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Cladding example of this is the bimetallic tubes used in strippers in urea strippers , which have an external stainless tube and internal zirconium tube for resistance to highly corrosive carbamate . Another example is the Omega bond TM tubes used in urea stripper tubes fabricated from an inner zirconium tube and an outer titanium tube .
Cladding thickness In general , design codes and company specifications do not permit using the clad thickness as additional thickness to resist pressure but rather treat it only as corrosion allowance . For weld overlay application , the cladding thickness shall be sufficient to have an acceptable depth of the required chemical composition ( 1.5 mm per API Std . 660 ) considering the dilution at the interface with the base metal . Cladding and weld overlay thickness of the tubesheet in shell and tube heat exchangers depends on the type of tube-to-tubesheet joints ( strength welded or expanded ). When the tubes are expanded only to the tubesheet , the required clad thickness is higher compared to the case when the tubes are welded to the tubesheet . This is to provide the proper distance to expand the tube to the cladding material and prevent the ingress of the corrosive media to the base metal of the tubesheet . Note : expansion grooves , when applicable , should be in the tubesheet base metal , not in the cladding layer . In API 660 , the cladding ( including weld overlay ) thickness at the tube side face of a tubesheet shall not be less than 10 mm when tubes are expanded only , and 5 mm when tubes are welded to the tubesheet . In TEMA , for classes R and B , the nominal cladding thickness at the tube side face of a tubesheet shall not be less than 7.8 mm when tubes are expanded only , and 3.2 mm when tubes are welded to the tubesheet . The nominal cladding thickness on the shell side face shall not be less than 9.5 mm . In Hydrogen service , special consideration for the required effective clad thickness which can reduce the effective Hydrogen partial pressure to desired values which can eliminate the risj of hydrogen attack to the base metal . Corrosion resistant alloy clad and weld overlay are not considered as pressure retaining . When calculating the thickness of the pressure retaining components only the base metal should be considered
Repair and joint preparation for clad and weld overlaid steel Cladding shall be stripped back for a minimum distance of 3 / 16 in . ( 4.8 mm ) from the edge of the weld bevel . The cladding edge shall be rounded with a minimum radius of 1 / 16 in . ( 1.5 mm ) or tapered at a minimum angle of 30 °. For carbon and low-alloy steel backing material the stripped-back area shall be etched with either a nitric acid or copper sulfate solution to ensure complete removal of cladding . Prior to back cladding butt joints , Care shall be taken to avoid cross contaminating the cladding and backing material weld metal .
Figure 5 . Cladded plate .
Conclusion Cladding applications in shell and tube heat exchangers offer several advantages , including corrosion and erosion resistance , avoidance of dissimilar welds , and resistance to hydrogen diffusion and nitriding . Various application technologies , such as hot roll bonding , explosion bonding , and weld overlay , are used to apply cladding . Special care is needed for applications in hydrogen service , and bimetallic tubing can be used for applications with various corrosion resistance requirements . Cladding thickness should be sufficient to provide the required chemical composition and prevent the ingress of corrosive media to the base metal of the tubesheet . Overall , cladding applications are a costeffective solution for enhancing the performance and longevity of shell and tube heat exchangers in various industrial applications .
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Power Leads
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Figure 6 . Typical H2 disbonding test cell , ASTM G146 .
Insulation
Heater Ceramic Spacers
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About the author
Baher Elsheikh , Lead Mechanical engineer in SABIC , has 20 years of experience in oil and gas and petrochemical industry , particularly in the engineering and integrity management of pressure equipment . Baher is an approved ASME instructor . www . heat-exchanger-world . com Heat Exchanger World April 2024
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