Gamma Scans on Trayed Towers

ScanTech's Gamma Scan also known as a Column Scan or Tower Scan, is a reliable and safe method of characterizing operating conditions in a trayed tower. It is a method of "viewing" the process fluids that is non-invasive to your process and equipment.

Generally a scan path is chosen across the active area of a tray while avoiding intersection of the downcomers. In multi-pass trays, scans can be performed across each active area as required. If the height of liquid in the downcomers is desired, a scan line through the downcomer instead of the active area can be chosen depending on the location of the downcomers.

Gamma Scans are external to your vessel and require no preparation by plant personnel. If site safety orientated, our personnel can usually be scanning within an hour of arriving on site.

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 column, it is always advantageous to compare multiple scans. A Gamma Scan can be compared between two operating conditions on the same day (low vs high feed rates) or to a baseline scan done when conditions were "normal" or the tower was clean such as after a shutdown.


	Gamma Scan - Trayed Towers
	(Tower Scans & Column Scans)
Gamma Scan Response to Process Conditions	A ScanTech Gamma Scan is a cost effective solution to many of your trayed tower problems in refineries, chemical plants and gas plants. Distillation towers experience a broad range of problems including sudden pressure surges, high delta Ps, gradual fouling, downcomer blockages, foaming and flooding, and secondary problems related to associated vessels and piping. Our Gamma Scanning crew will quickly and accurately determine what is occuring within your tower. Your process engineers and maintenance engineers can use the results from our Gamma Scan to verify theories, eliminate scenarios or introduce previously unknown possibilities in their distillation tower troubleshooting. According to Henry Kister, a leading authority on distillation troubleshooting, the Gamma Scan is an effective troubleshooting tool and is mentioned in many of his books and articles. Gamma Scans can be used on almost any trayed tower including columns containing sieve trays, valve trays, bubble cap trays and others. Trays can be single downcomer, two downcomer or more or some unique downcomer configuration used in your process. The Gamma Scan techniques works very well for almost any tray spacing 12 inches or more. Gamma Scanning your trayed towers can diagnose the following conditions: Location and condition of trays, distributors and demisters Height of aerated liquid on each tray Location of flooding, entrainment, weeping and foaming Height of liquid on chimney and draw trays Level of liquid in the base of the column

Performing the Gamma 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 a trayed tower. It is a method of "viewing" the process fluids that is non-invasive to your process and equipment. Generally a scan path is chosen across the active area of a tray while avoiding intersection of the downcomers. In multi-pass trays, scans can be performed across each active area as required. If the height of liquid in the downcomers is desired, a scan line through the downcomer instead of the active area can be chosen depending on the location of the downcomers. Gamma Scans are external to your vessel and require no preparation by plant personnel. If site safety orientated, our personnel can usually be scanning within an hour of arriving on site. 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 column, it is always advantageous to compare multiple scans. A Gamma Scan can be compared between two operating conditions on the same day (low vs high feed rates) or to a baseline scan done when conditions were "normal" or the tower was clean such as after a shutdown.			Gamma Scan Orientation for a 2 Pass Trayed Tower

Flooded Tower Gamma Scan Example		Fouled Trays Gamma Scan Example
A Gamma Scan was performed on a stripping tower at a refinery. Operations personnel suspected tray damage was resulting in flooding due to an increased delta P reading in the tower. A Gamma Scan was performed at a low feed rate (red line) with a normal delta P to determine the status of the trays. All of the trays appeared to be in position and holding some levels of aerated liquid. The feed rate was increased to determine the flood point within the trayed section and a second Gamma Scan was performed (blue line). It was determined that the flood point was not within the trays but originating from the bottom of the tower. The base liquids were not being drawn from the tower quickly enough resulting in the flood situation. The two Gamma Scans including the rate change took about 5 hours to complete. After the scans operations personnel could then concentrate on the base of the tower with associated piping and pumps rather than the trayed section.		A customer was experiencing low throughput with an absorber tower at a gas plant. Fouling of the trays was suspected but could not be confirmed through plant instrumentation. The customer did not want to stop production and perform a costly chemical wash unless it was deemed necessary. A Gamma Scan was performed (red line) and verified that entrainment was originating approximately halfway up the tower and continuing to the top. A chemical wash was performed on the tower with our scanning crew on site to monitor the status of the wash in real time. The customer's original plan was to wash for only 2 hours. With the aid of the Gamma Scan our personnel could determine that the trays were only effectively cleaned after almost 6 hours of washing. A second Gamma Scan was performed after product feed was established to the tower to verify the tray loading (blue line) .

If you have any questions or would like to discuss the Gamma Scan application on one of your vessels 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 can send these in PDF or hardcopy format.

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This site was last modified on Saturday, November 24, 2007

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