1 CLASSIFICATION OF HAZARDOUS AREAS PRODUCED BY MAINTENANCE INTERVENTIONS ON N.G. DISTRIBUTION NETWORKS AND IN PRESENCE OF OPEN SURFACE OF FLAMMABLE LIQUID . Riccardo Tommasini Enrico Pons Department of Electrical Engineering Department of Electrical Engineering Politecnico di Torino Politecnico di Torino C.so Duca degli Abruzzi, 24 C.so Duca degli Abruzzi, 24 Torino, TO 10125 Torino, TO 10125 ITALY ITALY riccardo.tommasini@polito.it enrico.pons@polito.it Abstract – The safety and protection of workers is a duty of their employer. In case of presence of hazardous areas due to the risk of explosion, an area classification has to be performed to identify the shape and size of the locations where an explosion may happen. Two typical cases that can produce hazardous areas are gas emissions from a containment system, because of normal operation or because of a failure, and vapors emissions from an open surface pool of flammable liquid. In this paper two studies are presented: the first deals with the problem of natural gas releases during maintenance work on the gas distribution network, the second with vapors emissions from a pool of flammable liquid. In the first case experimental measures have been performed to easily calculate the size of the hazardous area; in the second case computer simulations are used to derive a simplified model to determine it. The results of the two studies presented are examined and commented in the light of the International and national Standards. Index Terms — Hazardous Areas Classification, Natural Gas, Inflammable Liquid. I. INTRODUCTION For many years the risk of explosion and the consequent requirement for the classification of areas has been a concern worldwide [1] and in the beginning industry codes were mainly used [2]. In 1994 and 1999 the two ATEX directives were published by the European Parliament, respectively Directive 94/9/EC [3] and Directive 99/92/EC [4]. The first deals with equipment and protective systems intended for use in potentially explosive atmospheres; the latter with the safety and health protection of workers potentially at risk from explosive atmospheres. In particular, Directive 99/92/EC provides that the employer adopts adequate measures in order to prevent the formation of explosive atmospheres, avoid their ignition and mitigate the detrimental effects of an explosion. Among these measures is the classification of the places where explosive atmospheres may occur which is particularly important. The regulations provided by the second ATEX Directive have been adopted in the International and European Standard IEC EN 60079-10-1 [5]; in Italy, moreover, in February 2007 a new edition of the Guide CEI 31-35, “Guide for classification of hazardous areas” [6] has been published. Meanwhile in the United States other Standards, with similar methods, are used for the classification of hazardous areas [7]. The two main different sets of standards, NEC and IEC have also been compared for what concerns the protection methodologies [8]. This paper briefly outlines the area classification procedure, describes the approach adopted by the international standard [5], illustrates some experimental measures and simulations and illustrates some general equations and assessment methods developed by the authors and introduced in the new Italian national guide [6]. II. AREA CLASSIFICATION PROCEDURE BASED ON IEC EN 60079-10 METHODOLOGY Whenever dangerous quantities and concentrations of flammable gas or vapour may arise, one of the standards used to classify the area is IEC EN 60079-10 [5]. Hazardous areas (in which an explosive gas atmosphere is present, or may be expected to be present) shall be classified in zones on the basis of the frequency of occurrence and persistence of the dangerous atmosphere, as reported in Table I. TABLE I ZONE TYPES ZONE 0 An explosive atmosphere is present continuously or for long periods or frequently ZONE 1 An explosive atmosphere is likely to occur in normal operation occasionally ZONE 2 An explosive atmosphere is not likely to occur in normal operation but, if it does occur, will persist for a short period only In accordance with Table B.1 of Standard IEC EN 60079-10-1 [5] (Table II below) the type of zone can be evaluated, knowing three parameters: the grade of release, the degree and availability of the ventilation.