Convection zone monitoring using remote visual inspection

Transcription

1 Convection zone monitoring using remote visual inspection Savio REBELLO, Paresh GOSWAMI More info about this article: Abstract Reliance Industries Limited, Reliance Corporate Park, Ghansoli, Navi Mumbai Reliance Industries Limited, Village Mora, Surat-Hazira road, Hazira, Surat Paresh. Cracker furnaces in ethylene cracker units present a special challenge to the in-service inspection personnel due to the high operating temperatures, in excess of 1000 o C, and high alloy metallurgy (35% chromium 45% nickel) of the radiant tubes. Fortunately, these tubes are available for periodic inspection, non-destructive examination and testing during the furnace decoke outages. On the other hand, the tubes in different banks of the convection zone are very compactly arranged with no accessibility for entry and inspection. Only the top-most and bottom-most tubes are available for visual and nondestructive assessment. The convection zones in cracker furnaces are also much bigger, in order to recover the maximum heat from the high temperature flue gases. Revamp of the radiant and convection zone of the cracker furnaces at the site was planned to enhance capacity and change the feedstock. One of the jobs in the convection zone was replacement of the existing tube supports, based on licensor recommendations. The job would involve complete removal and re-instatement of the tubes and refractory in these banks, significantly affected the overall schedule of the major turnaround. This paper details how remote visual inspection, supported by in-house design review, was used to establish the tube support condition before-hand. Replacement of the tube supports was cancelled. Inspection and metallography assessment during the turnaround confirmed that the supports were in good condition Keywords Convection zone, tube supports, remote visual inspection, creep. Introduction Cracker furnaces are at the heart of a petrochemical unit, as they crack the incoming liquid or gaseous feedstock into its various constituents. These constituents are in turn the feedstock for the downstream petro-chemical and polymer units. Any unplanned outage of the cracker furnaces directly impacts the downstream units. Hence it is imperative to ensure the healthy condition of the cracker furnaces through periodic inspection, non-destructive examination and testing. Convection zones of these cracker furnaces have always presented special challenges due to the compact arrangement of different banks consisting of a total of more than tube rows and lack of accessibility for physical inspection. Radiant zones of the cracker furnace were to be taken up for modification of coil configuration due to a change in feedstock from liquid to gas. Upgradation of tube metallurgy of two of the convection banks was recommended by the process licensor due to change in fluid composition and expected increase in metal temperatures. 1

2 Replacement of the intermediate tube supports for these banks was also recommended based on past temperature trends and completed service life of more than 20 years. Refer to table 1 below for the intermediate tube support materials and flue gas temperatures. Design metal temperatures were not mentioned in the original datasheet, hence they are not included. A combination of in-house design review and remote visual inspection during the furnace decoking outage as well as visual inspection and in-situ metallography during the shutdown was used to assess the condition of the tube supports. It was found that the tube support condition did not warrant replacement. This helped to significantly reduce the time for furnace modification and the furnaces could be put back in service much earlier. 1.0 In-house Design review The original design calculations for the convection zone tube sheets / tube supports were not available. These furnaces use natural gas, which is a clean fuel. Expected oxidation rates were also very low considering the tube sheet metallurgy and metal temperatures. The only expected damage mechanism affecting these tube sheets was creep. Hence an in-house design check based on rupture stress was done using API 530 2] principles. Generally an increase in metal temperature by 12 o C or an increase in stress by 15% reduces the furnace component life by half 1] ; the reverse is also true. It was found that although the flue gas temperatures were higher than original design, the load and hence, the stress on the tube sheets was very low compared to the rupture allowable stress. Hence creep damage was not expected. Further one of the known reasons for the flue gas temperatures higher than design was the poor heat transfer due to refractory dust and other debris collected over the convection zone over the years. Cleaning of the convection zone was planned during the furnace modification outage. Hence the flue gas temperatures were expected to reduce after the shutdown. Table 1: Temperatures and tube sheet materials of the convection banks under review. 2.0 Remote Visual Inspection It was necessary to confirm the results of the design review by physical inspection of the tube supports as a pre-shutdown activity, so that there were no unexpected surprises during the shutdown. Cracker furnace radiant coils typically require periodic decoking to 2

