Tie Rods for a heat exchanger

[TheMehanic08]

Hi,

I am currently deisgning a heat exchanger which is in sour service for the shell side fluid.
The client has specified tie-rods of material SA-193 B7M for the same. Our proposal for SA-36 was not accepted in lieu of the stated material. Hence we have decied to build the tie rods of several small parts with male and female threads screwed into each other ot make one complete rod (reqd tie rod length ~6m)

Is this an acceptable practice? Any suggestions?

 

[fegenbush]

I do not have a problem with your approach other than the expense. However, NACE MR0175 lists only two acceptable materials for bolting in sour service: A193 B7M and A320 L7M.

One possible solution is to reference A.2.1.2.a (Hot Rolled Steel is acceptable) and A.2.1.7 (Cut Threads are acceptable) of NACE MR0175 as support for your position to use ASTM A36. You may require hardness testing if it has not been provided by the material test report and material traceability is recommended.

 

[SnTMan]

That is, to me, an unusual interpretation. I don't have a copy of MR0175, but are any alloys acceptable?

Regards,

Mike

 

[fegenbush]

Mike,

There are alloys that are acceptable, but the issues present are the same. Stress Corrosion Cracking is the concern of the NACE guidelines. Both of the bolting materials listed are heat treated after forming which minimizes residual stresses. The most economical solution is to cut the threads which leaves minimal residual stress in comparison to rolling.

The error lies in the customer requiring bolting material for tie rods instead of structural material with cut threads.

 

[SnTMan]

fegenbush, yeah, I know what B7M is, but haven't done any NACE units in a long time. Just thought there might be some alloy rod that would be acceptable.

And yeah, tie rods aren't bolting

Regards,

Mike