Introduction
Boiler reliability is crucial for power plants. Forced outages can cost a plant millions of dollars, especially during periods of peak power demand. Non-Destructive Testing inspections during planned outages can prevent many of these forced outages. A good practice is to study the history of the unit and determine where to focus the inspections in the boiler.
The goal is to identify potential tube failures and replace the sections during an outage. Another key is determining the cause of the failures in the tubes. The plant can then take corrective actions to prevent the defects from reoccurring.
Two specific cases of where Non-Destructive Testing was implemented to help reduce boiler tube failures will be discussed in this paper. The first involves the inspection of the water wall tubes where corrosion cells and hydrogen damage were occurring. The second is the inspection of horizontal reheater tubes that were experiencing out of service corrosion. TesTex Inc. used the Low Frequency Electromagnetic Technique (LFET) to perform these
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The 6 keys
There are several Non-Destructive Testing technologies and inspection tools that are able to improve boiler reliability.
There are 6 critical keys for using NDT to improve boiler reliability.
- The first key point is to explain your problems to the NDT Company. Be sure to describe the unit history and any major disruptions to the boiler operation, such as the unit overheated, ran without water or was laid up for an extended amount of time. Inform the company of the failure history including locations, frequency and suspected root cause of the failures. Any metallurgical reports also are useful.
- Provide the NDT Company with any drawings that you might have. Drawings of the unit with the problem areas identified are helpful in analyzing equipment needed and estimating the amount of time the inspection will take.
- Provide the NDT Company with any tube samples that you might have. Samples allow the company a piece for their equipment to inspect. This allows them to fine tune the procedure to the boiler's specific problems. The samples provide the technicians some insight on what the signals will look like for the particular failure mechanism the unit contains. The collected waveforms will help fine tune the calibration. Be sure to ask the vendor to detail their experience with your particular problem and the results of the past inspections.
- During the early part of the actual inspection, the contractor needs to communicate with the plant on their findings. Some suspected defects should be removed to verify the inspection process, improve calibrations and the accuracy of the calls. These samples will provide an actual comparison for the collected data. This also will provide confidence for the actual inspectors on what they are observing.
- Make proper repairs. Try to repair as many of the defects as time and budget allows.
- Take corrective actions to prevent and/or reduce future failures. Without taking corrective actions, the failure process might be reduced for a period of time, but the failures will resurface.
Remember that using NDT should improve your Time Between Failures, but it will not eliminate all tube failures.
TesTex Inc. has a proprietary technology called "Low Frequency Electromagnetic Technique (LFET) to inspect boiler tubes from the O.D. of the tube. LFET injects an electromagnetic signal into the test piece. The electromagnetic signal is measured and any changes in the signal are noted. The distorted signals are then compared to calibrations to determine the amount of wall loss. It detects and quantifies I.D. and O.D. defects in ferrous and non-ferrous materials. LFET is a dry, non-contact method based on the principles of electromagnetics and it is forgiving to uniform surface scale. This means the scanning requires the surface to be smooth, but does not need to be sandblasted down to bare metal. A high-pressure water blast is usually sufficient in coal burning plants. The technology is adaptable to different applications, which allows the inspection of different diameters of tubing, the inspection of bends and the inspection of tubes in space-constricted areas. A standard LFET water wall scanner has 8 pickup sensors and is able to inspect most of the hot side of the tube in one scan. The system is lightweight, modular and uses digital signal processing electronics while being operated with a laptop computer. The results are displayed in real-time with high resolution color graphics 3-D display.
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