Advances in Eddy Current Inspection of Tube and Shell Heat Exchangers - Advances in electronics and software have made eddy current inspection of heat exchangers faster and more cost effective

摘要

Eddy current testing (ECT) is a proven, reliable technique for the inspection of heat exchanger tubing. One of four techniques that comprise the electromagnetic test (ET) method, the ECT technique involves passing an eddy current probe through a tube in a heat exchanger bundle. The AC-driven probe creates a varying electromagnetic field around itself that interacts with the tube wall. Eddy currents generated in the tube wall travel in well-defined paths and at specific depths within the tube wall until they encounter changes such as cracks or corrosion in the material that they move across - Fig. 1. Eddy current testing instruments measure and record how the eddy currents react to these tube wall changes. The inspection process can be performed at a single test frequency or at multiple test frequencies to gather a wider range of information. Higher frequencies can detect small surface flaws while lower frequencies look further into the material. Trained operators can interpret the signal changes seen on the test instruments and usually relate the patterns generated to specific types and amount of damage. Plant engineers then use this information to make realistic, safety-based life assessment, repair, and replacement plans. Tubing and heat exchanger inspections vary greatly. Small heat exchanger inspections have very few tubes and are done manually by a one- or two-person team. Large condensers and nuclear steam generators may have thousands of tubes and require teams of inspectors - Fig. 2. Nuclear steam generator inspections or inspections in highly caustic environments involve the use of sophisticated robotic systems to position the probe. This increases the reliability and consistency of the data and limits the risk to the inspectors. Heat exchanger geometry also affects the inspection scope. Recirculating-type designs require eddy current probes that can pass through "U-bend" areas.