Magnetizing Systems
An MFL tool contains a system that magnetizes a length of the pipe wall. Typically, sets of magnets are used to provide coverage around the circumference of a pipe. Either permanent magnets or electromagnets are used. Permanent magnets have a constant charge, and they require no power to operate.
Electromagnets generate a magnetic field by passing a current (power) through a coil of wire. By turning the current on or off, the electromagnet can be turned on or off. Also, the power of an electromagnet can be adjusted by increasing or decreasing the current passing through the magnet's coil. Tool designers must weigh the advantages of an adjustable magnet strength against power requirements when selecting between electromagnet and permanent magnet systems.
The body of an MFL tool contains the magnets. A permanent-magnetic system, shown above, has pairs of magnets that are attached to backing bars and to metal brushes or magnet shoes that rub against the pipe wall. Ferromagnetic brushes or shoes are used to efficiently couple the magnetic field to the pipe body. The backing bar is made of a material that is selected to obtain a high flux level in the pipe wall.
An electromagnet system is shown above. Here, an electromagnet and the backing bar assemblies are combined. A magnetic field is generated in the backing bar by current passing through wire that is wound around the backing bars. The strength of the magnetic field is proportional to the number of coil turns and the current passing through the coils. A strong magnet can be produced by using many coil turns or a high current.
