captsmith, from your last description of the your problems, this sounds to me like erosion of the tubes due to fluid impingement, particularly if the wear is at the shell inlet end only. Stainless tubes may help with this, or, if you have room in the shell above the tube field, a plate say 1-2" larger than the nozzle ID mounted on the bundle below the nozzle can perhaps protect the tubes. An alternative arrangement could be to mount "impingement" rods above the tubes, probably two rows on a staggered pattern.
A third approach sometimes used is a distributor belt, a short, larger shell containing the inlet nozzle, mounted outside the main shell which has cutouts to admit the flow to the bundle. This increases the inlet area to the shell and reduces the velocity of the fluid. Sorry, I don't have any pics or so forth handy to post at this time.
Can you provide some further details of your designs, maybe tube layouts, shell and nozzle diameters, flow rates, etc? It might then be possible to make some calculations, etc. to compare to industry standards.
Do you have any data from the manufacturers you list, to compare your operating conditions to?
My guess (and a guess only) is kind of that your flowrates and therefore fluid velocities are above what the original equipment, which as I understand it, you are "copying", was designed for, leading to the erosion wear you mention.
Regards,
Mike