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Eddy Current Non-Ferrous Tube Inspection System

EDDY CURRENT TESTING is the most widely accepted method for the rapid inspection of non-ferrous heat exchanger tubing. The inspection service offered by Torch Integrated Inspection Services utilises the latest in digital, microprocessor based, multi frequency, phase analysis instruments.

Principles

An eddy current probe consists of one or two coils wound around a former. If the probe is passed along a tube while an alternating current is being applied to the coils, an eddy current field will be generated in the tube wall. The presence of defects in the tube will disturb the eddy current field and generate a signal as the probe passes them.

These signals have both amplitude and phase. In the case of tube inspection, the amplitude of the signal is related to the volume of material lost, whilst the phase can give important information regarding defect depth and origin.

During probe withdrawal the eddy current signals can either be recorded on a paper strip chart recorder or passed to the Automated Tube Inspection System (ATIS) for automated analysis and computerised storage of data.

Multi-frequency instruments allow defect signals to be differentiated from interfering signals which are commonly encountered during eddy current testing. A typical example is the detection of defects at baffle plates, which can mask the signals from defects during single frequency inspection.

Site Implementation

The equipment operates on 110 volts AC mains power. A compressed air supply at 80 – 100psi (5.5 – 7.0 bar) is also required when an air driven winch unit is used for probe transport. As with other tube inspection techniques, the tubes generally need to be cleaned prior to inspection. However, the standard of cleanliness required is not as high as for the IRIS method. In ideal conditions, defects as small as a 1.0 mm diameter through hole can be detected. Although eddy current testing will give a good indication of tube condition, the accuracy of wall loss measurement is not as high as that obtainable using the IRIS and cross-checking by tube pulling and sectioning or IRIS may be necessary for critical applications. Tube sizes from 9 mm to 50 mm ID can be inspected. Tubes outside this range can be appraised for suitability on request. Eddy current testing can be applied to most non ferromagnetic tube materials, including austenitic stainless steels. Production rates depend on factors such as tube cleanliness and tube length, but are typically in the region of 500 – 750 tubes per 12 Hr shift.

Applications

Eddy current inspection can be applied to most non-ferromagnetic tube materials encountered in the oil, gas, petrochemical and power generation industries.