ASTM E1248-90(1997)
(Practice)Standard Practice for Shredder Explosion Protection
Standard Practice for Shredder Explosion Protection
SCOPE
1.1 This practice covers general recommended design features and operating practices for shredder explosion protection in resource recovery plants and other refuse processing facilities.
1.2 Hammermills and other types of size reduction equipment (collectively termed shredders) are employed at many facilities that mechanically process solid wastes for resource recovery. Flammable or explosive materials (for example, gases, vapors, powders, and commercial and military explosives) may be present in the as-received waste stream. There is potential for these materials to be released, dispersed, and ignited within or near a shredder. Therefore, explosion prevention and damage amelioration provisions are required.
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Designation: E 1248 – 90 (Reapproved 1997)
Standard Practice for
Shredder Explosion Protection
This standard is issued under the fixed designation E 1248; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3.1.4 explosion venting—the provision of an opening(s) in
the shredder enclosure and contiguous enclosed areas to allow
1.1 This practice covers general recommended design fea-
gases to escape during a deflagration and thus prevent pres-
tures and operating practices for shredder explosion protection
sures from reaching the damage threshold.
in resource recovery plants and other refuse processing facili-
3.1.5 explosion suppression—the technique of detecting and
ties.
extinguishing incipient explosions in the shredder enclosure
1.2 Hammermills and other types of size reduction equip-
and contiguous enclosed areas before pressures exceed the
ment (collectively termed shredders) are employed at many
damage threshold.
facilities that mechanically process solid wastes for resource
3.1.6 inerting—the technique by which a combustible mix-
recovery. Flammable or explosive materials (for example,
ture is rendered nonflammable by addition of a gas incapable of
gases, vapors, powders, and commercial and military explo-
supporting combustion.
sives) may be present in the as-received waste stream. There is
3.1.7 shredder—a size-reduction machine that tears or
potential for these materials to be released, dispersed, and
grinds materials to a smaller and more uniform particle size.
ignited within or near a shredder. Therefore, explosion preven-
tion and damage amelioration provisions are required.
4. Significance and Use
2. Referenced Documents 4.1 Shredder explosions have occurred in most refuse pro-
cessing plants with shredding facilities. Lessons learned in
2.1 National Fire Protection Association Standards:
these incidents have been incorporated into this practice along
National Electrical Code
with results of relevant test programs and general industrial
NFPA 13 Sprinkler Systems
explosion protection recommended practices. Recommenda-
NFPA 68 Guide for Explosion Venting
tions in this practice cover explosion protection aspects of the
NFPA 69 Explosion Prevention Systems
design and operation of shredding facilities and equipment
NFPA 497A Classification of Class I Hazardous (Classified)
used therein.
Locations for Electrical Installations in Chemical Process
4.2 This practice is not intended to be a substitute for an
Areas
operating manual or a detailed set of design specifications.
3. Terminology Rather, it represents general principles and guidelines to be
addressed in detail in generating the operating manual and
3.1 Definitions:
design specifications.
3.1.1 deflagration—an explosion in which the flame or
reaction front propagates at a speed well below the speed of
5. Design Practices
sound in the unburned medium, such that the pressure is
5.1 Design Rationale:
virtually uniform throughout the enclosure (shredder) at any
5.1.1 Each of the following design features is better suited
time during the explosion.
for some types of combustible/explosive materials and shred-
3.1.2 detonation—an explosion in which the flame or reac-
ders than for others. The selection of a particular combination
tion front propagates at a supersonic speed into the unburned
of explosion prevention features or damage control features, or
medium, such that pressure increases occur in the form of
both, should be made with an understanding of the types of
shock waves.
refuse entering the shredder, shredder operating conditions, the
3.1.3 explosion—a rapid release of energy (usually by
inherent strength of the shredder and surrounding structures,
means of combustion) with a corresponding pressure buildup
and the operating controls for screening input materials and
capable of damaging equipment and building structures.
restricting personnel access during shredding operations.
5.1.2 Several of the following explosion protection design
This practice is under the jurisdiction of ASTM Committee D-34 on Waste
practices are effective for deflagrations but not for detonations.
Management and is the direct responsibility of Subcommittee D34.16 on Processing
Deflagrations usually result from accumulations of flammable
Equipment and Unit Operations.
gas-air, vapor-air, or powder(dust) air mixtures in or around the
Current edition approved Dec. 28, 1990. Published February 1991.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 1248
shredder. However, commercial explosives and military ord- oxygen concentrations for nitrogen and carbon dioxide inerting
nance usually generate detonations. A few flammable gases are as listed in Appendix C of NFPA 69.
