Dear all,

I am performing a design of an high pressure shell & tube heat exchanger with ASME VIII Div.1 Code .

I have an external flanged joint ( shell flange bolted to tubesheet) and the tubesheet welded to the channel barrel (TEMA TYPE "C").

I have to evaluate two joints:

Case 1)
both the Shell Girth Flange and Tubesheet in Stainless Steel material.

Case 2)
the Shell Girth Flange in Low Alloy steel while the Tubesheet in Stainless Steel material.

The design conditions/data (Design Pressure, Design Temperature and pretty bid inside diameter) would lead to install BIG SIZED BOLTS.

Using stainless Steel boltings (i.e. B8T, B8M, etc) I obtained several bolting with BIG DIAMETER and the consequent increase of Girth Flange Diameter and Thickness.

Do you think could the use of Low Alloy Boltings-B7 or B16, (even for the flange and tubesheet in SS - Case 1) be reasonable/suitable ??

The B16/B7 have higher allowable stress than B8 boltings.
This would help me in reduce the size and quantity of the boltings with benefit to the girth flange dimension.

My concer is about the different thermal expansion coefficient of the Stainless Steel compared to that one of the Low Alloy?
(stainless steel has the thermal exp. coef. higher than Low alloy).

Is it possible this difference would lead to the "bolts loosing" with the relating leakage problems???

Has someone already experienced a completely stainless steel joint with Low alloy or carbon Steel boltings??

Awaiting your precious your help I thank you everybody in advance.

regards

GENGI

Your concern with regards to differering thermal coefficient of expansion is quite justified. I am unaware with regards to the temperature conditions of your equipment, but if we are talking of fairly high temperatures e.g. > 300 deg.c you may consider using SA-453 designation fasteners. These are used when dissimiliar materials are required to be bolted. The + point is they have expansion coefficient properties comparable to austenitic steels and strength properties identical to ferritic fasteners.

If the temperature is not substantial then the additional expenditure of using these fasteners may not be justified, so please weigh pros and cons and decide accordingly.

Hope this helps.

M

I can tell from experience, that you don't have to worry about different thermal expansion when you use metal-to-metal contact flanges outside of the bolt circle (Appendix Y) with a C-ring metallic gasket with a wire spring inside, located in a groove in the flange. But beware, there is some error in the code formulae, the required thicknesses are obtained too thin. It takes a FEA check to ensure whether the tubesheet will withstand localised bending near internal diameter of the flange. Just use UHX to design the tubesheet with caution. Anyway, it's a very robust and cost saving type of design, I have designed 14" main flange of a HX smaller than B16.5 standard 6" flange at the nozzle this way!

_________________
Konrad Anikiel