Non-Destructive Testing - NDT Methods

Much like UT, Long Range Ultrasonic Thickness Testing utilizes sound energy and time measurements to calculate the condition of straight, long runs of pipe. The transducer used in conventional UT is replaced by a collar which fits around the pipe.

In LRUT, long runs of straight piping (up to 100 meters / 300 feet) are measured at one time. Insulation needs to be removed for collar placement, but may be left in place on the remaining pipe being measured. Pipe diameter is also a factor in LRUT effectiveness, with 4 inch diameter pipe being the at the lower limit in size which may be measured.

There are 3 types of LRUT: Teletest, GUL, Magnetostrictive Sensor (MsS)

Characteristics of LRUT

• Can provide wall thickness readings over great lengths of piping, resulting in faster measurements.

Limitations

• High risk of radiation exposure, barriers and operations break may be required.
• Limited to straight runs of pipe.
• Insulation must be removed for installation of the collar.
• Sensitivity ≈ 10% to 15% of pipe cross-sectional area.

Pulsed Eddy Current (PEC) works on low alloy carbon steel by briefly magnetizing the ferrous metal through the insulation.

PEC may be used on larger diameter piping and pressure vessels with excellent results. The wall thickness of piping and plates should be between 0.25” and 2.5”. Insulation thickness may be up to 8”. Jacketing thickness for stainless steel, aluminum, and galvanized steel should be less than 1mm.

When determining the best method for your industry and company requirements, there are many factors that should be evaluated. Examine the following characteristics of conventional industrial radiography testing on piping.

Characteristics of PEC

• There is no risk of radiation with PEC, it is 100% safe to resume operations while testing.
• No insulation (up to 8”) removal or surface prep required. Requires uniform insulation thickness for best results.
• Most appropriate for larger diameter vessel shell & heads.
• Accurately measures single wall on pipe above 6″ without isolated pitting.
• Technicians can inspect up to 100 locations per day.
• PEC is able to provide real-time results.

Limitations

• Insulation over 8” may require removal.
• Cannot measure water or ice in insulation.
• Results are affected by nearby flanges, valves, pipe supports, or other ferrous steel components. Cannot reliable measure nested piping.
• Not appropriate piping under 6″ diameter (including insulation and jacketing) or where isolated pitting is present.

Much like a familiar medical x-ray, conventional Industrial Radiography testing uses radiation to capture an image, or an x-ray, of the pipe. An x-ray film or a digital plate is located near the pipe to be inspected and an x-ray or gamma ray energy source is located some meters away. The film or digital plate is then exposed to the radiation, resulting in an x-ray image of a portion of the pipe.

For radiography testing, safety regulations require that a safe working area be established during the creation of the images. This often means that a barrier will be identified. All personnel inside of the boundaries of the barrier will need to be evacuated during the exposure, including those on floors above and below.

Radiography is often followed by a second NDE/NDT method to further investigate areas of vulnerability identified by the evaluation.

When determining the best method for your industry and company requirements, there are many factors that should be evaluated. Examine the following characteristics of industrial radiography testing on piping.

Industrial Radiography includes Direct (DR), Computed Radiography (CR), Conventional Radiography (RT), Real-Time Radiography (RTR)

Characteristics of RT

• No insulation removal or surface prep is required.
• Can identify water or ice in insulation and measure pipe wall thickness and schedule.
• Can measure up to 6” diameter pipe (including insulation and jacketing).
• RT provides results post-evaluation for future further investigation where necessary.
• Technicians can inspect up to 20 locations per day.

Limitations

• High risk of radiation exposure, barriers and operations break may be required.
• Cannot measure piping greater than 6” diameter (including insulation and jacketing).
• Accuracy of indications and measurements are dependent/sensitive to orientation of film and radiation.
• All pipe cannot be interrogated with Industrial Radiography. Nested piping, suspended piping and elbows cannot be tested.

In an attempt to address some of the limitations of Conventional Radiography (RT), some NDT vendors have created a hybrid technique. Initial scanning is applied with Real-Time Radiography (RTR). Results from the RTR testing determine which locations will receive follow-up testing with RT.

Once RT locations are determined using RTR, the same benefits and limitations of RT techniques apply. This approach results in fewer locations receiving RT application.

