THE TAMING AND HOUSEBREAKING OF

REFUSE SHREDDERS

FORREST B. PYLE Glaus, Pyle, Schomer, Burns and DeHaven, Inc.

Akron, Ohio

ABSTRACT

The intent is to offer a paper which explores

some of the basic considerations of shredder explo­sions and explores some definitive alternatives to

reduce the frequency and severity of explosions in solid waste shredders. This paper is the outgrowth

of perceived design improvements resulting from

analysis of Factory Mutual's basic paper on shred­der explosions.

THE PROBLEM

The Fall 1974 NCRR Bulletin listed some 109 shredders being used for processing or recycling of municipal solid waste (MSW). Additional installa­tions have come on stream or have been committed since this report.

Increasingly, shredding is being recognized as a key unit process in producing RDF, energy recov­

ery, and recycling facilities. All of these shredders

have the potential for the production of explosions and most of them have experienced one or more

explosions. In most cases damage has been minimal and generally has not disrupted operations. Some

of the explosions have been more violent, however,

and the potential for explosions resulting in exten­

sive damage, disruption of operations, and hazards

to personnel is inherent in shredder installations.

The lack of extended experience and unfamili­

arity with this type of hazard has resulted in insuf­

ficient attention being given to the explosion prob­

lem in most designs. The author does not represent

215

himself as an explosion "expert" but simply as one

who has had the opportunity to benefit from some

of the "first generation" experiences and to con­sider the means available to improve on them. It is the intention of this paper to call attention to the problem, to attempt to improve understanding of the nature of the problem and some methods which can be considered to reduce risk.

REASONS FOR EXPLOSIONS Size reduction by shredding generally employs

either horizontal or vertical hammermills to cut, beat and tear the heterogeneous MSW. Impact with various materials will produce sparks, so there is a definite ignition source almost continuously pres­

ent. If material of sufficiently low ignition tem­perature is present, the potential for fires and ex­

plosions exists. In fact, one operator reports almost continuous low grade fires at the shredder discharge.

Volatile liquids in the presence of air are poten­

tially explosive in this environment. Dusts from the

shredder operation can be explosive in themselves or can contribute to increased severity of the ex­

plosion. More rarely, true explosives will be found in the solid waste stream.

SOURCES OF POTENTIAL EXPLOSIVES

An understanding of the source of the poten­

tially dangerous materials can contribute to the

con'sideration of means of either minimizing the

amount of such materials in the waste stream or providing detection and special handling of them.

Industrial liquid wastes such as solvents, thin­

ners, paints, etc. present a difficult disposal prob­lem and are inherently unsuitable for disposal through shredders. Consequently, they should not

be permitted to be received in the facility. House­

hold wastes of this type, then, are the most likely source of liquids with explosion potential. These include paints, gasoline, and aerosols. It is unlikely that these liquids can be completely eliminated from reaching the shredders.

Actual explosives such as dynamite, gun power, (commercial explosives) are most likely to be dis­

posed of in the householder's waste stream as dis­

carded military souvenirs. These generally do not pose serious hazards, but the potential is there. A

number of shredder installations attributed their

earliest explosions to deliberate sabotage. Subse­

quent repeated incidents led them to look for other causes. Consequently, the reported incidence attri­

buted to commercial explosives, while not statisti­cally large, may well be larger than the facts to the extent not documented by adequate investigation.

This is too convenient and dramatic as an assump­

tion and defense against other liabilities. However, sabotage cannot be completely ruled out nor com­

pletely defended against.

Combinations of the above three sources are also probable.

