Sep-2020

Pre-turnaround scans enable better critical path planning

Tru-Scan™ and Tru-Grid™ Scan results help determine whether to enter a particular column during the turnaround outage or remove it from the critical path list.

Nicole Porter
Tracerco

Article Summary

The Approach - Tru-Scan™ and Tru-Grid™ Scans are non-intrusive and performed while the vessel is still in operation, providing a density profile of columns. They are able to provide information about the mechanical integrity of the internals, as well as identify several process degrading conditions. Vessels are typically scanned for troubleshooting and optimisation purposes, or to obtain data to assist with planning turnarounds and outages.

Utilising Tru-Scan™ and Tru-Grid™ Scan technology on distillation columns ahead of turnarounds are proven to be a very useful. Usually, pre-turnaround scans are performed on columns anywhere from six to eighteen months in advance of the scheduled turnaround. These scans can help determine any mechanical issues present within the columns, which allows for better planning as to how much time will be needed to replace the damaged internals. It also allows for part orders to be made in advance of the scheduled outage, in turn saving potential last minute or emergency shipping fees and the time for those parts to arrive. Sometimes, pre-turnaround scans identify operational issues within a column that do not require an outage to address, but instead are alleviated by changing operating parameters. Finally, some pre-turnaround scans reveal a totally healthy column, which if timing does not require an entry, can be removed from the turnaround list, allowing the overall turnaround time to be decreased or freeing up time for more critical projects. The following three field tests cover some scenarios where customers were able to realise savings due to performing pre-turnaround scans.

The Field Test 1
A gulf coast refiner had planned a major turnaround scheduled to begin in the near future. Following the original planning phases in preparation for the turnaround, many additional potential projects were being added, and they were in jeopardy of having to add additional down time, or cut other projects originally scheduled in order to get them all completed. One of the original projects was to enter and inspect one of the distillation columns in an Amine Unit. The column was not presenting any issues, but it had not been entered for quite a while and there had been several upsets to the unit over the years. As such, the refiner wanted to inspect it to give a clean bill of health, and make any repairs that might be needed.

The Analysis
They had not yet reached the time requirement to enter this tower, and if they decided to wait until the next scheduled turnaround, they would still be in compliance with those requirements, so they were contemplating removing this column entry from the turnaround to allow a more critical project to be performed. Therefore, they decided to contact Tracerco to perform a gamma scan of the column to help them to make that decision. This trayed column was scanned and the results are illustrated in Figure 1.

The Conclusion
The Tru-Scan™ results revealed a healthy column, with no signs of gross mechanical damage, and no process degrading conditions. Combining their knowledge of this column’s operation with the data from this pre-turnaround scan, they decided to remove this column entry from the turnaround list and utilise the time that would have been needed to complete another project. This contributed to the turnaround finishing within the budgeted amount of time, and they have not experienced any issues with this column since.

 The Field Test 2
A petrochemical facility was scheduled for a turnaround in one of their units and decided to perform pre-turnaround scans on some of their distillation columns to determine if they would need to be entered during the outage.
 One of the columns had been operating poorly following a recent upset, experiencing poor separation.

The Analysis
This column was scanned about six months ahead of the scheduled turnaround, and the results of the Tru-Scan™ can be seen in Figure 2. Tru-Scan™ results revealed that the top twelve trays of the column, trays above the feed to the column, indicated an abnormal profile. The trays exhibited low densities and reduced or no froth height loadings when compared to the trays below the feed. Trays 5, 6, and 9 indicated profiles consistent with not actually holding liquid, but tray debris. The profiles observed for the trays in the top half of the column was indicative of tray damage.

The profile exhibited in the column scan would definitely explain the poor performance and separation issues that it had been experiencing.

Because of these scan results it was decided to enter this column during the turnaround. Parts were ordered in advance to replace the trays in the top half of the column, and arrived onsite and ready to install when the turnaround began. This allowed the facility to save both time and money to repair this column. Had they waited until the turnaround to enter and inspect this column, they would have had to order the parts on an emergency basis, increasing the price, as well as extending the amount of time for them to arrive. Also, the other columns that were scanned ahead of the turnaround were given a clean bill of health and did not need to be entered.

The Field Test 3
A petrochemical facility was preparing for turnaround of a unit that occurs on a regular basis, every other year. As part of their normal planning, they perform pre-turnaround scans on several of their columns ahead of the turnaround, then perform baseline scans following start-up, if any changes are made to the columns during the outage. They use the data obtained from the pre-turnaround scans to help determine whether are not they will enter a particular column during the outage.

The Analysis
Figure 3 is a plot from one of the columns scanned ahead of a recent turnaround. This column had recently been exhibiting a higher than normal differential pressure as well. The scan results contain two scanlines, a red scanline representing the baseline scan that was performed following the previous entry into the column, and a blue scanline representing the current pre-turnaround scan. During the pre-turnaround scan, trays 21-23 indicating evidence of foaming, inflections seen in the froth area of those trays, and there was evidence of carry over above tray 24 up to the top tangent of the column. However, there was no evidence of any mechanical damage observed during the scan. The profile seen during the pre-turnaround scan would definitely explain the high differential pressure that they column had been experiencing.

The Conclusion
There are certain operational parameters for this column that can cause foaming. After reviewing the data from this scan, changes were made to those parameters and the differential pressure returned to normal. The facility decided that an entry into this column would not be needed during the turnaround and they were able to shorten its duration. This was an example of a situation where there was an operational issue with a column, and had the facility shut down and entered the column for an inspection, they would not have discovered the true issue.
 
The Conclusion
Pre-turnaround scans are very useful tool for facilities to utilise for planning turnaround projects. Maybe they will indicate some damage within a vessel, allowing for parts to be ordered and on-site ahead of time. Maybe they will indicate an operational issue, which can be corrected without an entry into the vessel. Or, maybe they will indicate that the vessel is healthy and no work is needed on it at this time. In any of these cases, the amount of money and time saved greatly outweighs the cost of the scans themselves.