Corrosion under insulation (CUI) is nowadays real threat to the on-stream consistency of many of today’s plants. This kind of corrosion can create failures in areas that are not usually of a major concern to an assessment program. The failures are frequently the consequence of corrosion resistant metals and not general wasting over a huge area. These failures can be disastrous in nature, or at least, have unfavourable monetary effect in terms of downtime and maintenance. It is considered that the piping code first published in June 1993, recognizes CUI as an exceptional concern. Typically, as happened with API 653 and the Clean Water Act, the API codes turn out to be an industry standard, and the rules demand that organizations uphold a program to meet that standard.
CUI is hard to find because of the insulation swathe that masks the intergranular corrosion until it is too delayed. It is costly to eliminate the insulation, mainly if asbestos is involved. There are a number of processes used today to examine corrosion under insulation. The main ones are profile radiography, ultrasonic spot readings, and also insulation exclusion. The other method now offered is real-time X-ray. Real-time x-ray has established to be a protected, fast, and effectual method for inspecting pipe in plant operations.
The crisis occurs in carbon steels and 300 sequence stainless steels. On the carbon steels, it evident as generalized or one can say localized wall failure. With the stainless pipes, it is frequently pitting and corrosion induces stress corrosion cracking (CISCC). Though malfunction can occur in a broad band of temperatures, corrosion becomes a significant concern in steel at temperatures between 32 F (0 C) and 300 F (149 C). Corrosion under insulation is caused by the access of water into the insulation, which entrap the water like a mop in contact with the metal facade. The water can come from rain water, outflow, deluge system water, wash water, or swearing from temperature cycling or low-temperature process such as refrigeration units.
Systems Susceptible to CUI
API 570 identifies to the following areas as vulnerable to CUI:
- Areas exposed to mist overspray from cooling water towers.
- Areas exposed to steam vent
- Areas exposed to surge systems.
- Areas subject to process spills, access of moisture, or acid vapours.
- CUI is mostly hostile where operating temperatures cause regular condensation and re-evaporation of atmospheric humidity.
- Carbon steel piping systems that on average operate in-service above 250 F (120 C) but are in an alternating service.
- Deadlegs and attachments that stick out from insulated piping and activate at a temperature dissimilar than the active line.
- Austenitic stainless steel piping systems that operates between 150 F and 400 F. These systems are at risk inspection to chloride pressure corrosion cracking.
- Vibrating piping systems that have a propensity to causes of corrosion damage to insulation jacketing offering a path for water entrance.
- Steam traced piping systems that may practice tracing leaks, particularly at the tubing fittings underneath the insulation
- Piping systems with worsen anti corrosion coating and/or wrappings.
- Locations were insulation plugs have been removed to permit thickness measurements on insulated piping should receive particular attention
After all that, mostly all the equipment will be shut down at some time or other. The length of time and the occurrence of the downtime exhausted at ambient temperature may well contribute to the quantity of corrosion under insulation that happens in the equipment. It would be an intimidating task to gather the resources needed to embark upon this widespread list of piping with the conventional quantitative risk assessment. This is where real-time X-ray provides a real benefit. Once the damaged areas are recognized, follow-up X-rays and ultrasonics can gauge the loss by exterior corrosion. These methods will not detect CISCC in stainless steels.
One of the major limitations of the system is considered as a C-arm. There are a couple of sizes of C-arms accessible these days. The manufacturer has had achievement in checking pipes up to 24 inches in width. These systems were not initially designed for the field but relatively for laboratory work. This limitation has been tackled and the systems accessible today are more vigorous. However, they still need a lot of care and consideration. There will always be some proportion of piping where real-time X-ray cannot be used. The major instance is the centre lines among firmly nested pipelines with slight clearance between the pipes. Finally, while the X-rays are low energy, they are still emission, and so the system must be used with tremendous concern.
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