DEPARTMENT OF EMPLOYMENT The disaster Report of the Court of Inquiry London: HerMajesty's StationeryOffice Crown copyright 1975 First Published 1975 ISBN 01 1 361075 0 ii Formal Investigation into Accident on 1st June 1W4 at the Nypro Factory at Flixborough Appointment of persons to hold investigation The Secretaryof State,in exerciseof his powers under Section 84 of the Factories Act 1961, herebydirects a formalinvestigation to be held under the saidSection 84into the accidentwhich occurredon the 1st June 1974 at the factoryof Nypro (UK) Limited at Flixborough. The Secretaryof State,in further exerciseof his said powers under the saidSection 84, hereby appoints: ROGER JOCELYN PARKER, ESQ, QC, (Chairman) JOSEPH ALBERTPOPE, ESQ, DSc, PhD, WhSch, CEng, FIMechE, (Deputy Chairman) JOHN FRANK DAVIDSON, ESQ, MA, PhD, ScD, FRS, CEng, FIChemE, MIMechE WILLIAM JAMES SIMPSON, ESQ to hold thesaid investigation. The Secretaryof State further appointsB~NARD MICHAEL O'REILLY, ESQ, FRIC, to be Secretaryof the saidinvestigation. Michael Foot Secretaryof Statefor Employment 27th June1974 Introduction l Explosion exerting downward forceon 20 inch assemblyand at the same timean upward force Scopeof theinvestigation I on the motorssufficient to blow themup andto the west Site and generalhistory of FlixboroughWorks 2 Vertical emissionfrom 8 inch rupturelasting for Layout of the works 3 shortperiod after themain explosion Organisationof the Company 3 General observationson the &inch hypothesis 29 The Board of Directors Damageconnected with the 8 inch line 30 EssentialManagement Fracturedpipe-hanger, spring and angleiron The EngineeringDepartment Orientationof zinc cracks in the8 inch line Section25A - The process andlayout 4 Deformationof elbow K and 40" pipe twist General descriptionof process Deformationof elbow G Detaileddescription 20-inch and &inch hypotheses compared32 Startup procedures 6 Miscellaneous 32 Eventsfrom 28thMarch to 1stApril 7 (a) Security (b) Safety precautionsgenerally (c) Storageof potentially dangerous substances Eventsfrom 1stApril to 29th May I I Lessonsto be learned 32 Eventsfrom 29hMay to explosion I I Introduction The explosion andaftermath 13 General observation Sourceof Main Explosion Immediate lessons to be learned from the disaster The problemstated 15 Specific lessons 34 Two-stage ruptureof the by-pass 15 Integrity of plant Eye-witness,film and photographicevidence 15 Management operation Evidenceof the laboratorywitnesses Generallessons 35 Witnessessaid to have seen somethingbefore Nitrogen supply the first event whateverthat event may have Nitrate stress corrosion been Zinc embrittlementof stainlesssteel Conclusions oneye-witness evidence Creep cavitationof stainless steel Films and photographs The 20-inchhypothesis examined Miscellaneouslessons 36 Explosionof vapour clouds in theopen air The &inchhypothesis I9 Investigationof the disaster Background The stepsin the hypothesis Mattersto be referredto the special committeeor otherbodies 36 Examination of individual steps in &inch Plant layout and construction hypothesis 22 Siting of plant Loosebolts Planning procedures Leaksand laggingfire Emergencyarrangements Gasket burst Licensingstorage The flame to produce thedesired results Clad mild steel Fire in areaof fins and embrittlementof enclosed pipes Summary 37 Costs 38 Appendix III Procedural history 50 Acknowledgements 39 Appendix IV List of representations52 Appendix I Experimental and otherevidence Appendix V List of witnesses 53 relating to rupture of 20 inch pipe and bel- lows assembly from internal pressure andAppendix V1 Reports 55 temperature 40 1 Reportsgiven in evidence Flixboroughtests by SMRE 2 Reports referred to in evidence Nottinghamtests onbellows 3 Reports madeavailable to the Court Bretby pipe test Dynamic failure of bellows and pipe Appendix II Damageto the 8 inch pipe and related metallurgicalinvestigations 46 Damageto the pipe Damageto pipe hangers,support bracket, gas- Abbreviations kets and pipe lagging AISI AmericanIron andSteel Institute Displacementof separators BS British Standard Metallurgical investigations BTU British Thermal Unit C Column Creep cavitation DSM Dutch StateMines Swelling of the pipe H Hanger HMFI Her Majesty'sFactory Inspectorate Zinc cracking NCB National CoalBoard Sourcesof zinc NRV Non ReturnValve PCT ProcessControl Technician Zinc transferto stainless steel R Reactor Bulge at the 3 inch split S (four fig. number) Separator S Section Pipehanger spring temperature SMRE Safety in Mines Research Pipe hanger rod temperature Establishment standardwire gauge Summary of metallurgical evidence as to Valve temperatures Mixer Works of Nypro (UK) Ltd beforethe explosion.View from the south-east. Works after theexplosion. View from the south-east. Works after