EXPERIENCE WITH EXPLOSIONS

The greatest volume of experience with this type

of shredder and with explosions can be found in the

more than 160 domestic installations which shred automobiles. This is an intensively competitive in­dustry and some reluctance to divulge significant information is typical as it is in most of the second­ary materials industries. However, in October of

1974 the University of Wisconsin held a two-day seminar on automobile shredding which was well

attended and informative. All automobile shredder installations reported

frequent explosions in the shredders and down­

stream ducts and cyclones. These explosions were

attributed to all three of the previously mentioned causes, with the most prevalent being gasoline from

incompletely drained tanks or cans in car trunks. A poll was taken of the operators present and about 60 percent made no particular provision to suppress such explosions, simply providing blow out panels. More recently, an automobile shredder in Long

Island was threatened with shutdown as a public

nuisance because of the frequency and severity of

explosions which became a neighborhood nuisance.

About 40 percent of the operators had installed

explosion suppression systems. These operators re­ported that there had never been an incidence of an

explosion on a shredder properly protected by explo­sion suppression systems (to be described later). However, they also reported a much higher inci­

dence of system activation than had been their ex­perience with unprotected systems, indicating that the systems were being activated by minor and noncritical explosions.

All operators made some effort to eliminate the

source of explosions. Most would not accept hulks with gas tanks attached. Some went so far as to re­move the trunk lid. However, none of them believed

that such control procedures could be expected to be 100 percent effective.

Factory Mutual Research prepared on excellent

report for ERDA entitled "Assessment of Explo­

sion Hazards in Refuse Shredders" dated April, 1976. The reader is encouraged to obtain a copy of this voluminous report for the wealth of data

which it contains. Some tables (VI, VII, IX, X, and XII from this report) are included hereinafter by permission. This report indicates an explosion

inciden t for every 85,000 tons (77 ,000 t) of waste

processed. This would mean a noticeable explosion

every 85 days for a 1000 tons per day (907 t/day) installation. A major explosion would be predicted

every 425 days at this same 1000 tons per day (907 t/day) rate. A major explosion is defined as one

resulting in $25,000.00 + damage, over one week down time. Table IX also indicates variations associated with venting and continuous water spray. While the cost consequences of this frequency of explosions are not particularly significant, the im­plications in terms of availability and potential per­sonnel hazard do make shredder explosions a ser­

ious problem.

CONTROL OF EXPLOSIVES

The first line of defense should be to attempt

to reduce or eliminate the offending articles from

the waste stream even though this cannot be ex­pected to be 100 percent effective. A combination of education and publicity can alert industry and the householder to the dangers of improper disposal

of the dangerous items. Pick-up crews should be educated to be alert for such items.

The tipping floor man should also be on the alert for dangerous or unacceptable materials, par­

ticularly closed containers which may contain

216

No. Locations

Total 45

c W/Venting 24 .2 .... 0

W/O Venting 21 v .... 0 ....

CL Continuous 8

TABLE VI EXPLOSIBLE MATERIAL

No. Shredders

66

37

22

9

Flammable Commercial Vapor, Gas Explosive

30 II

10 8

20 3

2 3

Undetermined Total

54 95

20 38

34 57

6 c£ Waterspray (acetylene)

Vertical 8 II 5 18 24 Grinder

c

26 47 Ol Horiz. 24 38 14 7 'iii v Hammermill

0 >-

co Vert. 15 17 II 3 10 24 Hammermill

Note: Total may not agree with summation of categories, as some locations have had more than one type of

shredder or protection for the shredder and are counted more than once.

Note: Tables VI, VII, IX, X, and XII are reproduced by permission of Factory Mutual Research Corporation.

TABLE VII SEVERITY AND NO. EXPLOSIONS

No. No. Locations Shredders Negligible Minor Moderate Major Total

Total 45 c

66 28 48 14 5 95 .2

W/Venting .... 24 37 10 21 4 3 38 al

.... 0 W/O Venting 21 33 18 27 10 2 57 ....

CL >-

Continuous 8 9 6 6 CO Waterspray

Vertical 8 II Grinder

10 13 0 24

c

.21 Horiz. 24 28 14 17 13 2 47 VI v Hammermill 0 >-

CO Vertical 15 Hammermill

17 4 18 24

Note: Total may not agree with summation of categories, as some locations have had more than one type of

shredder or protection for the shredder and are counted more than once.

