Rotating ultrasonic pulses scan the entire pipe wall from the inside. The probe head is rotated by means of water. The water also conducts the ultrasonic signal. The reflected ultrasonic signal is translated into an associated internal diameter, external diameter and consequently the wall thickness.
The Internal Rotary Inspection System (IRIS) is a technique that is extremely suitable for inspecting both ferrite and non- ferrite pipes. IRIS makes it possible to measure wall thicknesses very accurately. However, the method is far slower than eddy current inspection for example. Internal and external defects can be detected and distinguished, not only the depth but also the geometry.
Because the method is based on reflecting ultrasonic pulses, it is important that the pipes are very clean inside. Soiling in the pipe can adversely influence the accuracy of the measurement.
The internal rotary inspection system (IRIS) is an ultrasound technique used to measure thickness of ferrous and nonferrous tubing. This method allows the detection of general thinning such as corrosion and erosion, and of localized defects such as wear tear, scars, inside diameter (ID) and outside diameter (OD) pits. However, it cannot be used to detect pinholes and tight cracks. As long as tubes are clean enough to avoid ultrasound diffraction and attenuation losses, IRIS can accommodate a wide range of tube diameters and wall thicknesses. Typically, the pulling speed is quite slow, about 50 mm/s.
The IRIS probe consists of a rotating 45-degree mirror that redirects the ultrasonic beam into the tube wall. The mirror is driven by a small turbine that rotates at about 50 rev/s. The turbine is set in motion by the pressure of water pumped inside the probe. As the probe is pulled, the spinning motion of the mirror results in a helical scan path, so from one revolution to the next, the complete surface of the tube is covered.
The ultrasound beam is generated by the transducer; the beam hits the mirror and is reflected at a 45-degree angle. Because no mirror is perfect, a small echo is reflected back to the transducer. Other than that, ultrasound waves are partially reflected by the tube internal wall (ID) but also transmitted through the wall, and finally reflected by the tube external wall (OD). Using the speed of propagation of ultrasound in the tube material, the wall thickness can be calculated by using the time-of-flight difference between OD and ID echoes.
This series of reflections (echoes) would appear as shown beside on a time base A-scan view. The target pin echo (yellow) is generated by a pin attached inside the probe which produces an ultrasonic reflection each time the mirror passes it. This echo is used to synchronize the screen display.