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Salt Precipitation in Sales Gas Compressors

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Inexpensive modifications to gas scrubbers have minimized the danger of salt precipitation clogging sales gas compressors. A few years ago,  researchers and Kollsnes staff addressed the problem of liquid being transferred (carried over) from inlet separators (scrubbers) to Troll sales gas compressors. The performance of the five compressors began to falter early in 2001, and in August 2001 one of them broke down completely.

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  • When the disabled compressor was opened up and the interior bundle (rotor) dismantled, the culprit was discovered to be a layer of deposited salt.
  • Chemical analysis of the compound revealed that it was practically identical to salts that had become incorporated in mono-ethylene glycol (MEG) – a gas hydrate inhibitor used both in the Troll A pipeline and the Kollsnes gas treatment plant pipe work.
  • The cause was MEG/liquid droplets carried over from the compressor inlet scrubbers to the compressors themselves.
  • The MEG and water then evaporated due to the increased pressure, and the salt ions in the water/MEG droplets precipitated.
  • The entire gas treatment plant, including all of the separators in the processing chain, was evaluated, and several inexpensive modifications made to their internal workings.
  • These included the installation of Statoil-designed liquid protection (splash) plates in the compressor inlet scrubbers, which alone reduced the MEG carry-over by approximately 98%.
  • The expander inlet scrubbers in the gas trains were also modified by installing new splash plates, new mist mats (for liquid coalescence and drainage) and new drains from the inlet device.
  • Subsequent tests showed that the MEG carry-over had stopped and the carry-over of condensate had been drastically reduced.
  • The economic value of this ‘Troll process clean-up project’ lay in reduced maintenance, increased regularity and increased capacity
  • Prior to the modifications, each sales gas compressor was limited to a maximum throughput of 20 million standard cubic metres of gas per day. This was consequently increased to 28.