Deflagration And Detonation

ixContents7.5. Check List for Inspection1517.6. References1518Regulations, Codes, and Standards8.1. Regulations, Codes, and Standards Summaries1538.2. Comparison of Various Flame Arrester Standards and Codes1628.3. Standards and Codes in Preparation1658.4. References1658.1.1.8.1.2.8.1.3.8.1.4.United StatesCanadaUnited KingdomEurope and International1531581591609Illustrative Examples, Calculations, and Guidelinesfor DDA Selection9.1. Introduction1679.2. Example 1—Protective Measures for a Vent Manifold System1679.3. Example 2—Sizing of an End-of-Line Deflagration Flame Arrester 1699.4. Example 3—Calculation of Limiting Oxidant Concentration (LOC)1729.5 Example 4—Calculation of the LFL and UFL of Mixtures1729.6. Example 5—Calculation of the MESG of Mixtures1739.7. Determination If a DDT Can Occur1759.8. Typical Locations in Process Systems1759.9. List of Steps in the Selection of a DDA or OtherFlame Propagation Control Method1769.10. References17710Summary10.1. Status of DDA Technology179

xContents10.2. Recommended Practices18110.3. Why Flame Arresters Fail18210.4. Future Technology Development18210.5. References184Appendix AFlame Arrester Specification Sheet for Manufacturer Quotation185Appendix BList of Flame Arrester Manufacturers187Appendix CUL and FM Listings and Approvals191Appendix DSuggested Additional Reading193Glossary197Index209

1Introduction1.1. Intended AudienceThis “concept book” is intended for use by chemical engineers and othertechnical personnel involved in the design, operation, and maintenance offacilities and equipment where deflagration and detonation arresters(DDAs) may be required. These people are usually technically competentindividuals who are aware of, but not experts in, combustion phenomena.The facilities where such devices may be needed include chemical plants,petrochemical plants, petroleum refineries, pharmaceutical plants, specialty chemical plants, storage tank farms, loading and unloading facilities,and pipelines.This book will also be of use to process hazard analysis (PHA) teammembers and process safety and loss prevention specialists.1.2. Why This Book Was WrittenThere is a need in many chemical processes for protection against propagation of unwanted combustion phenomena such as deflagrations and detonations (including decomposition flames) in process equipment, piping,and especially vent manifold systems (vapor collection systems).1

21. IntroductionThere are different ways, both passive and active, to provide thisdesired protection against deflagrations and detonations. Methodsinclude DDAs, venting, pressure containment, oxidant concentrationreduction (inerting and fuel enrichment), combustibles concentrationreduction (ventilation or air dilution), deflagration suppression, andequipment and piping isolation. These are discussed in more detail inChapter 3.This book makes reference to flame arresters, deflagration flamearresters, and detonation flame arresters. “Flame arresters” is the genericterm for both deflagration and detonation flame arresters. Deflagrationflame arresters are used when a flame only propagates at subsonic velocity,whereas detonation arresters are used when a flame can propagate at allvelocities including supersonic velocities.One of the major reasons that this book was written is that nonspecialist chemical engineers know little about DDAs. Although DDAs have beenspecified and installed for many years, quite often they have failed becausethe wrong type of flame arrester was specified, or it was improperlyinstalled, or inadequate inspection and maintenance were provided.It is intended that this book will foster effective understanding, application and operation of DDAs by providing current knowledge on theirprinciples of operation, selection, installation, and maintenance methods.1.3. What Is Covered in This BookThis book covers many aspects of DDA design, selection, specification,installation, and maintenance. It explains how various types of flamearresters differ, how they are constructed, and how they work. It alsodescribes when a flame arrester is an effective solution for mitigation ofdeflagrations and detonations, and other means of protection (e.g., oxidant concentration reduction) that may be used. It also briefly covers someaspects of dust deflagration protection.Chapter 2 is a general discussion of the historical development ofDDAs, and an overview of the applicable standards and codes is presented.An overview of various prevention and protection methods againstdeflagrations and detonations is presented in Chapter 3.Chapter 4 presents an overview of combustion and explosion phenomena as this is vital to the understanding of the conditions under which aDDA must function.Chapter 5 is a comprehensive discussion of DDA technology, coveringthe various types of DDAs used in the chemical process industries, as well asselection and design considerations and criteria. Detailed information is

