1、Designation: E 1248 90 (Reapproved 2004)Standard Practice forShredder Explosion Protection1This standard is issued under the fixed designation E 1248; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A num
2、ber in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers general recommended design fea-tures and operating practices for shredder explosion protectionin resource recovery p
3、lants and other refuse processing facili-ties.1.2 Hammermills and other types of size reduction equip-ment (collectively termed shredders) are employed at manyfacilities that mechanically process solid wastes for resourcerecovery. Flammable or explosive materials (for example,gases, vapors, powders,
4、 and commercial and military explo-sives) may be present in the as-received waste stream. There ispotential for these materials to be released, dispersed, andignited within or near a shredder. Therefore, explosion preven-tion and damage amelioration provisions are required.2. Referenced Documents2.1
5、 National Fire Protection Association Standards:National Electrical CodeNFPA 13 Sprinkler SystemsNFPA 68 Guide for Explosion VentingNFPA 69 Explosion Prevention SystemsNFPA 497A Classification of Class I Hazardous (Classified)Locations for Electrical Installations in Chemical ProcessAreas3. Terminol
6、ogy3.1 Definitions:3.1.1 deflagrationan explosion in which the flame orreaction front propagates at a speed well below the speed ofsound in the unburned medium, such that the pressure isvirtually uniform throughout the enclosure (shredder) at anytime during the explosion.3.1.2 detonationan explosion
7、 in which the flame or reac-tion front propagates at a supersonic speed into the unburnedmedium, such that pressure increases occur in the form ofshock waves.3.1.3 explosiona rapid release of energy (usually bymeans of combustion) with a corresponding pressure buildupcapable of damaging equipment an
8、d building structures.3.1.4 explosion ventingthe provision of an opening(s) inthe shredder enclosure and contiguous enclosed areas to allowgases to escape during a deflagration and thus prevent pres-sures from reaching the damage threshold.3.1.5 explosion suppressionthe technique of detecting andext
9、inguishing incipient explosions in the shredder enclosureand contiguous enclosed areas before pressures exceed thedamage threshold.3.1.6 inertingthe technique by which a combustible mix-ture is rendered nonflammable by addition of a gas incapable ofsupporting combustion.3.1.7 shreddera size-reductio
10、n machine that tears orgrinds materials to a smaller and more uniform particle size.4. Significance and Use4.1 Shredder explosions have occurred in most refuse pro-cessing plants with shredding facilities. Lessons learned inthese incidents have been incorporated into this practice alongwith results
11、of relevant test programs and general industrialexplosion protection recommended practices. Recommenda-tions in this practice cover explosion protection aspects of thedesign and operation of shredding facilities and equipmentused therein.4.2 This practice is not intended to be a substitute for anope
12、rating manual or a detailed set of design specifications.Rather, it represents general principles and guidelines to beaddressed in detail in generating the operating manual anddesign specifications.5. Design Practices5.1 Design Rationale:5.1.1 Each of the following design features is better suitedfo
13、r some types of combustible/explosive materials and shred-ders than for others. The selection of a particular combinationof explosion prevention features or damage control features, orboth, should be made with an understanding of the types ofrefuse entering the shredder, shredder operating condition
14、s, theinherent strength of the shredder and surrounding structures,and the operating controls for screening input materials andrestricting personnel access during shredding operations.5.1.2 Several of the following explosion protection designpractices are effective for deflagrations but not for deto
15、nations.Deflagrations usually result from accumulations of flammablegas-air, vapor-air, or powder(dust) air mixtures in or around the1This practice is under the jurisdiction of ASTM Committee D34 on WasteManagement and is the direct responsibility of Subcommittee D34.03.02 onMunicipal Recovery and R
16、euse.Current edition approved Dec. 28, 1990. Published February 1991.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.shredder. However, commercial explosives and military ord-nance usually generate detonations. A few flammable gases(
17、for example, acetylene and hydrogen) are also prone todetonate when dispersed in highly turbulent, strong ignitionsource environments such as exist inside a shredder. Becausemany explosion protection design practices are not applicableto detonations, rigorous visual detection and removal ofdetonable
18、 material before it enters the shredder is particularlyimportant (6.1).5.1.3 In view of the difficulties in preventing and controllingall types of shredder explosions, it is important to isolate theshredder and surrounding enclosure from vulnerable equip-ment and occupied areas in the plant. This is
