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Mock-up of tube-to-tubesheet joints in shell and tube heat exchangers
The tube-to-tubesheet joints in a shell and tube heat exchanger are critical locations which in many cases govern the life of the exchanger or call for unplanned outages in order to be repaired or plugged . There are many factors which should be considered in order to ensure long-lasting reliable joints .
By Baher Elsheikh , Lead Mechanical Engineer - SABIC
Those factors include the type of the joint ( expanded only , welded and expanded , or welded only ); design of the joint ( weld size , expansion grooves size , configurations of the weld groove ); method of application ( mechanical expansion , hydraulic expansion , manual welding , orbital welding ); and , quality control and inspection techniques used during fabrication ( eg . tubesheet hole surface finish , bevel size , dye penetrant test for welding joint , hardness test , sensitive leak test by helium , hydrostatic test , tube expansion ratio measurement , ferrite content measurement , radiography examination ); in addition to the qualifications of the joint through the applications of the mock-up .
A successful mock-up A mock-up model is one of the important steps should be performed before the actual tube-to-tubesheet joint application , where confidence can be built for the design of the joint , quality of the tools and techniques applied , and qualification of the welder ( s ) and procedure . To ensure the successful application of the mock-up , a detailed procedure should be prepared including the sequence of application , sketch and dimensions of the used material , testing to be applied , acceptance criteria and template for the output results report .
General consideration and best practices to be considered while preparing the mock-up procedure :
• ASME BPVC Sec . IX QW-193 to be followed with additional requirements based on a case-by-case basis
• Mock-up required for each welding / procedure qualification
• The mock-up assembly should essentially duplicate the tube-to-tubesheet weld joint design to be used in production
• For tube-to-tubesheet welds to clad tubesheet , the cladding or overlay may be represented by a base material with a chemical composition that is essentially equivalent to the cladding composition
• The mock-up should consist of a minimum of 10 tubes for procedure qualifications and a minimum of 5 tubes for each welder qualification .
• Hardness testing to be considered for work with hardening materials ( ex . DSS and Ti ) or for service requirements ( e . g sour service )
• It is advised that the dummy tubesheet is made from the same material of the production tubesheet , to ensure that HAZ in the mockup ( dummy ) tubesheet will provide representative results
All mock-up procedures should consider the following : 1 . Check the conditions of the tubesheet holes and tube surface 2 . Measure and record the actual diameter of the holes , Tubes ’ inner diameter , tube thickness for all tubes in four points with a micrometer to have accurate measurements .
3 . Follow the same sequence of expansion and welding as agreed for the production .
4 . The tool will be used for expansion ( mechanical or hydraulic ) and its model and applications steps to be mentioned in the procedure
5 . Liquid penetrant for root pass and final pass
Mock up model for square layout . DPT for mock-up model .
32 Heat Exchanger World April 2023 www . heat-exchanger-world . com