theexplosion, seen from the north. The conflagration,seen from the south-west. The fires seenfrom the south-west. Model of section25A kept in the control roomand destroyedin the explosion. Reactor5 showingcrack which ledto its removaland replacement by the by-pass inMarch 1974. Reactors4 and 6 soon after theexplosion. Thejack-knifed 20 inch pipe with attachedflanges and 28 inch stubpipes. Thejack-knifed 20 inch pipe showingtrapped aluminium cladding and charredwood. Squirmedlower bellowsin the SMRE simulation test. Squirmedbellows in the NottinghamRig. Buckled pipe at Bretby. The 50 inch ruptureon the 8 inch pipe. Broken and bent pipehanger rod, and spring. View of the 8 inch pipewhile testingwater spray system during plantacceptance tests (pipe partly lagged). Typical water spraynozzle with air-operated sensorbelow. Photograph taken after theexplosion. REPRODUCED BY COURTESY OF HMFI, DR K GUGAN, "SUNDAY EXPRESS', "SCUNTHORPE EVENING TELEGRAPH", NYPRO, SMRE, C H WOOD (BFD) LTD AND CREMER & WARNER. vii List of Figures Simplifiedplan of Flixborough worksof Nypro (UK) Ltd. Cyclohexaneoxidation plant Section25A. Simplified plan and elevationof part of Section25A. Arrangementof 20 inch pipescaffolding (as deducedfrom theevidence). Section25A - chartused for recording oxidationsystem pressure. Sketchof pipe and bellows assembly showingshear forceson bellowsand bending momentson pipe. Principleof NottinghamTest Rig. Sketchof 8 inch lineas viewed from north-west. View of 8 inch line showingtypical lagging boxaround valves as viewed from the east. The damaged8 inch pipe. Stepsin the "8-inch pipe" hypothesis. To the Rt Hon Michael Foot, MP, Secretaryof Statefor Employment Introduction 1 At about 4.53 pm on Saturday1st June 1974the Flixborough Worksof Nypro (UK) Limited (Nypro) were virtually demolishedby an explosionof warlike dimensions.Of thoseworking on the site at the time, 28 were killed and 36 otherssuffered injuries. If the explosionhad occurredon an ordinary working day,many morepeople would havebeen on the site, and the numberof casualties would havebeen much greater. Outside the Worksinjuries and damagewere widespreadbut no-one was killed. Fifty-threepeople were recordedas casualties by the casualty bureauwhich was setup by the police; hundreds moresuffered relatively minor injurieswhich werenot recorded.Property damage extended overa wide area, anda preliminary surveyshowed that 1,821houses and 167 shops and factories hadsuffered to a greateror lesser degree. 2 On 27th June1974 you appointedus underSection 84of the FactoriesAct 1961to hold a formal investigationto establishthe causesand circumstancesof the disaster andto point out any lessons which we might considerwere immediatelyto be learned therefrom. 3 As requiredby Sub-Section3 of Section 84of the Act we held our investigationin open Court. The hearingsoccupied a totalof 70days. They openedat the Wortley Hotel in Scunthorpeon 9th September 1974.After five days, mostly devotedto the takingof eye-witness evidence,the hearings were adjourned to London. In all we heardevidence from 173 witnesses. 4 Prior to our appointment,investigations and an interim report had alreadybeen madeby Her Majesty's Factory Inspectorate(HMFI), and investigationswere beingpursued by many other persons andbodies. Betweenour appointment and the openingof the investigationat Scunthorpe the following stepswere taken: (a)we held a preliminary meetingfor the purposesof giving directions as to representation and procedure; (b) the taskof assembling theevidence for presentationat the hearing was undertakenby the Treasury Solicitor;(c) Messrs Cremer and Warner, Consulting Chemical Engineersof 140 BuckinghamPalace Road, LondonSWlW 9SQ, were appointedat our requestas technical advisers to the Court and instructedto seekand provideto the Treasury Solicitor evidenceas to the causesof the disaster; (d) thework of recovering, identifyingand examining wreck- age,and of conductingrelevant tests,was undertaken by the Safety inMines ResearchEstablishment at Sheffield(SMRE) to whom we wish to pay a specialtribute; (e) further investigationswere pursued by other persons andbodies. 5 During the courseof our investigationsa large numberof scientificreports were submittedto us and circulatedto all parties.A completelist of suchreports appearsin AppendixVI. Many of such reports althoughof greatvalue to the ascertainmentof the causeof the disasterhave no wider applica- tion. Others containscientific information of a more general kindwhich, we believe, wouldbe of value to publish. We havemarked the paperswhich fall into this categorywith an asterisk. Scope of the investigation 6 The evidencebefore us establishedconclusively at an early stage two basic facts. (a) thatthe cause of the disasterwas the ignition and