Ultrasonic Inspection Approaches for Measuring Corrosion Wall Loss in Process Piping
The direct and indirect cost of corrosion can be staggering.
A well-known study published in 1999 by NACE (the National Association of Corrosion Engineers) titled “The United States Cost of Corrosion Study” indicated that the direct cost of corrosion is more than three percent of the Gross Domestic Product (GDP). Similar studies report direct costs ranging from two to 4.5 percent of the GDP.
Given these enormous costs, it is not surprising that there are large industries centered on:
- Corrosion Prevention (such as additives in water systems, coating materials like paint for automobiles, etc.)
- Corrosion repair and maintenance
- Corrosion monitoring, detection and measurement
For this article, we will focus on ultrasonic corrosion monitoring, detection and measurement as it pertains to pipes and vessels fabricated from metals, with or without protective coatings.
Think of the industrial infrastructure of a plant, mill, refinery or manufacturing facility as a human body; the metal pipes and vessels are the circulatory system and organs. Pipes transport water, processed chemicals, and raw materials such as crude oil and waste products. Vessels, like organs, accept the materials brought to them, and manage temperature and pressure to circulate materials (product and wastes) back into the system for distribution.
The materials of the pipes and vessels are chosen by engineers to withstand as much as possible the elements that cause corrosion in the intended temperature, pressure and product environment. While these pipes and vessels are still subject to corrosion, the degrees and types can vary. Types of corrosion typically fall into three main buckets:
1. Predictable. If a given material is passing through a given pipe or vessel, at a given set of temperatures and pressures, general corrosion is expected and predictable. Materials in this category are most often under a comprehensive maintenance and inspection program. This case is generally well known and statistically significant inspection surveys will often suffice rather than full asset inspections.
In some cases, engineers should expect specific attack mechanisms such as MIC (microbial induced corrosion) or other pit developing process to take place. These conditions call for large area inspection techniques that can cover large areas quickly with precision measurements, such as phased array ultrasound. Spot checking will not reliably find pits, and digital radiography may not be a suitable option due to time, safety and defect size considerations.
2. Unpredictable, but expected. Use conditions may give rise to localized corrosion environments, such as:
a. CUI – corrosion under insulation. In the process of transforming raw materials to products, temperature is often a major control requirement meaning that pipes or vessels are insulated. In many cases, these process are carried out outside in the environment, so insulation can be damaged or degraded, allowing an ingress of water or product into the space between metal wall and insulation. It is not always easy to recognize the areas of concern, and it is costly to remove insulation, inspect for damage, and then re-apply insulation. During the time insulation is not on the pipe, the system must be shut down.
b. FAC – Flow Accelerated Corrosion has known locations of likelihood (a pipe elbow after a valve), but there is a low probability of knowing which elbows have had FAC, so again a quick reliable tool for elbows is needed.
3. Unpredictable. An example of unpredictable corrosion would be in the production of crude oil. If a well picks up significant sand or other abrasive material, it can cause very fast acting corrosion. The only reliable means of determining if this is happening is to fully monitor the pipe or vessel with installed sensors attached to the metal surface and under the insulation, if any.
Corrosion Monitoring and Pit Detection by Phased Array Ultrasound
Regularly scheduled inspections will validate corrosion rates and allow engineers and operators to better plan for maintenance situations. Ultrasound thickness (UT) readings can be of occasional use with regularly-scheduled inspections, but do not provide enough precision with the collection of manual thickness readings to adequately determine wall thickness losses from corrosion. Pitting cannot be reliably detected by conventional UT methods simply because the size of the defect is small compared to the area inspected.
Phased array ultrasound techniques can be developed to approach the needed precision and get great coverage quickly. Historically, the issues with the use of phased array ultrasound for corrosion evaluation and pitting detection have been:
- Availability of trained technicians
- Uniformity of testing (i.e. consistency between testing)
- Equipment costs
Availability of trained technicians is the most pressing issue; it has been much more difficult to find and train technicians on phased array systems than other inspection tools. However, this also is changing as more training institutes and colleges are incorporating phased array ultrasound technology courses, and the great benefits of phased array have resulted in more companies investing in training. Equipment manufacturers are allowing the experts in a company to easily customize the user interface of a phased array device so that a less experienced technician can much more easily learn to set up the device correctly and gather high value data.
