CUI – What Is Piping: All about Piping Engineering

What is Corrosion Under Insulation (CUI)? The Corrosion Under Insulation (CUI) war has been fought for many years in the chemical and petrochemical industry. Corrosion Under Insulation is severe localized corrosion damage that is caused by the moisture (intruding water) present on the external surface of insulated equipment. The corrosion processes are well understood but yet corrosion under insulation or CUI often goes undetected until the damage is significant which may lead to catastrophic failures, e.g. on equipment operating under high pressure. The industries which are highly impacted by CUI are RefineriesChemical industriesPetrochemical industriesOffshore industries andMarine and Maritime industries Materials prone to CUI Corrosion Under Insulation may occur on: Carbon steel (general/localized corrosion)Stainless steel (localized corrosion/stress corrosion cracking) Effect of Corrosion Under Insulation CUI (Corrosion under insulation) cost studies have shown that: 40 to 60 % of pipe maintenance costs are caused by CUINDE/inspection costs with a high confidence level for detecting CUI are equal to or exceed field painting costsApproximately 10 % of the total maintenance budget is spent repairing damage from Corrosion under insulation or CUl In case, corrosion under insulation is not detected in proper time, it can lead to failures like an explosion, leaks, downtime, equipment failure, etc. Corrosion Under Insulation Temperature Range General temperature ranges in which risk of Corrosion Under Insulation is prevalent is Carbon steel: -4 °C to + 175 °C: Risk of CUI (highest risk area: +60 °C to +120 °C)Stainless steel: +50 °C to +175 °CInsulation of process equipment is normally implemented when the outer steel temperature exceeds 50 °C (due to the risk of work-related injuries as well as heat loss). Mechanism of Corrosion Under Insulation CUI involves the requirement of the following three elements: Availability of oxygen. High temperature. The concentration of dissolved species. In general, with an increase in temperature corrosion rate reduces as the amount of dissolved oxygen decreases. But when it is covered by insulation, the moisture makes it a closed system and retains the dissolved oxygen. In some cases, chlorides and acids may present in the insulation which greatly promotes corrosion of the underlying surface in the presence of moisture. Factors affecting Corrosion Under Insulation The major factors that affect the corrosion process under insulation (CUI) damage are: Frequency and duration of the moisture exposureService temperature.Type of insulation and protective barrier condition.Cyclic operation modes.Corrosivity of the environmentType of climate where the system is installed.Proximity to humidity-causing equipment like cooling towers, etc Corrosion under Insulation of Carbon Steel Insulated carbon steel corrodes due to: Infiltration of water under insulation (rain, process liquids, firewater, etc.)Condensation waterIngress of external contaminants The insulation material may also contribute to CUI: Creates a crevice for water retentionMay absorb waterMay leach contaminants that increase the corrosion rate Temperature and oxygen - corrosion rate for CUI Fig. 1: Curve Showing Corrosion rate of Steel with respect to temperature. CUI of carbon steel Fig. 2: CUI of Carbon Steel Corrosion under insulation of Stainless Steel Stainless steel (austenitic or duplex) corrodes due to: Chlorides (or other halides) in presence of water are transported to the hot surfaceThe chlorides are concentrated by evaporation of the waterThe chloride concentration reaches a critical level causing stress corrosion cracking (SCC)(The critical chloride concentration depends on Temperature and alloy type) The insulation material may also contribute to CUI in the following ways:Creates a crevice for water retentionMay absorb waterMay leach contaminants (e.g. chlorides) that cause ESCC Fig. 3: Industry Accepted Chloride Stress Corrosion Cracking temperature Limits How is CUI avoided? By Avoiding that moisture entering the insulation material and the steel surface; corrosion under insulation can be prevented. These can be achieved by Correct selection and design of the insulation materialGood design of the item to be insulated – e.g. it is difficult to insulate around flanges, taps, flowmeters, supports, etc.The cover above the insulated item (against rainfall, etc). Application of corrosion protection On average, 60% of all insulation in service for more than 10 years will