SAE-AIR-1662A-1998-R2007.pdf

AEROSPACE INFORMATION REPORT AIR1662 REV. A Issued1984-10 Revised1998-09 Supersedes AIR1662 Minimization of Electrostatic Hazards in Aircraft Fuel Systems FOREWORD Changes in this revision are format/editorial only. INTRODUCTION The prevention of fires and explosions resulting from electrostatic discharges in aircraft fuel systems is of special concern to aircraft designers and operators. The purpose of this SAE Aerospace Information Report (AIR) is to assist in reducing the hazard by a review of the physics of electrostatic phenomena, a brief survey of electrostatic incidents and accidents, and a recounting of design practices which reduce the hazard. This document is not a complete design guide; extensive research and testing will be required to produce a successful design. An excellent review of the literature by Leonard (Reference 1) and a record of experiences and practices by the American Petroleum Industry (API, Reference 2) are useful supplements. The API has summarized the conditions necessary for an incendiary electrostatic discharge as follows: a.There must be a mechanism to generate electrostatic charge b.There must be a means to accumulate electrostatic charge in enough quantity to produce an incendiary spark c.There must be a means of discharging the accumulated electrostatic charge in the form of an incendiary spark, that is, a spark gap d.There must be a combustible vapor in the spark gap The hazard, then, may be eliminated by designing to prevent one or more of these necessary conditions. SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising therefrom, is the sole responsibility of the user.” SAE reviews each technical report at least every five years at which time it may be reaffirmed, revised, or cancelled. SAE invites your written comments and suggestions. Copyright © 2007 SAE International All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: 724-776-4970 (outside USA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.org Reaffirmed 2007-12 Copyright SAE International Provided by IHS under license with SAELicensee=MHI - NAGOYA related to 3944000/3944000013 Not for Resale, 01/23/2008 21:40:04 MSTNo reproduction or networking permitted without license from IHS -,-,- SAE AIR1662 Revision A - 2 - TABLE OF CONTENTS 1.SCOPE .3 2.REFERENCES .3 3.PHYSICS OF ELECTROSTATIC CHARGE GENERATION AND ACCUMULATION IN AIRCRAFT FUEL SYSTEMS .4 4.INCENDIVITY OF ELECTROSTATIC DISCHARGES8 5.CRITERIA FOR IGNITION8 5.1Metal-to-Metal Discharge - Ignition Criteria .10 5.2Dielectric-to-Metal Discharge - Ignition Criterion .14 5.3Influence of Temperature.14 5.4Effects of Fuel Mists.17 6.CAUSE AND PREVENTION OF ELECTROSTATIC HAZARDS IN AIRCRAFT.17 6.1Causes of Electrostatic Hazards18 6.2Techniques to Prevent Charge Accumulation20 6.3Other Techniques to Prevent Incendiary Discharges.22 7.DESIGN AND OPERATING RECOMMENDATIONS23 Copyright SAE International Provided by IHS under license with SAELicensee=MHI - NAGOYA related to 3944000/3944000013 Not for Resale, 01/23/2008 21:40:04 MSTNo reproduction or networking permitted without license from IHS -,-,- SAE AIR1662 Revision A - 3 - 1.SCOPE: This SAE Aerospace Information Report (AIR) provides background information, technical data and related technical references for minimization of electrostatic hazards in aircraft fuel systems. Techniques used to minimize the electrostatic hazard include: a.Reducing fueling rate into tank bays including use of multiple refueling inlet nozzles. b.Reducing refuel plumbing flow velocities. c.Introducing fuel into the tank at a low velocity near the bottom and directing it to impinge upon a grounded conducting surface. d.Avoiding electrically isolated conductors in the fuel tank. e.Using conductivity additives in the fuel. 2.REFERENCES: 1.Leonard, J. T., “Generation of Electrostatic Charge in Fuel Handling Systems: A Literature Survey,“ Naval Research Laboratory Report 8484, September 1981. 2.“Recommended Practice for Protection Against Ignition Arising Out of Static, Lightning and Stray Currents,“ American Petroleum Institute RP2003, Washington, DC, Fourth Edition, 1982. 3.Lewis, B. and von Elbe, G., “Combustion, Flames and Explosions of Gases,“ 2nd Edition, Academic Press, New York, NY, 1961. 4.Bridges, J. E., et al, “Gap Energy, a More Accurate Criterion for Ignition Threshold of Flammable Mixtures,“ 1975 IEEE Electromagnetic Compability Symposium Record, San Antonio, TX, October 1975. 5.Lyle, A. R. and Strawson, H., “Estimation of Electrostatic Hazards in Tank Filling Operation,“ in “Static Electrification, 1971,“ Institute of Physics, London, 1971. 