New Trend In Classification Of Hazard Of Explosive Atmosphere In Laboratory Workplaces

Transcription

1 New Trend In Classiication O Hazard O Explosive Atmosphere In Laboratory Workplaces Di Benedetto A. 1, Dolcetti G. 2, Grimaz S. 3., Russo P. 4, Salzano E. 1 1 Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche Via Diocleziano , Napoli, Italy 2 Dipartimento di Scienze e tecnologie chimiche, Università di Udine Via Cotoniicio, Udine, Italy 3 Dipartimento di Georisorse e territorio, Università di Udine Via Cotoniicio, Udine, Italy 4 Dipartimento di Ingegneria Chimica e Alimentare, Università di Salerno via Ponte don elillo Fisciano (SA), Italy Current EU legislation makes the classiication o workplaces in hazardous zones as compulsory, in order to prevent explosions. The procedure requires in-depth knowledge about the sources o ignition and the potential sources o risk as described in the ATEX 137 Workplace Directive 1999/92/EC. To this aim, the hazard o substances is typically classiied as unction o speciic physical and chemical properties which are relevant with respect to risk issues. However, some other parameters which may aect the overall hazard o substances in conined or semi-conined spaces, such as the ventilation rate and the total quantity o substance have to be take into account. In this work, we propose a new view o explosion hazard classiication as unction o the diusion and residence characteristic times other than physical and chemical properties. This classiication has been applied to the case o a research chemical laboratory where dierent chemical substances are used in small amounts. 1. Introduction ATEX 137 (Directive 1999/92/EC) is addressed to employers operating in actories and establishments where explosive atmospheres can occur, in order to prevent and provide protection against explosions. Employers have to take technical and/or organizational measures appropriate to the nature o the operation or: the prevention o the ormation o explosive atmospheres; the avoidance o the ignition o explosive atmospheres; the mitigation o the detrimental eects o an explosion so as to ensure the health and saety o workers. In chemical research laboratories, a plurality o lammable substances is generally used, oten in a very small quantity. Thereore, in order to survey the potential danger related to ATEX ormation, it is necessary to carry out a risk assessment based on dierent representative and simpliied scenarios. With speciic reerence to Italian guidelines and standards, the CEI EN (CEI 30-31; CEI and CEI 31-35/A) gives algorithms or the deinition and the classiication o the hazard zones in each workplace. Besides, CEI 31-35/A, in the Annex GF Chemical Laboratories, provides that i some particular management measures are taken, no meaningul ATEX conditions are determined and thereore no

2 ATEX 95 (Directive 1994/9/EC) requirements or equipment and protective systems are requested in those areas. However, only qualitative indications are given in these standards and directive. Hence, it would be useul to introduce simpliied tools which allow, by quantitative evaluations, to deine the boundary limit between ATEX and NO ATEX as a unction o both the typology and characteristics o substances and the operative and environment conditions. To these aims, we have run simulations according to the CEI in order to calculate the maximum allowable volume V o lammable substances in typical research laboratory conditions or the application o ATEX zones. The ATEX-GAS sotware o the TNE has been utilized or the cases described in the ollowing sections. 1.1 Criteria o CEI The primary source o potential ATEX is associated with the use o lammable liquids. Source releases can occur when the surace layer o these liquids is directly exposed to the. In most cases, the liquid temperature will be below the boiling point and the vapor release rate will depend principally on the ollowing parameters: a. Liquid temperature b. Vapor pressure o liquid at its surace temperature c. Dimensions o the evaporation surace d. Ventilation On the basis o this parameters the CEI standard introduces the concept o hypothetical volume V z or the deinition o the volume o lammable envelope rom a source o release. ore speciically, CEI standard provides that i Vz 0.1 m 3 than the extent o potential damage due to the sudden increase in temperature and/or pressure, as a result o the ignition o the explosive atmosphere, is negligible. For the calculus o V z the standard gives the ollowing equation: Vz (dg / dt) max TV0 dv0 / dt 293 k LELm = (1) where: is the eiciency o ventilation in terms o its eectiveness in diluting the explosive gas atmosphere, whit ranging rom =1 (ideal situation) to, typically =5 (impeded low) (dg/dt) max is the maximum rate o release at source (kg/s) LEL m is the lower explosive limit (kg/m 3 ) k is a saety actor applied ti the LEL m (typically k=0.25 or primary grades o release or k=0.5 or secondary grades o release) T is the ambient temperature (K) dv 0 /dt is the total low rate o resh through the volume under consideration (kg/s) V 0 is the entire volume served by the actual ventilation in the vicinity o the release being considered.

