boltings material choice 1

[gengi]

Dear all,

I am performing a design of an high pressure shell & tube heat exchanger.

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

 

[unclesyd]

We have numerous 48" SS flanges where the bolting is B7/B16 that operate in cyclic service and do so with no problem. Almost all our SS heat exchangers use B7/B16 except where corrosion would be a problem.

CS in lieu of SS Bolting
As long as the bolting is designed correctly and properly tightened you should have no problem as long as your bolt stays in its' elastic range.

With all said above there caveats or just good bolting procedures.
Torque measurement is usually required.
You have to maintain a torquing sequence on the fasteners.
Depending on you gasket seating stress you may have to hot torque in rare cases.
You have to maintain a solid landing area for the nut.

As you state one versus many.
One thing that we have done in recent years is to go with a larger number of small fasteners in lieu of a smaller number of large fasteners. This is design consideration and wan't work in all cases. But where possible it is a hit with the maintenance workers as well we are getting a better joint.

 

[gengi]

Dear unclesyd,

thanks a lot for your reply.

so you say that, with a good bolt up procedure there would be NO problem to use B7/B16 bolting even if the exchangers (i.e. flanges) are completely in SS.....???

we are going to tight that joints using an hydraulic bolt tensioners ( 4 tools at the same time) with our proper bolt-up procedure.

In addition please note that, the gaskets we have foreseen are WELDED LIP SEAL ( Y=0 m=0). This would let us more confident against any leakage problem.

The exchangers are in Sour H2S service ( severe and not severe, depending of the exchanger).
I think we should discuss with our Client in order to know if the Flanges will be insulated. In that case there would be a possible sour enviroment which would require suitable boltings (but in this case B7M material could be OK, so we are not obliged to use Stainless steel material.
I mentioned "a possible sour enviroment" but I think that this could happen with the use of conventional gaskets where some hydrogen can go out through the gaskets..BUT using WELDED LIP SEAL GASKETS the welding at the top of the gaskets shoul make a barrier.........

OF COURSE THE ABOVE IS A MY PERSONAL OPINION SO I AM NOT SURE TO BE RIGHT.......

What do you think?? Could you further help me??

Thanks in advance

GENGI

 

[unclesyd]

As you mention discussion with the owner/client that is one point that I think should be undertaken immediately.

You need to make sure that the owner is familiar with welded lip seal type gasket as the nomenclature can conjure up several different meanings.

Again as you state the gasket seating stresses are going to approach zero. If you are using flange design software watch you input values.

if you decide to go with SS fasteners watch out on any size above 1 1/4" as you can run into a problem with distance between bolts. It shouldn't be a problem as your gasket should accommodate any lost of compression.

Are you going to test with welded lip seal gasket?