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non-destructively tested

Pressure vessel inspection quickly and reliably with ultrasound

Ultrasonic testing is an important part of the non-destructive inspection of metallic pressure vessels and pressure gas cylinders made of aluminum and steel, such as medical gas cylinders or CO2 bottles. This applies to the basic inspection as part of the final production inspection, but also to the recurring inspection as part of the reuse of bottles.

As a certified and accredited testing center for non-destructive material testing, we support you in this task: With PROline testing systems developed for the inspection of pressure vessels or nondestructive testing services. Thus, we offer our customers single and serial testing of pressure vessels and cylinders in our test center in Burgwedel or at your site.

Inspection before commissioning

Ultrasonic testing of pressure vessels and thus also pressure gas cylinders is standardized by ISO 18119 (05-18) “Gas cylinders – Seamless steel and seamless aluminum-alloy gas cylinders and tubes”.  These are seamless steel cylinders or aluminum alloy cylinders.

Manufacturers of pressure vessels and gas pressure cylinders must already perform ultrasonic testing during production. For example, automated ultrasonic testing can be integrated as a fixed part of the production chain. In this way, a 100% inspection is realized in a cost-efficient and timed process. In particular, the volume of the cylinder shell and the wall thicknesses at critical points are inspected ultrasonically.

Operators of gas pressure vessels (e.g. CO2 gas pressure cylinders of water bubblers) must periodically inspect the gas pressure cylinders or pressure vessels to maintain product safety.  As an alternative to the pressure test, hydraulic test or visual inspection of the interior of the gas pressure cylinders, the components are automatically inspected using ultrasound. The cylinders are inspected vertically and at 3 angles (tangential and axial) for volume defects, primarily cracks. At the same time, a wall thickness measurement is also performed with ultrasound to ensure a minimum wall thickness of the shell and the bottom of the gas pressure cylinder.

Recurring inspection

Most gas pressure cylinders, such as CO2 gas pressure cylinders of water bubblers, are intended for reuse as reusable cylinders. At a time interval specified by the standard, usually every 10 years, these must be inspected for flawlessness using various methods.  Ultrasonic testing is often used as an alternative to pressure testing, hydraulic volume testing or visual internal inspection. The main focus is on the inspection for cracks and minimum residual wall thickness.

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Ultrasonic testing of pressure vessels

Ultrasonic testing is a method of non-destructive material testing and is suitable for detecting internal defects in almost all components. So-called ultrasonic testing systems are used for this purpose. Sound has the characteristic of being partially reflected at boundary surfaces. Such boundary surfaces are e.g. air, change from one material to another, i.e. back walls etc.. Ultrasonic testing makes use of this property. During testing, a probe connected to such an ultrasonic device transmits sound waves into the material. For sound transmission, a so-called coupling agent, usually water or gel, is required between the probe and the component.

Fig.: Ultrasound A-scan display with all echoes occurring in the event of a defect in the material

The sound passes into the component and is reflected almost 100% at the back wall and is received again by the probe. If there are defects (e.g. cracks or pores) between the material surface and the back wall, parts of the sound are also reflected at these. The inspector or the automated ultrasonic inspection system can see and analyze all these reflections in the so-called A-scan.

Fig.: A-scan during ultrasonic wall thickness measurement.

Wall thickness measurement with ultrasound works in exactly the same way. Here, the ultrasonic testing device calculates the wall thickness based on the position of the backwall echo and the time of flight of the sound.

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Typical test sequence for testing the complete cylinder jacket

The non-destructive testing of pressure vessels such as bottles can be performed with a manually or automatically controlled ultrasonic testing system. First, the data of the bottle is entered into the inspection system by a scanner (e.g. a barcode reader). The bottle to be inspected is then placed and positioned in the inspection system manually or mechanically.

The inspection system begins to rotate the bottle. During this process, a probe fixture with a compact probe arrangement consisting of five ultrasonic probes moves along the cylindrical surface of the bottle. The probes scan the material at different angles in order to be able to perform a comprehensive evaluation. Optionally, a phased array probe can be used for this purpose, which can performs this function electronically.

The inspection is performed using the local immersion technique. This means that the cylinder is only wetted with water, but not immersed. The advantage of this is that no water can get inside the cylinder in the case of gas pressure vessels without a valve. This prevents the risk of corrosion as far as possible.

Fig.: Set-up of the test construction for ultrasonic testing of the shell of bottle

In addition to the two-dimensional inspection of the cylinder shell, the wall thickness of the cylinder bottom can also be inspected selectively. Automated ultrasonic testing is performed in accordance with test specification DIN EN 1968:2002 + A1:2005 (D), Annex G or the more recent standard ISO 18119 (05-18) Gas cylinders – Seamless steel and seamless aluminum-alloy gas cylinders and tubes.

The OK / NOK test result is visually displayed on the screen. The automatic evaluation, test report generation and result archiving saves time and creates test reliability.