3 1.2 Input data o simulations For the simulation by ATEX-GAS, we have assumed a number o chemicals and laboratory conditions. Table 1 reports the list o the lammable substances investigated in this work and their main properties needed or the simulations. Table 1 Chemical properties o substances. Chemicals Flash Lower Flammability Diusion P sat at 20 C, point, C Limits, % coeicient, m 2 /h mmhg Acetone ethanol Hexane uite clearly, the exercise requires also the deinition o working environment, as reported in Table 2. The data were obtained at ixed ventilation conditions. Table 2 Laboratory and Hood conditions Laboratory Hood Volume, m Temperature, C Pressure, Pa _aria, m 3 /h Number o changes, h ethodology and Results Results o simulations carried out according to the methodology proposed in standard CEI are reported in Table 3 in terms o the maximum allowable volume V o spilled lammable substances in laboratory and inside ume cupboard (hood) as described in Table 2. Table 3 Results o simulations Chemicals V, m 3 Laboratory Hood Acetone ethanol Hexane Starting rom these results we tried to derive a general criterion in order to rationalize all the data in a classiication map. To this aim, we deine three characteristic times: the diusion time, the resident time and the dilution time. Details o these times ollows. The diusion time t D gives an estimation o the time required by the vapor to diuse in the surrounding environment: 2/3 V t D = (2) D

4 where V is the volume o the chamber and D is the substance diusion coeicient. The values o D are given on Table 1 or all the substances used. The residence time t r allows the requirement to renew the gas in the chamber: V t r = (3) where is the chamber ventilation volume low rate. Finally, the time required to keep the substance concentration, within the analyzed environment, as lower or equal to the lower lammability limit (dilution time t d ). V t d = (4) where is the volume low rate o the substance, which may be calculated rom the ollowing mass balance: ρ = ρ LFL + ρ LFL (5) where ρ is the density, is the molecular weight and LFL the lower lammability limit and subscripts and reer to uel and, respectively. From Eq.4) the expression o the lammable substance volume low rate becomes: ρ = ρ LFL ( 1 LFL) (6) By arranging all the data obtained by means o the sotware ATEX GAS, reported in the Tables 1-3 and calculating the diusion time (t D ), the residence time (t R ) and the dilution time (t d ), we obtained the data plotted in Figure 1, which represents a classiication map in which two zones (NO-ATEX and ATEX) may be clearly distinguished. An a-posteriori test was done here by adding the data obtained by the simulations according with CEI 31-30, utilizing the ATEX GAS sotware or xylene. The data o xylene are given in table 4. Table 4 Chemical properties o Xylene Chemicals Flash point, C LFL, % D, m 2 /h P sat at 20 C, mmhg Xylene From test results it was ound that 4 L o xilene can be considered a sae volume. Adding these points on the graph o Figure 1 we ound that they are placed in the NO- ATEX region, as it can be expected.

5 15000 Flammable substances - Lab & hood data Xylene data diusion time/residecne time NO_ATEX ATEX e-5 1e-4 1e-3 dilution time (h) Figure 1 ATEX / NO ATEX regions as a unction o diusion, dilution and residence time. 3. Conclusions Current health and saety legislation requires that employer has to demonstrate that he has taken all organizational and/or technical measures to reduce the risk o harm to employees and other workers rom hazardous material, so ar as is reasonably practicable. For the case in which the quantities o lammable substances manipulated are small as in the chemical laboratory it is necessary to evaluate i some ATEX areas are generated, and speciic equipments must be adopted. The method presented in this work deines the maximum quantity o lammable substance that doesn t determine ATEX 137 zones, or which ATEX 95 directive equipment is required. In other words it permits to know the maximum volume o spillage o same lammable substances that can be tolerated without ATEX certiied equipments. The results o the proposed procedure have shown that in many cases the volume determining ATEX conditions is larger than capacity o containers generally used in laboratories. Thereore, depending on the type o substance, i containers in laboratories are smaller than 2.5 dm 3, both laboratory and hood may be classiied as no-atex zones. The procedure is particular useul in the design o laboratories in order to deine the equipment and acilities requirements. oreover the procedure is useul to establish the exercise limits in the management o activities in laboratory and in particular to deine the maximum quantities o lammable liquids manageable without determining ATEX zone classiication.

6 In any case, where lammable liquids or gases are handled in laboratories, it is always necessary to consider sources o ignition, even i there are no hazardous areas ormally designated. 4. Acknowledgements Authors would like to thank Pro. Gennaro Russo or his encouragement and scientiic help. 5. Reerences ATEX-GAS sotware, Version 2.1, TNE Tuttonormel, Torino CEI EN (CEI 31-30): Costruzioni elettriche per atmosere esplosive per la presenza di gas - Parte 10: Classiicazione dei luoghi pericolosi. CEI 31-35: Costruzioni elettriche per atmosere esplosive per la presenza di gas - Guida all applicazione della Norma CEI EN (CEI 31-30) - Classiicazione dei luoghi con pericolo di esplosione per la presenza di gas, vapori o nebbie iniammabili. CEI 31-35/A: Costruzioni elettriche per atmosere esplosive per la presenza di gas - Guida all applicazione della Norma CEI EN (CEI 31-30) - Classiicazione dei luoghi con pericolo di esplosione per la presenza di gas, vapori o nebbie iniammabili: esempi di applicazione. Directive 1999/92/EC. ATEX 137. European Union Directive on minimum requirements or improving the saety and health protection o workers potentially at risk rom explosive atmospheres Directive 1994/9/EC. ATEX 95. European Union Directive on the approximation o the laws o ember States concerning equipment and protective systems intended or use in potentially explosive atmospheres.