Tube Inspection - Centest

Tubular exchangers, refrigeration units, condensers and other similar equipment are vital to the operation of plant. Regular inspection is the best means of assuring continued reliable operation.

Brief Description

The Centest 91 is neither an eddy-current nor a flux-leakage system. The Centest system works by measuring changes in flux density within a tube.

The probe contains a core and a magnetising coil, which creates a magnetic field within the tube. Also contained within the probe are detector coils that measure the flux density in the tube. The flux density increases as the thickness of the material decreases, consequently variations in material thickness can be detected and located. Results are recorded on a strip chart recorder, which provides a differential trace to locate localised defects such as pits; an absolute trace to locate large zones of damage such as erosion or fretting. A second differential trace serves to enhance the detection of fine pits where there are coarse variations in the tube material.

The electro-magnet in the probe has to be capable of achieving a certain level of magnetisation in the wall of the tube. As an approximate guide, this requires that the cross-sectional area of the bore of the tube is equal to or greater than the cross-sectional area of the tube wall. Although a large variety of probe sizes are available, it is not possible to manufacture probes to suit some sizes of tube.

Capabilities

Ferrous tubes, ranging in size from 15mm to 100mm outside diameter, can be inspected.
Provides 100% coverage of tube circumference.
Sensitive to both internal and external defects.
Typically, 40-50 tubes can be inspected per hour, dependant on tube length.
Defect position can be located in relation to tube length.
Data collected is stored on a 3-channel chart recorder.
In good quality, seamless carbon tubes 1mm diameter holes and 5% general loss of wall thickness can be resolved.
It is possible to scan around reasonably radius bends with the appropriate probe.

Limitations

Does not provide definitive measurements of wall loss, defect severity is estimated in comparison to manufactured defects within a calibration? tube.
Poor surface preparation or deposits may affect probe fit and consequently, results.
Will not detect tight, vertical planar defects.
Access may be required to both ends of the tube.
Optimum probe fit required for best results.
Some tube bore and thickness combinations cannot be inspected.

Preparation

The bore of the tube must be cleaned to bare metal along the full length for optimum results. This generally requires the tubes to be cleaned by high pressure water jetting.

Services Required