There can be fewer more risky industries and environments than those involving petrochemicals and power generation. Harsh, challenging and in constant danger of cataclysmic incidents, the focus on health and safety is understandably high. Harmonious working between man and materials is essential and, whilst much human endeavour can be observed and regulated, inert equipment is always prone to the unexpected. Minimising risk is very much a way of life.
Both industries rely heavily on arterial infrastructure; tubes of all dimensions and uses, being especially dependent on heat exchangers and boilers. Often tubes are bundled together making even visual inspection difficult or hazardous; in many cases normal ultrasonic and radiographic inspections fail to achieve results to an acceptable standard. Allied to these safety issues are economic factors relating to downtime costs and production shutdowns, frequently measured in weeks. Risky reliance on visual inspection and destructive testing of many small-bore tubes has led to past uncertainty and potentially expensive decisions as to whether and when to repair or replace.
We can offer the following tube inspection techniques:
Remote Field Eddy Current Tube Inspection
This technique is well adapted to the inspection of small-bore ferromagnetic tubes such as carbon steel and is now the industry standard inspection for boilers and heat exchangers due to its low frequency (typically 50-1000Hz). The probe consists of two coils in a send-receive configuration, which are inserted into the tube. The energised exciter coil transmits a signal to the detector coil located some distance away. This signal passes through to the outside tube wall returning to arrive at the detector coil. With wall thinning there is less shielding hence the return time (greater phase) and attenuation (greater amplitude) is shorter. Phase and amplitude traces are generated as the probe is pulled through the tube as recorded data identifies the metal loss. Flaw sizing is also possible with the Remote Field Technique enabling depth, length and circumference to be accurately calibrated.
Eddy Current Tube Inspection
Eddy current tube inspection is another electromagnetic technique, which has been adapted for small-bore tubing manufactured from non-ferrous metals (such as stainless steel, titanium, copper, brass, coppernickelalloys and Inconel). Eddy current tube inspection is based on the principles of electromagnetic induction and uses a test coil through which is passed an alternating or varying current known as the primary field. This induces varying secondary electromagnetic field in the test object, known as an eddy current. In this way characteristic changes in the test object such as conductivity, permeability and geometry cause the eddy current to change, which is reflected to the test coil. A number of frequencies in differential and absolute modes detect gradual wall loss. Multi-frequency is essential to differentiate between defects and metallic deposits and also enables defects under support plates to be detected.
Magnetic Flux Leakage Tube Inspection
This electromagnetic technique is applied to small-bore ferromagnetic heat exchangers made from carbon steel. An Magnetic Flux Leakage probe consists of a magnet with a coil type and hall element pickups. Coil type identifies the rate of change of flux and detects small defects that cause perturbations; the rate of change inducing an output voltage is read by the probe. The hall element picks up absolute flux with its sensor detecting gradual wall loss. Additionally, it can be used for sizing wall loss flaws.
Ultrasonic IRIS Tube Inspection
Ultrasonic Iris Tube inspection is an internal rotary inspection system which measures thickness using ultrasonic waves. The probe consists of an ultrasonic transducer that is lined up to the centre line of the tube and to a rotating mirror. The mirror reflects the beam in the radial direction as it rotates in the tube. As the probe is pulled along the tube it scans the entire internal circumference. Frequently used for carbon steel tubes IRIS is sometimes used with non-ferromagnetic tubes for defect verification. A highly accurate technique for thickness measurement, IRIS will not identify pinholes and cracks.
Other non-standard specialist techniques include:
- Eddy Current Array Tube Inspection
- Partial Saturation
- Swaged Entranced
- Flexible probe to inspect around bends
- Variable diameter inspection using expandable probes.