Page 216 - Pipeline Pigging Technology
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Ethylene pipeline cleaning
period at low pressure. A second low-pressure purge was successful in
removing all residual ethylene, and capital works commenced after a delay of
12hr.
Capital works
When decommissioning was complete on a section, capital works began
immediately. Maximum piping prefabrication and site assembly had been
done prior to the outage, leaving only the actual pipeline tie-ins. These tie-ins
were completed with very few problems. The first section was ready for
cleaning on day 4, and the last section was ready on day 10 of the shutdown.
The initial cut-outs of the pipeline clearly revealed the polymer build-up in
place. A thin film, l-2mm thick, of slightly sticky and very cohesive low-grade
polyethylene was observed. It could easily be wiped off the pipe with a simple
rub of the hand.
Cleaning operations
The first cleaning pig in the line determined that the polymer was
extremely easy to remove from the pipe wall. Although several progressive
cleaning runs were planned, it was found that the 'scout' pig removed
virtually all of the polymer. Even modified with more by-pass holes and
notched cups, the scout tool continued to remove the majority of the
polymer. In fact, the compacted polymer carried in front of the pig created
too much of a barrier, and resulted in two stuck pigs and pipeline cut-outs.
Lost time was quickly regained, however, by omitting some of the proposed
cleaning runs. It was found that, following the initial pig run, the line was
effectively clean and did not require as extensive a programme as originally
anticipated.
Fig.5 gives a listing of the cleaning tools per section, with pressures,
speeds, and comments.
Inspection operations
Inspection operations comprised a calliper vehicle, a profile vehicle, and
the corrosion inspection vehicle.
All calliper vehicles completed their runs without major incident, and no
bend or diameter restrictions were identified. The profile vehicles also ran
successfully, and further confirmed that the inspection vehicle should have
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