Cumene hydroperoxide (CHP) can release large amounts of thermal energy and result in high temperature and pressure during runaway reactions. Calorimeters and related methodologies have been made for preventing runaway reactions of CHP. However, protective measures such as flooding and emergency relief system have not been studied intensively to protect CHP systems from runaway reactions of CHP. In this paper, vent sizing package 2 (VSP2) was used to study the runaway reactions of CHP in 12 wt%, 28 wt % and 48 wt% concentration and CHP solution mixing with water under adiabatic and heat input conditions. Chemical systems according to runaway reaction of CHP in cumene have been identified as non tempered systems. However, tempering occurs at about 180 degrees C with pad pressure of 9 bar in open cell tests after water addition. The Design Institute for Emergency Relief Systems (DIERS) methods were used to size the relief system of 120 m(3) reactor with 60 t CHP solution content considering flooding. The required relief rate decreases significantly and the mass flow rate per unit flow area decreases slightly with the addition of water. So the vent diameter of reactors can be reduced when water has been added. The results show that the flooding and emergency relief system can be effective to protect 120 m(3) reactors from runaway reactions of CHP under fire scenario in the concentration around 28 wt% or less. It is suggested that the relief system of CHP reactors should be sized properly considering fire scenario. If emergency flooding measure is involved for relief sizing, the measure must be credible. The volume of reactor and maximum concentration of CHP should be defined properly. (C) 2014 Elsevier Ltd. All rights reserved.