The present corrosion under insulation detection methods are:
Profile Radiography
Exposures are made of a small section of the pipe wall. A comparator block such as a Ricki T is used to calculate the remaining wall thickness of the pipe. The exposure source is usually Iridium 192, with Cobal 60 used for the pipes of heavier wall. (See Figure 1).
Profile radiography is an effective evaluation method, but becomes technically challenging in piping systems over 10 inches (25.4 cm) in diameter and only offers the limited luxury of verifying relatively small areas. This technique will not detect CISCC in stainless steels. In addition, readiation safety can be a real concern. Nobody can work within the area while the inspection is under way, this can result in downtime and manpower scheduling conflicts.
Ultrasonic Thickness Measurement
This is an effective method, but limited to a small area (Figure 2). It is expensive to cut the insulation holes and cover the holes with caps or covers. It is not practical to cut enough holes to get a reliable result. The inspection holes cut in the insulation may compromise the integrity of the insulation and add to the corrosion under insulation problem, if they are not recovered carefully. This technique will not detect CISCC in stainless steels.
Insulation Removal
The most effective method is to remove the insulation, check the surface condition of the pipe, and replace the insulation. This approach will detect CISCC in stainless steels; may require eddy current or liquid dye penetrant inspection. This is also the most expensive method in terms of cost and time lost. The logistics on insulation removal will probably involve asbestos and its attendant complications. Process related problems may occur, if the insulation is removed while the piping is in service.
Infrared
In the right conditions, infrared can be used to detect damp spots in the insulation, because there is usually a detectable temperature difference between the dry insulation and the wet insulation. Corrosion is a distinct possibility in the areas beneath the wet insulation.
Neutron Backscatter
This system is designed to detect wet insulation on pipes and vessels. A radioactive source emits high energy neutrons into the unsulation. If there is moisture in the insulation the hydrogen nuclei attenuate the energy of the neutrons. The instrument's gauge detector is only sensitive to low energy neutrons. The count displayed to the inspector is proportional to the amount of water in the insulation. Low counts per time period indicate low moisture presence.
Real-Time Radiography
Fluroscopy provides a clear view of the pipes outside diameter trhough the insulation, producing a silhouette of the pipe outside diameter (OD) on a TV-type monitor that is viewed during the inspection. No film is used or developed. The real-time device has a source and image intensifier/detector connected to a C-arm (see Figure 3). There are two major categories of RTR devices on the market today; one using a X-ray source and one using a radioactive source. Each has its own advantages and disadvantages, however the X-ray systems deliver far better resolution than the isotope type equipment.
The X-ray digital fluroscopy equipment operates at a maximum of 75 KV, a low level radiation source, but the voltage is adjustable to obtain the clearest image. this allows for safe operation without disruption in operating units or even confined spaces. The radiation does not penetrate the pipe wall as more powerful gamma-ray or x-ray would, instead it penetrates the insulation and images the profile of the pipe's outside wall. The radiation is generated electrically so the instrument is perfectly safe when the power is off, whereas the Iridium 192 used in wall shots produces gamma-radiation constantly, even when shielded within the camera. Therefore the gamma-ray camera always needs careful supervision and control during all operations, including transportation and shipping. The systems with the electrically generated X-rays are far more convenient for shipping.
The new systems come with a heads-up, video display. The helmet-mounted, visor-type video-display fress the system operator's hands so that he can maneuver the C-arm, while keeping the image before the operator at all times. The heads-up display also improves interpretation by shielding the screen from the sun. The video images can be printed on site using a video printer or recorded using a standard VCR for evaluation later.
Performing the Inspection
Using the sorting criteria listed above it is possible to prioritize a list of piping for inspection that is manageable in a reasonable time fram. The CUI inspection crew then inspects the pipes iso by iso.
The "C" shaped arm is the actual device used to scan the pipe. A cathode ray tube on one side generates the x-rays, shooting them across to the receiver on the other side. The operator manipulates the arm around the pipe, guiding it by the black and white heads up display on his hard-hat. A typical scan will go up the pipe while moving the arm about 45 degrees to both sides of the track. The C-arm is then rotated 180 degrees and the pipe is scanned downward in a similar fashion. After rotating 90 degrees the up and down process is repeated.
Results
To the untrained eye, the image in the screen would appear to indicate very serious corrosion. However, what is being imaged is the exfoliation of the rust (See Figures 4 and 5.) Performing the inspection in this manner the inspector can inspect a considerable amount of pipe in a short time.
Limitations
One of the main limitations of the system is the C arm. There are a couple of sizes of C-arms available. The manufacturer has had success in checking pipes up to 24 inches in diameter. These systems were not originally designed for the field but rather for laboratory work. This limitation has been addressed and the systems available today are more robust. However, they still require a lot of care and attention. There will always be some percentage of piping where real-time X-ray can not be used. The prime example is the center lines among tightly nested pipelines with little clearance between the pipes. Finally, while the X-rays are low energy, they are still radiation, and so the system must be used with extreme caution.
Source:https://inspectioneering.com/content/1996-11-01/116/inspection-techniques-for-dete
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