2]7

TABLE IX EXPLOSION FREQUENCIES BY SHREDDER PROTECTION:

K-TONS PER EXPLOSION AND

K-TONS PER MODERATE OR MAJOR EXPLOSION

Vented Unvented Shredders Shredders

All wlo All wlo All Vented Continuous Un vented Continuous CWS plus

Shredders Shredders Waterspray Shredders Waterspray All CWS Vented

Total 8058 4217 1967 Solid Waste Processed (K-Tons =

000 Tons)

K-Tons 85 III 55 per Explosion

K-Tons 425 602 281 per Moderate or Major Explosion

F or Locations with Over 50 K-Tons Processed

Total 7548 3867 1617

K-Tons per Explosion 108 184 85

K-Tons 539 1289 539 per Moderate or Major Explosion

Note: For metric tons, multiply by 0.907.

liquids. In time, he will become experienced as to habitual offenders and can issue the proper fines or sanctions. (Assuming that legislation and/or opera-

.

tors rules so permit.) It is important to provide the shredder operator

with observation of the shredder infeed conveyor. At this point many of the bags will be b�oken and, as the material proceeds up the conveyor, he will have the opportunity to notice items which should be banned. Some tumbling is expected on the in­clined conveyor and this can expose such items. A grapple, under the control of the shredder operator, can then be used to facilitate removal. The relative­ly deep bed on the conveyors and personnel limita­tions preclude 100 percent effectiveness; however, the opportunity for this surveillance can be a help.

An effort should be made to identify the source of material removed so that follow up measures

218

3841 3707 2384 2250

67 70 397 1125

320 309

3681 3581 2350 2250

75 76 588 1125

335 326

can be taken to discourage recurrance. It must be emphasized tha t all of these measures

cannot be expected to completely control the prob­lem, but the data of Factory Mutual indicates that operator experience has a considerable effect on reducing the number of explosions.

EXPLOSION RELIEF

Provisions for explosion relief or venting gen­erally have not been particularly effective as origi­nally installed. This is perhaps understandable as the shredders are inherently very rugged and gen­erally need relatively little relief to keep pressures within acceptable limits for the shredder. However, this does not insure that damage to hoods, infeed conveyors, or adjacent structures will not occur.

NFP A Bulletin 68 provides guides to explosion venting and is an excellent reference. But differ­ent authorities disagree on the amount of venting required to achieve a specified maximum pressure by a factor of 2.S: 1. Consequently, explosion vent­ing cannot be considered an exact science. For a stoichiometric mixture of a specific flammable, the vent area required is a function of the con­tained volume and the limiting maximum pressure desired. The presence of moisture and the space being occupied by material being shredded should tend to be a conservative influence on the calcula­tions.

If venting is employed as all or part of explosion management, then it is necessary to carefully con­sider the point of vent discharge. The theoretical maximum explosion pressure is based on contain-

ment and complete combustion of a stoichiometric mixture of the particular flammable substance within the containment envelope. Venting permits release prior to complete combustion. Conse­quently, an air/fuel mixture with ignition estab­lished is released somewhere. The nightmare scen­ario is that this picks up flammable dust and trig­gers a massive secondary explosion which demol­ishes the building. The greater the volume of the secondary space, the less the resulting pressure. On the other hand, if the entire volume contributes flammable dust, then the problem is severely aggra­vated. Note the housekeeping implications of the secondary explosion phenomena. Despite the preva­lence of airborne and deposited dust in the area of shredder operations, there has been no implication of serious secondary explosions in any incidents re-

TABLE X EXPLOSION FREQUENCIES BY SHREDDER TY PE:

Total

Solid Waste Processed (K-Tons =

000 Tons)

K- Tons Per Explosion

K-Tons Per Moderate or Major Explosion

For Locations with Over 50 K-Tons Processed:

Total

K- Tons Per Explosion

K- Tons Per Moderate or Major Explosion

K-TONS PER EXPLOSION AND

K-TONS PER MODERATE OR MAJOR EXPLOSION

All All Horiz. Shredders Hammermills

8058 5556

85 118

425 347

7548 5311

108 148

539 483

Horiz. Hammermills

w/o CWS All Vert. or

Venting Hammermills

3306 1641

73 68

206 821

3306 1458

73 146

206 729

Note: For metric tons, multiply by 0.907.

