Oct-2015

On-stream refractory measurement

Precise assessment of refractory condition can avoid unwelcome discoveries during unit turnarounds

DAVE FERGUSON and CARL TIPTON
Tracerco

Article Summary

Every refinery invests a tremendous amount of time and money in planning a unit turnaround. The scheduling of the tasks, manpower, and equipment have to be detailed and thorough. To do this, knowledge is needed about the process system in order that critical tasks can be planned to meet turnaround time constraints. Some of this condition based information is gathered using tools such as gamma scans, radiography, ultrasonic probes, vibration monitors, acoustic sensors, and other non-destructive techniques whilst the process is still operating. Internal damage to distillation columns, heat exchangers, reactors, and other vessels has to be known to avoid surprises that can disrupt the turnaround schedule. Since each day of downtime is a day of lost production, it is very important that a turnaround is not extended due to unexpected repair work.

In 2014, Tracerco was approached by representatives of a major oil company that operates multiple sites with fluidised catalytic cracking (FCC) units. These units typically have a major turnaround every five years. One of the tools used to gather information about the condition of internal refractory linings of the FCC vessels is a thermal imaging camera. This device identifies locations on the surface of the vessels that are hotter than they should be, indicating that the refractory inside has been eroded or damaged in some way. Unfortunately, one inch of refractory remaining attached to the vessel wall is often enough to stop the heat, yielding a normal thermal scan. However, an area of refractory that is only one or two inches thick, when it should typically be four inches, will require remediation during a maintenance shutdown, once discovered.

In order to allow the accurate measurement of refractory thickness before shutting down a vessel, Tracerco has carried out an extensive research project culminating in the design and construction of a new inspection tool. The portable equipment, named InDepth, is able to measure the depth of refractory inside the external wall of a vessel or pipe whilst the system is operational.

Research
Tracerco’s R&D team of physicists and engineers carried out an extensive technical review including many laboratory trials and computer simulations to determine the technology platform of most interest. The research led to an adaptation of the gamma scanning science for which Tracerco is known.

A process vessel was simulated using steel plates with a thickness similar to that of a typical process vessel, and ceramic sheets to mimic various thicknesses of refractory. Results demonstrated that the technology was capable of accurately measuring the ceramic layer thickness located behind the steel. Further tests were then carried out using various thicknesses of refractory provided by the oil company interested in the technology.

Figure 1 shows a typical response with varying amounts of ceramic sheet and refractory. The data demonstrated that there was only a small change in response for ceramic or refractory thickness of one inch or less. However, between 1in and 3.5in there was a significant change in response, allowing accurate measurements to be made. As in the case of thicknesses less than 1in, between 3.5in and 5in of refractory, the change in response was small, demonstrating that below 1in and above 3.5in refractory thickness depth accuracy was reduced. The research team believes that experience in field work will allow us to improve the accuracy in these two areas of thickness.
 
Prototype testing
A prototype was built and tested on three field trials to establish if it produced the required results in the very hot conditions experienced in a typical FCC unit, and to determine the best deployment approach. Figure 2 shows an artist’s concept of how it would be deployed.

The first test of the prototype was on a piece of equipment from a refinery where Tracerco had previously performed a number 
of tomography scans (using ThruVision) on the FCC riser to monitor the build-up of coke just above the feed nozzles. During a turnaround in 2013, a 2.5m long section of the riser was removed from just below the feed nozzles. This piece was given to Tracerco 
to further our research. The InDepth prototype was tested on this section of riser where results could be compared to visual inspection.

The riser was originally commissioned with 2in of refractory. It was observed that, over the life of the run, some of the residual feed and catalyst had attached to the walls and formed a layer of coke and catalyst between 1in to 6in in thickness. Trials of the InDepth prototype on this section of riser showed the presence of the refractory with a layer of approximately 2in. In addition, it was also established that instrument response could differentiate and measure the depth of coke up to around 2in when knowledge of coke density was factored into the response, adding another potentially important measurement facet to InDepth’s capability.

The prototype was then taken to the operating FCC unit from which the riser section had been removed and replaced. The prototype was tested on the operating FCC regenerator and riser. On the positive side, the prototype was found to be yielding results that were consistent with the expected thicknesses of refractory. On the negative side, it was discovered that the tool was overheating due to vessel radiant heat and a better approach to protect it from the heat was required. It was also determined that the winch system was somewhat cumbersome.

The necessary modifications were made to the prototype to better protect it from the heat and the winch system improved. New trials were scheduled at two other refineries.

Refineries 2 and 3 each had a decommissioned riser in the laydown yard from recent turnarounds. Most of the refractory in the decommissioned riser from refinery 2 was still present (see Figure 3).

In Refinery 3’s decommissioned riser, there were places where the refractory had fallen completely away from the steel wall. Other places had various thicknesses of refractory still attached. Results of InDepth measurements that matched the actual thicknesses of the refractory in the old riser are shown in Figure 4.

At Refinery 3, one line of data was collected on the new riser that had been operating for about six months. There was no expectation of damage to this refractory and it was expected to be 4in thick. The data collected indicated that there was 3.75-4in of refractory in the operating riser (see Figure 5). Since it was assumed that the contractor had installed the refractory to the specification required and inspectors had verified a proper installation, results from the InDepth tool were less an indication of damage to the refractory since start-up but more an indication of tool measurement accuracy of +/-0.25in.
 
InDepth
Collating all of the learning gained during field testing of the prototype, a final design was agreed for the InDepth tool and the winch from which it operates (see 
Figure 6).

The tool’s measurement capability is available for use on refractory lined vessels. The two site requirements needed to use the tool include an electrical supply available to power the winch and a permanent or temporary platform directly above the side of the vessel or pipe to be investigated.