Take Bolts As an Example to Learn In-depth Knowledge of Eddy Current NDT

Ⅰ. Eddy current NDT is effective for testing bolts

In the petrochemical industry, various forms of tubular heat exchangers are widely used. Many of them operate in high temperature, high pressure and strong corrosive media. Due to the effects of media corrosion, erosion, fatigue stress, etc., it will cause various corrosion defects, damages, and thinning of the wall thickness of the tube wall will seriously threaten the safe operation of chemical equipment.

Therefore, regular inspections of these in-service heat exchanger tubes to grasp the existence and development of defects are the key to ensuring the long-term safe operation of the entire chemical equipment. Among the several common testing techniques at present, the eddy current NDT method has become an effective testing method for bolts due to the advantages of fast testing speed, high sensitivity, and non-contact testing.

But eddy current nondestructive testing is affected by many factors.

Ⅱ. Take bolts as an example to see what are the limitations of eddy current testing:

1. Noise caused by attachments on the surface of the threaded area. In order to prevent the bolts and threaded holes from being rusted and seized and difficult to disassemble after the reactor pressure vessel has been in operation for a period of time, a layer of anti-rust materials will be applied to the threads during on-site installation. These anti-rust materials need to be cleaned during eddy current nondestructive testing. If the cleaning is not thorough, the residual material in the threaded area will cause noise to the eddy current signal.

2. Noise caused by uneven material of the bolt itself. In the process of bolt manufacturing, if the material is not uniform, noise signals will be formed. The characteristics of this type of noise are generally relatively uniform, existing in the entire threaded area, and appearing in a continuous sine wave.

3. The lift-off effect caused by the peeling off of the local coating in the thread area. In order to prevent corrosion, a uniform phosphate coating is added to the surface of the bolt. However, during the installation and disassembly of the bolt, the coating will be damaged to a certain extent and cause uneven shedding. In addition, due to the high sensitivity of the eddy current probe, it is particularly sensitive to these changing areas, resulting in changes in electrical conductivity or magnetic permeability, and ultimately a larger lift-off signal.

4. Noise caused by external electromagnetic interference. The basic principle of eddy current non-destructive testing is electromagnetic induction, which has very strict requirements on the electrical environment, and must always provide "clean" power supply. If there is welding and grinding work around the eddy current testing equipment, external electromagnetic noise is likely to be formed, which will be more serious to interfere with the eddy current signal.

5. The noise caused by the damage of the probe of the eddy current testing equipment. Since the eddy current probe is a point probe, it needs to be rubbed with the thread for a long time during detection, and its life is not very long. After a period of time, the coil will be damaged, which will cause noise.

6. Noise caused by unstable eddy current testing equipment. At present, the universal bolt eddy current testing equipment in the world is vertical, that is, the bolt is placed vertically on the testing equipment. When the motor drives the bolt to rotate, the whole device shakes greatly due to the high center of gravity, which brings a lot of system noise to the entire device.

Grounding is an important technical measure to suppress noise and prevent interference, ensure the electromagnetic compatibility of the equipment, and improve reliability. Correct grounding can not only suppress the influence of interference, but also inhibit the equipment from emitting interference. The usual practice in testing is to ground the panel of the eddy current testing equipment or other electrical control box with a wire, but the other end of the wire should be connected to a metal structure connected to the earth as much as possible.