Many mining and mineral processing facilities extensively utilise pressure vessels in various applications. Rapallo recently supported a client to successfully investigate, temporarily repair then replace the affected flange and fully reinstate the nozzle integrity to better than the original condition. The site’s production, operational and statutory obligations were all met, without incurring any lost revenue.
Background
The vessel had been in service for a relatively short period of time when the client observed a small leak near the manway nozzle. The client recognised their Classified Plant obligations, notifying the necessary authorities, and isolating the vessel. They then emptied the contents, commenced an internal investigation, and requested assistance from Rapallo.
Based on initial photographs, vessel data and documentation, preparations were made to source appropriate resources to enable a thorough failure investigation and develop a repair strategy. Rapallo provided the expertise to accurately identify the cause of the failure, and located appropriate welding consumables for the task, whilst the client sought and mobilised a suitably qualified welder.
Once a detailed physical and design examination was completed with support from Rapallo off-site engineering and technical expertise, a detailed repair sequence was developed and refined on multiple occasions in accordance with the emerging evidence acquired during the assessments.
The initial condition is presented below.
As presented in this photo the corrosion of the fillet weld is the symptom that resulted in the ultimate vessel leakage. The extent of the issue and cause required a sequence of further tests and assessments.
This revealed that the corrosive product that had been flowing through the void and had caused considerable damage to the carbon steel backing flange.
Interim Solution
The vessels service duty expected lifespan and the operational limitations were all considered to ensure the best possible outcome for the client. Based on all the gathered evidence, further consultation, and discussion a final repair plan was agreed, albeit a temporal repair.
Different weld consumables were selected to firstly close any breaches as best possible, and secondly complete a weld overlay to the repaired areas only.
A representative photo of the completed temporary repair is below, prior to final NDT.
Following weld inspection including dye penetrant testing and in-service inspection to AS3788, the vessel was recommissioned while a permanent solution was developed.
Determination of the root cause of this failure was imperative to ensuring the vessel service life will be achieved.
Design Change/Permanent Repair
The original design of the manway nozzle flange and the sealing ring was inadequately executed on this replacement vessel during manufacture leading up to the in-service failure. Rapallo recommended simplifying the nozzle design and materials to match that of the main vessel and fabricating the replacement piece accordingly.
Rapallo developed multiple options for the design and consulted Rapallo’s specialist engineer for validation to select the most appropriate nozzle arrangement for this vessel. The issues, options and solutions were presented to the client. The preferred arrangement as agreed by all stakeholders, an entire manway nozzle flange with stub was then dimensionally verified. Quality commitments during the local manufacturing and independent Non-Destructive Testing expectations were clearly communicated.
Site installation was planned and successfully executed safely and within the expected timeframe. The goal of effecting a permanent repair to the vessel was successfully met with all engineering, inspection, and statutory obligations being met, without disrupting the site processing output. A photo and description of the final repair are presented below.
Rapallo successfully recorded all the activities logically to demonstrate compliance with Mines Safety and Inspection Regulations 1995Part 6, Division 3, – Classified plant, and the required Australian Standards that included; AS 1210, 3788, 4037, 3992 1796, 2062.
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