19、 best achieved bylocating the shredder outdoors or, if indoors, in a locationsuitable for explosion venting directly outside. Locations in ornear the center of a processing building are not desirable. If theshredder is situated in an isolated, explosion resistant structure,the structure should be de
20、signed to withstand the explosionpressures specified in NFPA 68.5.1.4 The shredder and all contiguous enclosures should beequipped with an explosion protection system consisting of oneor more of the following: inerting system (5.2); explosion vents(5.3); explosion suppression system (5.4). Water spr
21、ay systems(5.5), combustible gas detectors (5.6), and industrial fireprotection systems (5.7) should also be installed for additionalprotection. Adjacent structures and personnel should be pro-tected (5.8).5.2 Inerting Systems:5.2.1 An inerting system is intended to prevent combustionexplosions with
22、in a shredder (and contiguous enclosures) bymaintaining oxygen concentrations below the level required tosupport combustion.5.2.2 The following factors must be accounted for in de-signing a shredder inerting system: inert gas source anddistribution; operating controls and associated instrumentation;
23、leakage of inert gas from and entry of air into enclosures;maintenance and inspection constraints in an oxygen deficientatmosphere during normal operations; effect of inert gas onshredder materials and waste throughput; and contingencyplans for inert gas source supply interruption.5.2.3 Flue gas fro
24、m an on-site furnace or boiler can be asuitable inert gas providing there is a reliable means to preventflame propagation into the shredding system and providing fluegas conditioning is installed to maintain suitable temperature(to prevent steam condensation or spontaneous ignition) andflue gas comp
25、osition (including dew point, oxygen, carbonmonoxide, soot, and contaminant concentrations).5.2.4 Steam from an on-site boiler can be a suitable inert gasproviding the temperatures of the shredder and contiguousenclosures are sufficiently high (at least 180F (82C) toprevent steam condensation and th
26、e associated increase inoxygen and flammable gas concentrations.5.2.5 Oxygen concentrations in the shredder and all contiguous enclosures should be no higher than 10 % by volume,unless test data for the particular inert gas employed and thevariety of combustibles expected in the shredder demonstrate
27、that a higher oxygen concentration can be tolerated withoutgenerating a flammable mixture. Test data for maximumoxygen concentrations for nitrogen and carbon dioxide inertingare as listed in Appendix C of NFPA 69.5.2.6 Reliable oxygen concentration monitors should beinstalled, calibrated, and mainta
28、ined to verify that the maxi-mum oxygen concentration is not being exceeded in theshredder and contiguous enclosures. This will require multiplemonitors and sampling points depending on the extent anduniformity of flow in the enclosed volume. Provision forcleaning and clearing sample lines, as recom
29、mended in 5.4.5are needed.5.2.7 The inert gas distribution system should be designedin accordance with the provisions of Chapter 2 of NFPA 69.5.3 Explosion Venting:5.3.1 Explosion venting is intended to limit structural dam-age incurred during deflagrations by allowing unburned gasand combustion pro
30、ducts to be discharged from the shredder orcontiguous enclosures, or both, before combustion and theassociated potentially destructive pressure rise is completed.The effectiveness of explosion venting for a particular explo-sion depends on the rate of combustion versus the rate ofdischarge of gases
31、through the explosion vents. The rate ofcombustion in the shredder or adjacent enclosure depends uponthe composition of the combustible gas-air, vapor-air, ordust-air mixture, the size of the shredder/enclosure, and theturbulence level as determined by air flow rates and hammer tipspeed.5.3.2 In gen
32、eral, explosion venting is most effective withlarge vent areas, low vent deployment pressures, low ventpanel weight, and vent locations near the expected ignitionsource (which is often hammer impact sparks within theshredder). The following quantitative guidelines for thesefactors are intended to pr
33、otect against near worst-case flam-mable gas-air mixtures occupying the entire shredder internalvolume.5.3.3 Explosion vent areas should be sufficiently large tomaintain explosion pressures under the damage threshold valuefor the particular shredder installation. Previously publishedguidelines relat
34、ing peak pressure to vent area are not directlyapplicable to Municipal Solid Waste (MSW) shredders becauseshredder hammer velocities can increase the combustion ratewell above that considered in establishing previous guidelines.The following recommended relationship is based on propane-air explosion
35、 tests conducted in a full-scale large shreddermock-up, including rotating hammers (1).25.3.3.1 The vent area, Av, required to maintain explosionpressures under the shredder damage threshold (in units ofpsig), PM, is given by the equation:Av5 0.13V2/3PM20.4355 1 0.034vH!(1)where:V = shredder interna
36、l volume, andvH= hammer tip velocity, ft/s.The calculated vent area will be in the same units as V2/3.The metric equivalent, if PMis in bar, and vHis in m/s, is2The boldface numbers in parentheses refer to the list of references at the end ofthis practice.E 1248 90 (2004)2Av5 0.041V2/3PM20.4355 1 0.
