Eddy Current Crack Inspection and Multi-Laser OD Measurement
This proprietary eddy current and diameter measurement device which has been named as LEO-SCAN which stands for Laser Eddy-Current Outside has been in use for more than 20 years. The eddy current probes were developed by MP specifically to test reformer tubes with FULL penetration of the cross-sectional thickness up to 23 mm and is verified for use on all current centrifugally cast alloys. Unlike standard eddy current systems, these powerful probes from MP generate a magnetic field strong enough to completely penetrate the tube material. The most important requirement for testing these tubes is the need to look inside the tube wall for the formation of micro-fissures and cracks. Secondly, the test results must be repeatable from inspection to inspection, even over many years. As the EC probes do not require a couplant and operate electronically, the readings obtained are completely repeatable from test to test.
There are 3 different scenarios of tube damage:
- Cracking without tube expansion or creep
- Tube expansion, or creep, in combination with cracking
- Significant tube expansion, or creep, without cracking
The type of tube damage depends on the process conditions, upset conditions and cycles. For the most part, the predominant damage mechanism is creep.
The diameter measurement is accomplished simultaneously in combination with the eddy current crack detection system using up to four lasers. The lasers allow for maximum accuracy and repeatability. Diameter measurement of tubes is used as a secondary procedure revealing normal creep and also clarifying the probable conditions that cause any damage. Cracks can form without diametric growth. There can also be significant diametric growth without cracking. Cracking without diametrical growth, is generally caused by operational upsets and is known as accelerated creep. Operational upsets can also result in thermal shocks leading to catastrophic tube failures.
The complete inspection system is mounted on a unique tube crawler designed and built by MP. The special design of the crawler allows for passage from the furnace floor to the roof or arch, passing below any flue gas collection tunnels. The probes and lasers are mounted near the bottom of the crawler allowing readings to be taken in the most critical lower areas of down flow steam methane reformers. The device is also designed to test reformers with small tube to tube clearances as low as 20 mm. This allows the inspection of furnace designs with very tight clearances between the tubes. Other configurations are available to test up flow reformers. Tubes are inspected in less than two minutes each. No cleaning of the tubes is required.
Eddy Current Crack Inspection Creep Evaluation with Multi Laser Devices
The MIDREX Direct Reduction Process converts iron ore into a high-purity product for use in iron and steel making in addition to foundry applications.
The process direction in this furnace is up flow. The tubes have diameters between eight and twelve inches. For crack inspection, MP developed and built an eddy current technique which is used to test these tubes. Cleaning of the tubes due to scale on the tube surfaces is not required. Integrated multi laser devices accurately measure tube creep, which can be 10% or more of the tube diameter. The components of the mechanized inspection system are especially designed for the inspection of the MIDREX tubes which considers process flow direction, scaling, dual alloys and large diameters. One tube can be accurately inspected in less than two minutes.