It is allowed provided that the finished geometry is controlled. Problems can be excessive wall thinning on the elongated side, so you may need to start with more wall thickness than you need for pressure containment, wrinkling, strain hardening, cracks and excessive ovaling of the pipe. Some codes require a calculation on excessive wall thinning, others don't, provided the geometry is maintained within limits.
We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer
And you need to finish with the correct HT /mechanical properties.
Generally the the only difference between pipe and tube is the dimensions (pipe needs to conform to some particular NPS ).
