Boiler life extension requires accurate condition assessment
Modern evaluation techniques coupled with low-capital-cost modifications significantly improve demand capability
By Timothy B. DeMoss, Associate Editor
That power producers are uncertain about the future business climate is an understatement and a recurring theme in today`s power generation industry. However, electric power demand knows no time line and is unsympathetic to economic concerns. Many utility managers have realized that as electric energy needs grow, they can often rely on their older power plants to meet increased demand. Extending a plant`s life by improving its boiler availability, efficiency and reliability can provide power for as little as 10 percent of new plant capacity cost, measured on a cost-per-installed-kilowatt basis, according to Greg Nakoneczny of Babcock & Wilcox (B&W).
Critical to undertaking remedial actions to extend plant life is assessing the current boiler condition. In Phase I of a three-phase assessment approach followed by B&W, critical components are determined prior to a planned outage. B&W`s approach also includes evaluating maintenance and operation history and establishing an outage inspection and test plan during Phase I. According to Nackoneczny, the following historical data should be considered in this phase:
z unit operating hours,
z unit operation mode (i.e. cycling versus base load),
z cycling characteristics (frequency, ramp rates, hot, warm or cold),
z past failure history including failure analysis reports,
z maintenance history,
z replacement/upgrade history,
z construction materials,
z actual steam operating temperatures and
z specific design characteristics.
During the outage (Phase II), inspections and testing on indicated components provide data for the remaining life analysis, which is conducted after the outage is complete (Phase III). Table 1 lists the typical life expectancies for several key components subject to fatigue, corrosion, creep and overheating.
As indicated in the table, boiler tubes are typically the first boiler components to reach the end of their useful lives. Boiler tube failures have historically been the major cause of lost availability, according to Steve Paterson of Aptech Engineering Services. Fortunately, engineers have learned much about tube degradation prevention, correction and control in the past quarter century. Advances in on-line monitors and condition assessment tools now help utility engineers detect, locate and characterize tube damage in its early stages.
Conventional on-line monitoring tools include plant cycle chemistry monitors, pressure drop gauges, flow meters, thermocouples and expansion trams. More recent tools include flux domes, which measure heat absorption rates and gas-side tube temperatures, high-temperature strain gauges, boiler video cameras, infrared cameras and acoustic leak detection devices. Real-time damage rate monitoring systems should soon be available, according to Paterson.
Besides visual inspection, conventional condition assessment tools include ultrasonic wall thickness examination, tube sampling, magnetic particle testing, liquid penetrant testing, hydrostatic testing and radiographic examination for cracking, wall loss, deposit buildup and dissimilar metal welds. Advances in these tools have greatly increased the number of assessment options. More recent tools include videoprobe, ultrasonic oxide thickness examination, ultrasonic hydrogen damage classification, ultrasonic corrosion fatigue crack detection, eddy current testing for wall thickness, metallurgical replication and infrared wall thickness surveys.
Tube creep failure
Long-term overheating is one boiler tube failure mechanism responsible for large availability losses. Because tubes slowly deform during service despite stress levels below the tube material`s yield strength, engineers must design superheater and reheater tube banks with a finite creep life.
The objective in designing for the overheat mechanism is to slow the creep process as much as possible and replace the tubing just prior to the end of its creep life. The trick for plant maintenance engineers is to accurately predict when a tube`s creep life will be exhausted. This can be a difficult task considering the numerous factors that contribute to creep life reduction.
Erosion and corrosion associated with coal ash attack a tube`s outer diameter, thinning the tube wall and increasing tube stress. Excessive moisture can also erode tube wall material. In addition to tube material thinning, iron oxide deposits on tubes` inner walls increase tube metal temperature (Figure 1), leading to further decreases in creep life. Battling these additive life-reducing processes requires assessment tools which can reliably measure wall and steamside oxide thickness, improving accuracy in life prediction estimates.
Life prediction with NDE
With advances in non-destructive evaluation (NDE) methods that allow measuring both oxide and wall thickness, engineers can now make life predictions on every tube assembly and row directly in the region causing concern. Knowing oxide and wall thickness, and other changes in tube operating conditions, permits commercial life-prediction systems to account for these parameters in their calculations.
The typical cost for a tube bundle survey and life estimation, approximately $15,000 to $25,000 according to Paterson, is easily recovered if using the results prevents even a single boiler tube failure and subsequent forced outage. It is common in a tube survey to find only a few tubes with short estimated lives. This fact contributes to further savings because rarely in such a case would wholesale tube replacement be necessary.
By establishing baseline tube conditions and continuing to monitor tube degradation with on-line techniques or periodic assessments, engineers can determine with confidence the necessary actions to mitigate damage to individual tubes and to the tube system.
Mitigating tube damage
Assuming that life predictions are unacceptable and point to creep as the damage mechanism, Figure 2 depicts the options available to the engineer to correct the problem. Selecting the final option should be based on a cost/benefit comparison. One of the most successful means to increase tube bundle life is steam flow redistribution.
According to Paterson, in his paper "Minimizing the Life Cycle Costs Attributed to Boiler Tubing in Fossil-fueled Plants," this patented technology has been used to extend the life of six superheater and reheater sections in boilers ranging in size from 150 to 420 MW. The modified boilers have seen 16 unit years of service with no failures.
The key to the modifications involved welding steam flow controllers (Figure 3) with various tapers, lengths and inner diameters into selected inlet tubes. Engineers first mapped the tube temperatures and life distribution for the bundles and then selected those tubes with excessive remaining life which were also operating at low temperatures. Installing steam flow controllers in these tubes redirected the flow from the tubes, increasing tube temperature. As a consequence, and more important to life extension, increasing the flow into hot tubes reduced their metal temperatures and slowed the creep process. Optimizing steam flow controller placement achieved maximum benefit for bundle life. Figure 4 illustrates this modification`s effect on tube failures.
Improving overall performance
Because life extension requires capital investment, power plant managers often look for capacity increases as part of their plant upgrades. Although increasing boiler capacity may not be possible, upgrading to improve lost efficiency and availability often results in improved capacity nonetheless. Figure 4 shows how capital expenditures for life extension affect boiler availability.
Engineers can sometimes extend boiler limits to increase capacity because older designs were often conservative. In his paper, "The Role of Condition Assessment in Increasing Boiler Output and Economically Extending Boiler Life," Nakoneczny groups design parameters affecting capacity into two categories: combustion and circulation. By using today`s knowledge and techniques, an engineer can optimize burner and heating surface spacing, drum capacity, tube flow, heat input, convection pass flow, pressure drop and other design parameters to take advantage of built in conservatism, increasing capacity with little investment.
In addition to capacity increases, emissions control should be included in an upgrade to bring older plants up to current standards. Many retrofit technologies are now available to meet standards for SO2, NOx, CO2, particulates and air toxins. A fabric filter or electrostatic precipitator can control particulate emissions. Engineers can control NOx by installing low-NOx burners or by using selective or non-selective catalytic reduction after the combustion process. There are three basic options for SO2 removal: switching to low-sulfur coal, installing a fluidized-bed combustor or installing post-combustion process systems such as flue-gas desulfurization.
Extending boiler life and upgrading systems at aging power plants will continue to be priorities in North America for the immediate future. The economic benefits from life extension often outweigh those for building, operating and maintaining a new plant. Strict environmental regulations and open competition will ensure the need for older plants to follow this path. As more plants are forced to make decisions regarding life extension options, the industry will continue to see advances in the tools and systems necessary for older plants to keep up with electric power demand. z