The FPA Method for Analysis of Static Electricity Risks

Transcription

1 1 The FPA Method for Analysis of Static Electricity Risks Eyal Zadok Israeli Electrostatic Control Laboratory PO Box 108, Hila 24953, Israel phone: fax: Abstract The FPA (Failure Point Analysis) method, was developed to handle the static electricity phenomenon in various branches of industry. It is a multidisciplinary work tool that is aimed at three levels: 1) Future, predicting ignition risks or disruption of electronic systems; 2) Present, Location & identification of situations and system elements that may potentially be harmed; 3) Past, failure analysis in equipment and systems. The method combines scientific knowledge & data with laboratory testing tools, and over 25 years of practical experience in handling risks and disruptions caused by static electricity in both R&D and production facilities. The application of the FPA method in the process industry is presented for the Present level, emphasizing the analysis and synthesis of data, leading from the very beginning step in existing chemical plant to the end step of the conclusions about the most appropriate measures to be implemented, in order to achieve elimination or minimization of the possibility for electrostatic ignition. I. INTRODUCTION The phenomenon of static electricity is one of the oldest physical phenomena recognized in the world, particularly due to lightning storms. With the development of industry over the past hundred years, it has become clear that static electricity is manifested in many areas of industry, particularly as a negative factor: disruptive, corruptive, and even dangerous. It is difficult to list all of the types of industries that are at risk of damage by static electricity, but its negative impacts may be classified into three fields: 1. It may cause ignition in areas that contain a flammable atmosphere; 2. It disrupts the function of electronically based systems;

2 2 3. It affects attraction/rejection between bodies with small masses. Each one of these fields has direct implications regarding the safety of employees and the work environment, the quality and functioning of the product, the economic efficiency and business strength. The FPA (Failure Point Analysis) method, was developed to handle the static electricity phenomenon in various branches of industry. The method is a multi-disciplinary work tool that is aimed at three levels of treatment: 1. Predicting ignition risks or disruption of electronic systems in a new process or equipment that is on its planning stage; 2. Location and identification of situations and elements that may potentially be harmed in existing process or equipment; 3. Failure analysis in process or equipment in which a malfunction has occurred. The method is based on a unique approach combining scientific knowledge and technical data together with laboratory testing tools, and over 25 years of practical experience in handling risks and disruptions caused by static electricity in industrial R&D and production facilities. The method allows identifying and focusing on failure points, which may simultaneously contain a sensitive situation and electrostatic charge level above a threshold of sensitivity. The method has been applied for over 10 years in the process industry, as well as in other areas of industry, such as: electronics, microelectronics, electrooptics, clean rooms, and the aircraft industry. The following chapters present the principles and implementation stages of the method in the chemical industry, where flammable materials and atmospheres are present. II. UNIQUE APPROACH TO ELECTROSTATIC HAZARD ANALYSIS A. Insights into FPA method 1) The objects of the method are: identifying hazardous situations, understanding the mechanism of ignition in each situation, and presenting of specific recommendations to minimize the possibility for an ignition to occur. 2) Hazardous situation is a potential failure point in a process. 3) A potential failure point is defined as the suspicion for the existence of flammable atmosphere and electrostatic charges at the same time & place, in which the possibility of ignition to occur has to be analyzed.

3 3 4) Recommendations to minimize this possibility are derived from the mechanism of ignition, so that be feasible and effective. B. Flow chart of FPA method The method consists of analysis and synthesis of technical data, collected from and created through different sources. It contains three main stages, as follows. Failure Point Analysis Method for an existing chemical plant Review of process materials properties Review of equipment/process data Technical Laboratory & Plant Flow diagram, Literature field tests schemes procedures Electrostatic charge generation study, through process activities analysis List of activities Flammable atmosphere generation study, through equipment analysis List of tools Potential failure point list (p.f.p.) Discharge type analysis at each p.f.p.: spark, brush, p. brush, cone, corona Flammable atmosphere classification at each element or area Failure point list Quantitative analysis of discharge igniteability at each failure point, using : MIE, MIC, E DIS, T IGNI, E EQUI, etc. Failure mechanism of each point Effective & feasible recommendations for minimizing/eliminating ignition

4 4 III. STAGE I IDENTIFICATION OF POTENTIAL FAILURE POINTS The possibility for co-existence of flammable atmosphere and electrostatic charges in the plant should be analyzed using detailed technical data about the plant. This technical data pertains to the entire production process, including auxiliary tools and materials, and is therefore carried out in two phases: 1. Review of the materials in the process; 2. Review of the equipment and the conditions of the process. The properties of materials can be obtained from technical literature (e.g. m.s.d.s., standards, handbooks, reports), or through laboratory tests made on the materials and field tests made at the plant facilities. Care should be given to the data obtained, to reflect the relevant status of the materials in each process stage. Among these properties are: electric volume resistivity, dielectric constant, density, particle size, minimum ignition energy, minimum ignition concentration, auto ignition temperature, flammable limits, vapor density, vapor pressure. The review of the equipment and process data includes the construction of equipment in the plant through its floors, the construction materials of each tool, identification of conductive (metallic) and insulative elements, volume of vessels, diameter of pipes, rate of flow, entrance order of materials, materials accumulation state, transportation modes, manual handling of materials, ambient conditions in/out of equipment (temperature, relative humidity, pressure, atmosphere content, etc.). All these informative details are needed in order to answer two questions: 1. Which spaces (inside or outside tools) are liable to contain a flammable atmosphere, such as gas/vapors, dust/ fibers with air/oxygen? 2. Which process activities are liable to create an accumulation of electrostatic charges? The answers take the form of two lists: one for tools and the second for activities. These lists are considered together with process data about the timing of the creation of a flammable atmosphere and the accumulation of electrostatic charges, to provide one list indicating suspicious situations (spaces & elements) namely potential failure points (pfp).

