Power Engineering

Boiler life extension requires accurate condition assessment

boiler maintenance

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).

Condition assessment

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

Click here to enlarge image

Click here to enlarge image

Click here to enlarge image

Click here to enlarge image

Click here to enlarge image

Sponsored by FLSmidth

Related Articles

B&W to perform steam generator work at Michigan nuclear power plant

American Electric Power (AEP, NYSE: AEP) awarded a contract to Babcock & Wilcox Nuclear Energy, a subsidiary of The Babcock & Wilcox Co. (NYSE: BWC) to provide steam generator services for the D.C. Cook Nuclear Plant in Michigan.

B&W sued over faulty coal-fired boiler

The Arkansas River Power Authority (ARPA) on Feb. 28 filed suit against Babcock & Wilcox Power Generation Group (B&W, NYSE: BWC) alleging B&W supplied a coal-fired boiler that never met its emissions guarantees.

Follow Power Engineering on Twitter
Latest News

Westermost Rough offshore wind park enters operations

The 210-MW offshore wind power plant, Westermost Rough, has entered operations. The project w...

Duke Energy to upgrade energy storage system at Notrees Windpower Project

Duke Energy (NYSE: DUK), Samsung SDI and Younicos will work together to update Duke Energy's ...

Wisconsin Energy acquires Integrys, forms WEC Energy Group

Wisconsin Energy Corp. successfully acquired Integrys Energy - forming the WEC Energy Group (...

Regulator rejects Iberdrola plan to buy Connecticut utility

Connecticut regulators have taken a step toward rejecting a plan by Iberdrola, Spain's leadin...

Largest Southern Co. subsidiary enters rooftop solar market

The largest Southern Co. subsidiary will start selling and installing solar panels on homes W...

Wisconsin Supreme Court upholds wind turbine rules

The Wisconsin Supreme Court upheld the state's wind turbine construction rules Tuesday, sayin...

After mercury ruling, higher scrutiny of Obama climate rules

Sweeping pollution limits at the center of President Barack Obama's climate change plan are f...

Coal plant operator says Colstrip remains profitable for now

The formation of a new company to run Montana's largest power plant has raised questions amon...

Power Engineering Current Issue

Volume 118, Issue 3
Products Showcase
Dynamic Fluoride Ion cleaning DFIC of industrial natural gas turbines Hi-Tech Furnace Systems

Dynamic Fluoride Ion Cleaning of IGT Parts

The Dynamic Fluoride Ion Cleaning (DFIC) Process from Hi-Tech Furnace Systems is able to clean deep, narrow cracks of oxides by cycling between negative, atmospheric, and positive pressure.

Archived Articles

2000 | 2001 | 2002 | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 | 2009 | 2010 | 2011 | 2012 | 2013

Power Industry Wire News

SafeWorks Acquires DualLift GmbH (DL)

SafeWorks Acquires DualLift GmbH (DL)

NuTech Energy Resources Debuts Trading on OTC Markets

NuTech Energy Resources Debuts Trading on OTC Markets

MyDx to Host Virtual Roadshow on July 15, 2015

MyDx to Host Virtual Roadshow on July 15, 2015

Envision Solar's EV ARC(TM) Is Featured at the International Parking Institute Conference and Expo

Envision Solar's EV ARC(TM) Is Featured at the International Parking Institute Conferen...

EnerJex Resources Declares Monthly Cash Dividend on 10% Series A Preferred Stock

EnerJex Resources Declares Monthly Cash Dividend on 10% Series A Preferred Stock

Power Engineering

Article Archives for Power Engineering Magazine

Continuing Education

Professional Development Hours

To access a course listing associated to a specific topic listed below, click on the topic of choice from the list below.

Latest Energy Jobs

View more Job Listings >>