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The legally prescribed regular inspections of gas technology systems are a major challenge for every operator. Processes often have to be completely interrupted or even entire sections of the plant shut down for the duration of the inspection procedures. In addition to the loss of use and the resulting costs of the test media to be disposed of, this also poses risks when the system is put back into operation. Procedures in which the systems can remain in operating condition are a real alternative.

All plant and pipeline operators strive not only to operate their pipes, fittings and systems economically, but also to meet the highest standards in terms of environmental protection, occupational safety and availability. Bohlen & Doyen Bau GmbH has therefore developed a pioneering and innovative solution in close cooperation with TÜV NORD to counteract the disadvantages and risks of conventional testing techniques for pipelines and pressure vessels.

By using the latest testing methods, a solution has been designed that can increase the economic efficiency and product service life of systems while ensuring their safety and environmental compatibility. This new technology is already being used by several operators in the energy sector for maintenance and inspection.

Conventional testing procedures for pressure vessels and pipelines involve several steps, starting with the decommissioning of the vessel or pipeline section. The affected section is then depressurized and drained of the existing medium in a controlled manner and, in the case of gases, pumped around and/or flared accordingly. The originally gas-bearing sections are then usually flushed with nitrogen in order to be able to measure them gas-free. In the next step, the system section is filled with water and subjected to a pressure test. It is important to bear in mind that the tank or pipeline section was not always designed and built to be filled with the test medium water. This can lead to considerable stability risks and static problems with the underlying foundation or tank statics.

Alternatively, pressurization with the medium nitrogen is possible, but involves a high risk potential. Containers with a volume of more than 10 m3 are not uncommon, so that when filled with water, an additional load of more than ten tons acts on the system structure. Together with the system's own weight, this often results in a load of more than 20 tons on the foundation, which experience has shown is not always designed for this and can lead to considerable hazards for system safety. There is also the possibility of the test water causing corrosion. In the long term, this leads to damage to the inner surfaces of the tank or pipeline. After the pressure test with the test medium water has been carried out, it is removed from the test object and disposed of at great expense and effort. The tank or pipe section must then be dried and reconnected at great expense. Finally, the test object is commissioned.

Conventional testing methods: not only more cost-intensive, but also more risky
With regard to the process steps described in the course of testing pressure vessels and pipeline sections, these conventional methods have major disadvantages. They are often associated with high costs and a considerable amount of time. The risks that can arise in the course of such testing work are also sometimes underestimated. In addition to stability, risks also arise from the disposal of the test medium or flaring of the operating medium. Finally, the procurement of test materials and auxiliary equipment such as scaffolding, cranes, mobile flares, mobile compressors, test water, nitrogen and seals also entails considerable expense.

There are also particular challenges for occupational safety, as some personnel have to work with respiratory protection and carrying out the inspection is exposed to high risks. The dismantling of pipe components or connections requires additional working time and costs, making maintenance work more complicated and extending the necessary interruptions to operations.

An alternative to carrying out conventional testing procedures on pipelines and pressure vessels therefore also makes perfect sense from a sustainability perspective. A procedure for legally prescribed inspections of pipelines and pressure vessels that are in operation is therefore also preferable from the point of view of efficiency and resource conservation. In addition to more sustainable process handling, this also results in a longer system service life.

The new version of the BetrSichV has changed the regulations for pressure vessel system inspections since 2015. These stipulate an internal inspection every five years and a strength test every ten years. The entire system must be inspected during this period. The Ordinance on Industrial Safety and Health makes it possible to create a specific concept for testing a container. This allows the internal and strength tests to be replaced by taking defined parameters into account. If an equivalent inspection concept is submitted, it is possible to carry out the recurring measures without opening the container or taking the system out of operation. On this basis, it is possible to save on emptying, gas-free measurement, cleaning and disposal with a suitable test procedure. The respective methodology can also be used for the recurring tests on pipelines and segment or scissor closures. The integration of state-of-the-art safety technologies and procedures can thus ensure increased plant safety and availability. Potential safety risks can be identified and rectified at an early stage or preventive measures can be implemented and initiated.

