Sep-2020

PackView™ liquid retention scale is a quantitative analysis of packing maldistribution

PackView™ can be used to compare operational parameters with engineering design

Nicola Porter
Tracerco

Article Summary

Corrosion to vessel internals is a contributing factor to reduced efficiency, poor separation, and mechanical failures. Corrosion maybe be accelerated by elevated temperatures and lower pH levels. Water can also exacerbate the corrosion process if left unbalanced.

In this particular case study, a Tru-Grid™ Scan was performed for a customer in order to verify mechanical placement of internals as well as liquid distribution characteristics. The client had been observing an increase in heavy material in their overhead product and small metallic flakes in their downstream filters after running for several years.

The Field Test
A Tru-Grid™ Scan consists of four individual vertical scans of the vessel, oriented in a four-quadrant grid pattern crossing through the beds of packing and is typically used to investigate the quality of liquid distribution (Figure 1).

PackView™ liquid retention scale is provided on the Tru-Grid™ Scan plots to measure densities within the packed section. PackView™ is a calculated density scale based on the radiation attenuation through the bed. This tool enables the client to observe density variations within the bed to identify liquid retention, packing material and any other substances in the packing.

The Analysis
Scan results shown in Figure 2 revealed 0 lb/ft3 (0 kg/m3) in the upper 4 ft (1.2 m) of the bed. There was a density absorption at the 9 ft (2.7 m) elevation, which was consistent with the packing hold-down grid location. Below the hold-down grid, the packing density was 1.5 lb/ft3 (24 kg/m3) less dense than the manufactures dry packing density of 8.2 lb/ft3 (131 kg/m3). The remainder of the bed showed a slow density gradient with increasing density towards the bottom of the support grid. The Tru-Grid™ Scan also showed a density difference of 2.5 lb/ft3 (40 kg/m3) between all four scan cords signifying poor liquid distribution.
 
The Conclusion
Based on the results of Traceco’s Tru-Grid™ Scan findings and the use of the PackView™ liquid retention scale, the client was able to consult a packing manufacture about the lack of dense material in the upper half of the bed. They were able to order more resilient and corrosion resistant material to replace their deteriorated bed and preventing a re-occurance of the same issue, months before their unit shutdown. This increased preparedness and planning resulted in a much quicker unit turnaround, which in turn resulted in major cost savings to the plant.

Based on the scan results, the customer decided to shutdown and enter the tower. The client found the upper 4 ft (1.2 m) of the bed had corroded and the packing hold-down grid settled on packing that still had some ridged structure. The next 3 ft (0.91 m) of packing had corroded to about half its metal thickness. The remainder of the bed was a combination of corroded packing along with packing debris that had washed down from the heavy corrosion above. The structure of this packing material was also so corroded, that it could be crushed within the palm of the hand.

It is always easier to understand and discuss technical issues when quantitative information provided by PackView™ can be used to compare operational parameters with engineering design. This advanced analysis provides a new method for extracting quantitative information from Tru-Grid™ Scan data to diagnose and characterise the operation of distillation and separation towers, improving scan data and facilitating improvements in the operation of mass transfer equipment.