A very peculiar case.
Urea service, cabammate heat exchanger. Very high pressure
Base metal carbon steel channel side.
Double internal lining consisting in two sheets of SA 240 Gr.316L urea grade. Inner sheet 8 mm and the sheet in contact with the c.s. 2 mm thk.
The closing seam has no access from inside, that means the two sheets shall be applied before the closing seam in c.s is welded. Tell-tale holes pass through the 2 mm 316L sheet arriving to the back of the 8 mm thk 316/L sheet. It is evident the intent of the designer to avoid contact of the fluid with the c.s. Actually if the 8 mm sheet leaks the tell-tale holes notify it before the product enters in contact with the c.s. being stopped by the 2 mm sheet.
The problem comes at the time of the hydrostatic test. The resistance to the stress of the c.s. channel shell is verified but not its tightness because of the barrier of the two 316/L sheets.
I would consider this case very similar to glass lined pressure vessel where the glass lining is the watertight barrier. Note that Code hydrostatic test of glass lined vessel is only that after glass lining.
Does this concept apply to the h.e. in subject as well? I would say yes, the hydrostatic test may be made although the internal lining is the watertight barrier. Any adverse comment?
Mauro