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Cladding
applications in shell and tube heat exchangers
Cladding
Weld overlay and cladding are widely used applications for adding a protective metallic layer to a substrate base material , where the cladding material has higher preferable properties , mainly corrosion protection . This is a cost-effective solution that can replace fabricating the entire component from high-alloying material . The required material strength to withstand applied loads and required thermal conductivity and fabricability are ensured by a less costly substrate metal . In this article , the main aspects of the correct application of cladding will be explored , covering design , fabrication , inspection , and in-service considerations .
Baher Elsheikh , Lead Mechanical Engineer - SABIC
Advantages of using cladding applications Cladding applications offer several advantages , making them a popular choice in various industries . Here are some key benefits :
Corrosion Resistance Cladding directly addresses corrosion concerns by adding a layer of material with high corrosion resistance , avoiding the need to upgrade the full thickness due to cost or better fabricability reasons .
Erosion Resistance In specific situations , localized erosion can be a major concern , such as in the tube entrance area , especially in Flow Accelerated Corrosion ( FAC ) conditions . FAC is metal loss that occurs in carbon steel equipment when the normally protective magnetite ( Fe 3
O 4
) layer is dissolved into a flowing stream of water or water and steam . Cladding the tubesheet and using insert ferrules at the entrance of tubes can mitigate the threat of FAC and erosion at these critical locations .
Avoiding dissimilar welds ( DW ) between tubes and tubesheet In applications where the tube material is different from the tubesheet material , DW can be a concern . Cladding the tubesheet with a material matching the tube material can avoid DW issues , including thermal
Figure 1 . Cladded tubesheet .
stresses , galvanic corrosion , and quality assurance challenges during weld applications .
Avoiding the need for post weld heat treatment ( PWHT ) for in-service tube plugging For tubesheets fabricated from materials requiring PWHT , such as Cr-Mo steels , when tube plugging is required after in-service degradation , plug welds should be subjected to PWHT . This is a challenging activity due to the high heat sink from thick tubesheets and the susceptibility of the ligaments in the tubesheets to high thermal stresses . By applying a cladding layer to the tubeside face of the tubesheet , plugs can be welded to the clad , and plug material can be selected to match the clad material , avoiding the need for PWHT .
Resistance to hydrogen diffusion In hydrogen service , cladding reduces hydrogen diffusion to and from the substrate material . Although it does not eliminate hydrogen diffusion , it is an effective mitigation technique . The low diffusivity of hydrogen in stainless clad or similar overlay materials used for corrosion protection results in an effective pressure of hydrogen at the clad / overlay-to-base metal interface ( bond line ) that is lower than that of the process stream . This can result in a significant reduction in the effective hydrogen partial pressure experienced by the underlying steel below the cladding .
Resistance to nitriding Nitriding is the formation of a hard , brittle surface layer on some alloys due to exposure to high-
Figure 2 . Erosion and flow accelerated corrosion . www . heat-exchanger-world . com Heat Exchanger World April 2024
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