Detection of Corrosion Under Insulation and Blockages on Piping System Using Profiler System
CUI (Corrosion Under Insulation) has always been a challenge for plant operators, quality assurance/reliability engineers and equipment owners. It is hard to identify the problem until it has become an emergency situation, often leading to unit shut downs or even the whole facility shut down for emergency repairs.
As per industry statistics, next to leakages in flanged joints, the highest incidents of piping failure in process industry is caused due to corrosion in pipe, especially under insulation.
HOW CUI IS CAUSED:
An insulated piece of equipment can have trapped moisture by two means: moisture can become trapped due to cyclic equipment and condensation forming and being trapped under insulation. The second and more common reason is rain/ snow. Rainwater or water from melting snow will eventually enter weak points in the system and pour onto the surface under insulation. The difference when compared to a stack is that the water gets trapped because of the insulation and is not allowed to escape.
Thermal shock can also be a significant cause of CUI. Thermal shock is categorized by a dramatic rise or fall in the temperature of the equipment. Thermal shock may occur when a unit is turned on or off, during the normal cyclic conditions the unit may exhibit, or when the steel is exposed to water that has penetrated the jacketing.
CHALLENGES IN INSPECTION
CUI is difficult to find because the insulation covers the corrosion problem until it is too late. It is expensive to remove the insulation, inspect and then reinstate the insulation after inspection. Inspection of the covered areas without removing the coverings reduces the cost of carrying out an inspection. Therefore the development of non-destructive testing methods to detect corrosion in the above situations is therefore a major benefit to the industry. And same or worse is the case with erosion, especially when it happens at unexpected locations most often due to unusual operating conditions or turbulent flow in pipe lines and a variety of other reasons. And if this erosion is in an insulated line, the problem is multiplied many folds.
There are a number of methods used today to inspect for corrosion under insulation. The most common and straightforward way to inspect for corrosion under insulation is to cut plugs in the insulation that can be removed to allow for ultrasonic testing. The other commonly used methods are profile radiography, and complete insulation removal. More advanced methods now available includes real-time X-ray and low Intensity X-ray Imaging.
The Ultrasonic Thickness spot readings and profile radiography gives accurate values of remaining wall thickness but UT readings require the insulation to be removed and proper surface preparation. However, many times plugs removed for UT thickness gauging can itself be the source of moisture leakage. The main problem with this technique is that corrosion under insulation tends to be localized and unless the inspection plug is positioned in the right spot, the sites of corrosion can be missed. Radiography is time consuming and requires cordoning off the areas to be inspected. Both these techniques are reliable and economical only if you know the exact location of erosion and or corrosion, which is almost impossible.
CUI and erosion are mostly localised and often inspection results can be misleading as area as close as a few millimetres from corroded areas can be with normal thickness values.
Tangential X ray, a relatively new technique shows the outer surface of the pipe but gives little information about the wall thickness or any erosion from inside.
INTRODUCING THE PROFILER:
Low Intensity X-ray Image Profiling is an advanced inspection tool to assist reliability engineers / Quality control inspectors and life extension programs in process industry. Used in conjunction with other inspection techniques, this technique offers a cost effective, fast and reliable inspection program for most insulated and non insulated piping in industry both offline and online.
The profiler basically utilizes the principle of radiography. A collimated beam of Gadolinium 153 isotope is used as the source and the photons emitted from the source are received at a receiver. Custom designed application software converts the amount of photons received into thickness values.
The profiler uses a low intensity collimated gamma ray source of Gadolinium 153 mounted on a C arm. The principle of inspection is based on the absorption of gamma radiation in the tested material. The scanning is performed using a small radioactive source (Gd 153) and a sensitive electronic detector / receiver. The source and detector are kept external to the pipe and positioned on opposite sides at a fixed distance apart.
Gamma rays travel from the source through the pipe to the detector where the photons received are counted. A proprietary software specially developed for this equipment converts the photon counts and displays a graphical image that corresponds correctly to the pipe condition of the pipe being scanned.
Once calibrated to a known thickness value of the piping material, it can detect most anomalies in process piping that causes a change in density from one area of pipe to another. The Profiler may be used to inspect insulated piping without insulation removal, including straight runs of pipe, elbows, tees, reducers, weld-neck flanges, and most other piping components.
Profiler scan can reveal density variations caused by corrosion, welds, wet insulation, blockages, and any other indication caused by a change in density. The Profiler is capable of inspecting all known materials used in pipe fabrication, including carbon steel, stainless steel, cast iron, aluminium, plastic, and PVC.
