Tubesheet to shell joint in lethal service

[19750412]

Dear All

I have two questions in the filed of interpretation of ASME. Now we design heat exchangers working in lethal services, in meaning of ASME VIII Div. 1 UW-2 (a). Acc. to data sheets, exchangers shall be BEM type and for that we chosen corner joint type “i” of fig. UW-13.2. Such solution is also necessary to keep inside diameter of shell and further to keep guarantee heat exchange. Of course this type of weld acc. to point UW-2 (a)(1)(b) & (c), shall be type 1 or 2 of table UW-12, but due to above explanation can not. And therefore, we decide to treat theses joints as butt welds and to make 100% of volumetric testing what in our eyes shall fulfill requirements of UW-2(a) concerning 100% RT of butt welds. As you can imaging for such joint it is impossible to perform RT testing, but instead of it, we decided to do ultrasonic test of 100% welds. And now my questions:
- can we stay with such design solution or we shall redesign exchangers?
- can we replace RT test by UT test based on UW-11(a)(7)?

For this exchangers our client specified tube’s material acc. to SA-213 UNS S31254, for which ASME II part D dose not specified allowable stresses in different temperatures. But this material is specified in ASME II part A. Dose anybody know whether any code case allow to use it in temperatures different from room temperature.

Thanks for help.
Arti

 

[SnTMan]

19750412, I would not use your proposed corner joint for lethal service for two reasons: 1) You cannot eliminate or seal the crevice at the ID, 2) Code clearly does not permit it. Bite the bullet and redesign for a hubbed tubesheet.

Regarding your clients' tube material I do not see it even listed in Sec II, Part D, Table 1. Unless you find it is permitted by a Code Case, I think you have the duty to inform your client his choice of material is not a Sec VIII, Div 1 material and work with him to identify an acceptable alternate.

Regards,

Mike

 

[metengr]

Seam welded tube (SA 249) or welded/seamless pipe (SA 312) is permitted for UNS S31254. See pages 122, 123 in Table 1A of ASME Section II, Part D.

 

[19750412]

Thank you for your help.

My opinion regarding tube to tubesheet joint is similar but I would to compare it before I present it to our client.

Regards
Arti

 

[SnTMan]

19750412, best of luck

 

[MJCronin]

Have you considered a double tubesheet HX design with the interstitial space monitored/instrumented ? This might make your welding problem go away.

Although somewhat expensive, it only makes the HX slightly longer.

Some companies mandate these in certain processes.