Digital Radiography Goes Mobile for Non-Destructive Testing Above the Arctic Circle
Crunching over the snow to one of a dozen automated oil wellheads scattered over the tundra in a field far above the Arctic Circle, where ice is a regular thing even in summer and bitter cold turns steel pipes brittle in winter, the maintenance crew’s first order of business is to check the condition of every pipe, weld, joint, and valve. In the past, to assess the internal conditions of piping and fittings, they’d have had to set up a big, cumbersome film-based x-ray radiography system, and then bring exposed film back to their headquarters to develop. If flaws were revealed, they would then have to trek back out to the installation to make repairs. The process was time consuming, expensive, and very inefficient.
They no longer have to do that because Digital Radiography (DR) systems like the BoltX Pro from Vidisco Ltd. are now available for NDT teams to use in the field.
Responding to oil-and-gas producers’ need for truly portable NDT x-ray equipment, engineers at system developer Vidisco, working with imaging-equipment developers at Teledyne DALSA, shrank the usual large-footprint NDT setup into a package small enough to fit in a backpack (Figure 1). Based on the company’s 10 year experience with flat panel imagers for NDT, the new system provides instant feedback on the internal condition of any above-ground plumbing.
DR technology is familiar to anyone who’s had a medical x-ray – or even a dental examination – in the last decade. Instead of slow, messy, and frankly dangerous photochemically developed x-ray film, medical DR relies on the extreme sensitivity of solid-state image detectors (Figure 2) to reduce x-ray dosages for patients and speed results for doctors.
The issues for NDT are different. Rather than short exposures at low dosages, the premium is on penetration and lateral resolution – and portability. Can’t forget portability. When the “patient” is an oil-production facility frozen into tundra permafrost, the “patient” can’t go to the x-ray, the x-ray must go to the “patient” – the pipe?
Also, the NDT team doesn’t care if the exposure takes one second or twenty seconds – the pipe isn’t going to move. What counts is instant feedback. How fast can they get an image up on the display? How fast can it analyze what it sees to determine if something needs to be done to avoid equipment failures, and keep the facility producing at peak levels.
For NDT in remote locations, equipment ruggedness is paramount. It does no good to drive half a day to an installation, only to find the x-ray source has jiggled to pieces on the way. The equipment needs to operate in extreme conditions of heat and cold, be able to take mechanical shocks and vibrations, and still operate flawlessly.
Finally, as the saying goes, “There’s more than one way to skin a cat.” That’s as true for NDT x-rays as it is for feline taxidermy. Our maintenance team uses a vacuum-tube-based x-ray source, but others may prefer other solutions, such as radionucleotide isotope sources. Ideally, the system should work equally with different available x-ray sources.
These were the parameters engineers at Teledyne DALSA and Vidisco designed the BoltX Pro system to meet.
Digital radiography basics
Like all x-ray techniques, DR produces a shadowgraph image (Figure 3). That is, a test subject interposed between an illumination source and a screen casts, its shadow by absorbing radiation from the x-ray beam, preventing it from reaching the screen. The denser and thicker the material in the subject, the more radiation is absorbed, and the darker the shadow becomes.
Radiography is the technique of capturing the shadowgraph image in some permanent form. Digital radiography uses an array of x-ray-sensitive solid-state devices to capture measurements of x-ray intensity at millions of points across the image plane – the screen – and record them in a digital computer’s memory. DR’s advantages over other radiography technology are:
- DR x-ray detectors are vastly more sensitive, thus flaw detection is possible even with small amounts of x-ray exposure;
- Images can be captured, displayed, shared and archived in real time;
- Proprietary tools for image handling and analysis use readily available mainstream computer technology to capture, display, share, and archive data and results.
Two parameters characterize the x-ray source beam: intensity and wavelength. Intensity measures the amount of radiation – corresponding to the brightness of a light beam. Wavelength corresponds to a light beam’s color, and determines the beam’s ability to penetrate through material in the test subject. The shorter the wavelength, the more easily the x-rays “punch through” the subject.
There are many ways to produce x-rays, but the most common is to accelerate a beam of electrons at high energy toward a solid, usually copper, molybdenum, or tungsten target. These electrons have enough energy to pass through the atoms’ electron shells but are deflected when they pass close to the nucleus. Like comets passing close to the Sun, they make a sudden turn around the heavy nucleus’ positive charge.
