Economizer tubes

Water treatment monitoring and control is often a knife-edge operation and must be tailored to the overall operation of the boiler because waterside and gas-side problems usually are interlinked. Consequently (and as with other types of WT boiler), not only should the utility boiler FW be essentially free of dissolved oxygen to prevent waterside pitting corrosion of the economizer and other boiler components, but also the temperature must be high enough to prevent dewpoint condensation and subsequent acid attack on the gas side of the economizer tubes. 

Because economizer tube bundles produce very hot FW within the tubes, they may be subject to serious waterside corrosion and deposition problems, in addition to gas-side dust erosion problems and cold-end acid attack from sulfuric acid (derived from sulfur-containing fuels). 

Particularly critical areas for corrosive attack include economizer tube inlet headers, certain boiler tube areas, surface condenser tubes, and HP heaters. Problems occurring here may be related to EC or perhaps to the dissolution of copper or iron and its transport to some point elsewhere in the boiler system. If transported metal reaches certain critical areas (such as copper reaching turbine blades), very serious outages may occur. 

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. 

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... 

Dust from the fabric filter collection system has tended to accumulate on the sides of the hopper in a problematic manner. Noting the success of acoustics on the boiler ash that collected on the superheater id economizer tubes, plant personnel successfully trans erred that technology to the fabric filter hoppers periodic sonic blasts now maintain clean hopper sides.1... 

The most commonly used fuels, oil and coal, leave a residue on the boiler and economizer tubes. The composition of such deposits will vary, depending on the fuel and its source, but none have beneficial characteristics. When the boiler is taken off line, and the tubes cool, moisture can be absorbed by these deposits, leading to corrosion this can be particularly severe if sulfur is present. The burning of fuels low in sulfur is one way to help, but this by itself will not cure the problem. Since fireside corrosion causes thinning of the tubes, it may take several years to be noticed. By then, the problem is often widespread and severe. 

In comparing economizers with air preheaters, it can be said that a failing economizer tube leads to a forced shutdown, whereas leakage in an air preheater can be dealt with in a scheduled shutdown. A forced economizer shutdown could be... 

Failure of Economizer Tubes in Medina/Yanbu Plant.33... 

The temperature regimes in which this corrosion occurs are summarized in Figure 3.2. The data generally show that K2S2O7 will form from K2SO4 and SO3 at 400°C when SO3 concentration is at least 150 ppm as the temperature increases, the SO3 requirement increases, so that, at 500°C, at least 2000 ppm SO3 will be required to form liquid K2S2O7. Sodium pyrosulfate can form at 390°C with = 2500 ppm SO3, but at 485°C, 2% by volume SO3 will be required. Based on these results and the anticipated maximum level of 3500 ppm SO3 in a pulverized-coal boiler, Reid (1971) concluded that pyrosulfates can contribute to metal loss in the waterwall and economizer tubes but may not be a cause of corrosion in superheaters and reheaters in conventional systems (Natesan 2002). 

In a study by Andijani and Malik (2004), leakages were reported in the economizer tube in boiler 7. The tube was welded, and most of the corrosion activity was confined to weld or surrounding areas. The front side (A) and back side (B) have different morphologies. Side A Corrosion products are concentrated at the weldment, and metal is eaten away at the weld. Side B Micropits, pinholes, and macroholes were found at or in the vicinity of weldment. The surface is wrinkled, and there is marked reduction in thickness of the tube wall in the vicinity of weld. 

Photographs (Figure 3.10) show frontal and back-side views of the economizer tube, respectively. Figure 3.11 shows thinning of the tube wall of the economizer tube (back side). 

FIGURE 3.10 Photograph showing economizer tube 52, boiler 7, Medina/Yanbu Plant—frontal view (a) and back-side view (b). (Reproduced with permission.)... 

Photograph showing thinning of the tube wall of the economizer tube. (Reproduced with... 

During 2012, after approx. 3 years in operation, economizer tubes ruptured/thiimed out at inlet in all the four HRSGs. Figure. 3 4 shows ruptured/leaky tube location at inlet header. Figure. 5 6 shows close view of ruptured tube. Figure. 7 shows cut section of localized corrosion at tube bends. 

Qatargas is also having additional four HRSGs in QG3/4 facilities with different design, used for similar application/service/eycle chemistry. Detailed inspections of those HRSGs, in operation for 2 - 3 years, did not reveal any metal loss/eorrosion in economizer tubes. 

At higher elevations, there is fume and carry-over with the flue gases that cause deposits to form on the superheater, generating bank, and economizer tubes. Deposits may be molten depending on the concentrations of chloride and potassium. Superheater temperatures are usually between 300 C at the steam inlet and 500°C at the outlet. Although there are a variety of materials used in superheaters, ranging from chromium-molybdenum steels to stainless steels and nickel-base alloys, changing materials offers httle improvement when superheater deposits are molten. 

The 230°F water inlet may cause the flue gas temperature to drop below 450°E This will precipitate a weak H SO mist, which will aggressively corrode the fins or studs of the economizer tube bank. If low sulfur (less than 100 ppm HjS is being fired), H SO corrosion should not be a problem. 

These are heat exchangers used for recovering waste heat from the combustion of products after the superheaters and reheaters, but before the air heaters. The water temperature after the feedwater heat is increased and tempera-tme below saturation temperature and feedwater temperature is minimized. The waterside of economizer is an extension of boiler feedwater line where the heat input increases the temperature (Fig. 11.8). Tube bundles are placed in the flue gas and the water flow is counter to the flue gas flow. It is preferable for the water to flow up through the economizer tubes and for the flue gas to flow down across the tube. 

Chemical-resistant compositions of glass can be used as a coating for air-heater element plates, tank heating cods, air-heater tubing, economizer tubing, economizers, etc. 

Heating Surface. According to curve 4 of Fig. 23-9, the approximate economical tube length for such a still is about 20 ft. A cold velocity of 3 ft per sec indicates a tube size of about 4.75 outside diameter. The amount of radiant surface and the number of tubes for an effective tube length of 19 ft are... 

Fia. 23-9. Most economical tube lengths for some common arrangements of pipestills. 

Combustion systems Burner nozzles, reheater, super heater, economizer tube banks, boiler heat exchanger. In bed tubes, tube banks, etc. 5,6... 

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