contain corrosion-inducing moisture; which will result in corrosion under insulation. Carbon steel: The following steps are followed to prevent corrosion under insulation for carbon steel piping/equipment materials. Organic coatingsThermal-sprayed aluminum (TSA)Personnel protecting cages Stainless steel (austenitic or duplex) To prevent the CUI of stainless steel materials, the below-mentioned steps are taken: Organic coatingsThermal-sprayed aluminum (TSA)Al-foil wrapping (prevents ESCC and pitting)Personnel protecting cages. Organic coatings on carbon steel Experience shows that organic protective coatings on piping in CUI service range from 5-13 yearsOnce the protective life is reached, field re-painting is necessary (or periodic NDE in order to monitor CUI).Coatings may be damaged during handling and installation - leaving unprotected areas.Important parameters for coatings used for CUI prevention: - High permeability resistance (barrier protection) - High flexibility (for cyclic temperature service) Methods of CUI protection The following image (Fig. 4) shows the methods to prevent various types of corrosion under insulation. Fig. 4: Methods of protection Thermal Sprayed Aluminium (TSA) to prevent corrosion under insulation TSA may provide long-term corrosion protection at significant life cycle cost savings, however at higher initial costs.TSA provides atmospheric corrosion protection for more than 40 years.Has been recorded to provide 25-30 years of maintenance and inspection-free CUI service.Application by electric arc or flame spray.It requires high-quality surface preparation and strict application control. Advantages of TSA in CUI prevention: The coating is robustMinor pores/damages are protected by the surrounding coatingNo under-rustingUnlimited construction sizesNo risk of deformations (”cold process”) Disadvantages of TSA: No treatment in hollow spaces, inside pipes, etc.The treatment prescribes a certain structural designSharp edges must be chamfered/roundedOften large variations in layer thicknessQuality controlPrice Corrosion Under Insulation Prevention Strategy Data from operating facilities shows that water-free insulation is not practical in aging facilities. Thus a CUI prevention strategy is necessary in order to provide long-term and reliable prevention of CUI: A choice between strategies: Organic coatings on carbon steel; Ongoing re-painting; NDE (does not prevent CUI but predicts remaining life).TSA on Carbon Steel Stainless steel (+ TSA or Al foil wrapping) Initial, maintenance, and inspection costs need to be assessed for each choice in order to give the lowest total lifetime cost. Corrosion Under Insulation Inspection Various codes and standards provide guidelines for corrosion under insulation inspection methods as listed below: API 510 covers the CUI inspection in clause 5.5.6 of the standard that says inspection for CUI must be considered for externally insulated vessels and those that are in intermittent service.API 570, Piping Inspection Code provides guidance to determine the piping systems susceptible to CUI in clause 5.2.1 and clause 5.4.2.API RP 574, in clause 6.3.3 gives a reference to corrosion under insulation.API RP 583 provides a detailed overview of CUI.ASTM STP 880, provides information on corrosion problems on thermally insulated plant equipment and piping components.NACE SP0198 provides detailed guidance on Control of Corrosion Under Thermal Insulation and Fireproofing Materials. Corrosion under insulation is detected by any of the following inspection methods: Visual InspectionX-Rays or RadiographyReal-Time RadiographyComputed RadiographyDigital Detector ArrayUltrasonic Thickness MeasurementPulsed Eddy Current Susceptible Locations for Corrosion under insulation The areas that are more susceptible to CUI are: Damaged insulationAreas that tend to trap moistureEquipment nozzles and manways.Pressure safety valve flangesAreas exposed to steam vents, deluge systems, process spills, acid vapors.Areas where steam tracing is leakingSystems operating intermittently above 120 Deg C Few more useful Resources for you… An Article on Forms of Corrosion Corrosion under insulation: A Presentation Corrosion Protection for Offshore Pipelines Corrosion Monitoring Techniques & Surveys: A short Presentation Guide for Coating Selection for External Bolting to Reduce Corrosion Application of Anti-Corrosive Linings in Oil and Gas Industry Anti-Corrosive Composites for Oil and Gas Industry Piping Materials Basics Piping Stress Analysis Basics Piping Design and Layout Basics Few Jobs for you…