One of the greatest challenges facing many of the process industries; namely the petrochemical, refining, fossil power, and pulp and paper industries is: How to effectively examine their insulated piping?
While there are a number of failure mechanisms involved in various process piping systems, piping degradation through corrosion and erosion are by far the most prevalent. This degradation can be in the form of external corrosion under insulation (CUI), internal corrosion through a variety of mechanisms, and internal erosion caused by the flow of the product through the pipe.
Refineries, chemical plants and electrical power plants have MANY thousands of miles of pipe that are insulated to prevent heat loss or heat absorption. This insulation is often made up of several materials, with calcium based material being the most dense. The insulating material is usually wrapped with an aluminum or stainless steel outer wrap.
A partial sampling of the most popularly used detection techniques
are described as follows. As with anything, they have caveats,
benefits, and limitations. Check with your NDE specialist
to make sure you understand the availability, limitations,
and benefits of the various options.
Brute forcing
The least complex way to detect CUI,
brute forcing involves simply stripping the insulation off
of the equipment and examining it for corrosion. This is
a comparatively time-consuming, fairly expensive work
process, especially if the insulation contains asbestos, so
it may not be suitable for all situations.
Conventional radiography
This is the most common
NDE technique used for detecting CUI without insulation
removal. Conventional radiography involves a process
where radioactive rays are directed at the object to
be inspected, passing through it and capturing the image
on a silver halide film to be examined. It has numerous
advantages, including: it can be used with insulation of
any thickness or type, through many types of internal
product (some cases might require very high radiation
sources), on pipes of varying diameters, and on both
thick and thin wall pipes. Check with your NDE specialist
to make sure conditions permit a valid “shot”.
Digital radiography
As opposed to conventional radiography,
digital radiography relies on exposing reusable
storage phosphor screens as an alternative traditional
silver halide film. This allows the information to be
stored digitally, saving both time and storage space. It
also requires less radiation due to the phosphor film and
therefore has a reduced impact on the safety compared
to conventional radiography. Consult with your radiation
safety specialist to make sure you fully understand the
safety impact prior to establishing the Safe Zone.
Low intensity x-ray
The low-intensity x-ray imaging scope is a hand-held, totally portable fluoroscopic device utilizing a low-energy, low-intensity gamma source of Iridium. This can be a very quick way of qualitatively screening pipe for CUI. Iridum-192 is a typical radiation source for this technique.
Pulsed eddy current (PEC)
This method has been used in corrosion detection for several years and is highly useful in situations where an object’s surface is rough or inaccessible. Moreover, this method does not require surface preparation or the removal of insulation, thus it can be a quick and cost-effective solution for corrosion detection. The method works by sending out a pulsed magnetic field via probe coil, which penetrates through the non-magnetic insulation between the probe and the object being inspected. This will induce eddy currents that can be measured to determine whether or not corrosion is present.
Guided-wave ultrasonics (GWUT)
This method of testing involves sending guided waves out along the axial direction of a pipe and then measuring the reflections for echoes, which might be caused by corrosion. The main advantage of this method is that it is possible to inspect supports that are not directly accessible for visual inspection. The downside of the method though is that the actual accuracy of the results are strongly dependent on how good the inspector and the testing procedures used are; thus an inexperienced inspector can lead to inaccurate results.
Ultrasonic thickness measurements
This process can be used to determine the external condition of vessels and remaining thickness of piping components. It works by sending ultrasonic waves into the surface of the object and measuring the time taken by the wave to return to the surface. It is a fairly simple technique. However, it does require adequate contact with the material, so it is not viable for every situation.
Other techniques, such as neutron back scatter and infrared thermography, can help to find moisture under insulation, which may then help detect where CUI is occurring as well. These methods infer that there is potential CUI activity, as opposed to directly detecting metal loss or cracking
Source:https://inspectioneering.com/journal/1999-01-01/387/portable-pipe-wall-thickness-m
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