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The physics of ILI tools in gas pipelines
The excess of the pig's design travel speed in gas pipelines is due to the compressibility of the driving medium, the gas.
The tool acts like a cork in a champagne bottle: Once the driving gas pressure exceeds the cork friction it pops out at high and uncontrolled speed. The driving gas pressure behind breaks down as it is not able to build up again with the same speed rate as the tool moving forward.
As a consequence, the tool moves forward at changing velocities, quite often exceeding the design speed.
ILI tool speed over inspected pipeline length