this minimum thickness does not apply to the
inner pipe of double pipe heat exchangers nor to pipes and
tubes that are enclosed and protected from mechanical
damage by a shell, casing, or ducting, where such pipes
or tubes are NPS 6 (DN 150) and less. This exemption
applies whether or not the outer pipe, shell, or protective
element is constructed to Code rules. When the outer protective
element is not provided by the Manufacturer as
part of the vessel, the Manufacturer shall note this on the
Manufacturer’s Data Report, and the owner or his designated
agent shall be responsible to assure that the required
enclosures are installed prior to operation. Where pipes
and tubes are fully enclosed, consideration shall be given
to avoiding buildup of pressure within the protective chamber
due to a tube/pipe leak. All other pressure parts of
these heat exchangers that are constructed to Code rules
must meet the 1⁄16 in. (1.5 mm) minimum thickness requirements