High pressure natural gas pipeline companies conducting in-line magnetic flux leakage (MFL) corrosion inspection operations had to significantly reduce gas throughput velocity to accommodate MFL corrosion tool inspection speeds.

A large bypass, variable speed NPS 36 MFL corrosion inspection tool has been developed and run successfully in several high pressure natural gas pipelines without noticeable impact on operational throughput Active speed control enables the tool to run at speeds significantly lower than line velocity commonly experienced in high pressure natural gas pipelines. Unique mechanical innovations include large diameter flow bypass, an efficient speed control mechanism, variable drag backing bars and an independent bypass override system. A floating backing bar system ensures uniform sensor/wall contact for optimum data collection. Magnetic self-levitation of the backing bar results in reduced load on suspension and wheels providing more reliability and longer life to these components. Operating in higher line velocities infers higher possible tool speeds. This potential required development and construction of a more durable tool capable of higher speeds than typical MFL corrosion inspection tools. In this paper, development, testing and field operation of this tool is described.

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