Is a complete replacement of the heat exchanger tube bank (with material upgrade) requires redesign?
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Question: One of our company’s heat exchangers (a category II pressure vessel) has an outlet tube sheet made of 304L material. Due to frequent corrosion, we are considering replacing it with 904L material, while keeping all the structure and dimensions unchanged. Is it necessary to redesign the design in this case? Are strength calculations (finite element analysis), stress analysis, etc. required? If so, should these be carried out by the original design firm or by the manufacturing plant (which has the necessary design qualifications)?
In the case of fixed tube sheet heat exchangers, changing the material of the shell flange might have an impact on the tube sheet
It’s basically a new one, right? Redesign. Remanufacture it
This falls under the category of modification; it is necessary to comply with the requirements related to equipment modification as specified in the regulatory guidelines. The calculation documents can be prepared either by the original design unit or by a manufacturing unit with design qualifications, but whichever party prepares them is responsible for the safety of the equipment design.
The heat exchange tubes in the heat exchanger’s tube bundle are vulnerable components, and it is common to replace them at an early stage. When replacing the tube box, it is necessary to take into account the conditions of the tube bundle as well as how long the replaced tubes will be usable.
Seeking advice: For a heat exchanger in our company, the original material of the outlet tube box is 304L. [What material is the tube bundle made of? Isn’t it prone to corrosion? ], due to frequent corrosion, it is considered to replace it with 904L material, while keeping all structures and dimensions unchanged [Theoretically, this is impossible, as the mechanical properties of different materials vary; thus, the structural dimensions would change under given design conditions]. Is it necessary to redesign in this case? [Yes.] Are strength calculations, stress analysis, etc., required? If so, should they be conducted by the original design unit or by the manufacturer? [It’s sufficient to select a qualified design institute.]
It falls under major repairs of pressure vessels and shall be carried out in accordance with Chapter 5 of the Regulations on Pressure Vessels.
The tube bundle is made of 904L and has not suffered any corrosion
It needs to be redesigned. The material of the main components has changed, so the stress needs to be recalculated.
If the design temperature of the tube box is below 100 degrees, no calculation is required; if it is above 100 degrees, consider the remaining margin after calculation before making a decision