Corrosion under insulation (CUI) is a type of corrosion that occurs due to a moisture build up on the external surface of insulated equipment. The buildup can be caused by one of multiple factors environmental and application based factors. 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 process unit or an entire facility and in rare case it may lead to a process safety incident.
The first and simplest method of inspection is, of course, visual inspection. It involves removing the insulation, checking the surface condition of the pipe, and replacing the insulation. It’s also the most expensive and time-consuming method. The logistics of insulation removal occasionally involves asbestos and attendant complications. Process-related problems may also occur if the insulation is removed while the piping is in service. Nowadays, visual inspections are usually performed with portable visual scanners, which allows for precise, traceable sizing of surface corrosion at the outer diameter.
X-Rays or Radiography Visual Inspection:
Visual Inspection is the oldest and most basic method of inspection. It is the process of looking over a piece of equipment using the naked eye to look for flaws. It requires no equipment except the naked eye of a trained inspector.
Visual inspection can be used for internal and external surface inspection of a variety of equipment types, including storage tanks, pressure vessels, piping, and other equipment.
Visual inspection is simple and less technologically advanced compared to other methods. Despite this, it still has several advantages over more high-tech methods. Compared to other methods, it is far more cost effective. This is because there is no equipment that is required to perform it. For similar reasons it also one of the easiest inspection techniques to perform. It is also one of the most reliable techniques. A well-trained inspector can detect most signs of damage.
Visual inspection is used in a variety of industries, including:
- Welding inspection
- API tank, pressure vessel and process piping inspections
- Quality assurance during manufacture or repair
- Coating inspection
- Lift/aerial equipment inspection
- On-site surveillance
X-rays are used in a variety of ways in detecting CUI, such as real-time radiography (RTR), computed radiography (CR), and digital detector arrays (DDA; flat-panel detectors).
Ultrasonic Thickness Measurement:
This method (referred to as UT) is the non-destructive measurement of the local thickness of a solid element based on the ultrasound wave’s time of flight. This type of measurement is usually performed with an ultrasonic thickness gauge. This is an effective method, but limited to small areas. Just as with partial removal of insulation, it’s expensive to cut plugs out of the insulation and impractical to cut enough holes to get reliable results. The inspection plugs can compromise the integrity of the insulation and add to the CUI problem, if they are not properly recovered..
Pulsed Eddy Current
PEC is an electromagnetic method to determine the wall thickness of electrical conductors. The PEC probe is placed on an insulated pipe or vessel. A magnetic field is created by an electrical current in the transmitting coil of the probe. This field penetrates through the weather sheeting and magnetizes the pipe wall. Next, the electrical current in the transmission coil is switched off, causing a sudden drop in the magnetic field. As a result of electromagnetic induction, eddy currents are generated in the pipe wall. The eddy currents diffuse inwards and decrease in strength. The decrease in eddy currents is monitored by the PEC probe and is used to determine the wall thickness. The thicker the wall, the longer it takes for the eddy currents to decay to zero. Applied operating principles of PEC varies from system to system. Generally, to obtain a quantitative wall thickness reading, PEC systems use algorithms that relate the diffusive behavior in time to the material properties and the wall thickness.
PEC averages wall thickness over a relatively large foot print. As a result, PEC is suitable for general wall loss, but isolated pitting defects cannot be detected, making it a screening tool only.