The conditions under which the transient outflow from a punctured pipeline may be approximated as that emanating from a vessel using a simplified analytically based vessel blowdown model (VBM) is investigated in this article. The above addresses the fundamental drawback of long computational run times associated with the numerically based techniques used for simulating pipeline puncture failures. The efficacy of the VBM is tested by comparison of its predictions against simulation data obtained using a validated rigorous but computationally demanding numerical technique based on the method of characteristics. The results show that the accuracy of the VBM increases with decreasing puncture/pipe diameter ratio, line pressure, and increasing pipeline length. Surprisingly, the VBM produces more accurate predictions for two-phase mixtures when compared with permanent gases. This is found to be a consequence of the better applicability of the isothermal bulk fluid decompression assumption within the pipeline in the case of two-phase mixtures. (C) 2011 American Institute of Chemical Engineers AIChE J, 57: 3305-3314, 2011