(for example, acetylene and hydrogen) are also prone to 5.2.6 Reliable oxygen concentration monitors should be
detonate when dispersed in highly turbulent, strong ignition installed, calibrated, and maintained to verify that the maxi-
source environments such as exist inside a shredder. Because mum oxygen concentration is not being exceeded in the
many explosion protection design practices are not applicable shredder and contiguous enclosures. This will require multiple
to detonations, rigorous visual detection and removal of monitors and sampling points depending on the extent and
detonable material before it enters the shredder is particularly uniformity of flow in the enclosed volume. Provision for
important (6.1). cleaning and clearing sample lines, as recommended in 5.4.5
are needed.
5.1.3 In view of the difficulties in preventing and controlling
5.2.7 The inert gas distribution system should be designed
all types of shredder explosions, it is important to isolate the
in accordance with the provisions of Chapter 2 of NFPA 69.
shredder and surrounding enclosure from vulnerable equip-
5.3 Explosion Venting:
ment and occupied areas in the plant. This is best achieved by
5.3.1 Explosion venting is intended to limit structural dam-
locating the shredder outdoors or, if indoors, in a location
age incurred during deflagrations by allowing unburned gas
suitable for explosion venting directly outside. Locations in or
and combustion products to be discharged from the shredder or
near the center of a processing building are not desirable. If the
contiguous enclosures, or both, before combustion and the
shredder is situated in an isolated, explosion resistant structure,
associated potentially destructive pressure rise is completed.
the structure should be designed to withstand the explosion
The effectiveness of explosion venting for a particular explo-
pressures specified in NFPA 68.
sion depends on the rate of combustion versus the rate of
5.1.4 The shredder and all contiguous enclosures should be
discharge of gases through the explosion vents. The rate of
equipped with an explosion protection system consisting of one
combustion in the shredder or adjacent enclosure depends upon
or more of the following: inerting system (5.2); explosion vents
the composition of the combustible gas-air, vapor-air, or
(5.3); explosion suppression system (5.4). Water spray systems
dust-air mixture, the size of the shredder/enclosure, and the
(5.5), combustible gas detectors (5.6), and industrial fire
turbulence level as determined by air flow rates and hammer tip
protection systems (5.7) should also be installed for additional
speed.
protection. Adjacent structures and personnel should be pro-
5.3.2 In general, explosion venting is most effective with
tected (5.8).
large vent areas, low vent deployment pressures, low vent
5.2 Inerting Systems:
panel weight, and vent locations near the expected ignition
5.2.1 An inerting system is intended to prevent combustion
source (which is often hammer impact sparks within the
explosions within a shredder (and contiguous enclosures) by
shredder). The following quantitative guidelines for these
maintaining oxygen concentrations below the level required to
factors are intended to protect against near worst-case flam-
support combustion.
mable gas-air mixtures occupying the entire shredder internal
5.2.2 The following factors must be accounted for in de-
volume.
signing a shredder inerting system: inert gas source and
5.3.3 Explosion vent areas should be sufficiently large to
distribution; operating controls and associated instrumentation;
maintain explosion pressures under the damage threshold value
leakage of inert gas from and entry of air into enclosures;
for the particular shredder installation. Previously published
maintenance and inspection constraints in an oxygen deficient
guidelines relating peak pressure to vent area are not directly
atmosphere during normal operations; effect of inert gas on
applicable to Municipal Solid Waste (MSW) shredders because
shredder materials and waste throughput; and contingency
shredder hammer velocities can increase the combustion rate
plans for inert gas source supply interruption.
well above that considered in establishing previous guidelines.
5.2.3 Flue gas from an on-site furnace or boiler can be a
The following recommended relationship is based on propane-
suitable inert gas providing there is a reliable means to prevent
air explosion tests conducted in a full-scale large shredder
flame propagation into the shredding system and providing flue
mock-up, including rotating hammers (1).
gas conditioning is installed to maintain suitable temperature
5.3.3.1 The vent area, A , required to maintain explosion
v
(to prevent steam condensation or spontaneous ignition) and
pressures under the shredder damage threshold (in units of
flue gas composition (including dew point, oxygen, carbon
psig), P , is given by the equation:
M
monoxide, soot, and contaminant concentrations).