VOGT Ultrasonics offers you a PROline ultrasonic inspection system for the automated inspection of cylinders up to 500 mm in length and 150 mm in diameter.

strength counts

Typical test sequence for testing the minimum wall thickness during production

In the manufacturing process of smaller gas pressure vessels, different wall thicknesses can occur due to the oval shape of the cylinder shell. Non-destructive testing with ultrasound in the form of a line scan allows the cylinders to be inspected quickly and reliably. The ultrasonic inspection system used for this purpose has a rotating unit under which a probe with a water flow section (bubbler) is installed. In such a bubbler, a water chamber is installed above the probe, which is continuously filled with water. This causes a water dome to form above the water chamber, which transmits the sound into the material.

The bottle is placed manually or mechanically on the rotating device so that the water dome touches the area to be inspected, in this case the lower bottle shell. Now the component is rotated. The probe works with the pulse-echo method. The inspection system detects the residual wall thickness on the basis of the ultrasonic A-scan and also displays this visually for the inspector in the form of a color-coded C-scan and a result proposal. The entire inspection is performed within one rotation of the bottle.  Based on the result, the bottles are then sorted manually or mechanically.

The wall thickness of the bottle bottom can also be inspected in the same way

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Video:  Ultrasonic testing of gas pressure cylinders in the VOGT test center

Requirement for the testing of pressure vessels with ultrasound

For automated ultrasonic testing of gas cylinders, the test surface must be free of rust, non-adhering paint, dirt and oil (according to EN 1968:2002 + A1:2005 (D), Annex G, Ultrasonic testing or also ISO 18119 (05-18) Gas cylinders – Seamless steel and seamless aluminum-alloy gas cylinders and tubes. This is necessary so that the ultrasound can be transmitted into the material without interference, otherwise it will already be reflected at the surface.

Test report

The ultrasonic testing performed on components subject to monitoring must be fully documented. Compliance with the standard, i.e. the ultrasonic testing specification, must be proven. Thus, a test report must be generated. This contains the component data and unique identification, the ultrasonic setting parameters used as well as the results of the inspection.

Modern ultrasonic testing systems such as PROline automatically document the inspection results in accordance with standards and in comprehensive form. In addition, there is also comprehensive proof that the inspection system was correctly adjusted at all times. The inspection data of each inspection can also be stored in complete form. This means that a subsequent evaluation can be carried out at any time within the scope of product liability. In this way, proof of the function and proper performance of the ultrasonic inspection can be provided at any time.

Why are pressure vessel tests necessary?

Der Druck Inside gas pressure vessels, the pressure is higher than that of their surroundings. Defective or damaged gas pressure vessels pose a danger to people and the environment. If defects occur in the material during production or as a result of their use, these can lead to significant stress peaks. These can be the cause of a pressure vessel bursting. People and the environment are endangered by escaping gases (explosion hazard) and flying parts.

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the advantages

Advantages of ultrasonic testing of pressure vessels

Due to its properties, ultrasonic testing is the ideal testing method for gas pressure vessels. It allows the inspector to gain a comprehensive insight into the inside of the cylinders in just a few seconds without destroying them. Most tanks are made of steel or aluminum alloys, which have the best sound conduction properties. Due to their high homogeneity, they produce a clear ultrasonic image. Due to their rotationally symmetrical shape, these components are ideally suited for fast and automated ultrasonic testing. The handling of these vessels as well as their positioning in the ultrasonic testing system is simple due to their geometry. The application of ultrasonic testing for cracks and wall thickness reductions is not a major challenge of ultrasonic testing systems. This makes the testing safe and reproducible. Ultrasonic testing is a safe testing method. By rapidly rotating the gas pressure vessel in a rotary unit of the ultrasonic system and moving a probe fixture with all probes along the shell surface, ultrasonic testing is performed quickly with the aid of water as a coupling medium. This inspection is not as costly as the alternatives: Pressure test, hydraulic test and internal inspection of the pressure vessel.

if there are still questions

FAQ

Gas pressure vessels and pressure bottles are tested after production and on a recurring basis. Depending on the type and area of application, they may be subject to legal requirements. They are inspected for external and internal material defects (cracks, inclusions) as well as wall thickness variations and corrosion, which may affect their safe operation or safe use.

Ultrasonic testing of pressure vessels and gas pressure bottles can detect external and internal material defects (cracks, inclusions) as well as wall thickness variations and corrosion. In the production of the vessels, priority is given to ensuring that they are free of defects in volume as well as to ensure a minimum wall thickness. During the recurring ultrasonic inspection, cracks (inside, outside and in the volume) as well as wall thickness deviations due to corrosion are in the focus of attention.

Pressure vessels such as gas cylinders can be inspected in series during production or on a recurring basis. Cost-saving framework agreements are available for this purpose.

An alternative is the purchase of an ultrasonic testing system, which is integrated into the production process by means of automation. Both variants can also be combined cost-effectively, depending on requirements. Let us talk about your inspection task.

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