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

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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.

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  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.

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  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

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