Safety design of emergency relief system must consider adequate vent sizing whether the flow is a single phase or two-phase vapor-liquid flow and two-phase flow generally requires a larger relief area. It is an even more complex situation due to the effect of friction loss in a relief line with high viscous fluids. Generally, viscous fluids are polymer solutions or melts. The purpose of the present work is to study the effects of the viscosity of the liquid on the venting behavior and ascertain how well SAFIRE program can model the emergency venting of a non-reacting fluid. The fluids used in this study were water solutions made by adding a neutral polymer of PVP (polyvinyl-pyrrol id one) to water with 10 weight percent of the concentrations of the additive in the water. The viscosity was adjusted by selecting the different molecular weight with 40,000 g/mol and 3,60,000 g/mol of PVP in simulating the conversion condition of polymerization. At ambient temperature, the viscosity was estimated to be 0.007 and 0.113 Ns/m(2), respectively. The blow-down was performed by a purposed built bench scale reactor with volume of 1600 cc and a 0.1 m(3) pilot reactor at 150 degrees C where the relief pressure was about 5 bar. Venting experiment in the pilot reactor is important in simulating the large vessel in industry and avoiding the small pipe effect in bench scale. Simulation of the characteristics of two-phase flow was conducted by SAFIRE program. Experimental data of pressure and mass flow rate were compared with the results of simulation by using bubbly, churn-turbulent, and homogeneous flow model. The microscopic dynamic behaviors were studied by the fast photography technique in a 1600 cc reactor with two windows to allow the flow visualization. Experimental data of superficial vapor velocity and bubble rise velocity were directly measured in the fast photography in the blow-down. It is expected that the present study may contribute a better understanding of the dynamic behavior and the mechanism of flashing now. (C) 2008 Elsevier Ltd. All rights reserved.