Eddy Current Testing
Eddy Current Inspection, Examination & Testing
Examination of Tubes
Eddy Current Testing (ET) is a nondestructive test technique based on inducing electrical currents in the material being inspected and observing the interaction between those currents and the material. Eddy currents are generated by electromagnetic coils in the test probe, and monitored simultaneously by measuring probe electrical impedance.
The Eddy Current Testing Process occurs as follows: When a periodically varying magnetic field intersects an electrical conductor, eddy currents are induced according to Faraday’s and Ohm’s Laws. The induced current (known as eddy currents because of their circulatory paths) generate their own magnetic filed which opposes the excitation field. The equilibrium field is reduced resulting in a change of coil impedance. By monitoring coil impedance, the electrical, magnetic and geometrical properties of the sample can be measured. Eddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux. They normally travel parallel to the coil winding and parallel to the surface. Eddy current flow is limited to the area of the inducing magnetic field. It is mainly used for thin conductive nonferrous alloys such as: Copper, Brass, Copper-Nickel, Titanium, Stainless Steel and others. Remote field eddy current using low frequencies is good for testing thin wall carbon steel. Eddy current inspection can detect such defects as: cracks, voids, laps, seams, inclusions. Such applications as: sorting materials, measure of conductivity, coating, grain size, heat treatment and physical dimensions can be performed as well.
Eddy Current Testing is a perfect tool for inspection of tubular products such as:
- Heat exchangers
- Refrigeration systems
- Commercial chilled water air conditioning units
- Boiler tubes
- Ship condensers
- Stainless steal pipes
Why use Eddy Current?
- It saves time and money to run the equipment
- It is a good preventative tool to detect problems before they arrive
- Controlling and monitoring the corrosion rate, air leaks, inhibitors, water treatment and life expectancy
Early detection by Eddy Current Inspection would have prevented the following failures:
Fig. 1 Copper prime tube from evaporator section stress-corrosion crack
Fig. 2 Copper-Nickel skip-fin tube from generator section. Severe tube wear at support sheet
Fig. 3: Copper-Nickel prime tube from Hot water heat exchanger. O.D. corrosion on water side
Fig. 4: Copper tru-fin tube from condenser section. Massive I.D. corrosion
Fig. 5: Copper prime tube from Absorber section. I.D. corrosion due to improper water treatment
Fig. 6: Copper prime tube from Condenser section. O.D. corrosion at support sheet due to air leak
Enhanced Tubes
Yesterday's refrigeration systems and heat exchangers used externally enhanced (finned) and smooth surface (prime) tubes with internal smooth bore and tube wall thickness ranging from 0.028" to 0.049".
Most of today's new high efficient refrigeration systems and heat exchangers use internally and externally enhanced tubes. However, with internally enhanced tubes and thinner tube wall thickness, it's more important then ever to monitor tube condition for corrosion or mechanical wear. The new generation of internally enhanced tubes wall thickness range from 0.019" to 0.028", which is about 30% to 50% thinner. Internally enhanced tubes are harder to clean, making them more prone to under deposit corrosion resulting in quicker failures. Periodic Eddy current inspections are an important tool to extend the life of the tubes. Photo's 1 through 7 show different styles of enhanced tubes with common tube failures.
Fig. 1: shows Copper skip-fin internally enhanced tube from condenser section. This photo shows inner surface deposit.
Fig. 2: shows Copper skip-fin internally enhanced tube from condenser section. This photo shows the same location with the corrosion pit developed under deposit.
Fig. 3: shows Copper skip-fin internally enhanced tube from condenser section. This photo shows deposit in the groves.
Fig. 4&5: Shows internally enhanced tube with fine riffling. This photo shows the same location free from deposit. Magnified photo shows pit in the grove.
Fig. 6&7: Shows copper longitudinally corrugated tube from absorber section. Magnified photograph shows corrosion after removal of deposit by macro etching.
Manufacturer's Defects
It is recommended that new and re-tubed machines be Eddy current tested before charging or running the system. This can prevent catastrophic tube failures due to manufacturer related defects. Manufacturer defects can occur during the manufacturing process, shipping, tube handling, warehouse or transportation. Performing an Eddy Current Testing and Inspection of tubes before start-up, will identify tube imperfections and establish a base line for monitoring corrosion rate. Below are pictures showing manufacturer defects found by Eddy Current Inspection in new tubes.
Fig. 7: Copper prime tube with 'tear' type defect.
Fig. 8: Copper-Nickel prime tube with such defect as Lap (or fold) developed during drawing process.
Fig. 9: Copper tru-fin tube from the bottom row of Evaporator section shows burst crack due to freeze-up as result of improper drainage for storage.
Why use us for your eddy current needs?
Tri-State Nondestructive Testing, INC. has over 17 years experience in Eddy Current Inspection of tubular products. Our professional staff with over 30 years of experience is always ready to serve your needs. ASNT TC-1a levels II and III technicians have an excellent knowledge of equipment being inspected, giving fast reliable test results. Specially design by Tri-State NDT, Inc. computerized equipment and software records every tube tested. Each job is reviewed twice to insure the accuracy and quality of the Eddy Current Test results. Permanent recordings allow comparison with previous test results for estimating corrosion rate and identifying specific change in tube environment. Recordings are copied to a CD for storage and could be presented at your request for reviewing on your computer with our special software. Our typical report gives you complete information, which include color coded tube sheet layout, Tables, Strip-chart recordings, Comparing data with previous test and Recommendations.
- High Speed and Accuracy
- High Quality and Low cost
- Simple and Informative test report
- Quick response in emergency situations
- Results of Eddy Current Testing and examination can be given on the spot, allowing for a quick response
Please Contact Us to learn more about our Eddy Current Testing Services and how we meet your needs. We service the Eastern US from Maine to Florida. Our service area includes: New York, New Jersey, Massachusetts and Washington DC, as well as parts of Pennsylvania, Virginia, West Virginia, Vermont and New Hampshire.