BET shell tube heat exchanger tube side external design pressure

[abehong]

For BET type heat exchanger, the tube side floating head assembly including head , girth flange normally contacts with shell side process fluid , in another word, will experience external pressure same as internal pressure on shell side . I am working on the replacement job of BET type shell tube heat exchanger. The design condition for existing BET heat exchanger is 900 psig ( internal design pressure)/ 7.5 psig ( external design pressure) on shell side and 290 psig ( internal design pressure)/7.5 psig ( external design pressure) on tube side ( the HEX was designed and fabricated back to 1980's). Vendor conduct the design for the new heat exchanger using same design condition as existing one. For floating head assembly , they use external design pressure of 735 psig . Personally, I do not agree with it . As per UG21 and Appendix 2 in ASME VIII DIv 1 2010 edition, each element including flange shall be designed for at least the most severe condition of coincident pressure at temperature expected in normal operation. For this BET type heat exchanger, I think the external design pressure for floating head assembly shall be internal design pressure on shell side ( 900 psig) or at least maximum difference in pressure between the inside and outside of tube and shell. Just want to see if my understanding is correct or not.

 

[SnTMan]

abehong, I don't agree with it either it seems to me it should either be 900+7.5 or if differential design 900-290.

EXCEPT, the orginal designer could have done a diff P design at the noted pressure of 735. You have anything to document such a condition?

Regards,

Mike