The aim of the present paper is to provide new tools and criteria for conducting HAZOPs in continuous chemical processes (e.g., petroleum-refining processes). These are mainly based on five HAZOPs of different systems, conducted by different teams. Its scope covers the organizational step of the HAZOP study, which principally entails selecting the nodes to be analyzed and estimating the time required to examine them. These two aspects are focused on defining and planning the sessions that will be necessary to complete a HAZOP study. Part I of this paper focuses on developing tools, guidelines, and criteria for selecting and sizing nodes. A methodology for selecting nodes is illustrated and mathematical models for predicting the number of nodes to be selected and the time needed to select each node are presented. Part II focuses on developing a HAZOP time estimation model for predicting the time required to perform the whole HAZOP, from its preparation and organization up to the release of the first HAZOP draft report. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.