Magnetic Flux Leakage (MFL)
The non-destructive testing technique known as magnetic flux leakage (MFL) is used for the inspection of... View more
The non-destructive testing technique known as magnetic flux leakage (MFL) is used for the inspection of storage tank floors (mild steel plate). … When there is a velocity difference between the plate and the excitation system, eddy currents are induced opposing the applied magnetic field distorting its profile.
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Detail of MFL pipeline inspection tool
Detail of MFL pipeline inspection tool
Background and origin of the term “pig”: In the field, a device that travels inside a pipeline to clean or inspect it is typically known as a pig. PIG is an acronym for “Pipeline Inspection Gauge”. The acronym PIG came later as the nickname for “pig” originated from cleaning pigs (first designed pigs) that actually sounded like squealing or screeching pigs when they passed through the lines scraping, scrubbing and “squeegeeing” the internal surface. The name serves as common industry jargon for all pigs, both intelligent tools and cleaning tools. Pigs, in order to fit inside the pipeline, are cylindrical and are necessarily short in order to be able to negotiate bends in the pipeline. Many other short, cylindrical objects, such as propane storage tanks, are also known as pigs and it is likely that the name came from the shape of the devices. In some countries a pig is known as a “Diablo”, literally translated to mean “the Devil” relating to the shuddering sound the tool would make as it passed beneath people’s feet. The pigs are built to match the diameter of a pipeline and use the very product being carried to end users to transport them. Pigs have been used in pipelines for many years and have many uses. Some separate one product from another, some clean and some inspect. An MFL tool is known as an “intelligent” or “smart” inspection pig because it contains electronics and collects data real-time while travelling through the pipeline. Sophisticated electronics on board allow this tool to accurately detect features as small as 1 mm by 1 mm, dimensions of the wall of a pipeline as well as depth or thickness of wall (helps indicate potential wall loss).
Typically, an MFL tool consists of two or more bodies. One body is the magnetizer with the magnets and sensors and the other bodies contain the electronics and batteries. The magnetizer body houses the sensors that are located between powerful “rare-earth” magnets. The magnets are mounted between the brushes and tool body to create a magnetic circuit along with the pipe wall. As the tool travels along the pipe, the sensors detect interruptions in the magnetic circuit. Interruptions are typically caused by metal loss and which in most cases is corrosion and the dimensions of the potential metal loss is denoted previously as “feature.” Other features may be manufacturing defects and not actual corrosion. The feature indication or “reading” includes its length by width by depth as well as the o’clock position of the anomaly/feature. Mechanical damage such as shovel gouges can also be detected. The metal loss in a magnetic circuit is analogous to a rock in a stream. Magnetism needs metal to flow and in the absence of it, the flow of magnetism will go around, over or under to maintain its relative path from one magnet to another, similar to the flow of water around a rock in a stream. The sensors detect the changes in the magnetic field in the three directions (axial, radial, or circumferential) to characterize the anomaly. The sensors are typically oriented axially which limits data to axial conditions along the length of the pipeline. Other designs of smart pigs can address other directional data readings or have completely different functions than that of a standard MFL tool. Oftentimes an operator will run a series of inspection tools to help verify or confirm MFL readings and vice versa. An MFL tool can take sensor readings based on either the distance the tool travels or on increments of time. The choice depends on many factors such as the length of the run, the speed that the tool intends to travel, and the number of stops or outages that the tool may experience.
The second body is called an Electronics Can. This section can be split into a number of bodies depending on the size of the tool. This can, as the name suggests, contains the electronics or “brains” of the smart pig. The Electronics Can also contains the batteries and is some cases an IMU (Inertial Measurement Unit) to tie location information to GPS coordinates. On the very rear of the tool are odometer wheels that travel along the inside of the pipeline to measure the distance and speed of the tool.
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