| 摘要 |
A pig or traveller for use in a pipeline contains a magnetic device having poles disposed close to, but out of contact with, the inside walls of the pipe so as to be capable of magnetically saturating the walls and creating a magnetic field outside the pipe which can readily be detected, e.g. by means of a sensitive magnetometer. Pole pieces in the form of radial flanges may be used with an axially extending magnet assembly. Alternatively, the magnetic device may be rotatably mounted in the pig and provided with an impeller which employs pipeline pressure to rotate the magnetic device, and so creates a rotating magnetic field, if the pig becomes stuck. By establishing detector stations at intervals along the pipeline, the progress of the pig can be monitored.
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