These custom interfaces can be ordered into a seamless workflow as the less experienced technician works his way through setting up his device, calibrating it, developing gain compensation curves and getting ready to collect data. These serialized custom interfaces allow the input of photos, documents, and other aids to help the technician be sure he is correctly set up. If a technician does not understand a step or a data signal, he can push a button on the device and his screen is immediately shared on the PC, phone or tablet of his boss or customer—as long as both are in a wireless area. These new developments mitigate the hurdles in taking advantage of the precision and productivity gains allowed by using phased array UT.
Selecting the Right Probes, Scanners and Accessories for Corrosion Mapping
Ultrasonic phased-array corrosion mapping assists in quickly scanning, detecting, profiling and sizing of pitting and erosion in pipes and tanks. Encoded scanning data allows a high degree of repeatability and ensures 100% coverage to compare asset conditions and track corrosion rates over time.
Conducting accurate, precise corrosion inspection takes more than just an ultrasonic flaw detector or A-scan thickness gage—selecting a scanner, probe and accessory configuration for your application is a critical part of inspection planning. The right setup can make or break your inspection.
The NDT equipment industry offers a variety of scanners and probes to improve inspection productivity when mapping large surfaces for wall thickness and metal loss. This is a snapshot of the technologies available and their recommended applications.
Robotic and Motorized Scanners
Robotic or motorized corrosion scanners are particularly useful for inspection in confined spaces or overhead pipe runs, where it is unsafe or impractical to introduce a live inspector. During a maintenance turnaround where time is critical, it is often desirable to avoid the time delays and costs associated with ventilation, rigging, scaffolding or hole watch. The use of a robotic scanner or crawler can pay for itself in a single turnaround.
When selecting a robotic scanner, it’s important to ensure compatibility with your existing UT flaw detector, and to ensure that the device is certified as intrinsically safe when it will be operating in highly hazardous environments.
Manual scanners operate in much the same way as a motorized scanner, but with the requirement of a live inspector to physically move and operate them. Because they have fewer moving parts they can be very reliable. However to ensure accurate encoding of the scanned surface area, they require strong magnets and precision mechanics. It is advisable to train operators on manual scanning carefully to ensure consistent procedures.
High quality UT instruments are compatible with a variety of commercially available aftermarket scanners and robotic systems to meet virtually any corrosion mapping need.
DM Array Probes
For corrosion inspection and mapping, it is recommended to use a multi-element array probe. They provide comprehensive linear coverage and excellent resolution of backwall corrosion and pitting. Curved or flat wear bars can be added to the probes to extend probe life, ensure alignment, and aid acoustic coupling to the inspection surface. The larger surface area of multi-element probes allows inspectors to cover more surface area faster, and an advanced C-scan image display provides a visual representation of wall thickness loss.
Because corrosion mapping often takes place in challenging field conditions or at high temperatures, it’s important to consider the quality of probe construction when choosing a probe supplier.
Conventional DM Probes
When initial corrosion mapping of a pipe or tank wall identifies irregularities, inspectors are often called on to conduct more specific point inspections or thickness measurements using conventional UT. To maximize inspector productivity, the best phased array UT instruments are equipped with a conventional channel so these spot-checks can be conducted with the same instrument.
Conventional dual element transducers are recommended for spot-checking and measuring remaining wall thickness in corrosion application. Dual element transducers generate sound waves with one element and receive with another—in a ‘V-path’ orientation, which increases sensitivity when examining corroded or pitted back walls.
For productive and precise corrosion inspection, software can be just as important as probe and scanning hardware. Today a new generation of “app-based” UT flaw detectors operate much like a smartphone, allowing users to customize their device interface and build error-reducing guided inspection procedures. These “apps” can automatically identify compatible probes, improve calibration consistency and lock-out unnecessary device parameters, ensuring more reliable and repeatable inspection data. In addition, remote collaboration and live streaming via wireless internet streamlines reporting and allows experts to offer second opinions on tough inspection calls.
The direct and indirect cost of corrosion can be staggering. With the right combination of probes, technology, scanners, and software, today’s NDT inspectors are speeding up UT corrosion inspections and improving accuracy.