5 5 IV. STAGE II VERIFICATION OF FAILURE POINTS In order to confirm or negate failure point validity it is necessary to evaluate the two main components of each p.f.p.: 1. Type of electrostatic discharge, so as to decide whether or not its nature matches the type of the corresponding atmosphere. For example, a corona discharge is considered as not capable of igniting gas or vapors, while a propagating brush discharge is capable of igniting gas, vapors and dust. The analysis of type of discharge considers the construction of each element, the materials it is made from, the charge density or electric capacitance, the vicinity to other elements. Using mathematical calculations and/or electrostatic simulations, assessment is made for the discharge type. 2. Classification of flammable atmosphere for zoning and drawing the boundaries of a three-dimensional area, so as to decide whether or not the electrostatic ignition source is enclosed within this space. The assessment of the classes is made according to standard documents such: - NFPA 497 (Gas/Vapors) - NPPA 499 (Dust) - IEC 79 (Gas/Vapors) The output from these two examinations is summarized in a list that contains detailed information for each valid failure point. This failure point list indicates the real hazardous situations existing in the plant. V. STAGE III QUANTITATIVE ANALYSIS AND FAILURE MECHANISM The understanding of the failure mechanism is the key for credible and appropriate measures to minimize the danger of ignition. Quantitative analysis of the electrostatic behavior at each failure point is conducted through mathematical calculations for the amount of releasable energy expected in the specific discharge type for a certain failure point. The released energy calculation should be based upon the concept of Equivalent Energy for charged insulating material systems, capacitance/field intensity for charged conducting material systems, as well as charge densities and the arrangement of the charge donor/acceptor. The calculations should indicate the preliminary (threshold) conditions needed for a discharge to take place, as well as the discharge path. If possible, an electrostatic simulation of the discharge set-up (charge donor/acceptor) could assist in identifying the exact path. Knowing the conditions, energy and path of the

6 6 discharge, a scenario of the ignition occurrence is than recorded (sometimes more than one scenario is possible). Going through this scenario, reasonable ideas for the prevention of ignition are raised and discussed. The discussion of these approaches is done together with the plant team, so the chosen solution is most likely to be both feasible and effective. Using the FPA method over the last ten years the author conducted over 100 projects in the process industry, including examination of plants at the design stage, existing processes and plants, and investigation of fire and explosion events. The most significant benefit of this method manifested itself in the fact that following the implementation of the recommendations no recurring events were encountered in the processes examined. REFERENCES [1] N. Gibson and F.C. Lloyd, Br. J. Applied Physics, Vol. 16, pp , [2] W. M. Bustin, and W. G. Dukek, Electrostatic hazard in the petroleum industry (Book style), Research Studies Press Ltd, England, [3] H Kramer and K. Asano, Incendivity of sparks from surfaces of electrostatically charged liquids, Journal of Electrostatics, 6 (1979), pp [4] P. Tolson, The stored energy needed to ignite methane by discharges from a charged person, Journal of Electrostatics, 8 (1980), pp [5] E. Heidelberg, The ignition of explosive mixture by static electricity, Proceedings of the 1 st Intrnational Conference on Static Electricity, Vienna, 4-6 May, [6] T. V. Selivanova, V. A. Olishevets and V. D. Koshevaya, Electrification of the powders used to make titanium carabide by self-propagating synthesis under conditions of sieving and pouring, Plenum Publishing Corporation 1984, Translated from Fizika Goreniya I Vzryva, Vol. 19, No. 5, pp , September-October, [7] B. Maurer, Discharges due to electrostatic charging of particles in large storage silos, Ger. Chem. Eng. 4 (1979) pp [8] J. A. Cross, Electrostatic problems in powder handling, Wolfson Applied Electrostatics Advisory Unit, University of Southampton, Paper No. 1, Session E, 9 th March, [9] M. Glor, Electrostatic hazards in powder handling, Research Studies Press Ltd, England, [10] L. G. Britton, Avoiding static electricity hazards in chemical operations, (Book style), American Institute of Chemical Engineers, New York, USA, [11] S. Ose and S..R. Silva, Electrostatic characterization of powders, The Second Israel Conference for Conveying and Handling of Particulate Solids, Jerusalem, Israel, [12] M. Bailey, P. Hooker, P. Caine, and N. Gibson, Incendivity of electrostatic discharges in dust clouds: the minimum ignition energy problem, 2 nd Internet Conference of Safety in Process Industry, March, [13] J. E. Owens, B. E. Schorn, Electrostatic ignition hazards with flammable liquids, IEEE Transaction on Industry Applications, Vol. IA-16, No. 6, November-December, [14] H. Strawson, Electrostatic explosions and fires, Tribology Practical Reviews, 1 March, 1973

7 [15] API 2003 Protection against ignition arising out of static, lightning and stray currents, 5 th edition, American Petroleum Institute, [16] Model Code of Practice for Petroleum Industry: Part 1, Electrical safety code, Institute of petroleum, U.K., [17] British Standard 5958, Code of practice Control of undesirable static electricity, BSI, U.K., [18] CENELEC Technical Report CLC/TR Electrostatics code of practice for the avoidance of hazards due to static electricity, June, [19] NFPA 77, Static Electricity, USA, August, [20] E. Zadok, Static electricity as an ignition source in the process industry, The 8 th National Conference of The Israel Society for Quality Assurence, Tel-Aviv, November,