Dual Diameter Pig Testing with Gas

Pipeline Research was involved in nitrogen testing of the pig motion and integrity of a 9-inch x 16-inch dual diameter wheel pig (Varipig from FTL Technology). The tests were performed with nitrogen upstream and downstream at 38 bars (550 psi) outlet pressure: –

  • The pig was a three-module wheel supported pig with 9-inch seals and guides and 16-inch seals with the ability to negotiate 3D 9-inch bends and 16-inch Y-pieces;
  • The test facility used high-pressure nitrogen input (up to 10000 scfm) with inlet flow control and outlet pressure control.
    Pigging in a gas pipeline can lead to high accelerations, velocities and possible damage to pigs. To understand the motion of the pig during the tests, the Pipeline Research PIGLAB Compressible model was used to aid in the test facility design and to fill in the gaps in our understanding during the tests. The basis of the model was a follows: –
Figure 1 – Overview of test facility model setup

The pig differential pressure was established from the tests and the pig position was determined from pig signalling. The model was then used to mimic the test, understand the peak velocities achieved and get to the detail of how the pig negotiated the test pipes. Aspects such as sudden reversals in the 16-inch, peak velocities into bends and understanding of the motion of the pig as it negotiated the transition from 9-inch to 16-inch were analysed and allowed more understanding than could be delivered by the closed-in test facility: –

Figure 2 – PIGLAB output with pig velocity profile (top) and pig differential pressure (lower graph)

The output shows the pig velocity as modelled and verified by the tests. Velocities of up to 14.5 m/s (47.5 ft/sec) were determined which matched up with the output from the tests. The lower graph shows how the model predicted a potential reversal of the pig after it enters the 16-inch section (negative pig differential pressure, DP). This was also observed on site.

The PIGLAB Compressible model can then be used to scale-up and determine the motion of the pig in the actual pipeline. This is particularly useful for inspection pigging (maintaining the pig velocity within a speed range), determining the best operating conditions and providing accurate predictions of pig arrival times.

PIGLAB Compressible is an in-house tool developed by Pipeline Research Limited. We provide consultancy and advice on pigging using this tool and others. Contact us at info@pipeline-research.com to discuss how we could help you.

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