The acid plant is attached to a smelter and cleans furnace off gas and produces sulphuric acid; two vessels were to be inspected and assessed the Final Absorption Tower (FAT) and the Drying Tower (DT), this case study will focus on the works carried out on the FAT. The FAT is a large process column (10 m diameter x 24 m high) of carbon and stainless steel construction divided into two chambers with the lower chamber lined with refractory bricks and the upper chamber filled with 300T of ceramic packing.
The vessel shell was in poor condition with extensive patch work and previous repairs, visual inspections showed evidence of shell deformation and bulging. A critical area of concern was the internal refractory lining integrity as this forms the main structural support of the vessel with the shell’s primary function being to containing the process fluids, however the refractory could not be directly assessed while the plant was operational. A knock on consequence of any unscheduled shutdown of the acid plant would be a shutdown of the associated smelter.
An extensive external inspection program was undertaken while the vessel was in service. Ultrasonic thickness testing was done over the entire shell with a maximum grid spacing of 150 mm, this required extensive scaffolding and was initially presented in a spreadsheet. Thermographic imaging of the shell was done yielding hundreds of images initially presented in a written report. Extensive 3D laser scanning of the vessel shell was done to determine nozzle locations, strake weld positions, repair patch locations and to analyse the shell deformation from a design baseline.
Discussions with the client engineers resulted in a plan to re-process all inspection data and create deliverables that would present the data on a common reference grid enabling correlation of artefacts from the different inspection methods. A vessel shell development was drafted with a corresponding reference grid being established, all data was then transposed onto the same grid system. Samples of the final mapped deliverables are presented in below.
Other deliverables produced included 3D point cloud data in various formats, and as-built 3D models of the Vessels. All deliverables were presented on the local plant coordinate system with recovery survey marks left on site to enable coordination of any subsequent measurements and to allow any precision construction set-out.
The project and deliverables were well received by the client and enabled their integrity engineers to easily correlate the different inspection results, improving the interpretation and comparison between the different data types. The project outcome aided the client in their failure analysis and planning for maintenance and further focused inspection. As a footnote some research yielded the term “Multimodal NDT Data Fusion” for describing the process of data manipulation and deliverable format.
Project Details Part II – Internal Vessel and Refractory Inspections
Approximately 18 months after the initial inspections and surveys Fyfe were contacted by the client to submit a proposal to conduct a 3D laser scan of the FAT’s internal chamber. Criteria for the proposal were to be extremely challenging and the time frame was extremely short with field work needing to commence just two weeks after the initial contact. The principal objective was to assess the condition of the refractory brick lining, providing input information to a vessel entry risk assessment and planning for any necessary repairs.
A survey methodology had to be devised, planned and estimated very quickly and immediately implemented. Factors influencing the survey approach included the short time framing requiring immediate availability, an extreme operating environment of 40 – 50 deg C, 99% sulphuric acid dripping from above, high acidic humidity and zero lighting. The measurements would have to be captured very quickly and data streamed from the instrument as it was not clear how long the instrument would survive the environment, an air cooled instrument was not a viable option. The instrument would have to be operated remotely as survey personnel would not be allowed within a 10m exclusion zone of the vessel’s open manway, which also meant the instrument could not be levelled before taking measurements. The combination of factors and restrictions lead to the selection of a Leica HDS6000 terrestrial 3D laser scanner as the most suitable instrument, availability of a locally sourced instrument lead to its procurement for the project.
Planning and logistics were critical to success and client also needed to review the results on site on the same day as the survey. A scanner support mount and bracket need to be designed and fabricated, the instrument had to be shipped to site and a survey team trained in its use. The scanner would be installed into the manway by the client fire and rescue team wearing full PPE and breathing apparatus, operation would be done remotely by Fyfe surveyors via an Ethernet wired connection.
Careful planning and preparations lead to a successful field campaign with data visualised during capture and quickly prepared and presented during a vessel risk assessment meeting on the afternoon of the survey. The entire refractory brick lining of the lower chamber could be inspected, visualised and measured during the onsite meeting. Due to active laser measurement, no ambient light was required to gather the measurements, also the instrument proved to be quite resilient to the conditions so extremely high resolution data was able to be captured. Upon inspection it was instantly evident there had been partial collapse of the refractory brick work, the accurate scan data enabled a detailed “virtual” inspection including checking and measurement of brick alignments.
Ultimately the scan data enabled the client to make decisions on safe vessel entry and necessary corrective actions for implementation during the shutdown. The data was also further analysed post shutdown against external data captured previously which provided further detail on shell deformations, refractory thickness and future areas to focus detailed inspections and potential repair works.