United States Environmental Protection Agency Municipal Environmental Research Laboratory Cincinnati, OH 45268 /r,v\ Research and Development EPA-600/S2-82-102 Mar. 1983 &EPA Project Summary Determination of Explosion Requirements for Solid Waste Shredders Venting Municipal Robert G. Zalosh, J A gas explosion conducted in a n mock-up of a mun test program was shredder. The 61 -m! (2200-ft3) mock- up simulated a hor zontal-shaft ham- mermill (including r< tating shaft, discs, and hammers, but v ithout trash) with a large, inclined fe id hood. Varying amounts of propane were injected into the shredder, and concentrations gen ğrated by rotor-in duced mixing were propane explosion t ists were also con- ducted with varyinj stoichiometric pro and various hamme 'mill shaft speeds. Tests indicated tha t venting through the top of the shredc er effectively kept pressures under 4' shredder shaft spe< ds of 250 to 660 rpm with 16 hamm srs; but pressures reached 69 to 103 I Pa (10 to 15 psig) at a shaft speed of hn P. Coll, and David M. Goertemoeller alistic, full-scale cipal solid waste the resulting gas measured. Eight volumes of near- >ane-air mixtures kPa (6 psig) at 900 rpm with 48 hammers. The pressures gen erated with a ham- mermill shaft speed of 900 rpm and 48 hammers were much larger than would have been expected on the basis of current guidelines for explosion venting design. New guidelines are suggested that include a quantitative relationship between peak pressure and shaft speed. The recommended guidelines also dis- cuss the effects of vent ducting, vent covers, and blast waves emitted during a shredder explosion. This Project Summary was developed by EPA's Municipal Environmental Re- search Laboratory, Cincinnati, OH, to announce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Introduction In recent years, shredding of municipal solid waste (MSW) has become a common processing step before land disposal, re- source recovery, or incineration. Because it is virtually impossible to screen the input stream thoroughly, potentially explosive materials such as gasoline, propane, paint thinner/cleaner, gunpowder, etc. occa- sionally enter the shredder. An explosion can occur from ignition of these materials by impact sparks or hot spots generated during shredding (hammering). Explosion venting is the most popular method used to control shredder explo- sions. The principle of explosion venting is to prevent pressure buildup in the shredder by allowing an incipient pressure rise to actuate blowout panels or curtains so as to vent unburned gas and combus- tion products. Existing explosion venting guidelines may not be adequate for the more challeng- ing shredder explosion applications. Pre- vious explosion venting design criteria are based on tests involving simple structures such as rooms or spherical or cylindrical pressure vessels. But MSW shredders represent a more severe explosion environ- ment because of the effects of rotor windage/turbulence and internal obstruc- tions (shaft, hammers, breaker plates, trash, etc.). The objective of this project was to de- velop and test explosion venting require- ments for MSW shredders. The approach was to perform gas explosion tests in a realistic, full-scale, mock shredder outfitted with a typical explosion vent configuration ------- employed at several MSW shredding facilities. Shredder Mock-up The full-scale mock-up of a large, hori- zontal-shaft hammermill was constructed at the Factory Mutual Research Test Center in West Glocester, Rhode Island. The mock-up, which had the approximate size and shape of the Williams 680* shredder was 8.23 m (27 ft) high with a total internal volume of 62 m3 (2200 ft3), including a 19-m3 (660-ft3) inclined inlet hood (Figure 1). The shredder structure consisted of a steel frame with 3.8-cm- thick (1.5-in.-thick) plywood walls. The frame and sheet-metal-clad walls were designed to withstand an internal quasi- static explosion pressure of 34.5 kPa (5 psig). Some of the 1.2- x 1.2-m (4- x 4-ft) plywood panels were fastened with collap- sible washer-type explosion vent fasteners so that the panels could blow off at a prescribed static overpressure during the explosion tests. In most of the tests. * Mention of trade names or commercial products does not constitute endorsement or recommendation for venting was accomplished by deploying four panels on top of the shredder. The total vent area of the four panels was 5.1 m2 (55 ft2). Additional venting capacity was available through the inlet hood and the bottom discharge areas. The hammermill shaft on the mock-up was outfitted with 24 91-cm-diameter (36-in.