Failure of CNG Cylinder

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  CNG cylinder burst in a bus during gas filling – Lesson learned G. Bhattacharjee, S. Bhattacharya, S. Neogi, S.K. Das * Department of Chemical Engineering, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India a r t i c l e i n f o  Article history: Received 23 July 2009Received in revised form 11 January 2010Accepted 5 May 2010 Keywords: CNG cylinderBus accidentHuman error a b s t r a c t An accident leading to bursting of a compressed natural gas (CNG) cylinder fitted to a passenger bus inIndia resulted one person died and four persons injured. This paper presents the incident, the human fac-tor involved, safety issues and lesson learned.   2010 Elsevier Ltd. All rights reserved. 1. Introduction Compressed natural gas, CNG, is the natural gas compressedinto very high pressure of usually 3000–3600 psi (Ahmad, 2004).Natural gas is lighter than air, and mixtures of air and naturalgas are inflammable only in a fairly narrow range of gas concentra-tions between 3.7% and 17% by volume ( dated 20.7.09).Natural gas is an environmentally clean, plentiful, low-cost fuelfor motor vehicles. Chemically, it normally consists of over 90%methane with smaller amounts of ethane, propane, butane, carbondioxide and other trace gases. The high methane content gives nat-ural gas its high octane rating (120–130) and clean-burning char-acteristics, allowing high engine efficiency and low emissions.CNG has the following safety features that make it an inherentlysafer than petrol, diesel or LPG (Ahmad, 2004),(1) It is lighter than air so if it leaks, it just rises up and dissi-pates into the atmosphere.(2) It has a self-ignition temperature of 700   C as against 455   Cfor petrol.(3) CNG has to mix with air within a small range of 3.7–17% byvolume for combustion to take place. This is a far narrowerrange than for petrol.(4) CNG cylinders are designed and built of special materials towithstand high pressures, therefore, are far safer than ordin-ary petrol tanks.It is being used for many years in vehicles in countries likeArgentina, Italy, Pakistan, Brazil, USA and New Zealand. CNG usedas fuel for vehicles in only late 1990s in India. In Delhi (May 2001),the number of CNG buses rapidly increased from 900 to 7000,which represented perhaps the largest city CNG bus fleet in theworld (Erlandsson and Weaver, 2002). In general, the major prob-lem of the CNG bus in India is it is not suitable for travel at least200 miles at a time due to non-availability of the CNG filling sta-tion all over the country. The danger of fire in case of leakageCNG would be greatest near the ceiling and for the liquid fuel itis always near the floor. The strength of the natural gas cylindersand fuel system generally avoids any leakage or fire. The accidentaldataare available in diesel powered vehicles but fatal accidentdataand experience are limited for CNG buses as it started recently inIndia. The major causes for CNG bus accident are due to the gaspipe disconnection, accident due to driver’s errors, related to fillingoperation and structural failures. Time has proven that natural gasvehicles safe in actual operation all over the world. This paperdeals with incident that leads to burst one cylinder and killedone person and injured four persons is based on data collection,examination of material of construction of burst cylinder, scientificanalysis, interviewed the eye witness and the role of human factorfor the incident. 1.1. Human factor  The human factor defined by Health and Safety Executive as‘‘Human factors refer to environmental, organizational and jobfactors, and human and individual characteristics which influ-ence behavior at work in a way which can affect health and safety”( dated 0925-7535/$ - see front matter    2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssci.2010.05.002 *  Corresponding author. Tel.: +91 33 2350 8386x247; fax: +91 33 2351 9755. E-mail address: (S.K. Das).Safety Science 48 (2010) 1516–1519 Contents lists available at ScienceDirect Safety Science journal homepage:  6.7.09). Human errors generally categorized into several types asoperating errors, design errors, maintenance errors, fabricationerrors, inspection errors and contributory errors (Meister, 1962).Lincon (1960) classifies five distinct types of errors in operatingequipment and are as follows:1.  Attention errors  – operator fails to pay attention to things whenrequire attention.2.  Memory errors  – operator’s memory fails to perform a requiredtask as specified in the operating procedure.3.  Interpretation errors  – misunderstands the meaning of giveninformation and takes action accordingly by the operator.4.  Operation errors  – incorrect action by the operator to achievethe desired effect.5.  