Heat Exchanger tube switching material from CS to SS
Heat Exchanger tube switching material from CS to SS
(OP)
Got a request to evaluate the mechanical integrity assessment for changing the tubes of two exchangers ( AXM and NEN type) from carbon steel to stainless steel. Because of difference in heat transfer coefficients of CS and SS, our process group wants to reduce the SS tube wall thickness from that for the CS. Currently, the carbon steel tube that we need to replace is size at 3/4" with 0.083" thickness. The process engineer propose to offset heat transfer change by decreasing the wall thicknesses from 0.083" to 0.065" of SS material ?. But this need to be confirm by mechanical engineer if there is any issue on mechanical strength. I did verify the minimum required tube wall thickness due to internal and external pressure. It turns out that the thickness of 0.065" 316 SS tube material is acceptable . But for fixed end tube design, I am not sure how to justify tube to tube sheet joint load because SS tube obviously will experience higher thermal expansion stress than CS. In addition to tube to tube sheet joint load, Do I need to verify tube sheet design as well because of tube material change ?
It will be appreciated if you can give me quick response.
It will be appreciated if you can give me quick response.





RE: Heat Exchanger tube switching material from CS to SS
RE: Heat Exchanger tube switching material from CS to SS
Regards,
Mike
RE: Heat Exchanger tube switching material from CS to SS
worse thermal expansion, worse thermal stress+ lower yiled stress= more fatigue damage
better corrosion resistance
lower youngs modulus may imply vibration worsens- may need closer spaced supports, ditto thinner wall + lower section modulus
may need to anneal tube bends
dissimilar metal weld at tube sheet
RE: Heat Exchanger tube switching material from CS to SS
I am a bit concerned about galvanic corrosion of the tubesheets.
This is actually a very common change.
The only risk of vibration is from the thinner walls, the modulus is about the same.
The biggest issue is that you are lowering the back pressure on the system.
If there are centrifugal pumps used the pressure/flow will change.
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Plymouth Tube
RE: Heat Exchanger tube switching material from CS to SS
I have typically seen wall thicknesses for SS tubes less than that of CS tubes because of the higher corrosion resistance for SS. That depends however on the fluids in question and expected corrosion rates.
RE: Heat Exchanger tube switching material from CS to SS
Beware of SCC lurking in the bushes whenever 300 SS is proposed as a magic bullet.