Corrosion Testing of Coal Fired Boiler Tubes

Unscheduled shutdowns, repairs and power replacement due to boilers failure in electric power generation plants cost the utility industry over $5 billion per year in the US alone. NDT using A-scan flaw detectors with conventional piezoelectric transducers used to measure the wall thickness and determine the degree of corrosion is a long established technique to predict the life of boiler tubes. However, such tests require clean, rust free surfaces and all tubes must be sand-blasted and cleaned prior to the inspection. Since no other form of maintenance can be conducted inside the boiler while these tubes are being cleaned, then this process is directly adding to the outage duration, where just one extra day of shutdown can cost $500,000 for a typical 500MW plant. It is easy to see the cost benefits in using an NDT technique which does not require tube cleaning. This report outlines a fast growing technique in the coal fired power stations using EMATs. EMAT Principle For a piezoelectric transducer an elastic wave propagates in the piezoelectric element and is then introduced into the test piece via a coupling agent. The nature of the coupling is therefore purely mechanical, requiring physical contact. Electromagnetic Acoustic Transducers (EMAT) work on electromagnetic principle. It consists of an RF coil and a permanent magnet. The pulse generator excites the RF coil which sends out an electromagnetic wave. When the coil is placed close to a conductor the wave is absorbed near the surface producing an eddy current field. In the presence of an applied magnetic field a force is exerted on the conductor causing the elastic wave to be propagated into the test piece. Providing the corrosion layer is not a conductor, the EMAT transducer will measure the thickness of the metal alone. In their quest to find the best combination of EMAT probe and digital ultrasonic flaw detector the power generation companies in the UK have performed exhaustive trials, including the use of expensive pre-amplifiers needed to generate sufficient power with certain manufacturer’s flaw detectors. Furthermore they have determined that the EMAT can only be used with certain flaw detectors due to the recovery time of the initial pulse. Most flaw detectors do not have the facility to reduce the pulse width sufficiently to eliminate the standing echo caused by this recovery time, and cannot measure below 2mm. The only equipment which PowerGen deem suitable for this application are the flaw detectors from Sonatest Masterscan Series.