Identification errors  – incorrect identification of an item and sothe subsequent decision is incorrect.The operating errors are highlighted in Fig. 1. Human errors arealso categorized as 1.2. UnintentionalSlips  – are failures during the actual execution of actions,speech, etc. (i.e. ‘actions-not –as-planned’). The plan itself may or may not be acceptable. Lapses  – are failures at the storage or recall stage (i.e. forgotteninformation, planned actions, etc.). Again, the plan may or maynot be acceptable. Mistakes  – are errors in the selection of objectives (or faultyintentions) or the means to achieve them (the plan). Sointended actions may proceed as planned, but fail to achievetheir intended outcome. Not understanding properly howsomething works or an error of diagnosis or planning. 1.3. Intentional Violations – are situations where operators deliberately carry-out actions that are contrary to organizational rules, operating pro-cedures, etc. 2. The gas cylinders The bus srcinally has 12 nos. of cylinders and was conformingIS: 15490:2004 specifications(IndianStandard Code).These 12 cyl-inders were cascaded and had a single filling point. These cylinderswere adopting sound engineering practice. The valve was of  IS:3224:2002 standard. Among 12 cylinders, six cylinders have 80 L water capacity and other six have 50 L water capacity. The ownerhad also installed three additional gas cylinders of 80 L watercapacity which two were conforming the IS: 15490:2004 specifica-tions and the others were not. The two cylinders (confirming ISspecification) were installed by local vender, had one filling pointand the valve is satisfying the IS: 3224:2002 standard. The othercylinder, not confirming IS specification, had separate filling point.This cylinder and its fittings were not confirm the IS specifications.The automobile fuel station was granted the licence in Form Gof  Gas Cylinder Rules 2004 by the Chief Controller of Explosives,Nagpur. The licensee had to use only one filling point for CNGbus. But unfortunately, the owner of the bus and the filling stationoperator always used portable filling assembly with multivalvearrangement for the additional three cylinders. 3. The incident Ataround 14 = 20 h, first week of July,2007, the bus reachedtheauthorized filling station for refuelling. The chassis fitted srcinallywith 12 nos. of cylinders were filled by the CNG gas. The bus hadone portable type of filling assembly and was used to fill otherthree cylinders. The two cylinders (confirming the IS specification)were also filled by the CNG gas. During the gas filling in the lastcylinder, a burst took place at around 14 = 40 h. This cylinderwas unable to withstand high pressure. Due to the burst, this cyl-inder was totally dislodged from its grouting and found lying inpieces in the ground. Seats and windowpanes of the bus were alsobadly damaged. Clamp for holding this additional cylinder fitted tothe bus was found at the scene of occurrence. The CNG intake linefrom these cylinders (three additional cylinders) to the engine wasfound damaged. Twelve nos. CNG cylinders with valves and pipingof srcinal CNG cylinders cascade fitted to the bus were found to-tally intact. The pressure of CNG in the cascade fitted to the bus Fig. 1.  Human errors – highlighted operating errors. G. Bhattacharjee et al./Safety Science 48 (2010) 1516–1519  1517  was found to be 170 kg/cm 2 (g). All the tyres and tubes of the buswere found intact. The facilities in the dispensing station with CNGcascade, CNG compressor and two numbers of CNG dispensers,canopy, sales room, electric transformer room, CNG pipelines andboundary wall of the premises were found intact.  3.1. Damage to the property The CNG cylinder, not confirming IS specification, is damagedinto pieces, the windowpanes and rear seats of one passengerbus received substantial damage due to bursting. Portable fillingassembly with multi valve system was makes also found to bedamaged.A. Injured: four persons.B. Died: one person (on the spot due to head injury). 4. Research of technical failures 4.1. Analysis of the burst cylinder  The sample piece of burst cylinder was examined – a compara-tive statement showing the test result of the burst cylinder andstandard quality as required under CNG cylinder specificationsare shown in Table 1. 4.2. Examination of the burst cylinder  Twelve cylinders were installed in the bus by manufactureradopting sound engineering practice and cylinders were of  IS:15490:2004 standard. These 12 cylinder cascade has one fillingpoint. Two cylinders of  IS: 15490:2004 standard were installedby the bus owner through local vender. This cylinder had one fill-ing point and was filled by one portable filling assemblyusing mul-tivalve and it violates all the engineering practice.One cylinder fitted by local vender of unknown standard withone additional filling pointhad been fitted to the bus and it violatesall the engineering practice. From the test result, it is evident thatTensile Strength and Yield Strength, Carbon, Chromium, Molybde-num and Manganese content are low compared with the standardcylinder. This cylinder is not fit for CNG service. Thus, it can beemphatically that the burst cylinder of unknown srcin was totallyunfit for CNG service. 