1.5. Units of Measure3presented on how to select a DDA for various operating conditions andapplications (e.g., deflagration versus detonation conditions, end-of-lineversus inline, vent manifold/vapor recovery systems). During the description and discussion of various types of DDAs, some application examplesare presented.Subsequent chapters cover installation considerations (Chapter 6),inspection and maintenance practices (Chapter 7), regulations, standards,and codes, including certification test protocols (Chapter 8), and someillustrative examples (Chapter 9). Chapter 10 provides a summary of thepresent state-of-the-art and what other information and research isneeded, followed by appendixes, a glossary, and suggested additionalreading.This book does not provide specific recommendations for maritimeoperations (e.g., ship and barge loading and unloading). The requirements for these are covered in the U.S. Coast Guard regulations, which areoutlined in Section 2.3.1 and in Chapter 8.1.4. What the Reader Should Learn from This BookAfter reading this book the reader should1. be aware that it is not possible to design flame arresters from basictheory;2. be more conversant with deflagration and detonation phenomenain process equipment and vent manifold systems;3. know when a flame arrester is an effective solution for combustionhazards;4. be able to select an appropriate flame arrester and have it properlyinstalled;5. know what needs to be done to keep a DDA functional;6. be able to work with and ask the proper questions of “experts” andmanufacturers.1.5. Units of MeasureThe equations given in subsequent chapters are presented as they appearin the original reference source. Some may have mixed units (English andmetric) and, therefore, the numerical constants are not dimensionless.

2History and State-of-the Art2.1. Historical Development of Flame ArrestersThe forerunner of the present-day flame arrester is the miner’s safetylamp. In the early 1800s candles and oil lamps were used in coal mines andwere responsible for many disastrous explosions. Sir Humphrey Davy wasrequested to find a solution to this problem, and in 1815 he presented apaper to the Royal Society of London entitled “On the Fire-Damp of CoalMines, and on Methods of Lighting a Mine so as to Prevent its Explosion.”This resulted in the invention of the famous Davy lamp that uses a finemetal gauze as a flame arrester. He demonstrated that a metal gauzehaving about 28 openings per linear inch would cool the products of combustion so that a flame would not ignite flammable gas on the other side ofthe gauze. To avoid danger resulting from failure of a single gauze cylindersurrounding the flame, he found it necessary to use two concentric cylinders, one slightly smaller than the other. The lower edges were fittedsnugly to the bowl containing the fuel, and the upper ends of the cylinderswere covered by disks of similar gauze.Also in 1815, but before Davy presented his first lamp to the public,George Stephenson (one of the pioneers in the development of the steamlocomotive) quite independently was also working on a safe miner’s lamp.He discovered during his experiments that flame produced by a particulargas at a given concentration will not pass through a tube smaller than a certain diameter. While most people have heard of Davy’s lamp (it seems thatSir Humphrey received all the credit), it was actually Stephenson’s discov5

