(4e) Troubleshooting and Solving a Gas Treater Downcomer Unsealing Problem

ADGAS have commissioned the Integrated Gas Development (IGD) project that transfers 900 MMSCFD of dry gas from the gas wells to the Habshan Industrial site through a 30” pipeline. The project comprises a gas receiving station, three gas drying units (300 MMSCFD each), three GT driven compressors, and a fuel gas treatment (FGT) system with the capacity of 25 MMSCFD of sweet gas. The FGT system was designed by an engineering contractor from Japan and the tower internals by a major tray vendor.

Since the start-up/commissioning of the FGT system it was observed that when the plant operates above 10 MMSCFD, the absorber column became very unstable. A sudden increase in pressure drop in the Fuel Gas Treater column occurred, and that resulted in a sudden level rise in FG KO drum, tripping the whole system. Changes to the MDEA flow rates did not appear to have much influence. An attempt to fix the problem by the contractor and vendor failed to provide a significant improvement.

A thorough troubleshooting investigation by ADGAS, together with hydraulic analysis by Fluor, yielded a correct diagnosis and a successful solution.

Downcomer seal loss in moving valve trays was the root cause of the flooding and instability problem. The constant dry pressure drop held by the valve tray as valves close off at reduced rates can make downcomer sealing difficult in moving valve trays, especially with large clearances. Large downcomer clearances should be avoided at low liquid rates, both with valve trays and other trays.

The valve trays in the tower were replaced by the new bubble cap trays. The tower has been performing well ever since with no signs of flooding or instability, even well above the design rates.