Tower Scans

Tower scans are a useful tool to identify both mechanical abnormalities and process problems on packed and trayed towers. Because the scans are performed while the tower is operating, we are able to spot the issue while the problems are being experienced.

Trayed Towers

TRAYED TOWER ANALYSIS ORIENTATION AND BENEFITS

Distillation column gamma scans have proven to be useful in diagnosing process related problems such as maldistribution, entrainment, flooding, foaming, weeping and damage. Any of these issues can result in contaminated product streams and reduced capacity. Sometimes the symptoms suggest various causal factors which can be less than obvious. Using a small radioactive source and a sensitive detector we will develop an on-line density profile of your column which can be used to identify the integrity of internals and column operating conditions. We will typically take readings across the active area of the tray (see diagram to the right). We can also scan through the downcomers to evaluate downcomer liquid loadings. Our proprietary scan system greatly reduces the inherent reliability, energy discrimination, and data collection issues associated with wireless systems used by some scan companies. We utilize state of the art electronics and radiation detection equipment – the same used by our National Defenses and Homeland Security. This equipment allows us to use smaller size radioactive sources. Our equipment also provides enhanced energy selectivity capabilities resulting in increased definition of process anomalies such as entrainment and foaming. The density profile, or data plot, is depicted with column elevation on the Y-axis and radiation intensity, or relative density, on the X-axis. The relative density decreases from left to right on the logarithmic scale. We will then interpret the resulting profile. A gamma scan is an effective diagnostic tool enabling a quick resolution to your issues. With our proprietary software we can immediately download, print, or email you a report and plots while at your site. Tower scans are beneficial to:
  • Increase Production
  • Reduce Downtime
  • Verify/Rule Out Issues
  • Identify Bottlenecks
  • Optimize Performance
  • Save Time and Money During Turnarounds
View Graph

NORMAL OPERATION

In a trayed column, each tray should sustain an adequate liquid loading based on spacing and hydraulic rates and a liquid free vapor region above the aerated liquid level, referred to as the disengagement zone. In the example to the right, the plot is labeled with a clear vapor bar and a liquid level line. The clear vapor bar corresponds to the lowest density observed in the column and is assumed to be liquid free. This clear vapor bar is then used as a reference for normal liquid disengagement between trays. The liquid level line provides a means for determining any gross variations in individual tray hydraulic loadings. View Graph

DAMAGE

One common reason for conducting a gamma scan is to determine the integrity of column internals. A gamma scan measures the relative height of liquid on a tray. If a particular tray, or trays, is unable to sustain an adequate froth due to damage, this will be revealed on the scan as a lightly loaded, or dry tray. The example to the right shows how damaged trays appear on a scan. View Graph

FLOODING

Flooding can often be attributed to some form of liquid restriction in the column due to a mechanical limitation or inaccurate readings from instrumentation. A gamma scan can help verify the presence and extent of flooding, and in many cases help isolate the cause. Notice in the example to the right that any potential damage to the trays in the flooded area is not readily discernable. Since tray debris could possibly be the cause of the flooding to begin with it may be necessary to alleviate the flooding in order to evaluate tray integrity. View Graph

ENTRAINMENT

Entrainment occurs when the aerated liquid is carried upward from tray to tray effectively reducing the disengagement zone. A gamma scan illustrates this phenomenon as a sharp point in the curve adjacent to the clear vapor bar rather than the more rounded curve characteristic of normal liquid disengagement. Entrainment can also cause an increase in each tray’s aerated liquid level. In addition, the degree of entrainment can be estimated based on the percentage of departure between the clear vapor bar and liquid level line. The example to the right shows the various degrees of entrainment in relation to the clear vapor bar and liquid level line. View Graph

FOAMING

Foaming is a common problem in many services. Foaming has a very characteristic appearance in a scan where one or more subtle hitches are observed above the normal aerated liquid on a tray. In instances where foaming is indicated, follow up scans are extremely useful to monitor the effectiveness of antifoam treatments. The example to the right is of a Sponge Oil Absorber, where the red scan illustrates a severe foaming condition from Tray 9 up to Tray 20 and possibly carrying overhead. The blue scan reveals the foaming condition to be alleviated after an anti-foaming agent was introduced. View Graph

WEEPING

A weeping condition can be caused from damage or below design operation and results in liquid essentially raining down from tray to tray. Similar to entrainment, weeping produces a departure from the clear vapor bar; however, rather than the sharp point, weeping is observed as a more flattened curve in the disengagement zone. Likewise, the severity of weeping can be estimated based on the percentage of departure from the clear vapor bar. View Graph

PRE-TURNAROUND AND BASELINE SCANS

In addition to being an effective troubleshooting tool, pre-turnaround scans are an effective way to identify issues ahead of your scheduled turnaround and avoid unwanted surprises. Identifying and quantifying missing or damaged trays in advance allows replacement hardware to be ordered without incurring short notice expediting fees or impacting your turnaround schedule. Baseline scans provide benchmark operating information that serves as a reference for future troubleshooting. Subsequent scans can be compared to baseline data and become much more meaningful.

Packed Towers

PACKED TOWER ANALYSIS ORIENTATION

Gamma scans yield valuable information for packed towers. In order for a packed bed to function as designed, the countercurrent liquid and vapor flows must maintain as homogeneous a mixture as possible. A grid scan is one method of quickly verifying and quantifying the liquid/vapor distribution quality through a packed bed. A Grid-Scan is comprised of four individual scans in a grid pattern across four equal quadrants. A typical Grid-Scan source and detector placement diagram is shown to the right. View Graph

NORMAL OPERATION

Good liquid/vapor distribution in the packed section will generate a uniform density pattern in which all four scan lines overlay with each other. The amount of shared or overlapping area of the scan lines provides a relative measure of the quality of liquid/vapor distribution in the beds. View Graph

MALDISTRIBUTION

Maldistribution can occur due to fouling material, poor initial liquid and/or vapor distribution, or inadequate design, to name a few. Maldistribution will reveal itself in a Grid-Scan as varying densities of the four scan lines within the packed bed elevations. Any spread among the four scan lines is a sign of maldistribution with a liquid bias being associated with lower radiation intensity (left side of the data plots) and vapor bias associated with higher radiation intensity (right sideof the data plots). View Graph

MISSING/DAMAGED PACKING

Gamma Scans on packed columns can reveal whether packing material has been damaged or dislodged. This information will allow the plant to have on-hand the proper amount of replacement packing and eliminate expedite costs or warehouse storage space for excess. In some cases, a single scan may be sufficient to evaluate missing packing. View Graph

FOULING

By design, packed beds provide a greater surface area for liquid/vapor contact in relation to trays, which is particularly beneficial in vacuum services. One disadvantage of this greater surface area, however, is the susceptibility of fouling material to accumulate if certain conditions are favorable. Grid-Scans will identify the presence of fouling in packing. The fouling typically reveals itself as a flooded section of the packed bed. The example to the right shows flooding in the upper section of a packed bed due to fouled packing. View Graph

TURNAROUND PLANNING

By having a more realistic and comprehensive insight of the hardware replacement needs in critical path vessels, we can help you to better define the scope of your turnaround. The practice of gamma scanning towers early in the turnaround planning stage has gained much acceptance in recent years. This pre-turnaround scan data not only provides the information to help define the labor and material needs, but can also identify areas where the plant can improve performance in the months leading up to the shutdown.

These are just a few example of issues that gamma scans will identify in distillation columns. Contact us to discuss the many other capabilities of gamma scans

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