2/3 20.435
A 5 0.13V P ~5 1 0.034v !
v M H
5.2.4 Steam from an on-site boiler can be a suitable inert gas
(1)
providing the temperatures of the shredder and contiguous
enclosures are sufficiently high (at least 180°F (82°C)) to
where:
prevent steam condensation and the associated increase in
V 5 shredder internal volume, and
oxygen and flammable gas concentrations.
v 5 hammer tip velocity, ft/s.
H
2/3
5.2.5 Oxygen concentrations in the shredder and all contig The calculated vent area will be in the same units as V .
uous enclosures should be no higher than 10 % by volume, The metric equivalent, if P is in bar, and v is in m/s, is
M H
unless test data for the particular inert gas employed and the
variety of combustibles expected in the shredder demonstrate
that a higher oxygen concentration can be tolerated without
The boldface numbers in parentheses refer to the list of references at the end of
generating a flammable mixture. Test data for maximum this practice.
E 1248
2/3 20.435
5.3.4.2 If the pressure increases shown in Fig. 1 are intol-
A 5 0.041V P ~5 1 0.112v !
v M H
(2)
erable, a duct with a diverging cross-section area should be
used. Apparently, there have not been any published test data
5.3.3.2 If the shredder discharge is at least 3 ft (0.91 m)
on how much divergence is required to prevent significant
above an unenclosed discharge conveyor, half the discharge
pressure increases above the unrestricted vent values given by
area can be credited toward attaining the required vent area, A
Eq 1 and 2. Even with large divergence angles, the vent duct
v. The difference should be made up with unobstructed explo-
should be designed to withstand a pressure equal to the
sion vents. No credit should be taken for the inlet area which
shredder damage threshold pressure.
is usually too obstructed to be an effective vent.
5.3.4.3 It is desirable to prevent flammable gas from enter-
5.3.3.3 To illustrate the use of Eq 1 and 2, consider a
3 ing and accumulating in a vent duct during normal shredder
hypothetical shredder with an internal volume of 1000 ft (28.3
operation. Although this is difficult to achieve, two possible
m ), including the portion of the inlet hood directly above the
approaches are use of a sturdy vent cover (5.3.5), or vent cover
hammermill. Let us suppose that structural calculations indi-
and projectile deflector to separate the shredder from the vent
cate that the weakest structural member can withstand an
duct; or, as a less desirable alternative, use of air sweeping of
applied load equivalent to a hydrostatic pressure of 10 psig
the vent duct by the induced draft of the shredder or by a
(0.70 bar). At the design shaft speed in this shredder, the
high-capacity dust collection or pneumatic transport system, or
hammer tip speed is 250 ft/s (76.2 m/s). Substitution of these
both. These systems should be equipped with their own
values into Eq 1 and 2 results in a calculated required vent area
2 2 2
explosion protection systems.
of 64 ft (5.95 m ). If the shredder discharge area is 20 ft (1.9
2 2 2
5.3.5 Vent covers are usually needed either (preferably)
m ), an explosion vent of at least 54 ft (5.0 m ) area should be
directly on the shredder, or at the far end of the vent duct.
installed on the shredder.
Without these covers, dust and debris generated during the
5.3.4 The explosion vent opening should discharge combus-
shredding process would be ejected and would possibly create
tion gases and flame into an unoccupied outdoor area. If the
a health and safety hazard to nearby personnel. Since impact
shredder is situated inside a building, vent ducting will be
forces from large ejected debris could prematurely open the
needed to channel gases and flame out of the building. This
vent cover, deflection gratings, heavy chain links, or wire rope
ducting, which should have a strength at least equal to the
are often employed to rebound these missiles back into the
shredder itself, should be kept as short as possible in order to
shredder.
avoid further burning and gas compression during venting.
5.3.5.1 The opening pressure of the vent cover should be
5.3.4.1 Vent ducting of any length will cause the pressure to
low in comparison to the shredder structural damage threshold,
increase significantly above the value expected for unrestricted
P . Based on the explosion tests described in EPA Report
venting. The increased pressure can be related to the unre- M
M2052 (1), it is recommended that the static deployment
stricted (no duct) vented explosion pressure through Fig. 1. The
pressure be no more than P /5 since the cover will open at a
parameter in Fig. 1 that determines this relationship is the ratio M
somewhat higher press
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