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Centest 91 Ferrous Tube Inspection System

The Centest 91 is an electromagnetic system for the rapid inspection of ferromagnetic heat exchanger tubing.


The CENTEST 91 is based on partial saturation magnetically biased eddy current principles.

Conventional eddy current testing cannot be applied to ferromagnetic tubes because of the effect that the high levels of magnetic permeability have on the depth of penetration of the eddy current field and on background noise levels.

These difficulties have been overcome in the CENTEST 91. A probe on the end of a cable is pushed and pulled through the tube to be inspected. The probe contains DC magnetising coils and AC Eddy Current sensor coils. The magnetising coils partially magnetise the tube wall and the presence of defects in the tube influences the magnetic field. As the probe is withdrawn from the tube the Eddy Current sensor coils detect variations in the magnetic field due to the presence of defects. Signals from the sensor coils are passed to an electronics unit for processing and recording. Results can either be recorded on a 3 channel paper strip chart recorder or transferred to the Automated Tube Inspection System (ATIS) for automated analysis and computerised storage of data. The system is sensitive to both internal and external defects and to pitting and more general loss.


Site Implementation

The equipment operates on 110 volts AC mains power. A compressed air supply at 80 – 100 psi (5.5 – 7.0 bar) is also required. 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. In ideal conditions, defects as small as a 1.5 mm diameter through hole can be detected. However, the test signal amplitudes are more closely related to volume of metal lost rather than to the depth of the defects. The system can therefore give a good indication of general tube condition, but not accurate wall loss information. Where more accurate information is required then a small number of tubes selected on the basis of CENTEST 91 results can either be pulled and sectioned or inspected using IRIS to draw up a correlation between defect depth and signal amplitude. Tube sizes from 12.5 mm to 50 mm ID can be inspected, providing that the cross sectional area of the tube wall does not exceed the cross sectional area of the tube bore. Tubes outside this range can be appraised for suitability on request. Production rates depend on factors such as tube cleanliness and tube length, but are typically in the region of 500 – 750 tubes per 12 hour shift.


Typical applications include the inspection of fin-fan coolers, condensers, coolers, boilers, reboilers, feed water heaters and chemical reactors as found throughout the oil, gas, petrochemical and power generation industries.