Pressure vessels in the chemical industry generally have top-mounted pressure relief valves (PRVs) sized to handle fire exposure and other possible scenarios, Designers usually assume that the fire scenario causes liquid to boil and vapor to vent, regardless of the initial liquid level. Under some circumstances, however, a high liquid level, together with thermal expansion, can result in a vessel being full of liquid when the PRV opens. If so, the initial fluid discharged through the PRV would be a two-phase gas-liquid stream. This paper analyzes non-reactive phenomena occurring during the beat-tip and venting process in non-agitated liquid-filled pressure vessels, A new criterion is developed to determine if the vapor-venting sizing assumption is justified. Results from several cases show that pressures in most liquid filled vessels sized for vapor-only flow will be below the ASME Code-allowable values during fire exposure, Hence, the common industry practice of ignoring two-phase flow when sizing fire cases is usually justified.