Boiler Tube Failures (BTF’s) represent the largest portion of availability loss in fossil fired steam generators and is estimated to be as high as three percent according to EPRI* investigations. The second largest cause of tube failures is due to fly ash particulate impact (FAE). Impact erosion at off-design conditions, similar to 'sand-blasting' often leads to tube ruptures and unplanned outages, (see Photos).
In addition to BTF’s, FAE to non-pressure-part elements can result in costly O&M activities for ductwork, structural members, heat exchangers and ash separation equipment.
Utility and industrial power plants across the United States, Canada and overseas have completed fly ash erosion (FAE) control programs. In the USA alone, 20 utilities, representing 40 units and all major boiler manufacturers, and 7 industrial sites have participated in some form of FAE/plugging control programs. Overseas participation increased as the positive benefits were demonstrated.
* Electric Power Research Institute, Palo Alto, CA, Charlotte, NC
To reduce chronic FAE, fly ash particulate and local flowrates must be modified, usually reduced to design conditions. Flow modification devices include vanes, baffles and expanded metal screens. The underlying assumption for the use of flow modification is that FAE is localized and not occurring widely across any given region of the convective pass. If widespread FAE exists, other means would be required to correct the problems; such as: derating, fuel change or redesign of pressure parts or other effected components.
Selection and placement of particular flow modification baffles is based on the unit’s FAE history (Boiler Assessment) and some type of testing to measure gas flow (see sequence flow chart at left). Testing most often means a Cold Air Velocity Test (CAVT) in the unit at ambient conditions. In lieu of a CAVT, a physical or computer model can be used when unit testing is not possible. Computer modeling would be used to evaluate ash particulate distribution and redistribution when needed.
Check FAE Program Conduct to learn about options for running a program at your site.
Sometimes, ash can plug a boiler section and cause erosion or other maintenance problems. There are methods for holding up or deflecting this material to reduce or eliminate the associated problems. If you are interested in controls for reducing backpass plugging, see Ash Plugging Controls for more information.