3 remove the coke deposits formed inside the tubes. The short decoke outage can also be used for any required quick maintenance / inspection. The intermediate tube sheets cannot be accessed by man entry unless a number of tubes are removed and the refractory-lined casing plates are cut. Further man entry would also require positive isolation / spading of the furnace based on safety requirements. This was not feasible in the short decoke outages. Hence it was decided to carry out remote visual inspection (RVI), using a specialized crawler type video camera. 2.1 Inspection procedure A pan and tilt type colour camera with 10X optical zoom, light sensibility of 1 lux, and suitable for insertion through 4 pipe was mobilized through a local NDT vendor. This camera was inserted through the 4 cleaning nozzles provided between each bank of the convection zone on both side walls. Refer figure 1 below for the video camera used. There were four such nozzles available at each elevation on each side wall. Thus the top and bottom of each of the intermediate tube sheets could be inspected. Such inspection was done during decoke outages for two of the furnaces. 2.2 Inspection results The video camera inspection did not indicate any major cracks or deformation in any of the tube sheets, including at the bottom side, which sees higher flue gas temperature. Minor damage due to oxidation was observed at the thinner edges of the tube sheets, away from the main load bearing area. Convection zone refractory (castable) in this area was also found in good condition. This video inspection vindicated the results of the design review. The replacement of the tube sheets was not necessary. Refer to figure 2 below for sample photographs from the video camera inspection. Figure 1: Photograph of the camera head used for inspection (crawler arrangement not shown) 3.0 Inspection during the furnace modification in shutdown 3.1 Visual inspection after tube removal Removal and replacement of tubes in two convection banks was carried out in shutdown, as mentioned earlier. The removal of two banks of convection tubes provided a very good opportunity to closely inspect all the intermediate tube sheets in these banks. The RVI carried out non-shutdown had 3

4 indicated the condition of the top and bottom of each tube sheet. After tube removal, the entire height and width of each tube sheet, including each tube hole and ligament area, was closely inspected. This visual inspection in shutdown supported the earlier findings of the design review and camera inspection. No significant deformation or cracking was observed on any of the tube sheets. Refer to figure 3 below for sample photographs from the complete tube-sheet inspection during the shutdown. Figure 2: Sample photographs taken during actual inspection Left: Top; Right: Bottom. Figure 3: Photographs of the complete tube sheets after tube removal in shutdown; photograph on the right also shows the locations where replica was taken for in-situ metallography 3.2 In-situ metallography In-situ metallography on the intermediate tube sheets was another inspection activity carried out during the shutdown after removal of the tubes. Remote and close visual inspection had shown no visible cracks, which would have indicated advanced creep damage. However inspection of the replicas taken during in-situ metallography confirmed that the microstructure of the tube sheets had not significantly degraded with time. Initial stages of creep such as voids and micro-fissures were also absent. Refer to figure 4 below, which shows the extract from the metallography report at one of the locations. 4

5 Figure 4: Metallography location (left) on CS (A216 WCB) tube sheet; Microstructure at 100X (right) 4.0 Summary of inspection and NDE done The pre-shutdown design review and remote visual inspection (RVI) using a video camera indicated that the intermediate tube sheets were in good condition and replacement was not necessary. The close visual inspection and in-situ metallography on the tube sheets during the shutdown opportunity confirmed the pre-shutdown findings. Thus the replacement of the intermediate tube sheets, which was a potential show-stopper and critical path activity, could be cancelled without impacting the furnace integrity. We intend to repeat similar remote visual inspection every two years, so that any initiation of damage can be detected well in advance. 5.0 Conclusion Convection zones of cracker furnaces are difficult to physically inspect by man entry due to the closely packed tube rows. However as demonstrated in this paper, the recent availability of the latest video cameras and other advanced NDE techniques such as drone-based thermography and intelligent pigging allow much better condition assessment of this previously un-inspectable area of the furnaces. 6.0 Acknowledgement The authors acknowledge the support received from the management of Reliance Industries Limited for the presentation of this paper. 7.0 References o 1. API RP 571, Damage mechanisms affected fixed equipment in the refining industry, First Edition (December 2014), p o 2. API 530, Calculation of heater tube thickness in petroleum refineries, Sixth Edition (September 2008). 5