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.
-------
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
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Fees Paid
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