Tube Inspection

Internal rotating inspection system (IRIS)

– The purpose of IRIS method is to provide a generic approach for determination of tube wall thickness, using ultrasonic immersion pulse-echo principles in conjunction with the internal rotating inspection system (IRIS).
– The inspection is largely applied for the quantification and assessment of internal and/or external tube wall corrosion, pitting, scarring and fretting etc. As a guide and for the purpose of this method, the minimum readable wall thickness is approximately 0.5 mm. It is noted that the method is not suitable for the detection of cracking.
– The extent and scope of the inspection will be that as defined as per the plant owner’s applicable work scopes. Where no defined work scope is available, the extent and scope of the inspection will be as agreed between the plant owner and the inspection provider.

Eddy Current (ET)

The purpose of Eddy Current (ET) is to provide a generic approach for the ET inspection of in-service for Non-Ferrous Heat Exchanger tubes using Bobbin Coil configurations and the Post Analysis Model.

Remote Field (RFT)

– The RFET technique differs from conventional eddy-current inspection techniques, as its probe comprises of a transmitter and receiver coils. The space between the transmitter and receivers is usually more than twice the outer diameter of the tube.
– In this configuration, the field produced by the transmitter can reach the receiver using two distinct paths namely the direct and indirect path. In the direct path, the field is rapidly attenuated in the axial direction due to circumferential eddy currents induced in the tube.
– However the field has difference in the indirect path. Firstly, it diffuses across the tube wall and undergoes attenuation and phase shift during its passage.
– Once on the outside, the field flows along the tube and then re-diffuses into the tube, undergoing another attenuation and phase shift.
– Since attenuation is greater than the direct path, the residual field detected at a certain critical distance from the transmitter originates from the indirect component of the field.
– This critical distance marks the beginning of the remote field area located approximately at twice the distance of the Outer Diameter of the tube. When the receivers are placed in the Remote Field zone, they sense a signal that has gone through the tube wall twice.
– This signal is representative of the discontinuities present in both portions of the tube penetrated by the field and enables complete inspection of the tube wall. Defects corresponding to loss of matter (erosion, corrosion, pits) will reduce the phase shift and field attenuation. These variations can be used to determine the volume and depth of a defect using Phase and Amplitude Analysis.

Near Field Technique (NFT)

– The Near Field Technique is relatively a new technology and has been developed as an alternative to Magnetic Flux Leakage for the inspection of carbon steel fin fan cooler tubes.
– The technology relies on a simple driver pickup eddy current probe design. The
method is limited to ferromagnetic materials.
– The basic probe is made with one driver coil and three receiver coils. The driver coil operate within the same frequency range as a remote field testing (RFT) probe.
– The receiver coils are placed relatively close to the driver, in a region where the direct magnetic field and eddy currents are restricted to the tube ID. Two coils form the differential signal and one coil the absolute signal.
– The amplitude of the absolute and differential signal is proportional to the defect depth and size.
– Inspection involves calibration of the system on a tube sample similar to the one being inspected.
– This helps in obtaining the necessary information that serves as a comparator /baseline for quantifying flaws on the actual tubes being inspected