37、112vH!(2)5.3.3.2 If the shredder discharge is at least 3 ft (0.91 m)above an unenclosed discharge conveyor, half the dischargearea can be credited toward attaining the required vent area, Av. The difference should be made up with unobstructed explo-sion vents. No credit should be taken for the inlet
38、 area whichis usually too obstructed to be an effective vent.5.3.3.3 To illustrate the use of Eq 1 and 2, consider ahypothetical shredder with an internal volume of 1000 ft3(28.3m3), including the portion of the inlet hood directly above thehammermill. Let us suppose that structural calculations ind
39、i-cate that the weakest structural member can withstand anapplied load equivalent to a hydrostatic pressure of 10 psig(0.70 bar). At the design shaft speed in this shredder, thehammer tip speed is 250 ft/s (76.2 m/s). Substitution of thesevalues into Eq 1 and 2 results in a calculated required vent
40、areaof 64 ft2(5.95 m2). If the shredder discharge area is 20 ft2(1.9m2), an explosion vent of at least 54 ft2(5.0 m2) area should beinstalled on the shredder.5.3.4 The explosion vent opening should discharge combus-tion gases and flame into an unoccupied outdoor area. If theshredder is situated insi
41、de a building, vent ducting will beneeded to channel gases and flame out of the building. Thisducting, which should have a strength at least equal to theshredder itself, should be kept as short as possible in order toavoid further burning and gas compression during venting.5.3.4.1 Vent ducting of an
42、y length will cause the pressure toincrease significantly above the value expected for unrestrictedventing. The increased pressure can be related to the unre-stricted (no duct) vented explosion pressure through Fig. 1. Theparameter in Fig. 1 that determines this relationship is the ratioof vent duct
43、 volume to shredder volume. In the example in5.3.3.3, the use of only a 5.5-ft (1.7-m) long duct attached tothe 54-ft2(5.0-m2) vent area would represent a duct volume of300 ft3(8.5 m3), corresponding to a duct/shredder volume ratioof 0 to 3. According to Fig. 1, an explosion pressure of 10 psig(0.7
44、bar) without the duct would be increased to about 21 psig(1.5 bar) with a duct/shredder volume ratio of 0 to 3.5.3.4.2 If the pressure increases shown in Fig. 1 are intol-erable, a duct with a diverging cross-section area should beused. Apparently, there have not been any published test dataon how m
45、uch divergence is required to prevent significantpressure increases above the unrestricted vent values given byEq 1 and 2. Even with large divergence angles, the vent ductshould be designed to withstand a pressure equal to theshredder damage threshold pressure.5.3.4.3 It is desirable to prevent flam
46、mable gas from enter-ing and accumulating in a vent duct during normal shredderoperation. Although this is difficult to achieve, two possibleapproaches are use of a sturdy vent cover (5.3.5), or vent coverand projectile deflector to separate the shredder from the ventduct; or, as a less desirable al
47、ternative, use of air sweeping ofthe vent duct by the induced draft of the shredder or by ahigh-capacity dust collection or pneumatic transport system, orboth. These systems should be equipped with their ownexplosion protection systems.5.3.5 Vent covers are usually needed either (preferably)directly
48、 on the shredder, or at the far end of the vent duct.Without these covers, dust and debris generated during theshredding process would be ejected and would possibly createa health and safety hazard to nearby personnel. Since impactforces from large ejected debris could prematurely open thevent cover
49、, deflection gratings, heavy chain links, or wire ropeare often employed to rebound these missiles back into theshredder.5.3.5.1 The opening pressure of the vent cover should below in comparison to the shredder structural damage threshold,PM. Based on the explosion tests described in EPA ReportM2052 (1), it is recommended that the static deploymentpressure be no more than PM/5 since the cover will open at asomewhat higher pressure under rapid explosion loads thanunder static test conditions. The vent opening pressure shouldalso be higher than pressures developed by air motion
