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Corrosion Under Insulation (CUI)

Corrosion Under Insulation (CUI)

Corrosion under insulation (CUI) is a severe form of localized external corrosion that occurs in carbon and low alloy steel equipment that has been insulated. This form of corrosion occurs when water is absorbed by or collected in the insulation. The equipment begins to corrode as it is exposed to water and oxygen. CUI is common in refineries and process plants that typically operate equipment at high temperatures.

In the simplest terms, corrosion under insulation (CUI) is any corrosion that occurs due to a moisture buildup on the external surface of insulated equipment. The buildup can be caused by one of multiple factors that are detailed below. The corrosion itself is most commonly galvanic, chloride, acidic, or alkaline corrosion. If undetected, the results of CUI can lead to the shutdown of a processing unit or an entire facility, and in rare cases, it may lead to a process safety incident.

Corrosion Under Insulation Processes
CUI is a blanket term that refers to several different types of corrosion processes. According to the National Board of Boiler and Pressure Vessel Inspectors, the main CUI processes are:

Generally, results from wet insulation with an electrolyte or salt, which allows a current to flow between different metals (e.g., the insulated metal surface and the outer jacket). The extent and severity of the attack on the less noble metal depend on the difference in potential between the two metals and their relative areas.

Alkaline / Acidic
Results from moisture and an alkali or acid in certain fibrous or granular insulations. Where service temperatures exceed 121ºC (250ºF), most of the water evaporates, condenses at the edge of the insulation, and then dissolves the alkaline or acidic chemicals there, which results in corroding the jacket.

Chloride Occurs on 300-series austenitic stainless steel surfaces when the insulation contains leachable chlorides and moisture at temperatures above 60ºC (140ºF). A concentration of chloride ions usually results from the evaporation of rain, water used in fighting fires, or process water. Velosi Help Client identify CUI through:
  • Pulsed eddy current (PEC)
  • Guided waves (GW)
  • Real-time digital radiography
  • Thermography
  • Visual examination

  • VELOSI Expert and Unique Solutions to CUI
    PEC: PEC equipment is based on electromagnetism and helps effectively determine the average wall thickness (AWT) of low-alloyed carbon steel components through their insulation, enabling the detection and localisation of CUI. PEC is successfully used in the industry for detection and semi-quantitative sizing of CUI. The main advantage of PEC is its ability to ascertain general wall thinning in insulated objects without removing the insulation and without any object surface preparation.

    Guided waves measurement is one of the key complementary technologies for the assessment of corrosion under insulation for long lengths of pipe. Typically, it uses low-frequency guided ultrasonic waves that propagate along the pipe wall and can detect external or internal corrosion. The technology requires insulation to be removed, however, only in a very small circumferential location. This part is used to attach the ring with the ultrasonic sensors onto the pipe. This way, long and difficult to access pipes can be examined from a single location with minimal preparation and without further removing the insulation.
    A test ranges of 50m (25m in each direction) or more is commonly obtained from a single transducer position. Our guided waves technology is often used to assess CUI on sleeved road crossings, buried pipe, wall penetrations, pipe racks, corrosion under supports, offshore risers and caissons, previously unrecorded weld locations, subsea lines and similar.

    Real-Time Digitial Radiography
    This technology makes use of a radiation source and an X-ray detector (often a linear array of radiation detectors). These are positioned in such a way that an on-stream X-ray inspection can be conducted, resulting in real-time imaging of potential anomalies under the insulation. With this technology, both internal and external corrosion defects on stretches of pipe can be detected while the pipe remains in service and without removing any insulation. As the unit and/or scanner are positioned onto the pipe, X-ray data is acquired and displayed on the monitor in real-time. Straight piping segments as well as locations with bends and elbows can be inspected using this technology. The system is sensitive enough to detect corrosion defects as small as 0.25 inches in diameter and 0.05 inch deep.

    Thermography and thermal imaging has been successfully used in the petrochemical industry for inspecting and monitoring system performance as well as the integrity of insulated pipelines and vessels for many years. Thermal imaging easily shows temperatures of materials and objects, and it is an effective method for the detection of energy waste. It, therefore, contributes to determining the insulation quality condition. Thermography can help locate water penetration and even leakages and any anomalies in insulation performance.

    Visual Examination
    Visual examination is an effective method for locating CUI and it helps identify potential CUI hazards resulting from damaged weatherproofing, cladding and/or insulation layers. Weatherproofing protection in the form of cladding dents, sealing, junctions and overlapping are potential penetration points for rain water; therefore this barrier needs to be monitored for effectiveness.
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