Through continuous monitoring and analysis during operation, potential problems are detected and rectified at an early stage. This leads to improved system performance and a reduction in downtimes.

New testing method with more effective ultrasound technology
The innovative testing concept is based on high-resolution ultrasonic methods such as phased array and the time of flight diffraction (TOFD) method. Phased Array Ultrasonic Testing (PAUT) is an advanced technique that is widely used in weld seam testing. It enables a thorough and accurate examination of welds to identify defects such as cracks, pores or inhomogeneities that could compromise structural integrity.

In contrast to conventional ultrasonic testing methods, PAUT uses special ultrasonic probes with a series of small, individually controllable elements. These elements generate ultrasonic waves that can be precisely directed and focused by phase control. By adjusting the phases of the individual elements, the ultrasonic beam can be electronically directed and focused to inspect different areas of the weld. This enables targeted inspection and localization of potential defects. Another advantage is the ability to cover multiple inspection angles simultaneously, which shortens inspection time and increases efficiency. In addition, PAUT offers higher resolution and sensitivity compared to conventional methods, allowing for more precise characterization of defects.

The TOFD test uses two ultrasonic transducers, a transmitter that generates a short ultrasonic pulse and a receiver that detects the pulses of defective reflected ultrasonic waves. The time it takes for the ultrasonic waves to travel from a transmitter to a receiver is measured. This time is called the time of flight and depends on the thickness of the material and the location of the defect. By analysing the TOFD data, various defects such as cracks, pores or volume changes in the weld seam can be precisely localized and characterized. The high sensitivity and accuracy of TOFD make it a preferred method in industries such as shipbuilding, aerospace and oil and gas. TOFD offers several advantages, including fast inspection with high resolution and accuracy, as well as the ability to perform precise depth measurements. In addition, TOFD enables complete coverage of the weld seam without dead zones, which increases the reliability of the inspection.

Application requirements specified by the BetrSichV
In consultation with TÜV NORD, Bohlen & Doyen uses the testing concept in the field of above-ground pipeline and pressure vessel testing. The BetrSichV specifies clear requirements for the safe operation of systems and pipelines. In principle, pipeline and plant operators are obliged to regularly inspect their pipelines for safety and functionality. Especially for companies that work with pressure vessels and pipelines, the implementation of inspection programs is of crucial importance in order to ensure safety and comply with legal requirements.

According to the current BetrSichV, operators and those responsible have a certain amount of leeway when selecting suitable test methods and techniques. This can include visual inspections, pressure tests, ultrasonic tests and other non-destructive methods. All are ultimately tailored to the specific characteristics of the test specimens, the operating conditions and the legal requirements. Carrying out inspections, tests and maintenance work is a prerequisite for identifying and minimizing potential risks. The regular inspection and maintenance work must be properly documented in accordance with BetrSichV.

The companies involved are obliged to keep precise records of the work carried out and to record the results and any defects identified accordingly. This documentation not only ensures legal compliance, but also the reproducibility of previous inspections. An essential component in the development of such individual inspection programs for pipelines is the risk assessment. This involves identifying potential sources of danger and evaluating the possible effects of malfunctions in a risk assessment. Based on this assessment, appropriate inspection intervals are defined to ensure that the pipelines can be properly maintained and inspected.

The technology offers improved accuracy as it records numerous signals at once and thus also reduces the number of misdiagnoses.

The possibility of in-process and in-production testing is a key part of Bohlen & Doyen's approach to increasing the efficiency of systems. Through continuous monitoring and analysis during operation, potential problems are identified and rectified at an early stage. This leads to improved plant performance and a reduction in downtime. In addition, the technical methods help to conserve resources and reduce environmental risks.

