where is the cases/applications you propose double eccentric butterfly valves or concentric ? and why ?
am talking about (( TSE lines - potable water lines - sewage plant , etc .. sizes from 300mm to 2000MM
A tip to you (and others) is to use the eng.-tips forums own search engine to see what is already given in previous answers (push the search button beside the forum button under the questions headline).
Relevant thread could for instance be
thread408-374003
thread408-289032
thread408-373637
thread408-321955 ... and others.
I will be happy to try to answer if you have additional questions