Accidental heating caused by chemical runaway reactions or engulfing fires can lead to dangerous overpressure in chemical reactors and vessels containing high liquid vapour pressure or tempered systems. Therefore, emergency pressure relief for chemical reactors is often provided by relief devices like safety valves or rupture disks at the top of the reactor. After the activation of the pressure relief system the reactor content is blown down from a pressurized boiling state leading to a fast depressurization of the reactor. A part of the content flashes which leads to two-phase flow in the reactor with a relative low velocity. Possibly, the rising liquid surface reaches the top of the vessel resulting in a critical two-phase flow through the pressure relief system. In this context the fluid phase viscosity has significant influence on the vapour/liquid disengagement inside the reactor and therewith on the required cross-sectional area for the pressure relief equipment. A better understanding of the underlying physical processes occurring in high viscosity emergency pressure relief systems will help the process industries to improve the safety of their plants and prevent releases of hazardous materials to the environment.