-diameter), 2.5-cm-thick(1-in.thick) plywood discs. Two simulated hammers in the form of 38-cm-long (15-in.-long) aluminum bars can be fastened to each of the discs. In the first seven explosion tests, only 16 hammers were installed. In the last test all 48 hammers were installed. Most of the tests were conducted with a 2.2-kW (3-hp) motor driving the shaft by means of a variable speed drive unit to generate shaft speeds in the range 250 to 690 rpm. In the last explosion test, the 3-hp motor was replaced by a 30-hp motor with a fixed speed transmission driving the shaft at 900 rpm. No trash was put into the shredder mock-up. Placing trash in the shredder would have caused an obstruction in the inlet and discharge areas. This was simu- lated in the mock-up by covering the 5.5- m2 (59-ft2) inlet area and the 2.76-m2 (29.7-ft2) discharge area with polyethylene sheets in many of the tests. Procedures Gas Mixing and Flow Visualization Tests Before the explosion tests, flow visual- ization and gas mixing tests were conduct- ed to determine how flammable gas-air mixtures might form during a shredder explosion. The procedure involved placing an intact flammable vapor container in the shredder and allowing it to be broken by the hammer impact Flammable vapor released from the broken container is diluted by the rotor-induced airflow. Flow visualization tests were designed to reveal induced air-flow patterns causing gas di- lution. The gas mixing tests were designed to determine the spatial and temporal extent of flammable gas-air mixtures formed during this scenario. In the flow visualization tests, a chemical smoke candle placed nearthe hammermill shaft was lit and the resulting smoke pattern was observed. In some of these tests, a translucent polyethylene covering replaced the plywood panels on the front wall of the shredder. Most of the smoke remained in the vicinity of the mill area for 1 to 3 minutes before diluted smoke began to exit through the discharge grating at the bottom of the shredder. PTA 136"PlywoodDiscs 24 Discs II 4 Hammers per disc 46"x93" Discharge Area Figure 1. Shredder mock-up. ------- A preweighed quantity of propane was used for the gas mixing and explosion tests. The gas mixing tests were con- ducted by rapidly injecting the propane into the hammermill portion of the shred- der. Three differnt injection locations (I, I', and J in Figure 1) were used. Injection at I' was achieved with a 81 -cm (36-in.) hori- zontal extension from I. Liquid phase propane was used in all but one of the tests. Propane concentrations were mea- sured with an Anarad AR-400 infrared gas analyzer with a calibrated range of 0% to 8% propane by volume and a response time of 5 to 1 5s, depending on sample location. (The latter are designated as locations A B, C, D, D' (extension of pipe D), and E in Figure 1). Peak concentration data for all gas mixing tests are listed in Table 1. Explosion Test Procedure and Instrumentation Explosion tests in the shredder mock- up were conducted with propane-air mix- tures of varying size and concentrations in the range 3.5% to 4% by volume. An electric match was used to ignite the gas mixture in the first few tests; later tests were fired by a 12-joule condenser spark discharge. Gas mixtures for the first two tests were formed by rotor-induced mixing with open inlet and discharge areas. But this unre- strained mixing resulted in a very weak explosion in the first test and in no explo- sion at all (after three attempts) in the second test Subsequent tests were there- fore conducted by confining the gas mix- ture with polyethylene sheets. In the first seven explosion tests, pres- sures were measured with two Dynisco Model PT 321 strain gage transducers with a calibrated range of 0 to 69 kPa (0 to 10 psig). In the last test, Celesco P2805 pressure transducers with a range of 0 to 103.5 kPa (0 to 1 5 psig) were used. One transducer (labeled Gage A) was mounted on one side wall of the shredderO.61 m (2 ft) belowthe top (location PTAin Figure 1). The other transducer (labeled Gage B) was mounted on the other side wall of the shredder 104 cm (41 in.) directly above the shaft (location PTB in Figure 1). In all but the last test, the four plywood panels on top of the shredder were used for explosion venting. They were outfitted with collapsible washer fasteners so that the nominal panel release pressure was in the range 1.4 to 2.8 kPa (0.2 to 0.4 psig). In the first two tests, pressures did not reach these values, so the vent panels did not deploy. In subsequent tests, the panels did deploy, but not until the pres- sure reached 