Materials Performance

JUN 2019

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

Issue link:

Contents of this Issue


Page 55 of 80

53 MATERIALS PERFORMANCE: VOL. 58, NO. 6 JUNE 2019 through visual and ultrasonic thickness checks using an Olympus 37DL Plus † . It was observed that the segments of piping were badly affected by corrosion under insula- tion (CUI) as well as thinning below the retirement thickness. Certain portions of pipe were punctured due to the combined effect of CUI and thinning. The segment of pipe was running hori- zontally, without any adjacent vertical line, bend, or closed loop, under the grating por- tion of the platform and such portions are not usually considered as "prone to CUI" during inspection reviews, unless they are small bore or exposed to rain. Undoubtedly, seeping of external moisture into the insula- tion resulted in its wetting, thereby causing CUI. An investigation was carried out to reveal the reason for moisture ingress inside the insulation. Quality control dossiers were consulted to check the construction records, inspection reports for insulation and cladding, as well as evidence of non- conformances that occurred in the past. No relevant evidence was found that could help to identify reasons behind the moisture ingress. Later, operator log books revealed that frequent water spills occurred on the platform right above that pipe segment; this might have resulted in the water ingress into the insulation. The corroded pipe spools were demol- ished and new spools were installed, fol- lowed by welding, nondestructive examina- tion (NDE), hydrotesting, and insulation works. The start-up milestone of the whole facility was delayed by approximately three days, resulting in extra repair costs and huge production loss. Results Evidently, failure of the pipe segment due to CUI at the time of plant start-up resulted from simultaneous overlooking of two different risk events. The first was ignoring the neighborhood conditions and relevant logbooks. Consideration of these could likely reveal the risk of dripping water from overhead platforms that led to mois- ture ingress and subsequent CUI on the pipe segment. The second ignored factor was criticality of this line in terms of safety as well as start-up of the whole refinery. The wall thickness of the pipe material was 23 mm (as per ASME B36.10 Classification Schedule 160), and wasn't readily available in stock, and even with local vendors. So, it took two more days for ordering and the delivery of the pipe material on site. This eventually led to the increased production downtime and costs resulting from subse- quent site works (comprising demolition and removal of corroded pipe spools, instal- lation and welding of new spools, nonde- structive examinations, hydrostatic pres- sure testing, painting, and re-insulation). Moreover, the platform segments right above the lines were secured from dripping water (if any) by replacing the grating with checkered plates. Discussion/Remarks The original design thickness of the pipe was ~23 mm and the expected corrosion rate was ~1.5 mm/y. On the other hand, higher deterioration rates due to unantici- pated CUI led to early retirement of the pipe segment. CUI monitoring is a crucial factor in the integrity of piping and equipment, as there could be a high level of uncertainty about the presence of CUI. CUI monitoring requires stripping off insulation or other NDE techniques (infrared thermography or neutron backscattering) that are gener- ally not viable due to high associated costs and longer logistical time. Recently, an- other NDE has been proposed that relies on a robotic crawler to inspect for CUI un- derneath insulated and cladded surfaces of various process equipment, like tanks and vessels. Still, there are challenges associ- ated with this technique in terms of opera- tion at uneven/damaged insulated sur- faces (with air gaps), compatibility of the robot with ser vice, feasibility of inspec- tions for the complex, inaccessible piping loops, as well as small bored piping, etc. 4 Moreover, cost and time for availability of this service is a point of concern for oper- ating process facilities. Finally, selection of inspection points for a CUI check by the crawler still requires the assessment, ex- pert's judgment, and decision making by the inspection personnel. A possible preventive solution would be relocation of pipe segments from the loca- tions that are prone to moisture ingress and trigger CUI. This would, in turn, require reviews from process engineering to inves- tigate the impacts of the line's re-routing on process conditions, including pressure, velocity, and flow rate, as well as feedback from operations personnel about the acces- sibility of valves and gauges for field opera- tors. Also, the review of engineering per - sonn el w ould b e cr ucial to ch eck th e interference of proposed new loops with the neighborhood as well as associated cost due to increased bends, field welds, and fab- rication works. If the rerouting or reloca- tions of the pipe to avoid CUI from dripping water are not viable due to undesirable pro- cess changes and consequent cost increase, then the use of checkered plates above the piping segment, as is done in the previously m ention ed case, w oul d b e on e vi able option, provided it does not jeopardize the integrity of the platform under the intended loads. During installation of cladding sheets, the orientation of closing edges (called laps) should be positioned in a way to ensure adequate accessibility for insulation appli- cators as well as inspectors, and minimize the probability of any moisture ingress from rain or other neighborhood conditions. Other than CUI, certain secondary damage mechanisms can be triggered due to the condition of insulation material. For the equipment and piping of Type 304L stain- less steel (UNS S30403) containing older calcium silicate insulations, a high proba- bility exists for chloride stress corrosion cracking. So, the selection of the right insu- lation with adherence to applicable insula- tion specs and recommended practices is the first step to counter CUI. 5 Inspection checklists must incorporate visual inspections of missing or damaged sealants, as the absence of sealant could be the gateway for potential moisture seeping into insulation via cladding. † Trade name.

Articles in this issue

Archives of this issue

view archives of Materials Performance - JUN 2019