No, it is not acceptable to make any such assumptions. The appropriate calculations must be performed. After the calculations are done, then you will know.
You know what they say about the word ASSume...
That said, there is a good likelihood that the calculations will pass, provided the D/t ratio is not too high, nor the L/D ratio, and the temperature is not excessively high. You do understand that the failure mechanisms are very different between internal and external pressure, and hence a simple assessment is not possible?
