c) In the case of welding with a backing ring, smaller values
of misalignment are to be obtained so that the space
between the backing ring and the internal walls of the
two elements to be assembled is as small as possible;
normally this space is not to exceed 0,5 mm.
d) The elements to be welded are to be adequately secured
so as to prevent modifications of their relative position
and deformations during welding.