Gamma Scans on Packed Towers - Random and Structured Packing

ScanTech's Gamma Scan also known as a Column Scan or Tower Scan, is a reliable and safe method of characterizing operating conditions in your packed tower. It is non-invasive to your process and equipment and gives you reliable information on the status of internal components and / or fluid distribution.

Gamma Scans on packed towers can be either single scans or Grid Scans. A single gamma scan can determine everything listed above with the exception of the levelness of components and liquid maldistribution.

A Grid Scan is required to evaluate the levelness of vessel internals and determine the amount and location of liquid maldistribution in the packing. To effectively determine annular maldistribution or obtain a comprehensive density profile across the bed a CAT Scan is required.

Gamma Scans are external to your vessel and require no preparation by plant personnel. A single Gamma Scan is oriented across the bed such that the scan path will intersect all objects of interest.

A Grid Scan is a series of 4 scans through the packed bed as shown in the figure to the left. The four scan orientations are chosen symmetrically around the center of the tower. The density profiles obtained from each scan can be superimposed on each other to determine maldistribution or out of level internals.

Prior to starting the scan our personnel will need a safe work permit, access to the top platform on the tower and a drawing of the column (showing column height, shell thickness, tray elevations, and downcomer orientation). The results from the scan are printed and discussed with the customer at the completion of the scan.

When a customer is experiencing problems with a packed column, it is advantageous to compare the current scan to a baseline scan taken just after the tower has been started up with clean packing.


	Gamma Scan - Packed Beds
	(Tower Scans & Column Scans)
Gamma Scan Response to Packed Towers	Packed towers can be optimized and investigated using ScanTech's Gamma Scan technique. The basic technique is the same as that used with your trayed distillation towers. A single Gamma Scan can be used to quickly evaluate the tower and determine the extent of any existing problems. A regular scanning program can be used to monitor your packed beds over time to allow you to take corrective action or schedule maintenance only when needed. If the only section of interest is the packed section, the entire tower does not necessarily need to be scanned. Packed bed scans benefit greatly if there is an existing baseline scan to compare to the current scan. Gamma Scans can be used on a variety of packing types including both random and structured packing. Packing material can be plastic, ceramic or metal and the shape of the packing does not afftect the scan results. The best results are obtained when scanning beds containing less dense packing such as plastic rings or saddles. Gamma Scanning your packed distillation towers can diagnose the following conditions: Location and condition of distributors and demisters Location of the top and bottom of the packing including support plates Damaged, crushed or settled packing Location and extent of fouling of the packing Liquid height and levelness of distributors and draw trays Level of liquid in the base of the column Location and extent of maldistribution within the packing

Performing the Gamma Scan and Grid Scan
ScanTech's Gamma Scan also known as a Column Scan or Tower Scan, is a *reliable* and safe method of characterizing operating conditions in your packed tower. It is non-invasive to your process and equipment and gives you reliable information on the status of internal components and / or fluid distribution. Gamma Scans on packed towers can be either single scans or Grid Scans. A single gamma scan can determine everything listed above with the exception of the levelness of components and liquid maldistribution. A Grid Scan is required to evaluate the levelness of vessel internals and determine the amount and location of liquid *maldistribution* in the packing. To effectively determine annular maldistribution or obtain a comprehensive density profile across the bed a CAT Scan is required. Gamma Scans are external to your vessel and require no preparation by plant personnel. A single Gamma Scan is oriented across the bed such that the scan path will intersect all objects of interest. A Grid Scan is a series of 4 scans through the packed bed as shown in the figure to the left. The four scan orientations are chosen symmetrically around the center of the tower. The density profiles obtained from each scan can be superimposed on each other to determine maldistribution or out of level internals. Prior to starting the scan our personnel will need a safe work permit, access to the top platform on the tower and a drawing of the column (showing column height, shell thickness, tray elevations, and downcomer orientation). The results from the scan are printed and discussed with the customer at the completion of the scan. When a customer is experiencing problems with a packed column, it is advantageous to compare the current scan to a baseline scan taken just after the tower has been started up with clean packing.			Grid Scan Orientation for Packed Tower

Grid Scan Example on Packed Tower
A Grid Scan was performed on a tower containing two packed beds at a petrochemical complex. Operations personnel suspected maldistribution through the beds possibly from an uneven distributor. A Grid Gamma Scan was performed across the two beds with the first Gamma Scan extending to the bottom of the tower to cover the active area of the 4 trays below the beds. The 4 scans were superimposed on each other next to a schematic of the tower. From the Grid Scan, it was determined that the top distributor was level. The packed bed below the top distributor was functioning properly with all of the curves from the 4 scans overlaying each other to display uniform density. Below the top bed we saw an increased density for the caps on the chimney draw trays and then approximately 16 inches of liquid on the chimney tray. The distributor above bed 2 appeared to be level. The packing in Bed 2 was fouled at the top of the bed indicated by the increased density on the Grid Scan. There is some maldistribution that originates in the fouled section of the bed. The cause of the maldistribution could not be determined from the scan as it could be either a blocked distributor or the fouled packing itself. The maldistribution becomes worse as preferential fluid flows on the red and green scan lines. The separation of the scan curves through the middle section of the bed indicates gross maldistribution. The blue and purple curves display the density of the packing only below the fouled section indicating that little liquid is flowing through this section. The purple scan line begins to return to a higher density indicating that the packing is trying to distribute the liquid through the lower section of the bed. Vapour is flowing up one side of the bed as indicated by the blue scan line density with fluid flowing down the other side. The four trays are in position in the bottom of the tower and holding levels of aerated liquid. Tray 1 has a slightly increased liquid load possibly due to minor fouling of the tray deck resulting in increased vapour velocity. The liquid level in the base of the tower was at the correct position as indicated by the vessel gauge. From the information obtained in the Grid Scan the customer was able to shut down the tower and inspect the lower distributor and replace only the lower packed bed saving considerable downtime.
		Grid Scans on Disitllation Towers at Petrochemical Plant
If you have any questions or would like to discuss the Gamma Scan application on one of your packed towers please contact one of our personnel. If you need brochures of our services or business cards for your files, please call or email and we will send these in PDF or hardcopy format.

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