Compliance with energy and environmental management standards such as DIN EN ISO 50001 and DIN EN ISO 14001 as well as the specified CO2 balance of the DEHSt (German Emissions Trading Authority) is ensured through efficient processes and innovative technologies. The creation and implementation of the innovative testing concept for pressure vessels and pipelines takes place in several steps. First, the container documentation and the associated risk assessment are intensively examined and evaluated in order to identify potential weak points in the systems in operation and to prepare a comprehensive analysis. A suitable inspection procedure is then determined in close consultation with the operator and the approved inspection body (ZÜS). In most cases, this includes an ultrasonic test and a surface crack test of the highly stressed seams of the system. The scanning of the material volume and the weld seams using a phased array inspection is carried out by specially trained and certified specialist personnel. The test head is guided along the identified weak points and the material volume is digitally recorded in three dimensions.

Compared to conventional ultrasonic testing systems, the phased array method has the advantages of being easier to use, faster and safer. A phased array probe is significantly more robust and easier to handle than conventional single-element probes. The technology offers improved accuracy as it captures numerous signals at once, thus reducing the number of misdiagnoses.

PAUT test strategies can be further optimized by using them in conjunction with simulation. Phased array technology provides a permanent record, generates no radioactive radiation and can be used for multiple applications. This allows defects to be detected not only on the surface, but also in the volume of a weld seam. This also provides information about the lateral position of a defect (depth and height). Even small defects that could lead to material failure are detected by this high-resolution ultrasonic method, so that a conclusive and very reliable assessment of the integrity of the test object can be guaranteed. As an innovative testing method, it is used to identify discontinuities in order to determine material integrity.

Conclusion and outlook
The non-destructive testing method of phased array technology enables high-resolution and precise diagnosis of the material condition of pressure vessels and pipes without having to take them out of service. The reproducible documentation of results from Bohlen & Doyen enables well-founded planning and implementation of maintenance measures. This in turn improves planning reliability and the efficiency of maintenance and repair processes. There is also added value for the operational management of systems thanks to the early detection of damage and defects without having to open the tank or pipe section.

In the future, hydrogen will also open up more and more areas as a medium. Unfortunately, hydrogen-induced corrosion poses a particular threat to the integrity of plants, especially when operating tanks and pipelines. This type of corrosion can lead to serious damage that not only jeopardizes safety, but also results in significant financial losses and environmental damage. Hydrogen-induced corrosion occurs when hydrogen penetrates the material and triggers reactions that lead to embrittlement of the material, resulting in cracking and ultimately failure of the material. This can occur in particular with metallic materials that are in contact with aqueous media, such as in pressure vessels and pipelines used for transporting liquids. The effects of hydrogen-induced corrosion can therefore be devastating, ranging from localized surface damage to severe structural damage such as leaks and, in the worst case, ruptures. This not only poses a risk to the safety of employees and the public, but also risks significant business interruption and financial loss.

Consequently, the aim is to detect and monitor the cause of hydrogen-induced corrosion at an early stage and to take countermeasures. However, this can hardly be achieved with conventional testing methods, meaning that the defects can only be detected and localized at an advanced stage. Overall, the issue of hydrogen-induced corrosion remains an ongoing process that requires close cooperation between companies, regulatory authorities and technology providers. The use of state-of-the-art testing methods with the advancement of techniques and systems by companies such as Bohlen & Doyen, together with TÜV NORD, can help to minimize risk and damage and create a safer and more sustainable future for the industry.

The future of pressure vessel and pipeline testing will be characterized by increasing integration of innovative technologies, increased data analysis and continuous improvement of materials and standards.

Holger Harms
Bernd Saathoff
Bohlen & Doyen Bau GmbH
Hauptstr. 248
26639 Wiesmoor
Phone: +49 (0) 4944 9456

Thorsten Jacobs
TÜV NORD Systems GmbH & Co. KG
Große Bahnstr. 31
22525 Hamburg
Phone: +49 (0) 160 888 4116 

Published in
bbr - June edition 2024