Boiler tube failures, especially in coal-fired power plants, is the major cause of boiler forced outages. However, if a tube leak is detected early more severe damage to surrounding tubes and tube banks can be prevented. Common methods for determining tube leaks in their early stage are on-line acoustic monitoring and off-line non-destructive testing. Software is also available for documenting and analyzing the problem of premature tube leaks.
Although most of the older coal-burning plants were designed for baseload operation, many of these plants are now being operated as cycling units. Unfortunately, because the units were generally not designed for cycling operation a variety of problems have occurred with pressure parts and steam/water systems. A change in heat absorption from switching fuels can also cause tube problems.
Cycling can result in various problems:
Poor load following
Increased downtime and maintenance costs
Tube overheating and limited boiler capacity.
In addition, there is the potential for excessive water deposition due to low flow, feed-water control fluctuations, and header cracking and distortion.
High Temperature Headers
According to B&W, because superheater and reheat outlet headers are exposed to temperatures greater than 900 F, they present some of the most challenging design issues. Fortunately they also provide some of the best opportunities for improving boiler operations and efficiency.
Under normal boiler operation superheat and reheat headers experience high temperature creep that reduces the life of the headers. In addition, thermal and mechanical stresses from cycling operations, when combined with creep, can lead to failure of the headers much sooner than creep alone.
To overcome header problems from high temperatures a combination of design and material changes can improve the life and performance of the boiler and headers. Using forged instead of welded outlet nozzles is one option. Likewise increasing ligament spacing by spreading the header penetrations around the header circumference, instead of clustering them together, will reduce high stress areas.
Superheaters, Reheaters and Economizers
Because not all coals have the same ash characteristics, fuel switching to reduce operating costs and/or emissions can result in excessive surface slagging and fouling of superheaters, reheaters and economizers.
Since increased slagging increases the furnace exit gas temperature it can cause superheater overheating and excessive attemperator flow rates. According to B&W, increased attemperator flow rates reduce overall cycle efficiency and in some cases limit the boiler`s load from insufficient spray capacity.
Fortunately there are solutions to reduce and/or eliminate slagging and fouling. In the low-pressure temperature area of a convection pass, the economizer can be upgraded to reduce fouling, pressure loss, ash plugging and limit corrosion and erosion. Some of the upgrades include redesigning the tube spacing and arrangements, modifying the gas velocity and adding erosion barriers and soot-blowing shields. Similarly, redesigning the furnace arch will allow a more uniform gas flow distribution.
Furnace Wall Upgrades
Although stratification is a major problem in some boilers it is possible to rectify the problem by modifying the furnace`s geometry. Foster Wheeler`s Tim Ristau, executive vice president, said a northeast client experienced stratification problems in an area of their boiler where gas leaves the furnace. This condition resulted in abnormal erosion of pressure parts and subsequent tube failures.
Fireside boiler tube erosion, especially in the burner throat of low NOx installations, can be a major problem. However, this problem can be resolved by adding a 360-degree weld overlay to the tubes in the burner throat area. This upgrade requires removal of the old walls and their replacement with new tube walls with 360-degree weld overlay. The weld overlay extends just above the top air-port opening. A Midwest utility has upgraded two of its units with weld overlay furnace walls, Figure 3. Additional units are in the process of being modified.
Furnace Surface Cleaning
