By Douglas J. Smith IEng, Senior Editor
Reducing boiler downtime — particularly from tube failures — demands constant vigilance.
Boiler tube failures, especially in coal-fired power plants, continue to be 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.
According to Babcock & Wilcox (B&W) the operating problems from boiler cycling are very complex. A seemingly small pressure drop or flow change in one area can result in significant deterioration in other parts of the boiler. Nonetheless, before these problems can be rectified it is recommended that utilities conduct a detailed circulation analysis of the boiler's existing design.
Overheating of wing wall boiler tubes at a large western utility drum boiler rendered the boiler unable to operate at full load. After conducting a detailed engineering evaluation and model study of the boiler's original design, B&W determined that the overheating was due to low flow velocities and flow imbalance. As a consequence steam/film boiling occurred in the tubes.
After installing low pressure drop drum internals to increase flow, redesigning the wing walls to increase heat absorption, increasing the angle of the panel tubes and installing ribbed tubes to prevent film boiling, the problem was resolved. Figure 1 compares the original boiler design versus the modified boiler circulation upgrades.
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.
According to Ristau, the stratification problem was resolved by the installation of a nose section which redirects gas flow for the rear wall of the boiler, Figure 2. Since installation of the nose section, stratification within this area of the boiler has been eliminated. The modification has also helped to reduce vibration of the rear screen tubes, reduce overheating and erosion of the first pass reheater, and reduce erosion on the rear screen tubes and the furnace floor near the sidewalls. Due to the success of this project Foster Wheeler is working with other utilities to realize similar benefits utilizing this approach.
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.
Figure 2: Furnace wall exit nose. Photo courtesy of Foster Wheeler.
To reduce boiler problems at cycling units originally designed for baseload operation, Foster Wheeler proposes use of a higher chromium alloy material for headers and tubes. Using a higher chromium material allows the headers and the boiler tubes to be made with thinner lighter weight material, says Ristau. Since the thinner walls reduce expansion problems, they also significantly reduce stress on the boiler during cycling operation.
Furnace Surface Cleaning
Keeping the surface of a boiler's furnace tube walls clean is essential for optimizing the output and efficiency of operating units. AEP recently completed a performance test of Clyde Bergemann's SmartCannon furnace cleaning system to improve boiler efficiency. The SmartCannon system was tested on Gavin power plant's Unit 2, a 1,300 MW Babcock & Wilcox wall fired boiler burning eastern bituminous coal. According to Randy Sheidler, senior coordinator, AEP Gavin, the SmartCannon performance test had three objectives:
- Reduce furnace exit outlet temperature
- Reduce NOx emissions
- Improve plant efficiency
To provide cleaning of the furnace from the nose elevation to just above the bottom slope tubes, four SmartCannons were installed above the top burners on the 9th floor elevation of Unit 2. Two cannons were installed on the front wall and two on the rear wall. The sidewalls of the boiler are divided into two sections.
In order to provide feedback on the cleaning efficiency of the cannons, 24 heat flux sensors were installed. A pump provided water for the cannons at various flows and pressures. Similarly, data from high temperature thermocouples, installed at the 13th floor elevation of the boiler, was used to evaluate the cannon's performance.
During the three-day test period, March 11-13, 2003, NOx reduction averaged 5.1% while the heat rate was reduced an average of 25 Btu/kWh. The heat rate improvement came from improved heat absorption in the furnace, which in turn reduced the attemperation spray flow controlling the reheat temperature. Although only a small increase in boiler efficiency was observed, the furnace gas exit temperature was reduced by approximately 124 F.
The cannons installed for the test program are still being used on Unit 2. Four months after the cannons were installed on Unit 2 the plant went ahead and installed them on Unit 1. However, because of some reliability problems with the cannons AEP is not installing the water cannons, on other units until the problems have been resolved, says Sheidler.
Tracking and Documenting Tube Leaks
In many cases records of boiler tube leaks and how they were rectified are kept at individual plants and not always shared with other plants in a utility's fleet. As a result, individual plants tend to only analyze the root cause of the tube leaks at their particular plant. Unfortunately, where electric utilities have no standard procedures for analyzing, trending and reviewing prior maintenance, they cannot generate integrated reports showing the history of failures and repairs carried out on multiple boilers.
To tackle this problem Automation Technology, Inc. (ATI), San Jose, CA. has developed a boiler tube failure software program called "Aware Boiler Module." The program enables plant engineers and a utility's central engineering group to constantly monitor and analyze boiler tube leaks at all of a utility's power plants. A common database for all of the plants is accessible from any web browser within the utility's intranet. There is no need to install the software on individual plant computers.
Figure 3: Weld overlaid burner throat opening. Photo courtesy of Foster Wheeler.
Standard tube failure and inspection reports are provided for all of the boiler pressure parts. The data includes a record of all tube failures and repairs, ultrasonic test readings, their location and cause of the failure. According to ATI, the benefits from installing the boiler software program include:
- A 50% reduction in forced outages caused by tube failures
- A reduction in the time to manually analyze and search for data
- A 50% reduction in the time to write reports
- Less time spent justifying repairs and replacements to upper management
- The potential to save $5,000/plant/year in the cost of collecting historical data for condition assessment studies.
A graphical interface can be used to superimpose the data on component specific AutoCAD drawings. The graphical display tends to give a better perspective of boiler tube failures. Figure 4 is a typical superheater tube failure screen.
Dominion Generation has installed the web-based Aware software and graphical interface in its corporate office. Access to the program though the company's intranet is available to all of their power plants. According to the utility all forced outages due to boiler tube failures that occur in their plants are now documented in the system. The ability to centralize boiler tube problems throughout their fleet has allowed the utility to streamline and standardize boiler tube failure internal reporting and analysis from one central location.
Onsite and central office boiler engineers can also share information on boiler tube failures for all the utility's boilers. Using this information the engineers are able to trend tube failures for boilers of the same design and with the same operating conditions. Results from analyzing the data are also used by Dominion to determine the scope of work for outage planning and management.
According to Frank Timmons, boiler engineer, Dominion Generation, the graphical interface has been used to develop CAD drawings showing dimensions, tube counts and elevations. This information is passed on to the outage planner who is then able to have a replacement panel or pendant loop pre-fabricated.
Another advantage of the program is for budgeting. There is no longer the need to make assumptions. It is now possible to prepare, recommend and justify maintenance requirements and expenses to upper management, says Timmons.
Each Dominion plant has a boiler tube failure coordinator who is responsible for identifying the tube failure and inputting all of the data into the historical database. This individual also makes sure the plant is in compliance with all ASME and National Board code requirements.
Utilizing this program Dominion anticipates reducing forced outages by at least one per year while improving the reliability of electricity supply during the summer peak months. Long term, Timmons believes that the program will allow Dominion to reduce downtime and maintenance costs.