RTR (Real-Time Radiography) can be an effective pre-screening tool for RT, as long as the equipment positioning difficulties and limited scanning area of RTR do not prevent comprehensive inspection. However, RTR cannot interrogate all piping, evaluates only the outside diameter (OD) of pipe and cannot measure pipe wall thickness.

This is in a category of radiography which uses real-time imaging systems. These systems have an x-ray tube on one end of the device which is aimed at a detector which produces an image.

The RTR system is moved along the pipe and the image updates in real time. Insulation is not removed to use these systems and there is no pipe preparation required. RTR systems are not powerful enough to x-ray through the pipe as RT systems do.  The image produced is of the outside edge of the pipe, through the insulation. Each scan of the system along the pipe is producing images of one edge of the pipe.  If you wanted the top, bottom, front and back edge of the pipe scanned, that would require 4 scans with the equipment.

Corrosion in ammonia piping generally occurs at the top and bottom of the pipe. However, there is no guarantee that this is the case. Due to gravity, wet insulation typically occurs in the bottom half of the pipe. In other industries, RTR systems are typically used to locate weld locations. If the pipe has severe enough corrosion or significant rust scale buildup, it may be visible in the image.

Characteristics of RTR

• Results are real-time.  Images are reviewed as they are produced, allowing for immediate review and additional inspection, if warranted.
• RTR system can be used with the insulation in place and do not require preparation of the pipe surface.
• Significant pitting and corrosion located on the pipe section may be evident if scanned with the RTR system.

Limitations

• Additional inspection results, such as confirmation of pipe size and schedule, location of ‘lost’ piping components and severely water-soaked insulation are not typically evident in the x-ray image analyzed.
• RTR systems typically need to be positioned perpendicular to the pipe run and positioned so that the beam and detector align with the tangent edge of the pipe, as viewed in profile. Pipe run construction or other obstructions, may limit the ability of the test personnel to properly position the RTR system for imaging.
• Pitting and corrosion located on the pipe section not at the pipe tangent edge being imaged will not be evident.
• In order to address higher radiation exposure levels with the RTR equipment in use, recent regulatory changes in some locales now require RTR equipment to be staffed and utilized as RT equipment – requiring radiation safety barriers and larger crews.

Magnetic Particle Testing is an NDT method used to detect surface and near-surface discontinuities in ferromagnetic materials.

MT involves magnetizing a ferromagnetic material and then applying fine ferromagnetic particles to the surface. Discontinuities in the material, such as cracks or voids, disrupt the magnetic field, creating a leakage field. The ferromagnetic particles are attracted to these leakage fields, forming visible indications of the discontinuities.

Characteristics of MT

• There is zero radiation risk
• Tests and average of 50 locations per day
• Can test all piping and vessels

Limitations

• Used to identify cracks and holes, it cannot measure wall thickness and corrosion is identified by visual inspection
• Insulation must be removed and surface cleaned before testing
• Prone to erroneous results on pitted or freezing pipe due to couplant
• Test area is max 1″ diameter on a single side

Liquid penetrant testing (PT) is an NDT method that uses specially formulated liquids to reveal surface discontinuities in solid and nonporous materials.

The process involves applying a penetrant liquid to the surface of the test part, allowing it to seep into any surface-breaking defects, and then using a developer to draw the penetrant out, creating a visible indication of the discontinuity.

This method is highly effective for detecting surface cracks, seams, and other defects that may not be visible to the naked eye.

Characteristics of PT

• There is zero radiation risk
• Tests an average of 30 locations per day
• Can test all piping and vessels

Limitations

• It cannot measure wall thickness or detect erosion
• Insulation must be removed and surface cleaned before testing

Visual inspection is the evaluation and assessment of equipment, a component, a system, or a portion of a system using human senses such as vision, hearing, touch, smell, or a combination thereof.  While visual inspection is non destructive, it is qualitative, and differentiated from testing.

Testing is a procedure that commonly employs instruments, devices, and/or gauges to determine the operational status of equipment, a component, or a device in the system. In addition to employing full-system integrity testing, NDT is often also employed to provide quantitative (definitive) metrics on areas identified as suspect by a visual inspection. Inspection and testing should be performed by two different parties.