A process called bremsstrahlung (Figure 4) – a German word meaning “braking radiation” – the rapid acceleration of the electrons as they swing around the nucleus generates short-wavelength electromagnetic radiation: x-rays. The faster the electrons are traveling which is the beam energy, the shorter the wavelengths of the x-rays. The larger the electrical current is in the beam, the greater the resulting beam’s intensity. Thus, the important parameters of the x-ray source are its current and voltage. Current controls the beam’s brightness. Voltage controls its penetrating power.
The test subject interacts with the beam by absorbing energy from it. Literally, it absorbs x-ray photons individually as they pass through. The energy absorbed mostly goes into heat, slightly raising the material’s temperature. It can also go into breaking chemical bonds, which is important for medical patients but insignificant for the non-living subjects of NDT procedures. Masses of iron and steel don’t care. The important parameter for NDT is the percentage of the x-rays removed from the beam as it passes through the subject: the subject’s x-ray density. Density is measured on a logarithmic scale where a density of 2 represents removing 99% of the radiation from the beam.
On the receiving end, intensity and wavelength are important, as well. The detector absorbs individual x-ray photons and converts the energy to electric signals. How it does this depends on the detector technology. X-ray film stores the energy in chemical changes. Charge-coupled-device (CCD) detectors store it as electrical charges in tiny capacitors. CMOS detectors – such as are used in the new BoltX Pro system– produce pulses of electrical current. The significant is that the different detectors have different sensitivity to different x-ray wavelengths, and all detectors reach a saturation level where they cannot absorb any more x-ray energy.
In the end, what the radiographer wants out of the system is an image mapping the x-ray shadow’s depth (measured as a percent of the detector’s saturation level) as a function of position across the screen. He or she can then find flaws, such as cracks or erosion, by looking at the shadow pattern.
Comparison of DR to film for NDT
The shift from film-based radiography to DR is motivated by three factors, which all boil down to improvements in mobility and speed. We’ve already talked about the advantages of DR’s use of computer technology. DR also removes the toxic, time-consuming, costly, and inconvenient step of chemically developing film-based x-ray images and storing all the developed films.
Of extreme importance, however, is the fact that DR detectors are as much as 60X more sensitive than film at appropriate wavelengths. Table 1 compares the two technologies’ sensitivities measured in a landmark study conducted by Vidisco scientists. It compares the dosage, measured as beam current vs. beam energy, required to achieve 99% detector saturation with x-rays passing through a 5 mm thickness of iron.
Note that the relative sensitivity of DR vs. film varies with x-ray energy. This effect is due to variations in the two technologies’ individual spectral sensitivities. At the most penetrating, high-energy end of the data set, film’s sensitivity – already low – falls off more rapidly with increasing x-ray energy than that of the DR detector.
Graph 1 compares the sensitivity of DR and film for various thicknesses of iron typically seen in oil and gas NDT studies. Film’s non-linear behavior (shown as a straight line in this semi-log plot) makes it an even worse choice for denser subjects than better-behaving DR.
Portable DR use in NDT
X-ray NDT inspections, (Figure 5) in the petrochemical industry and at oil refineries are required to detect tiny cracks, defects and corrosion even under insulation and coatings. To be effective, the facility needs an ongoing maintenance routine that includes regularly repeated NDT pipeline inspections. Advanced portable DR systems are designed to instantly provide top-quality, precise images at remote NDT inspection sites. The proprietary software helps teams exactly repeat inspection procedures on a routine basis and to monitor the changes in results over time. Results to the smallest details can be clearly seen (Figure 6).
Vidisco optimized the BoltX Pro system for detection of corrosion, erosion, cracks, corrosion under insulation, wall thickness measurement in refineries, oil and gas pipes, and chemical or pharmaceutical factories. In general, Portable Digital Radiography (DR) Systems support current industry standards: ASTM E2422-05, ASME Boiler and Pressure Vessel Code, Section V, Article II, ASTM E2736 (ASTM E2698, ASTM, E2737), BSS 7044, BSS 7075, ISO/DIS 17636-2, ISO/DIS 10893-6 and ASME E2597-07.
Using the system during service operations makes it possible for teams to immediately obtain high quality, high contrast X-ray images. DR can easily reveal not just corrosion and erosion, but it allows NDT technicians to precisely measure pipes' wall thickness and weld quality. They can continue using the same isotopes and industrial sources they have been using previously when shifting to Vidisco’s portable DR systems.