Inspection Trends / Spring 2011 25 of heat exchangers, including fossil fuel boilers (especially in water wall and generator bank tubes), black liquor recovery boilers, shell and tube exchangers, and air fin coolers. Remote field testing operates at relatively low frequencies and is a noncontact technique, so the probes have minimal friction with the pipe wall and require no couplant. The accuracy for remote field testing in the straight part of the tubes is about 10% of wall thickness for general wall loss. Accuracy is generally less (20% of wall) for highly localized discontinuities and near external conducting objects because of the changes in magnetic properties of the tube in that area and because of shielding effects of external objects. Remote field testing is also equally sensitive to inside and outside surface discontinuities but usually cannot discriminate between them without the help of near-field coils. It is relatively insensitive to scale and magnetic debris. Deployment of a Robotic Wall Crawler Using RFT It is very difficult to obtain access to the inside of boiler tubes so that an inspection tool can be inserted; therefore, all inspection must be performed from the outside of the tubes, inside the boiler. In this case, it is desirable to have an external tool that can detect corrosion or wall thinning without exhaustive cleaning of the surface, or removal of coatings. The traditional method of inspecting boiler water wall tubes for loss of wall thickness is by taking many thousands of ultrasonic thickness readings spaced several feet apart in elevation — Fig. 2. In order to do this, the boiler must be scaffolded and the tubes must be cleaned to bare metal where the readings are to be taken. Scaffolding and cleaning costs often exceed $100,000, and the ultrasonic inspection can cost the same amount again. If the boiler will be scaffolded anyway, the tubes can be inspected rapidly with a hand-held scanning tool that delivers the equivalent of up to 2000 thickness readings per foot, at a scanning speed of up to 10 ft/min — Fig. 3. For boilers that are not scaffolded, a magnetic “wall crawler” can be used to carry the external pipeline integrity tool (E-PIT™) RFT probe up the water wall. The crawler can handle water walls up to 200 ft in height and tube sizes from 1.5 to 3.5 in. in diameter. Inspection speed is 10 ft/min so an entire wall that is 100 ft high and 100 tubes wide can be inspected in less than three 12-h shifts. The E-PIT probe inspects the flame side of the tube to within 3⁄8 in. of each web, using 12 detection coils for high precision — Fig. 4. Pits, as small as 1⁄8 in. diameter, can be detected. Case Study Unit #1 of the generation station at ABC Power in Canada uses Orimulsion™ as fuel. Orimulsion is a bitumen-in-water emulsion produced from the vast reserves of the Orinoco belt in Venezuela. The emulsion contains 70% natural bitumen and 30% water. This liquid fuel, resembling a black latex paint, has relatively high energy content on a weight basis (i.e., about 110% of coal and 70% of heavy fuel oil). The scale deposition on the tubes and web in such boilers is worse than coal-fired boilers. Figure 5 shows an example of scale. Note that the crown is often scale free but the spaces between the tubes always have heavy scale. A Vertiscan™ system from Russell NDE Systems, Inc., Edmonton, Alb., Canada, was used to inspect the water wall tubes in boiler #1 in the fall of 2006 — Fig. 6. The system consisted of a TubeCAT™ magnetic crawler with Fig. 3 — For scaffolded boilers, inspectors can use a hand-held tool to perform up to 2000 thickness readings per foot at a scanning speed of up to 10 ft/min. Fig. 4 — An example of a robotic crawler being used to inspect boiler tubes. Fig. 5 — Heavy scaling. Fig. 6 — The Vertiscan system in place.
Inspection Trends - April 2011 - Spring
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