DR for Pipes Inspection
Digital Radiography or DR is an advancement of traditional Radiography. This technique utilizes DDAs (Digital Detector Arrays) instead of Film or CR (Computed Radiography) in order to create an instant Image. The Radiation reaches the DDA, which has passed through the object, converted by a Scintillator into visible light and then translated into a digital Image. The physics (Angles, Penetration, technique etc.) remain similar and only mild changes are required to make the transition to Digital Radiography.
Why do we inspect Pipes?
Pipes, whether in service, in production or during installation, have a variety of potential problems which can lead to failures. Typical inspections of pipes are performed in order to inspect the welds, measure wall thinning, Corrosion and clogging due residue build-up.
What would I gain by using Portable DR for pipe inspection?
The advantages are enormous in almost every aspect. Starting with the time needed to acquire an image, from setup until the interpretation stage, no need for returning to site for re-shoots, the added safety due to significantly lower dose and exposure time and the fact that consumables no longer take part.
It is no longer necessary to use neither a Dark room nor Chemicals with this technology.
Where are Pipe inspections performed?
Inspections are mainly done at the facilities where the pipes are in service;
In the Oil and Gas Industry we are talking about all the stages of Midstream and Downstream operations, hence starting from the Transportation stage of the Crude Oil (or Gas) up until the final product is produced. Locations could be around Oil wells, in Refineries and in Power generation stations.
Do I need to use special sources of Radiation with DR?
No, all the sources of Radiation (X-ray and Isotopes) which currently exist at your workshop are suitable for use with DR. In fact, with Isotopes you are now able to extend the life cycle since DR requires lower activity (ci) thus replacing them less frequently.
Are there any DR standards for Pipe Radiography?
Yes, the main one being the European ISO 17636-2, and the well known ASME Section V (article 2) which permits the use of DR with mild modifications to the inspection technique.
One of the most problematic parameters to “convert” was Film Density. Due to the fact that with DR there is no equivalent parameter (the closest is Grey levels) other methods had to be developed in order to verify the image quality. Some of these are: SNR (Signal-to-Noise Ratio) and CNR (Contrast-to-Noise Ratio).
In a nutshell, ASME Section V states: qualification of the digital radiographic system requires a demonstration of the image quality indicator (IQI). The demonstration of the IQI requirements shall be considered satisfactory evidence of compliance with the procedure. In other words, no changes need to be applied to the technique.
The ISO 17636-2 requires, in addition to the Wire type IQI, a Duplex Wire IQI in order to measure the Basic Spatial Resolution (BSR). The Standard also requires measurements of the SNR (Signal-to-Noise Ratio) and CNR (Contrast-to-Noise Ratio) both of which are included in our software.
What is a Duplex wire IQI?
Duplex wire IQI is used to evaluate and measure the BSR (Basic Spatial Resolution) or total Image Un-sharpness in a Digital image. The IQI consists of 13 tungsten wire pairs housed in rigid plastic. The wires are exactly spaced to correspond to the diameter of each pair. The level of un-sharpness is indicated by the number of wire pairs which can be seen. As un-sharpness increases, the wires merge to form a single image and the spacing cannot be identified. Measurement is not evaluated visually; it is evaluated mathematically using a Line Profile tool. By pulling the Line Profile Tool over the wires, a plot is formed of distance vs. grey levels (or DDR).
Duplex Wire IQI
Normally, a 20% dip is required in order to determine that the wire is “seen”. Then, after determining that the wire is “seen”, we go to a conversion table and “translate” the wire number into an un-sharpness value. This number determines the DDA’s effective resolution.
Would the fact that Digital Panels are rigid affect the image quality?
Being rigid has absolutely no effect on Image quality however, it does have an effect on the number of shots for medium size Pipes. For small bore pipes we need to take two shots just as with Film. In large diameters, the curvature of the pipes does not cause significant effect on the un-sharpness (Ug) in the Image therefore the number of shots remain the same. Moreover, Digital Radiography standards such as ISO 17636-2 state the same number of shots as with Film.
Just as with conventional Radiography, Pipes are inspected for three main purposes:
Pipe inspections using a DR system
As opposed to conventional Film Radiography, with DR we are able to utilize a safer alternative to Isotopes (for thin walled Pipes) which is a Pulsed X-ray source. This is mainly due to the dose sensitivity of the Detectors which require a lower dose / energy compared to conventional Film Radiography. However, using an X-ray source for wall thickness measurements requires applying a slightly different methodology due to the differences between the Spectrum of an X-ray source and the Spectrum of an Isotope. Using an Isotope requires only one exposure in order to visualize the inner wall while not “burning” the outer wall; using an X-ray source requires taking two shots at two different exposures (low exposure and high exposure) since it is not possible to visualize the inner wall without “burning” the outer wall. In other words, with an X-ray source we will take a low exposure “shot”, in order to see the outer wall, and a high exposure “shot”, in order to see the inner wall. These two images are then combined, processed and presented as one easy-to-interpret image.
The DWT (Double Wall thickness) Technique
The DWT technique is complementary to the Tangential technique. When having thick walled pipes it becomes difficult to perform wall thickness measurements due to the very large cross section which needs to be penetrated. The DWT technique requires penetrating only twice the wall (front and back) thus allowing using lower Energies or lower exposure times.
With this technique we actually convert grey levels into material thickness (mm or inch) as opposed to counting the number of pixels in the Tangential Technique.
When using DWT we must know or calculate the absorption coefficient (µ) which is specific for each material as a function of the energy (kV). This is less complicated than it seems since the software will do the calculation for us (with a little bit of our help).
With the Automatic measurement tool the technician can easily perform high accuracy measurements with one or two screen taps. A line profile is stretched
over the defect or pipe and with a simple tap, the software will automatically detect the edges of the crack or, the end & start of the wall thus providing the user with a measurement.
Automatic Wall thickness and defects measurements
Some DR standards require having the capability to make Image statistics, usually in a region of Interest (ROI) and not on the entire Image. The main parameters being measured are the SNR (Signal to Noise Ratio) and CNR (Contrast to Noise Ratio). SNR is parameter which indicates the level of Noise in the image which in turn, determines the sensitivity or minimum defect size which can be identified in the current setup. CNR is a measure of image quality based on the contrast vs. noise, rather than on the raw signal.