5. Research of human failures 5.1. Provision of additional (2 nos.) filling point in the bus The bus operator with the intension of running the bus for moredistances has decided to install additional cylinder with portablefilling facility. The bus operator got additional cylinders fittedwithout obtaining the technical approval of bus manufacturerand also it violates of Rule 45 of Gas Cylinders Rules 2004. Humanerrorshave been generally recognizedas the unsafe acts performedby the bus operator. 5.2. Unsafe human activities The automobile fuel station licensee has failed to use only oneapproved filling point in the bus. Instead, they used additionaltwo portable filling assembly to fill additional three cylinders.1. Portable filling assembly with multivalve was used by operatorof the fuel station. This is against the design of CNG refuellingsystem.2. Owner of the bus should not have additional fuel cylinderswithout the prior technical approval of bus manufacturer.3. Additional cylinders installed in the bus are unapproved andviolation of Rule 45 of Gas Cylinders Rules 2004.4. Before filling CNG, no inspection was carried out by the fuel sta-tion operator.5. The filling operator does not have proper training and commit-ted the unsafe activity of refuelling with two portable fillingpoints to fill three additional cylinders in the bus which isagainst the commitment by the licensee of the fuel station. Itis the violationof Rule 16 of  Gas CylinderRules2004 committedby the filling station owner. 6. Probable cause of the accident On the examination of the physical and chemical analysis of material of burst cylinder, it may be concluded that physical andchemical property of the material do not meet the requirementof the material for CNG cylinder including the cylinders of  IS:15490:2004 standard. The ill-fated cylinder was unable to with-stand high pressure and the bus operator had no knowledge aboutCNG cylinder specification. Thus, the failure of material of the cyl-inder operating error is the cause of this accident.  Table 1 Chemical and mechanical properties of the burst cylinder and compared with the standard CNG cylinder. Sl. no. Name of the test Burst cylinder specification Standard CNG cylinders specification1 Micro structure Uniform distributed fine grained, ferrite andpearlite structure are normalizedUniform distributed fine grained ferrite andpearlite structure are normalized Chemical compositions (% by weight) 1 % Carbon 0.101 0.34–0.372 % Silicon 0.226 0.15–0.353 % Manganese 0.393 0.65–0.854 % Phosphorus 0.016 0.020 (MAX)5 % Sulphur 0.015 0.020 (MAX)6 % Nickel 0.112 0.15 (MAX)7 % Chromium 0.116 1.00–1.208 % Molybdenum 0.019 0.15–0.95 Mechanical properties 1 Yield strength (MPa) 349 7852 Tensile strength (MPa) 448.7 9283 % Elongation 22 144 Hardness (HRB) 83–84 106–1095 Bend test Satisfactory Satisfactory1518  G. Bhattacharjee et al./Safety Science 48 (2010) 1516–1519  7. Human errors Analysis is shown that an operating error was the main cause of this accident. The nature of the error can range from unintentionalways where mistakes occurred by the bus owner and subsequentlyby the bus operator. Twelve cylinders were installed in the bus bythe bus manufacturer adopting sound engineering practice andcylinders were IS: 15490:2004 standard. Additional three cylinderswere installed by the bus owner and two were IS: 15490:2004standard and the other was unapproved unknown standard. Instal-lation of these three cylinders was supplied by the local unap-proved vender. The three additional cylinders were two separatepotable filling facilities which were against the Rule 45 of Gas Cyl-inder Rule 2004. After examination of the sample piece of the burstcylinder, the result is evident that Tensile Strength and YieldStrength, Carbon, Chromium, Molybdenum and Manganese con-tent are at variance with higher degree with the standard cylinder.This cylinder is not fit for CNG service.In an intentional way, the violations occurred where the busoperator deliberately carries out actions that are contrary to orga-nizational rules, operating procedures, etc. The violations workedas routine violations to become the normal way of working be-cause the busowner and operatorinstalled unapproved threeaddi-tional CNG cylinders with two portable filling facilities which is theviolation of Rule 45 of  Gas Cylinder Rules 2004. Worker of the fill-ing station had not properly trained by the management and com-mitted unsafe activity on refuelling, which is violation Rule 16 of Gas Cylinder Rules 2004. Gas supplier had also failed to imparttraining to their dealer.The bus operator and the worker in the filling station are over-loaded thus under stress due to work load may neglect the normalprocedures of the CNG filling operation. 