Heat Exchanger World Magazine April 2023 | Page 33

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Macro examination with 3.5 % magnification of cladded tubesheet .

6 . Visual examination : complete fusion , free of visual cracks , porosity indications , no evidence for tube burn through

7 . Measure and record the actual tube ID after expansion and calculate wall reduction ratio for all tubes

8 . Macro examination : section the mock-up through the centre and the 4 exposed surfaces to be smoothed or etched . Use magnifications 10 X to 20 X ( preferred 20 X for better visibility ) to check the following :

• Minimum Leak Path ( MLP )

• No cracking

• Complete fusion

• Complete penetrations 9 . Calculate and record the actual expansion ratio for each tube . All values should be tabulated and compared to the target expansion ratio

10 . In hydraulic expansion the applied pressure should cause the tube to take exactly the shape of the hole including the grooves and the imperfections or scratches ( should they exist )

11 . Mechanical roller must be clean as well and free from scratches or imperfections . Its lubrications , temperature and performance should be monitored during application .

Tubesheet hole preparation Holes to be drilled in the dummy tubesheet which will be used in the mock-up should have the same quality and fabrication requirements applied as the actual tubesheet holes drilling . Below is a list of the main requirements :

• Preparing the tube holes in the mock-up tubesheet must be done using the same technique to be used for drilling the actual tubesheet to have a clear representative view about the quality of the drilled holes

• Holes should be free from radial or longitudinal scratches or imperfections

• The inner surface of the tubesheet holes should be in as machined conditions ( no need for polishing )

• Tube holes and tube external surfaces should be cleaned

• Longitudinal scratches are of higher risk as it would create leak pass across the joint , please check the schematic sketch for elaboration

• The outer tube surface at the interface with hole should be also free from scratches or imperfections

Minimum Leak Path ( MLP ) Minimum Leak Path ( MLP ) is an important parameter for the strength weld type tube-to-tubesheet joint . MLP is the minimum path where the media can leak and can be defined as the minimum distance in any direction from the root of the tube-to-tubesheet weld to the nearest surface . MLP will depend on the type of the joint ( fillet / groove / Fillet + groove / … etc .) and the actual shape of the applications ( concave / convex / actual geometry / … etc .). In API 660 , MLP is required to be no less than two-thirds of the nominal tube wall thickness . While in ASME BPVC section IX , it is required to meet the design , but there is no specific requirements for how to be identified . The sketch below shows the MLP for different configurations of strength welded tube-to-tubesheet joints , colored red . In order to achieve the requirement of ASME Sec . IX , below are two demonstrative examples where the strength weld size is calculated as per rules of ASME BPVC Sec . VIII div . 1 – UW . 20 and then to be used for calculating the MLP length , as defined in the red lined marked sketches . Calculated values will be compared with the two thirds of the tube thickness as per API 660 .

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Radial imperfections . Internal three point expanding dial caliper .

MLP = the radius of the largest circle totally inscribed in the weld whose centre is situated at the root of the weld . www . heat-exchanger-world . com Heat Exchanger World April 2023

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