ACFM high temperature in-service detection

1 Ac electromagnetic field technology principle:

The technology of Alternating current field measurement (ACFM) is to excite the coil to induce uniform alternating current in the workpiece. The induced current generates disturbance in crack, corrosion and other positions. Based on the principle of spatial magnetic field distortion caused by electric field disturbance, The detection sensor is used to measure the distortion signal of space magnetic field, so as to realize the detection and evaluation of defects.

Detection schematic diagram

Detection depth diagram

ACFM detection system

ACFM uses relatively low-frequency AC signals to generate electromagnetic fields, so the detection depth and lifting height have certain advantages over conventional electromagnetic detection technology. ACFM detection defect depth varies according to the specific material, carbon steel 3mm, stainless steel 5mm, aluminum plate 10mm or so. The maximum distance from the surface of the workpiece to be checked is up to 10mm, without direct contact with the workpiece to be checked, can penetrate the coating detection. Temperature has little influence on the electromagnetic induction performance of the material, and for ACFM technology, in principle, the influence of temperature on the detection result can be eliminated.

By replacing the electrical components that operate normally at high temperatures and thermal protection of the probe, ACFM detection below 350℃ was realized and successfully applied in the field.

The testing effect proves that ACFM instrument can be tested at high temperature without stopping the equipment. Compared with high temperature magnetic powder and penetration technology, it has higher temperature resistance. It does not need to remove the paint layer on the pipe surface during detection, avoiding the grinding work required for ordinary magnetic powder and penetration detection, and improving the detection efficiency. No consumables, cost savings.

2 Application of ACFM technology online detection:

Steam valve field inspection

1, the high temperature steam pipeline valve of a power plant online detection

(1) Quantity: 11

(2) Material: carbon steel

(3) Temperature: 220-340℃

(4) Specification: DN125/DN175/DN22

There are anomalies in the application process of high temperature steam valve. ACFM technology is used for in-service detection, and the valve temperature is controlled below 350℃ by current limiting method. A number of defects were found in the testing process, mainly surface cracks and internal sand holes, and the test results were verified after grinding, as shown in Figure 5 and 6.

Valve defect signal

Valve defect image

2, a refinery enterprise atmospheric pressure storage tank, weld surface defects in service detection

(1) Quantity: 11

(2) Material: carbon steel

(3) Temperature: 20-40℃

The storage tank is directly tested during normal operation, and the protective paint on the surface of the storage tank does not need to be polished, only the insulation layer can be removed, and the detection efficiency is high. A number of defects were found in the inspection, and one of the defect detection images is shown in Figure 7 and 8.

Tank weld defect signal

Image of tank weld defect

3. In-service detection of weld surface defects of pressure vessel in a refining and chemical enterprise

(1) Quantity: 9

(2) Material: carbon steel

(3) Temperature: 200℃

The surface of the pressure vessel is seriously rusted, and the temperature is about 200℃, and the test is carried out in the state of unstopped and unpolished. Two defects were found in the inspection, one crack and one porosity, which were polished for on-site verification. The detection image of one defect was shown in Figure 10.

Container weld inspection site

Container defect detection image