8. Lessons learnt and recommendations This accidental risk is greater with a bus than with a smallervehicle, since the large volume of fuel stored on bus increasesthe volume of the inflammable mixture, and the structure of thebus body can trap gas underneath where it is exposed to ignitionsources. Both government and industry could not explain this typeof CNG burst incidents to assuage public fears ( dated 11.5.09). The maincause of this accident is human error, resulted one person died andfour persons injured. The following are the suggested measures.(1) Traffic accidents and driver errors (human errors) can neverbe eliminated completely, but the appropriate design canreduce the probability of uncontrolled release of gas. Sosome combination and high-quality safety maintenancedesigns are required.(2) CNG cylinder should undergo a detail safety inspection atleast every 3 years or 36,000 miles or when they have beeninvolved collisions, accidents, fires or other damage (ANSI/CSA NGV2-2000 Sections 4.1.4 and 4.1.5).(3) CNG cylinders are requiring periodic inspection particularlythe cyclic loading and corrosion tests as specified by ANSI(American National Standards Institute) standard.(4) It is responsibility of any test agency to issue a type approvalcertificate when a vehicle meets the specified norms.(5) To reduce human error (knowledge based mistakes), the busowner should provide the necessary instructions about peri-odic inspections, safety checks and ‘‘dos and don’ts” to thebus operators.(6) To train periodically to the dealer/bus operator/bus ownerby the gas supplier for the safety aspect of CNG.(7) To provide the bus operator necessary training, tools andgadgets to carry out the installation, periodic inspectionand maintenance procedure.(8) Responsibility of owners and also the bus operator to followthe instructions while filling CNG gas to be carry out leakagetest periodically at least one in a year. Appropriate traininggiven to the bus operator.(9) Each and every converted bus should undergo inspection toassure that the CNG installation is safe and proper by theappropriate authorities.(10) Emissions certification of a particular CNG kit for a particularengine should apply to all applications of the same enginemodel in different vehicle types.(11) The approval authority, ARAI (Automotive Research Associa-tion of India) the only test agency presently in operation inIndia, should issue guidelines on how to interpret currentlaws, regulations and other relevant documents and elimi-nate unnecessary and duplicative tests. 9. Conclusion In India, during refuelling CNG, an unauthorized cylinder burstin a passenger bus and resulting in death of one person and injuryto four persons. The seats and windowpanes of the bus were foundbadly damaged. After investigation, it is shown that the bus oper-ator got additional cylinder fitted without obtaining the technicalapproval of bus manufacturer. Human errors, both unintentionaland intentional errors, occurred. In unintentional way, mistakesand intentional way violation occurred. At last, it is concluded thatto avoid such type of accident, reduce human error and providenecessary safety instructions about handling CNG cylinders tothe bus owner and also the bus operator. References Ahmad, R.A., 2004. CNG, Presented in IAAAE 24th Annual Convention, 6–7 March.ANSI/CSA NGV2-2000. Basic Requirements for Compressed Natural Gas Vehicle FuelContainers, CSA, <> (11.1.2010).Gas Cylinder Rules, 2004. The Gazette of India, Extraordinary, Part-II Section 3-Sub-section(i) New Delhi, Tuesday, 21st September, 2004, Ministry of Commerceand Industry (Department of Industrial Policy and Promotion).Erlandsson, L., Weaver, C. 2002. Safety of CNG buses in Delhi, findings andrecommendations, August 2, Centre for Science and Environment, New Delhi,India, <> (11.05.09).<>?ID=185971 (20.7.09).<> (6.7.09).IS:15490:2004. Cylinders for on – Board Storage of Compressed Natural Gas as aFuel for Automotive Vehicles-Specifications. Bureau of Indian standards, ManakBhavan, 9 Bahadur Shah Zafar Marg, New Delhi-110002.IS:3224:2002. Valve Filling for Compressed Gas Cylinders Excluding LiquefiedPetroleum Gas (LPG) Cylinder-Specification (3rd Revision). Manak Bhavan, 9Bahadur Shah Zafar Marg, New Delhi-110002.Lincon, R.S., 1960. Human factors in the attainment of reliability. IRE Trans. Reliab.9, 97–103.Meister, D., 1962. The problem of human-initiated failures. In: Proc. 8th NationalSym. Reliability and Quality Control, USA, pp. 234–239. G. Bhattacharjee et al./Safety Science 48 (2010) 1516–1519  1519
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