Heat Exchanger tube switching material from CS to SS

[abehong]

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.

 

[SNORGY]

It's worth looking into in light of the additional consideration that you will also have more mechanical strength lost in the tube-to-tubesheet joint (assuming expanded tubes) after you achieve the 8% to 10% (I think that's about where it will end up) wall reduction after rolling.

 

[SnTMan]

abehong as these are both fixed tubesheet exchangers you absolutely need to evaluate the proposed designs with Part UHX calculations or something similar. Due to the change in metallurgy differential expansion may present problems that did not exist before the change. There is more involved that the tube wall and tube-tubesheet joint. Tubesheet thickness, stresses in integral cylinders, tube and joint strength all come into play. As for categorizing the joint strength Appendix A contains factors to account for different coefficients on thermal expansion. For stainless tubes in CS tubesheets, I would not normally consider the joint strength to decrease due to thermal effects.

Regards,

Mike

 

[davefitz]

issues are:
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

 

[EdStainless]

What material are the current tubesheets? and what is their condition?
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.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[TD2K]

I would doubt that going to CS from SS will have much effect on the overall heat transfer coefficient when you look at film and fouling coefficients effects in addition to the effect of the tube material. Reducing the wall thickness in terms of maintaining overall heat transfer doesn't make sense to me.

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.

 

[Guest102023]

Austenitic SS is implied by the CTE concern, but is ferritic SS possible?

Beware of SCC lurking in the bushes whenever 300 SS is proposed as a magic bullet.