Sep-2020

Tru-Scan™ identifies abnormal process conditions and helps optimise vessel efficiencies

Tru-Scans™ provide valuable data aiding operations in achieving 115% of unit capacity

Nicloa Porter
Tracerco

Article Summary

The Approach - Gas plants play a critical role in cleaning raw natural gas after being extracted from subterranean deposits. This process isolates hydrocarbons, non-methane fluids, hydrates and impurities to produce dry pipeline quality gas, which is then transported to the end user. These plants are generally low maintenance compared to oil refiners and most chemical processing complexes. However, they have limited options in the event of disruptions, degrading conditions or incoming raw gas flows. If these conditions do arise, Tracerco can provide online diagnostic testing using our Tru-Scan™ technology to both identify abnormal process conditions and to optimise unit/vessel efficiencies.

The Field Test
A customer contacted Tracerco after they recently replaced their trays with a more efficient design, however, the newly installed trays were carrying liquid overhead at only 80% of production rates. The newer design was installed so they could obtain an additional 15% over their previous 100% production rates. The customer had previously shutdown and entered the vessel after start up assuming the carryover would have been caused by solids or debris left behind during tray installation or startup. After entry, they found no material or limitation that would have caused the liquid carryover. They closed the vessel and started back up, but again found themselves carrying liquid overhead at only 80% of design load. This cost the plant three days of down time and production losses, not including several days of running at 80% of plant capacity prior to the shutdown and entry.

The Analysis
An initial Tru-Scan™ was conducted near 60-70% capacity where the customer was not experiencing issues or increased pressure drop (Figure 1). This scan (blue scanline) showed the bottom 2 trays were in a flooding state. A second scan (red scanline) was then completed at 80% capacity, which showed all the trays to be in a flooded state with liquid, well within the top head of the vessel. These scan results concluded that the initial liquid flood point originated at the bottom tray.

The customer re-evaluated their newly installed tray designs and found they had adjustable outlet weirs on the trays. They decided to re-enter the vessel a few days later and found that the weirs where positioned to their maximum height. After adjusting the weirs to correct design specifications they were able to safely operate the tower with 115% of unit capacity without increased pressure drop or liquid carryover. A baseline scan (Figure 2) was requested and conducted at their current operating capacity. The baseline scan (black scanline) showed all trays to be operating between 75-88% tray spacing with no indications liquid buildup of flooding.
The Conclusion

Tru-Scan™ results have proven to be an effective screening tool for providing detailed information about the hydraulic performance of trayed towers. Data results from the scan identified the initial liquid flood point helping the customer to safely operate at 115% unit capacity.