6.Grenich, A. F. and Tolle, F. F., “Electrostatic Safety with Explosion Suppressant Foam,“ AFWAL-TR-83-2015, Wright-Patterson Air Force Base, OH. 7.Eggers, D. F., et al, “Physical Chemistry,“ Wiley and Sons, New York, NY, 1964. 8.Desmarais, L. A. and Tolle, F. F., “Integrated Aircraft Fuel Tank Inerting and Compartment Fire Suppression System,“ Vol. I, AFWAL-TR-83-2021. 9.Kuchta, J. M., “Fire and Explosion Manual for Aircraft Accident Investigators,“ AFAPL-TR-73-74, August 1973. 10. “Procedures for the Use of Fuels for Turbine Powered Aircraft,“ Department of Transportation Order No. 8110.34A, March 1980. Copyright SAE International Provided by IHS under license with SAELicensee=MHI - NAGOYA related to 3944000/3944000013 Not for Resale, 01/23/2008 21:40:04 MSTNo reproduction or networking permitted without license from IHS -,-,- SAE AIR1662 Revision A - 4 - 3.PHYSICS OF ELECTROSTATIC CHARGE GENERATION AND ACCUMULATION IN AIRCRAFT FUEL SYSTEMS: When a hydrocarbon liquid such as jet fuel flows past a surface, positive and negative electrostatic charges can be separated along the surface. While the precise nature of the charging mechanism is not known, it appears to be associated with the presence of minute quantities of ionic impurities in the hydrocarbon. Ionic impurity mechanisms for charge generation on either metallic or non-metallic surfaces have been proposed by Leonard and others (Reference 1); whatever the actual mechanism, it is an observed fact that a hydrocarbon liquid flowing over a surface can acquire a charge, with the contact surface acquiring the opposite charge. Figure 1 shows a pipe wall with an affinity for negative charge, resulting in a net positive charge in the body of the fuel. If the fuel is set into motion and charge is separated, the immediate re-association of the separated charges is hindered by the very low electrical conductivity of highly refined hydrocarbon fuels. Charge is, therefore, convected away by the liquid flow, in opposition to the electric field between the liquid and its surroundings, creating a potentially hazardous condition. Whether the fuel becomes positively charged (as shown in Figure 1) or negatively charged depends on the combination of fuel, impurities, and containers involved. FIGURE 1 - Proposed Charge Generation Mechanism (Reference 1) Copyright SAE International Provided by IHS under license with SAELicensee=MHI - NAGOYA related to 3944000/3944000013 Not for Resale, 01/23/2008 21:40:04 MSTNo reproduction or networking permitted without license from IHS -,-,- SAE AIR1662 Revision A - 5 - 3. (Continued): One of the main charge generators during aircraft refueling is the ground refueling equipment dirt filter/ water coalescer-separator unit (Figure 2); the separator may add to the charge produced by the filter- coalescer (Figure 2A) or tend to neutralize it (Figure 2B); with another fuel, the net charge produced might be of opposite sign. In general, any filter has the potential to be a prolific charge generator because of the very large surface areas involved. FIGURE 2 - Charge Separation in a Filter/Separator (Reference 1) As charged fuel flows through refueling hoses and pipes and enters the aircraft tanks (Figure 3), charges have an opportunity to be eliminated by migrating to the walls and re-associating with the opposite charge, a process described as relaxation. The process of the recombination of charges can be described as a function of time. If the fuel flows into a grounded container, and if the flow is terminated after a total charge Qo has been accumulated in the container, experiments show that the mutual repulsion of the like charges in the fuel tends to cause them to migrate in the direction of the liquid fuel boundaries which are the normally grounded container walls and the electrically isolated fuel-air interface. The charge remaining in the container after a time from the start of an observation interval can be approximated as shown in Equation 1: (Eq. 1) Q Qo -e k o - = Copyright SAE International Provided by IHS under license with SAELicensee=MHI - NAGOYA related to 3944000/3944000013 Not for Resale, 01/23/2008 21:40:04 MSTNo reproduction or networking permitted without license from IHS -,-,- SAE AIR1662 Revision A - 6 - FIGURE 3 - Generation and Neutralization of Electrostatic Charge During Aircraft Fueling (Reference 1) 3. (Continued): where: Qo = Initial charge (Coulombs) Q = Charge at time (Coulombs) = Elapsed time (seconds) k = Fuel rest electrical conductivity (Siemens/m) = Relative dielectric constant compared to vacuum (for hydrocarbons, 2) o = Dielectric constant of vacuum (8.854 x 10-12 ampere seconds/volt meter) It is customary to quote a characteristic “relaxation“ time for electrical charges, normally the time required for the original charges to be reduced by a factor 1/e as they flow to the grounded walls. Defining the relaxation time as , it follows that: (Eq. 2) k o - -1 or o k - - 18 x 10 12 k -= Copyright SAE International Provided by IHS under license with SAELicensee=MHI - NAGOYA related to 3944000/3944000013 Not for Resale, 01/23/2008 21:40:04 MSTNo reproduction or networking permitted without license from IHS -,-,- SAE AIR1662 Revision A - 7 - 3. (Continued): It is also customary to describe the electrical conductivity of fuel in terms of conductivity units (CU): 1 CU= 1 picoSiemen/meter (pS/m) = 10-12 Siemen/meter = 10-12 ohm meter-1 Jet fuel electrical conductivity without antistatic additive normally ranges from 0.1 to 20 CU (Reference 1), so that typical relaxation times range from 180 to 0.9 s according to Equation 2. In actuality, the relaxation time for fuels of very low conductivity is shorter than the prediction of the equation, and accepted practice is that 30 s is sufficient to remove most of the charge from fuel after the end of a filtering operation (Reference 2). During fuel flow into a receiver such as an aircraft fuel tank, charge may be introduced with the fuel at the same time that the charge already present is relaxing. If the charging rate is more rapid than the rate of relaxation, then charge will continue to accumulate in the tank and create potentially hazardous situations. As shown in Figure 4, the residual charge which migrates toward the fuel-air interface tends to produce a charged region near the surface, causing strong electric fields which can lead to discharges to nearby grounded objects. FIGURE 4 - Discharge From Fuel Surface to Tank (Reference 1) In some military aircraft which contain explosion suppressant foam in their tanks, charge may also be produced within the fuel tank depending on how the refueling flow is introduced into the tank. Even though the fuel may not be charged as it enters the tank, if the incoming fuel impinges on the foam with substantial flow rate or velocity, the foam presents a very large surface area on which charges can be separated. One charge type will reside on the foam, while the opposite charge will be convected away as the fuel percolates through the foam. In this case, the charge residing on the foam is the principal hazard, and can be of either polarity depending on the type of foam used. Copyright SAE International Provided by IHS under license with SAELicensee=MHI - NAGOYA related to 3944000/3944000013 Not for Resale, 01/23/2008 21:40:04 MSTNo reproduction or networking permitted without license from IHS -,-,- SAE AIR1662 Revision A - 8 - 4.INCENDIVITY OF ELECTROSTATIC DISCHARGES: The strength of the electric field produced in the vapor space (ullage) above a fuel is roughly proportional to the quantity of charge present. If the electric field strength reaches a critical value, called “the breakdown value“, a discharge will occur. In case a discharge occurs from a dielectric surface (such as fuel explosion suppressant foam, or a convenient experimental analogue such as plexiglas) to a grounded metal object in the vapor space, only the charge on or near the dielectric surface will be involved, and further, only limited areas on the charged surface will participate (Figure 5); the reason is that the mobility of charge in the dielectric is very low because of the low conductivity of the dielectric and because of the short duration of discharges. It has also been observed that the discharge at its origin at the dielectric surface is diffuse, tending to coalesce into a bright filament in the vicinity of the metal object. In a number of instances, simultaneous multiple discharges have been observed in experiments with plexiglas; indirect measurements suggest the same phenomenon occurs in discharges from fuel. Under some circumstances, a dielectric can discharge to a metal object with little hazard. In these circumstances, a design which prompts a low intensity discharge or discharges which can relax the electric field may be beneficial. Under other consequences the discharge may be incendiary. The difference between the circumstances may be due to more than a single variable in practical fuel system designs. Discharges in fuel tanks may also be from one metal surface to another. If a fuel or foam surface comes in contact with an ungrounded metal object in the fuel tank, some of the charge contained in the fuel or foam will migrate into the metal object (Figure 6). If the accumulation of charge on the ungrounded metal object causes its voltage to reach a sufficient value, it can in turn discha