Recent technological advances have produced “intelligent pigs” that can perform multiple inspection functions from inside of the tube. These devices utilize immersion-based ultrasonics to measure tube inside diameters in both ferritic and austenitic heater tube coils. Simultaneously, the same ultrasonic transducers measure the tube wall thickness. The instruments are outfitted with multiple ultrasonic transducers (typically between 50 – 200 sensors) to permit  a high density sampling. These inspection tools are useful in addressing creep, corrosion, erosion or pitting-type damage mechanisms.

These devices are suitable for welded carbon and stainless steel convection section, radiant section, and cross-over piping coils.  Radiant section coils with radial insert plug headers can also be inspected using these devices.  These devices are not suitable cast materials, or radiant section coils with flat insert plug headers.  Though no scaffolding is required to utilize these devices, intelligent pigs operate in a bi-directional mode and require only a single point of entry to the coil’s ID to launch/receive the intelligent pig.  Intelligent pigs are capable of inspecting coils which contain changing diameters throughout the length, and varying thicknesses/schedules.

Intelligent pigs are self-contained units and are capable of navigating the short radius 180 degree x 1D U-bends. This is a significant advantage for inspection of convective section tubes that can not be accessed. Generally, convective tubes only receive a rudimentary visual inspection and their condition is estimated from inspections performed on other components of the heater like the convective tube U-bends and radiant tubes. The pigs are propelled through the tubes using a liquid medium (i.e. water, soda ash solution, diesel, glycol, etc.) to act as both couplant and hydraulic vehicle and allow a thorough, complete inspection of a tube pass. The pig also contains a axial positioning capabilities which enables precise location of the damage mechanism to be identified. Users are cautioned to understand the capability of the intelligent pig to accurately measure the wall thickness and tube inside diameter of U-bends and other fittings. The complex geometry of these locations can make conventional inspection practices difficult.

Other technological advances for in-line devices have produced various other pig or crawler technologies. One method is an internal ultrasonic pig that uses an internal rotary inspection system. The pig is designed to maneuver around bends being pushed along by motive water force. The device has a spinning ultrasonic immersion transducer (such as a transducer aimed at an angled-spinning tungsten mirror used for reflection or a specially designed membrane that allows transmission and protects an angled spinning centrally located transducer head). This method is accurate if there is no debris or scale on the tube ID and can very accurately determine the extent of ID and OD corrosion.

Various crawlers have been designed to navigate the ID of a tube along the length and around bends. These crawlers have been equipped with spot ultrasonic devices, spot pit depth gages, and digital video cameras.

No comments: