ASTM E1248-90(2009)
(Practice)Standard Practice for Shredder Explosion Protection
Standard Practice for Shredder Explosion Protection
SIGNIFICANCE AND USE
Shredder explosions have occurred in most refuse processing plants with shredding facilities. Lessons learned in these incidents have been incorporated into this practice along with results of relevant test programs and general industrial explosion protection recommended practices. Recommendations in this practice cover explosion protection aspects of the design and operation of shredding facilities and equipment used therein.
This practice is not intended to be a substitute for an operating manual or a detailed set of design specifications. Rather, it represents general principles and guidelines to be addressed in detail in generating the operating manual and design specifications.
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: E1248 − 90 (Reapproved 2009)
Standard Practice for
Shredder Explosion Protection
This standard is issued under the fixed designation E1248; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3.1.3 explosion—a rapid release of energy (usually by
means of combustion) with a corresponding pressure buildup
1.1 This practice covers general recommended design fea-
capable of damaging equipment and building structures.
tures and operating practices for shredder explosion protection
3.1.4 explosion venting—the provision of an opening(s) in
in resource recovery plants and other refuse processing facili-
the shredder enclosure and contiguous enclosed areas to allow
ties.
gases to escape during a deflagration and thus prevent pres-
1.2 Hammermills and other types of size reduction equip-
sures from reaching the damage threshold.
ment (collectively termed shredders) are employed at many
3.1.5 explosion suppression—the technique of detecting and
facilities that mechanically process solid wastes for resource
extinguishing incipient explosions in the shredder enclosure
recovery. Flammable or explosive materials (for example,
and contiguous enclosed areas before pressures exceed the
gases, vapors, powders, and commercial and military explo-
damage threshold.
sives) may be present in the as-received waste stream. There is
potential for these materials to be released, dispersed, and
3.1.6 inerting—the technique by which a combustible mix-
ignited within or near a shredder. Therefore, explosion preven-
tureisrenderednonflammablebyadditionofagasincapableof
tion and damage amelioration provisions are required.
supporting combustion.
3.1.7 shredder—a size-reduction machine that tears or
2. Referenced Documents
grinds materials to a smaller and more uniform particle size.
2.1 National Fire Protection Association Standards:
4. Significance and Use
National Electrical Code
4.1 Shredder explosions have occurred in most refuse pro-
NFPA 13 Sprinkler Systems
NFPA 68 Guide for Explosion Venting cessing plants with shredding facilities. Lessons learned in
these incidents have been incorporated into this practice along
NFPA 69 Explosion Prevention Systems
NFPA 497A Classification of Class I Hazardous (Classified) with results of relevant test programs and general industrial
explosion protection recommended practices. Recommenda-
Locations for Electrical Installations in Chemical Process
Areas tions in this practice cover explosion protection aspects of the
design and operation of shredding facilities and equipment
3. Terminology
used therein.
3.1 Definitions:
4.2 This practice is not intended to be a substitute for an
3.1.1 deflagration—an explosion in which the flame or
operating manual or a detailed set of design specifications.
reaction front propagates at a speed well below the speed of
Rather, it represents general principles and guidelines to be
sound in the unburned medium, such that the pressure is addressed in detail in generating the operating manual and
virtually uniform throughout the enclosure (shredder) at any
design specifications.
time during the explosion.
5. Design Practices
3.1.2 detonation—an explosion in which the flame or reac-
5.1 Design Rationale:
tion front propagates at a supersonic speed into the unburned
5.1.1 Each of the following design features is better suited
medium, such that pressure increases occur in the form of
for some types of combustible/explosive materials and shred-
shock waves.
ders than for others. The selection of a particular combination
of explosion prevention features or damage control features, or
both, should be made with an understanding of the types of
This practice is under the jurisdiction of ASTM Committee D34 on Waste
Management and is the direct responsibility of Subcommittee D34.03 on Treatment,
refuse entering the shredder, shredder operating conditions, the
Recovery and Reuse.
inherent strength of the shredder and surrounding structures,
Current edition approved Sept. 1, 2009. Published November 2009. Originally
and the operating controls for screening input materials and
approvedin1990.Lastpreviouseditionapprovedin2004asE1248–90(2004).DOI:
10.1520/E1248-90R09. restricting personnel access during shredding operations.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1248 − 90 (2009)
5.1.2 Several of the following explosion protection design unless test data for the particular inert gas employed and the
practices are effective for deflagrations but not for detonations. variety of combustibles expected in the shredder demonstrate
Deflagrations usually result from accumulations of flammable that a higher oxygen concentration can be tolerated without
gas-air,vapor-air,orpowder(dust)airmixturesinoraroundthe generating a flammable mixture. Test data for maximum
shredder. However, commercial explosives and military ord- oxygenconcentrationsfornitrogenandcarbondioxideinerting
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
5.1.3 Inviewofthedifficultiesinpreventingandcontrolling are needed.
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-
ment and occupied areas in the plant. This is best achieved by
5.3 Explosion Venting:
locating the shredder outdoors or, if indoors, in a location
5.3.1 Explosion venting is intended to limit structural dam-
suitable for explosion venting directly outside. Locations in or
age incurred during deflagrations by allowing unburned gas
near the center of a processing building are not desirable. If the
and combustion products to be discharged from the shredder or
shredder is situated in an isolated, explosion resistant structure,
contiguous enclosures, or both, before combustion and the
the structure should be designed to withstand the explosion
associated potentially destructive pressure rise is completed.
