Dead Zone

    The guided wave testing (GWT) equipment has a dead zone just as all UT equipment. The GWT is performed mostly with a pulse-echo mode using the same probe for transmitting and receiving. The dead zone is a short time at which the transmitted signal blinds the received signal due to electronic noise inside the equipment or the electromagnetic interaction of the probe itself. The length of dead zone is determined from the length of the transmission pulse. The pulse is decided by the number of cycles and operating frequency. Depending on the equipment, it can be extended due to the recovery time of the receiver circuits once the transmission burst has ceased.

Near Field Zone

    The near field is the distance from the probe at which the transmitting ultrasonic wave front becomes the symmetric wave (torsional mode or longitudinal mode) in guided wave testing of pipeline. The following figure shows the near field lengths when a discrete or continuous probe is used for generating symmetric guided waves. The near field of a continuous MsS probe is zero length, but the near field of a discrete probe ring or belt is about 4 to 5 ft along each pipe direction of pipeline depending on the operating frequency and the gap between each transducer along the pipe circumference.

    The maximum non-detecting length of dead zone and near field zone along each pipe direction is the length in which guided wave testing is not performed. For example, the magnetostrictive sensor (MsS) system has the maximum non-detecting length of 5 inches with 2-cycle and 128-kHz probe. Thus 5 inch from the probe in both direction, in total 10 inch, cannot be inspected using the guided wave testing. If the maximum non-detecting length is 5 ft with a discrete ring or belt, the total non-detecting length arond the probe will be 10 ft including the probe. Therefore, inspectors should know the non-detecting length of the guided wave testing system and probe. The best thing is to check the non-detecting length of GWT system with a mock-up sample of 10-ft-long, 2-inch-OD pipe which includes several defects.

    Guided wave testing for corrosion under insulation (CUI) requires testing of many short sections of pipe between many geometric features such as an elbow, clamp, and pipe support. The non-detecting length should be considered for deciding on the probe installation location. A properly trained service provider can identify and report the non-detecting zone with the data analysis software.

    If one is assessing cased crossings, the probe must be placed such that the non-detecting length does not extend into the casing. This is important because a majority of indications in casings are typically located within the first few feet.

    To properly assess the non-detecting zone the inspector can move the probe and conduct an additional inspection of the non-detecting zone. An alternate method of obtaining valid readings in the non-detecting zone is to use B-scan ultrasonic equipment and visual examination of the external surface. It is advised to ask the inspection provider what the non-detecting length of the guided wave systems is because the guided wave testing can have a long non-detecting zone depending on the guided wave equipment and probe used.