7.6 to 1 7.2 kPa (1.1 to 2.5 psig). In the last test, the plywood panels were replaced with a 0.10-mm-thick (4- mil-thick) polyethylene film designed for a nominal tear pressure of 2.1 kPa (0.3 psig). Explosion Test Results Test conditions and peak pressure data are outlined in Table 2. Seven tests were run, not including the misfire in Test 2. Peak pressures ranged from a low of 1.0 kPa (0.15 psig) in the first test to a high of 106.3 kPa(1 5.4 psig) in the last test Peak pressures measured by Gage A at the top of the shredder were consistently higher than those measured by Gage B in the hammermill section of the shredder. The test sequence involved generally increasing explosion severity. In the last test (Test 8), the unexpectedly high peak pressure pro- duced significant damage to the shredder. The violent explosion caused weld failures, deformation of members of the steel frame, and bolt fastener failures, which allowed the plywood panels to blow off the stucture. Conclusions Test results lead to the following con- clusions: 1) The probability and severity of an MSW shredder explosion depend greatly on the amount of flammable gas released, the presence of obstructions in the inlet and discharge area, and the ham- mermill shaft speed and number of hammers. 2) Explosion venting effectiveness is quite sensitive to shredder turbulence level as determined by shaft speed and number of hammers. 3) If existing explosion vent design guide- lines are used, little or no credit should be taken for venting through shredder inlet and discharge areas. Appendix A of the full report contains guidelines for shredder explosion venting based on test results, analysis, and a review of other published explosion venting design criteria. The full report was submitted in fulfill- ment of Contract No. 68-03-2880 by Factory Mutual Research Corporation under the sponsorship of the U.S. Environmental Protection Agency. Table 1. Gas Mixing Data Sample Location Test No. 1 2 3 4 5 6 7 8 9 10 anart Speed (rpm) 260 680 690 480 690 660 660 660 660 660 Injector Location 1 1 r I U U U U U Ut total wt of fuel (Ib) 1 1 1 1 2 3 2 2 2 2 A C* max >8 3.25 - 2.75 4.0 8.0 2.6 3.6 7.5 1.6 Tt 25 10 - - 18 31 6 20 15 - B C Cmax * Cmax ' 0.5 - 2.0 - . . . . . 0.5 - - . - D Cmax 1 1.6 - 0.8 - 1.5 - 1.0 - 1.0 - - 1.6 - . . - D~ Cmax T . 0.9 - 1.1 - 1.0 - - - - . . - E Cmax . 5.5 0.5 - - - - . . - £ T . 10 - - . - - . . - ~)pen or Closed Bottom Open Open Open Open Open Open Open Closed Closed Closed *Cmax = maximum concentration (vol %). tT= duration of flammable concentration(s). t Propane gas injected in the gas phase (top injection} in the last test and in the liquid phase (bottom injection/ in the first eight tests. ------- Table 2. Shredder Explosion Test Data Tesf No. 1 3f 4 5 6 7 8 Propane Concentration 2.4 3.5-4.0 4.0 3.8 3.6 3.7 3.9 Mixture Volume (ft3) (% of Shredder)" Uncontrolled Mixing 700 (44) 700 (44) 700 (44) 1600 (100) 1600 (100) 1600 (100) Shaft Speed (rpm) 690 690 438 660 250 660 900*" Vent Areaf (ft2) 0 55 55 55 55 55 55 Vent Release Pressure (psig) Static 0.4 0.4 0.3 0.3 0.2 0.2 0.3 Actual 2.5 1.1 # 1.3 1.75 1.7 Pmax It Gage A (Top) 0.15 2.7 1.3 2.0 4.8 15.4 vsig)t Gage E (Mill) 2.6 1.1 1.7 3.1 4.3 9.5 * Percentages of shredder volume are based on volume excluding inlet hood. f Vent area does not include shredder discharge area or inlet hood area. t psig = 6.9 kPa. f Test 2 did not produce an explosion because the uncontrolled mixing resulted in the ignitor firing a few seconds too late. # The actual vent release pressure is not known for Test 5 because the oscillograph was started too late. ** 48 simulated hammers were installed on the shaft in Test 8; only 16 hammers were used in Tests 1-7. Robert G. Zalosh and John P. Coll are with Factory Mutual Research Corporation, Norwood, MA 02062; and the EPA author David M. Goertemoeller is with the Municipal Environmental Research Laboratory, Cincinnati, OH 45268. Carlton C. Wiles is the EPA Project Officer (see below). The complete report, entitled "Determination of Explosion Venting Requirements for Municipal Solid Waste Shredders," (Order No. PB83-149 088; Cost: $ 10.00, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Municipal Environmental Research Laboratory U.S. Environmental Protection Agency Cincinnati, OH 45268 ftU.S. Government Printing Office- 1983-659-017/7024 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Postage and Fees Paid Environmental Protection Agency EPA 335 Official Business Penalty for Private Use $300 RETURN POSTAGE GUARANTEED U S FNViR PROTECTION AGENCY RtGiON b LIbğAĞ?Y 230 S DMK CHICAGO IL ------- |