Tubesheet thickness based on ASME or TEMA?

Both methods are based on a theory developped by K.A. Gardner more than 50 years ago, however ASME takes into account some additional effects (stresses in channel and shell connected to the tubesheet) not considered by TEMA. Note that Gardner's theory is the basis of many other design standards, such as PD 5500 in UK, CODAP in France, VSR in Italy and also the European harmonized standard EN 13445.3 (however this standard contains also a completely new alternative method). 

Although the theoretical basis of all these standards is the same, the differences in thicknesses may be very important, due to the difference in design criteria: allowable stresses to be used for thermal stresses due to differential thermal expansion in fixed tubesheet exchangers, allowable buckling load on tubes, etc. These criteria sometimes lead to unnecessary increases of tubesheet thicknesses, made only for the purpose of avoiding supposed overstress of other components connected to it (shell, channel, tubes). 

Note that Gardner's theory, whichever is the interpretation given to it by each one of these standards, is generally very conservative, particularly in fixed tubesheet exchangers: firetube boilers, which are also fixed tubesheet exchangers, are designed with very simple formulas which give much thinner tubesheets (see ASME Section I). Therefore using TEMA instead of ASME or vice versa has no substantial impact on safety: when Clause UHX of ASME VIII division 1 did not exist, the TEMA method was the only possibility for a heat exchanger where both ASME and TEMA were specified. In the latest edition of TEMA the tubesheet calculation has been displaced into an appendix, to be used only in case of a design standard not containing a suitable calculation method: therefore when both ASME and TEMA are specified, ASME UHX must be used, except in the case of different contractual agreements (that may be taken only if the ASME Code is not compulsory). 

In Europe the tendency is now to replace the standard method of EN 13445.3 (based on Gardner's theory) with the new alternative method, which is being implemented in order to take into account all the possible tubesheet types: in fixed tubesheet exhangers with a limited number of cycles the new method gives tubesheet thicknesses very close to the ones obtained with the boilers' formula.