Corrosion Under Insulation (CUI) is destructive, often becoming manifest at a time when it is least expected and very often without any forewarning.
CUI is usually prevalent within the operating temperature range from -12°C to 175°C. However, the threat generally increases for equipment which is operated discontinuously or at varying temperatures. In cases where the operating temperature fluctuates, condensation can form in the insulation material and the resulting aqueous accumulations may reach the external surfaces of the equipment thereby creating conditions which may then facilitate the onset of corrosion. In coastal and marine environments there is a potentially more elevated threat as the aqueous accumulations may also contain chlorides and sulphates which serve to increase the corrosivity of the aqueous phase, should these develop within the insulation systems that are in use.
Insulated Systems in ‘Cold Service’
The rate of corrosion in wet insulation systems is generally dependent on temperature. Increasing temperatures often leads to an enhancement of the rate of corrosion. This, however, does not mean that insulated systems which operate in ‘cold service’ are necessarily immune from CUI damage or are considered to constitute a significantly lower CUI threat.
In ‘cold service’ conditions, it is the difference in temperature between the cold medium and the warmer ambient environment that is a cause for concern. The salient issue is that this can result in a difference in vapor pressure which can then act on the insulation from the outside. The water vapor can then condense onto the insulation system and become absorbed by it. This can not only result in the deterioration of the insulating properties of the insulation system but it also increases the propensity for the onset of the corrosion of the underlying component.
The schematic illustration below is for an insulated pressure vessel operating at -45°C and serves to highlight the areas that are deemed to be the most ‘at risk’ from CUI.
The obvious caveat in the context of the CUI threat under ‘cold service’ conditions is that for CUI to occur, the conditions must be conducive to the existence of water in liquid form. In very cold conditions where ice formation occurs, this precludes the onset and progression of CUI damage. However, in operating conditions where temperature variations can lead to freeze-thaw cycles or in deadlegs where ambient warming can occur, CUI risk can be high.
The AIE Way
AIE recognizes that CUI is a threat to the integrity of insulated ‘cold service’ pressure systems equipment. However, we would also contend that the integrity threat and consequent risks are manageable nevertheless. The successful management of CUI risk warrants the development of an asset-specific risk-based strategy and one which focuses on maintaining CUI risks to as low as reasonably practicable (ALARP) levels. Using our in-house Veracity software modules and our wealth of corrosion and integrity experience, we work closely with clients to develop bespoke CUI management services which:
- Identify the most ‘at risk’ areas which may suffer future CUI damage, by making use of the relevant process information and predictive algorithms for CUI
- Provide robust insulated equipment inspection schedules where inspection extent and frequency shall be based on the assessed CUI risk status
- Utilise the latest inspection technology to evaluate facilities in the most efficient manner
- Evaluate CUI damage and perform detailed fitness for service analysis which minimises the impact of plant downtime and repair costs
- Provide guidance on optimal insulation design and installation procedures
- Provide consultancy services and training to optimise operating regimes which minimise global CUI risk
AIE works closely with its clients to ensure that areas of insulation damage which may be observed during plant inspections are targeted for remediation. Such an approach shall serve to ensure that any such damage will likely be repaired before any significant and costly CUI damage occurs. In addition, in instances where CUI has become manifest, AIE also works to ensure that the root cause of CUI damage is correctly identified and that the necessary steps for its future avoidance are planned and implemented.
AIE’s processes for managing CUI are centered on iterative learning and continual improvement, thereby driving CUI risk downwards and increasing plant safety and reliability.