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 As the probe is withdrawn from the tube the Eddy Current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.
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.