

Depending upon the source of the cmde oil, the refining process, and the fuel grade, varying amounts of sulfur may be present in different types of fuel oils. Combustion of the sulfur containing fuel oils produces sulfur oxides, which pollute the atmosphere and cause corrosion problems in boiler equipment. They may form sodium and vanadium complexes, and such deposits on external surfaces of superheater tubes, economizers, and air heaters cause equipment corrosion and loss of thermal efficiency. 

Following the early failures of tube-to-tubeplate welds in the two superheaters and the reheater no further failures occurred in the austenitic units until 1986, when a superheater tube leaked while the unit was being pressurised with steam prior to being put on line. This incident is described below. 

Combustion of coal generates very corrosive media, particularly near the superheater tubes of the boilers. In the boiler tubes suffering severe fireside corrosion, sulfate salts concentrate at the deposit/scale interface and become partially fused since these salts contain alkali metals of sodium and potassium. In the combustion systems, much of the sodium and potassium is volatized from the mineral matters in the flame to form Na20 and K2O vapors (Chawla et al. 2011). 

The structure of deposits on the superheater tubes in pulverized coal-burning boilers is shown in Figure 3.3. Above 700°C, the corrosion rate is decreased significantly due to decomposition of these complex liquid trisulfates. Once the liquid phase has been removed, the corrosion is due to oxidation in contact with flue gas. 

FIGURE 3.3 Structure of deposit on superheater tubes in pulverized coal-burning boilers. (Reproduced from Natesan J., High-Temperature Corrosion in Power-Generating Systems, http //www.ipd.anl.gov/ anlpubs/2002/05/43234.pdf, 2002. With permission.)... 

In a study by Koripelli et al. (2010), one finishing superheater tube sample, from a coal-fired unit, was received for metallurgical analysis. The tube was specified as a 1.875-in. outer diameter (OD) x 0.330-in. medium wall tubing (MWT), SA-213 T22 Cr-Mo steel. It had been in service for 23 years. Figure 3.4 illustrates the as-received tube sample and the internal diameter (ID) view. Significant wall thinning was observed on the flanks of the tube, characteristic of coal ash corrosion. Very hard scale was observed on the tube OD. On the steam side, there was a thick oxide. 

The critical regions in oil-fired furnaces are the evaporator tubes, steam superheater tubes, air heater, and channel. Evaporator tubes are affected by hot gas corrosion from hydrogen sulfide as the temperature exceeds 280°C. Superheater tubes suffer from sulfate/sulfite corrosion induced by molten sulfates above 620"C. Superheater tube holders undergo vanadic corrosion caused by molten vanadate species in the range of 550 C-600 C. Air heaters are subject to low-temperature corrosion from liquid sulfuric acid at 100°C-140°C. Flue gas channels suffer from acid deposits at the dew point (Balajka 1980). 

Content of Corrosive Components in Superheater Tube Deposits... 

Srivastava SC, Godiwalla KM and Baneijee MK, Fuel ash corrosion of boiler and superheater tubes, J Mater Sci, 1997, 32(4) 835-849. 

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