For ASME UHX heat exchangers, the design and analysis of thick flanged and flued expansion joints need to be carried out using FEA. Though the user can take various different approaches to achieve this, the analysis methodology recommended by TEMA (RCB-8) has become sort of standard in the industry. TEMA analysis guidelines are very well documented and the analysis is fairly simple and straightforward, however it can be fairly tedious to carry out analysis for all different cases.
Sample this – For a typical heat exchanger there will be 8 load cases (4 design, 4 operating) for each of startup, shut-down, operation, upset conditions etc. Also the analysis needs to be carried out for corroded and uncorroded models for minimum as well as nominal thickness of the expansion joint. For all these conditions TEMA recommends to check for stresses across 10 to 15 SCLs (stress classification lines) as shown below.
8 load cases * 4 design conditions *4 models (nominal uncorroded, minimum corroded etc) * 13 SCLs say = 1664, which means evaluating and checking stresses against allowable for a phenomenal 1664 cases.
Sometimes users wonder that carrying out analysis in the minimum corroded condition shall be conservative. This however may not be true always. A thinner expansion joint leads to higher pressure stress in the longitudinal (PD/4t) and hoop direction (PD/2t), however at the same time thinner joints are more flexible (smaller spring rate) and thus allow displacement loads much easily with lower axial stress. Hence design of expansion joints are always an optimization problem between strength and flexibility and the thickness for these joints shall not be too high than required to sustain pressure loads.
The FSE software comes in handy in analyzing all the requisite cases fairly quickly and without much FEA know-how for the user. I have made it publicly available for free for both educational as well commercial use.