ABSTRACT

This paper describes a methodology for assessing and addressing Corrosion Under Insulation (CUI) with respect to refinery pressure vessels. It has been of particular concern in managing the integrity of aging refinery pressure vessels where internal corrosion is not life limiting. Inspection methods and field experience with these methods are discussed. Options for corrosion mitigation are discussed in detail. Review of organic coatings, and metallic sprays such as thermal spray aluminum (TSA) is attempted. The paper provides a number of successful and unsucessful case histories of vessel CUI mitigation by field applied coatings. Success factors for coatings applications under field conditions are also highlighted.

INTRODUCTION & CORROSION MECHANISMS

CUI is a long term damage mechanism that occurs on the external surfaces of insulated pressure vessels operating in a temperature range from -4° to 150°C1. This is essentially an atmospheric corrosion mechanism resulting from water soaked insulation in contact with unprotected carbon and low alloy steels. As with any atmospheric corrosion mechanism, corrosion and metal loss rates are affected by a number of factors that involve the amount, duration, and frequency of wetness and other factors such as chloride or other chemical contamination, presence of industrial pollutants like SOx, operating temperature, and cyclic operation. External CUI corrosion rates can therefore vary by a significant amount, from nil in arid environments to upwards of 1 mm/y (40 mpy) in severe conditions. This paper will not address other damage mechanisms like external chloride stress corrosion cracking (ClSCC) on insulated austenitic stainless steels. See Reference 5 for more details on this topic.)

Original construction details can contribute to the likelihood of wetness and can also affect the rate of corrosion. Use of water absorbent (hygroscopic) insulation materials is also a key factor. When absorbent insulation becomes saturated, water is held in direct contact with the steel for very long periods. This not only increases the probability of corrosion, but also renders the insulation ineffective, allowing significant increase in energy loss. It is important that water which gets into the insulation is allowed to drain away from the equipment. Insulation design details should feature water shedding lap joints in jacketing, sealing of joints at protrusions by caulking (e.g. around vessel nozzles/flanged joints, or ladder and platform clips). Insulation support rings should be designed to allow water to drain down behind the insulation and not trap water.

The insidious nature of CUI is a result of the long term nature of the corrosion. It may take a significantly longer period of time for a thick-walled pressure vessel, as compared to thin-walled piping, for the pressure boundary to leak or become a significant risk from localized thinning. Furthermore, coating applications and field erection damage are often difficult to manage, let alone determine when actual corrosion truly begins. It has been our experience that external surfaces of pressure vessels in very low to non-corrosive service from the process side have often been given insufficient attention by refinery inspection groups.

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