62. History and State-of-the Artery that was extremely important, because it provided the basis for the concept of the “quenching distance,” which in turn led to the concept of“Maximum Experimental Safe Gap” (MESG). The MESG is extensivelyused today to classify gases and gas mixtures for the purpose of selectingflame arresters and electrical equipment. For further discussion of historical developments, see Smiles (1975).Flame arresters for chemical process equipment and flammable liquidcontainers have been available for over 120 years. A US patent was issuedas early as 1878 for a “spark-arrester” (Allonas 1878), while another“spark-arrester” was patented in 1880 (Stewart 1880). Numerous US patents have been issued for various designs of flame arresters, with one asrecent as 1995 (Roussakis and Brooker 1995). In Germany, patents wereissued in 1929 and 1939 for flame arresters that contained shock absorberinternals upstream of the flame arrester elements. This innovation madethem suitable as detonation arresters (Wauben 1999).The crimped metal ribbon flame arrester element (see Chapter 5),which is used in both deflagration and detonation flame arresters, was theconcept of Mr. Swan, an RAF Engineering Officer, who worked at theRoyal Aircraft Establishment at Farnborough, England (Binks 1999). Heneeded a flame arrester for use during purging of gas bags of dirigibles,which then used flammable hydrogen rather than the inert helium usedtoday. In this application it was used as a deflagration flame arrester,although it was also used as a detonation flame arrester for Group D and Cgases (Group IIA and IIB in Europe). These classifications will be discussedin Section 5.3.1. Mr. Swan’s crimped metal ribbon arresters were licensedto IMI Amal in the UK and were first manufactured in the late 1910s orearly 1920s. There were many applications for crimped metal ribbon flamearresters during World War II in aircraft and motor torpedo boat engines,but they were mostly used as deflagration flame arresters. Their widespread use in detonation flame arresters occurred after World War II.It was not until the 1950s that detonation flame arresters made ofcrimped metal ribbon elements were developed and began to be usedmore frequently (Binks 1999). The major impetus for the use of crimpedmetal ribbon detonation flame arresters in the US was the enactment ofclean air legislation (Clean Air Act of 1990) which inadvertently created asafety problem by requiring reductions in volatile organic compound(VOC) emissions. To do this, manifolded vent systems (vapor collectionsystems) were increasingly installed in many chemical process industryplants which captured VOC vapors and transported them to suitable recovery, recycle, or destruction systems. This emission control requirement hasled to the introduction of ignition risks, for example, from a flare or viaspontaneous combustion of an activated carbon adsorber bed. Multiple

2.2. Case Histories of Successful and Unsuccessful Applications of Flame Arresters7connections to a flare header greatly increase the variability of the mixturecomposition and the probability of entering the flammable range. In addition, the long piping runs in manifolded vent systems contribute to thegreater potential for deflagration-to-detonation transitions. Therefore,awareness of the need for detonation flame arresters for in-line applications did not gain widespread acceptance until the early 1990s.2.2. Case Histories of Successful and UnsuccessfulApplications of Flame Arresters2.2.1 Successful ApplicationsSuccessful case histories of flame arrester applications are not commonlyreported since no damage to equipment or loss of life occurred. However,one case history was found in the technical literature, which is presentedbelow.Sutherland and Wegert (1973) reported details of an acetylene decomposition incident in which a hydraulic flame arrester was used. The incident involved two plants that were connected by an 8-inch pipelinetransporting acetylene at a pressure of about 14 psig. The acetylene distribution network consisted of approximately 2500 feet of 6-inch pipe in oneplant; 5.5 miles of 8-inch pipe running between the two plants; and about6000 feet of 8-inch, 6-inch, 5-inch, 4-inch, and 3-inch pipe within the otherplant. A fire developed in the plant receiving the acetylene and resulted inan acetylene decomposition within the acetylene distribution network,which rapidly developed into a detonation. This progressed throughoutthe 7 miles of acetylene distribution piping in approximately 6 seconds.Fortunately, hydraulic flame arresters had been installed in the acetyleneproducing unit distribution line, in all user branch lines in both plants, andin the line to the flare stack in the plant supplying the acetylene. Except fordisruption of flows, little mechanical damage was done to either plant asthe hydraulic flame arresters functioned as designed. Also, the design ofthe acetylene piping to withstand detonations contributed to the mitigation of damage. However, because acetylene decomposes, forming hydrogen plus soot, the aftermath of the incident involved substantial cleaningand recommissioning efforts.Kirby (1999) reports two successful applications of deflagration flamearresters. In one incident, a deflagration flame arrester was installed nearthe junction of a collection header from an ethylene oxide process unitwith a flare stack. Although this type of flame arrester was really inappro-

82. History and State-of-the Artpriate for this service, the flame arrester satisfactorily interrupted a flashback from the flare stack into the header. A loud noise was heard, but anexplosion did not propagate upstream of the arrester. The mixture in theheader was barely within the flammable limits. The second incidentinvolved a deflagration flame arrester installed near an incinerator, whichsatisfactorily stopped flame propagation back into a header containingacrylonitrile and air–nitrogen atmosphere. A standing flame occurred atthe outlet of the flame arrester, and a thermocouple automatically actuateda valve which interrupted flow so that the flame went out.2.2.2. Unsuccessful ApplicationsCase histories of unsuccessful applications of flame arresters are foundmore often in the open technical literature. Ten flame arrester (parallelplate and crimped metal ribbon) failure incidents in the Canadian petroleum industry are presented and their causes examined by Roussakis andLapp (1989). According to the authors these incidents, which occurredbetween 1979 and 1989, represent only a small fraction of explosions thathave taken place every year in the province of Alberta and were not prevented because the flame arresters were faulty. All of the ten flame arresterfailures discussed occurred in sour oil vapor flaring operations. Nine ofthem involved oil production sites, and one involved a disposal facility forsalt water contaminated during sour oil production. Eight of the ten flamearrester failures resulted in explosions and fires in oil storage tanks. Onefailure, involving flashback into a knockout tank, sent the gauge float soaring into the air as a high velocity projectile. Another incident resulted inthe explosive destruction of a pair of concrete saltwater holding tanks.Eight of the incidents took place during startup, one took place duringemergency shutdown caused by a power outage, and the other occurredduring shut down for repairs. The reasons for the failures were categorizedas follows:1. Some arresters were used under conditions that exceeded their testlimitations.2. Some were not subjected to any official testing.3. Some units failed because they had working channels that were distorted by the flame front overpressure.4. Some failed due to structural or design flaws that provided a flamepathway through the element.

2.2. Case Histories of Successful and Unsuccessful Applications of Flame Arresters9Howard (1988) reported a flame arrester failure in a major chemicalplant with a large partial oxidation unit. The desired product was recovered in an absorber, and the off gas containing traces of organics was sentto an incinerator. To prevent flashback of flame from the incinerator intothe upstream equipment, the engineering contractor designed andinstalled a horizontal flame arrester with conical inlet and outlet sectionsand a flame arrester section filled with packing. For the 4-foot-diameterpipeline to the incinerator, the flame arrester was a 12-foot-diameter cylinder with conical inlet and outlet sections, and the arrester element was a 3foot-wide section filled with 2-inch Pall rings. The total included angle ofthe inlet and outlet cones was about 150 . The design was flawed since itwas known from experiments that 2-inch Pall rings will not stop a flamefrom combustion of hydrocarbon–air mixtures that are well within theflammable range. Also, the wide cone angles of the arrester are not conducive to good flow distribution over the entire cross section of the Pall ringpacking. When a process upset caused flashback from the incinerator, theflame arrester failed to stop the flame, which resulted in damage to theequipment on the other side of the arrester.A chemical plant flare experienced a series of three consecutive deflagrations resulting in severe damage to the flare water seal flame arrester(Desai 1996). The deflagrations occurred during process startup after acomplete unit shutdown. The first flashback from the flare is believed tohave tilted the water seal internals such that it lost its effectiveness as aflame arrester. After operators reset both the natural gas and snuffingsteam interlocks, the methane flow was reestablished

Packed Bed Flame Arrester 95 5.2.12. Velocity Flame Stopper 96 5.2.13. High Velocity Vent Valve 97 5.2.14. Conservation Vent Valves as Flame Arresters 98 5.3. Selection and Design Criteria/Considerations 98 5.3.1. Classification According to NEC Groups and MESGs 98 5.3.2. Reactions and Combustion Dynamics of