pressures specified in NFPA 68.
The effectiveness of explosion venting for a particular explo-
5.1.4 The shredder and all contiguous enclosures should be
sion depends on the rate of combustion versus the rate of
equippedwithanexplosionprotectionsystemconsistingofone
discharge of gases through the explosion vents. The rate of
ormoreofthefollowing:inertingsystem(5.2);explosionvents
combustionintheshredderoradjacentenclosuredependsupon
(5.3); explosion suppression system (5.4). Water spray systems
the composition of the combustible gas-air, vapor-air, or
(5.5), combustible gas detectors (5.6), and industrial fire
dust-air mixture, the size of the shredder/enclosure, and the
protection systems (5.7) should also be installed for additional
turbulencelevelasdeterminedbyairflowratesandhammertip
protection. Adjacent structures and personnel should be pro-
speed.
tected (5.8).
5.3.2 In general, explosion venting is most effective with
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;
maintainexplosionpressuresunderthedamagethresholdvalue
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
applicabletoMunicipalSolidWaste(MSW)shreddersbecause
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
flamepropagationintotheshreddingsystemandprovidingflue
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 510.034v (1)
5.2.4 Steamfromanon-siteboilercanbeasuitableinertgas ~ !
v M H
providing the temperatures of the shredder and contiguous
where:
enclosures are sufficiently high (at least 180°F (82°C)) to
V = shredder internal volume, and
prevent steam condensation and the associated increase in
oxygen and flammable gas concentrations.
5.2.5 Oxygen concentrations in the shredder and all contig
The boldface numbers in parentheses refer to the list of references at the end of
uous enclosures should be no higher than 10 % by volume, this practice.
E1248 − 90 (2009)
of 0 to 3.According to Fig. 1, an explosion pressure of 10 psig
v = hammer tip velocity, ft/s.
H
(0.7 bar) without the duct would be increased to about 21 psig
2/3
The calculated vent area will be in the same units as V .
(1.5 bar) with a duct/shredder volume ratio of 0 to 3.
The metric equivalent, if P is in bar, and v is in m/s, is
M H
5.3.4.2 If the pressure increases shown in Fig. 1 are
2/3 20.435
A 5 0.041V P 510.112v (2)
~ !
v M H
intolerable, a duct with a diverging cross-section area should
be used. Apparently, there have not been any published test
5.3.3.2 If the shredder discharge is at least 3 ft (0.91 m)
data 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
areacanbecreditedtowardattainingtherequiredventarea,A .
v
Eq 1 and 2. Even with large divergence angles, the vent duct
The difference should be made up with unobstructed explosion
should be designed to withstand a pressure equal to the
vents. No credit should be taken for the inlet area which is
shredder damage threshold pressure.
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
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
values into Eq 1 and 2 results in a calculated required vent area both. These systems should be equipped with their own
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
m ), an explosion vent of at least 54 ft (5.0 m ) area should be
5.3.5 Vent covers are usually needed either (preferably)
installed on the shredder.
directly on the shredder, or at the far end of the vent duct.
5.3.4 The explosion vent opening should discharge combus-
Without these covers, dust and debris generated during the
tion gases and flame into an unoccupied outdoor area. If the shredding process would be ejected and would possibly create
shredder is situated inside a building, vent ducting will be
a health and safety hazard to nearby personnel. Since impact
needed to channel gases and flame out of the building. This forces from large ejected debris could prematurely open the
ducting, which should have a strength at least equal to the
vent cover, deflection gratings, heavy chain links, or wire rope
shredder itself, should be kept as short as possible in order to are often employed to rebound these missiles back into the
avoid further burning and gas compression during venting.
shredder.
5.3.4.1 Vent ducting of any length will cause the pressure to
5.3.5.1 The opening pressure of the vent cover should be
increase significantly above the value expected for unrestricted
low in comparison to the shredder structural damage threshold,
venting. The increased pressure can be related to the unre-
P . Based on the explosion tests described in EPA Report
M
stricted(noduct)ventedexplosionpressurethroughFig.1.The
M2052 (1), it is recommended that the sta
...
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