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Microfocus radiography by using Ready packed Kodak AA400 Rollpack A A Lottering, Head of NDE Section, SASOL Chemicals Industries Inspection Authority, Polifin Limited - a subsidiary of SASOL Limited Contact

Microfocus Radiography is a powerful Non-Destructive Examination Technique, which is frequently used in Europe on tube to tube sheet welded joints.

Tube to tube sheet welded joints radiography using Microfocus X-Ray with ready packed AA 400 rollpack (with no intensifying screens), correct designed lead collimators with the examination technique can give the highest detection of micro indications such as micro cracks and can result in significant cost savings.
In this paper, a brief description will be given of the highly effective development of the technique to evaluate and assess the overall integrity and its advantages of flush and stick out tube to tube sheet welded joints on tubular heat exchangers manufactured in accordance with relevant manufacturing codes.

1.0 INTRODUCTION - BACKGROUND:

Polifin plant had its share of failures on shell and tube heat exchangers and this led to investigating means to detect and resolve these failures. During the investigation it was apparent that most of the failures to these exchangers could be attributed to the tube-to-tubesheet weld joint. With the purchasing of microfocus X-ray equipment, Polifin did not only resolve the reason for failures but also pushed the boundaries of quality on these types of equipment into the next millennium.

Fig A: Microfocus x-ray system type MF-160/200

Weld related failures to shell and tube heat exchangers was, and still is, a controversial issue in that blame is, in many cases, placed on quality control departments, both from the manufacturer and the third party inspection authority. Before the implementation of microfocus X-ray (and still) a test block is prepared by the manufacturer and by doing destructive testing the welding parameters are qualified. Only after acceptance of the test results by the third party inspection authority can manufacturing of the heat exchanger start. During this period the quality departments have no means of checking the weld quality of the critical tube-to-tubesheet joints other than by deliberately destroying some joints and a final dye penetrant test before hydraulic pressure testing.

This paper was prepared and presented by A A Lottering, Head of NDE Section, SASOL Chemicals Industries Inspection Authority, Polifin Limited - a subsidiary of SASOL Limited

2.0 NEW TECHNOLOGY INNOVATION

2.1 MICRO FOCUS X-RAY SYSTEM
A state of the art "open gun design" high definition vacuum x-ray system (see figure A), which can produce a source of under 5 microns. This allows high magnification and detection of microscopic defects. The micro focus design is of a modular form, and can be upgraded. A range of various types of target can be quickly fitted, dependent on applications. Target application for tube to tube sheet Radiography is a Backwards Throw Panoramic Aluminium Target, cooled by forced air.

Fig B: Microfocus x-ray machine film set-up with kodak AA400 film, collimator and heat exchange(side view)

Fig B: Microfocus x-ray machine film set-up

In applications where a directions head is used, it is possible to do in excess of 2,000 shots on a single target. This is done by using a remote target indexing head, which is controlled from the x-ray control panel. A read out on the panel will count the position of the index head. This function is particularly useful in high production where down time is important. It also allows for applications to reduce time where high current is required, and therefore more target burn is expected. Micro focus equipment range from 20kV up to 450kV with focal spot control that allows adjustment from less than 5 microns to 200 microns.

2.2 Advantages of using micr o focus x-ray equipment include:


• Reduction in film cost due to use of large grain film and use of higher speed film: such as KODAK AA 400 Rollpack and without lead screens.
• One of few methods to do non-destructive examination on tube-to-tubesheet welds successful. (See Figures B for film set-up technique)
• Inspection of microscopic defects with geometric enlargement of over 100 times.
• Showing geometric unsharpness near zero.
• Reduced scatter on film radioactivity. (Films without lead screens can be used).
• Reduction in screen unsharpness due to scintillator coating size.

Fig C: Rod anode with kodak film and collimator

2.3 GEOMETRIC UNSHARPNESS:
As can be seen in Fig. 1, with conventional x-ray equipment, geometric unsharpness is dependent on the distance from the object to the film. The further the film is from the object, the greater the unsharpness in normal radiography, but in the case of micro focus, Fig. 2, the source is a microscopic point. This allows geometric enlargement of over 100 times with virtually zero unsharpness.

2.4 Disadvantages
The disadvantages are far less than the advantages:

• The application is limited to work shop conditions. (See Figure D)
• Magnetic fields affect the Micro focus x-ray beam and can sometimes cause re-shoots of films, i.e. welding machines close to unit.

Fig D: Heat exchange in workshop

Fig E: Small bore rod anode

3.0 STRATEGIC MERIT FRAME WORK:

3.1 Embodiment Merit
The assessed value of the physical form of the smaller rod anode is a winner to x-ray small bore Heat Exchanger tubes. The unsealed Micro focus x-ray tube is also an improvement for maintenance. Polifin NDE section designed a small bore rod Anode to x-ray tube to tube sheet welds as small as an outside diameter of 19mm and 1,6mm wall thickness.

Fig F: Technique set-up to x-ray tube to tubesheetwelds(side view)

3.2 Operational Merit
The effect of the innovation on the companies' existing business is a positive contribution factor. With the Micro focus x-ray machine an almost defect free tube to tube sheet weld Heat Exchanger can be manufactured, even small micro cracks can be revealed and eliminated during the manufacturing process.

3.3 Other applications of Micro focus Radiography


• Solder joint inspection on printed circuit boards,
• Die bonding and wire bonding inspection on integrated circuits
• Inspection of active and passive components, electronic subassemblies and hybrid modules
• Inspection of electromechanical components, such as sensors, relays, fuses and coils etc. for short circuits or missing parts, especially when these components are in encapsulated condition.
• Examination of plastics, ceramic materials, light metal alloys and steel for material inclusions or pores/air inclusions.
• Examination of laboratory samples in experimental medicine and biology.

3.4 Market Merit
Market merit determines a business advantage. A possibility of bigger volumes of product to more customers and bigger export markets. The plants run for longer periods of time before failures occur. Higher volumes mean that the same profit or even bigger profits can be made on products at reduced prices.

The business advantage of this new technology is a must to implement this successful innovation.

CONCLUSION

This technical innovation is highly suited for the chemical corrosive environment on our plant to eliminate defects in welds and to cut downtime failures.

This will have definite benefits, high quality products and productivity implications.

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