Please read the scope description of ASME B31.3.
There are there no specific code requirements for 100% NDT or PWHT contained within the code for Ammonia
OR ANY OTHER specific process service.
B31.3 is not that specific,.... nor could any process piping code address
ALL of the unique and special piping design issues for
all of the many process liquids.
ASME assumes that the code is being used and implemented by experienced process piping engineers that are familiar with the specific requirements of their system. These engineers may or may not require 100% NDT and PWHT.
Regarding the classification of Category M fluid service, the answer is the same. ASME puts all responsibility on the piping engineer.
Having said that, please note that there has been a lengthy and heated series of discussions over the years on the
eng-tips fora regarding B31.3 Category M Service
let us know about your final decision ...
MJCronin
Sr. Process Engineer