“A stitch in time saves nine” says the old proverb; a wet insulation identified and corrective action taken in time can save the pipe from getting corroded. Imagine the advantages of preventing corrosion rather than taking corrective action after the pipe is corroded. Periodical scheduled scans can detect wet insulation and corrective action can be taken even before corrosion starts eating up the pipe walls.
Results of a Profiler inspection are graphically displayed in real time to the operator and do not need days or weeks of waiting for a film to be processed or detailed analysis of inspection data by another interpreter. Corrosion or other problems like leakage, wet insulation etc are found in real time, and the pipe may be marked for additional remedial action or further investigation using complimentary NDT methods.
Compared to more conventional inspection methods like ultrasonic thickness testing and industrial radiography, the Profiler is capable of collecting much more data in a short period, thus making the profiler a much more productive inspection tool.
The Profiler is simple to use, portable & works on a rechargeable battery and can perform in most process environments to provide quick, reliable results.
The major advantages of Low intensity x-ray imaging can be summarised as:
- No need of costly insulation removal
- No need of cordoning off the areas as required in radiography
- No physical contact is required with the piping and so even high temperature pipes can be inspected online.( No temperature, weather or surface condition limitations)
- No need of surface preparation.
- Even non magnetic, non metallic pipes can be inspected
- Can detect wet insulation which in turn will lead to CUI ( prevention is better than cure)
- Can detect internal erosion / corrosion and gives absolute values of remaining wall thickness.
Whether bare piping or insulated piping, The Profiler system is capable of detecting/identifying:
- Internal Corrosion
- External Corrosion
- Weld Line identification
- Heat Affected (HAZ) Corrosion
- Blockage and Fouling
- Changes in Pipe Schedule
- Determination of product level in piping
- Wet insulation
- Water or Ice in the insulation without a costly insulation removal and reinstallation process.
Like any other NDT technique, the Lixi profiler also has its limitations too. The largest C-arm of the profiler has an opening of 18 inches and so piping with a maximum diameter of 18” including insulation can only be inspected using Lixi Profiler.
As the C-arm straddles the pipe being inspected, piping that has access to both sides can only be inspected using this tool.
The maximum double wall thickness of 30 mm of steel ( or equivalent) can only be inspected with precision and accuracy using this technique, even though spot readings can be taken for up to 50 mm of double wall thickness.
It should also be noted that this tool is NOT recommended for weld inspections or detection of cracks or pin holes.
However most pin holes are associated with deterioration mechanisms causing localised thinning!
Moreover extensive training and experience is required for the interpretation of the scan data acquired by the profiler.
Case Study 1:
The profiler has been successfully used in the oil sands industries in Canada for detection of hardened bitumen built-ups/blockages in radiant heater coils and caustic lines. In one case, radiant heater coils were blocked with hard bitumen due to coke formation at elevated temperatures. The maintenance team had tried to flush it out with high pressure water/steam and, even attempted to send a pig through the line to open up the blockage. Eventually the pig was lost in the serpentine coil and unable locate the pig for several days. That is where our team came in with The Profiler system and it took less than few hours to locate the exact location of the blockage and even the extent of blockages at though out the entire length of the coil.
Axial Scan Full
The profiler equipment was calibrated with steel step wedge to match the density of the steel pipe and double wall thickness of empty pipe was checked. The empty pipe was found to have a double wall thickness of 17mm which matched the manufactures data of the pipe schedule.
On scanning the tubes, different readings ranging up to 25mm was found. The area with the highest reading of 25 mm was cut and examined. It was found to be tightly packed with coke that corresponds to full blockage. Areas with intermediate readings were found to have partial blockage or a layer of coke deposit around the tubes’ inside wall. The areas of full blockage were cut out and cleaned. Areas with partial blockage were cleaned using cleaning pigs as there was enough room for the pig to navigate through. The conventional method of repair which would have been cutting out 27 return bends and cleaning them welding them all back, the job was completed by cutting just 7 “U” bends and thus saving a lot of down time of the equipment and additional work. More than identifying the blockage locations, the profiler is able to estimate the extent of blockage too.
Case Study 2:
In another case, while conducting corrosion survey on boiler tubes in an oil sands facility in Alberta, a section of the tubes were found to have increased in wall thickness on a section of the coil. On further verification it was found to be tubes of higher schedule, an error in the drawing and the as built drawing was updated accordingly. The profiler is an ideal tool to verify actual wall thickness or schedules of pipes and exact location of the transition, welds, even under insulation.
Axial Scale on Radiant