219

Vert. All Vert. Grinders Grinders w/o CWS

861 730

36 33

861 730

779 679

32 31

779 679

ported to date. The possibility does exist, however, and must be considered in venting. Venting to atmo­sphere is probably preferable, but even here, con­sideration must be given to arrangements which do not endanger personnel. Also, some venting will necessarily take place through the shredder inlet no matter how large the outside vent may be. Conse­quently, it should be recognized that shredders will vent to their enclosure space, if any.

CONTAINMENT

The need for inlet and outlet feed openings pre­cludes containment as an explosion management possibility for shredders. The combination of con­tainment and venting requires that attention be given to the relative strength (high) of containment structures compared to that (low) of the relief area.

BUILDING CONSTRUCTION

Damage from shredder explosions can be the re­sult of direct or reflected pressure waves, or they can result from flying objects. Both types of dam­age should be considered in building design. Pres­sures at various distances can be at least approxi­mately calculated for given shredder volumes and vent areas. Walls, floors, roofs and glazing should be at least adequate for the calculated pressures. In addition, it is advisable to provide a generous vent­ing panel designed to relief to a safe point by fail­ure at a lower pressure.

In some cases, flying objects can be contained by blast mats. Plywood panels loosely suspended under the floor or roof over the shredders can con­tain and spread the loading to prevent penetration by thrown objects in these locations. Styrofoam in

TABLE XII CO MPARISON OF RECO MMENDED VENT AREA

CORRELATIONS APPLIED TO THREE HA MMER MILLS

NFPA 68(5)

Williams

Model 680* 40

(Volume 1500 ft3) (3.n 42.5 m

Modified Hazemag** 45

(Volume 1800 ft3) (4.2) 51.1 m

Heil

Model 420*** 10.3

(Volume 150 ft3) ( I .0) 4.3 m

Recommended Vent Area (ft2) (m2)

Yao ( 18) Bartknecht ( 10) Howard and Russell (20)

36 53.8 23

(3.3) (5.0) (2. 1 )

42 65 26

(3.9) (6.1) (2.4)

7.8 9.7 5.1

(0. n (0.9) (0.5)

• Horizontal Hammermill + input hood (LID 3) Horizontal Hammermill (LID 2) Vertical shaft Hammermill (LID 2)

220

appropriate locations can be used to prevent rico­chets. Shatter-resistant plastic glazing should also be considered for control room and other adjacent

windows. Mullions should be adequate to with­

stand predicted pressures and the "grip" on the glazing should be deep enough and of proper design to permit the required deflection. A safety bar in­

side the mullion can provide extra insurance against the window imploding as a whole in the event of overpressures.

ISOLATIONS

At least until extended operating experience

with the system utilized indicates freedom from explosions, it is desireable that personnel should not be permitted in the vicinity of the shredders during operation.

INERTING

The explosion of all but commercial explosives,

which contain their own oxidant, cannot take place in an inert atmosphere which, for all practical pur­poses, consists of less than 6 percent oxygen. Al­though flue gas may normally contain 7-8 percent

oxygen, it greatly reduces the oxygen available to the fuel and will drastically reduce both the fre­quency and resulting pressure from explosions. There is an additional diluting and cooling factor,

.and, for all practical purposes, inerting with flue gas may provide an effectively inert atmosphere.

Figures 3 and 4 indicate flue gas inerting sys­

tems which are contemplated for current applica­

tions. They will be push-pull systems in order to

control the pressure in the shredder and will be ad­justed to maintain a slight negative internal pres­

sure. This should have the additional beneficial effect of reducing the dust in the shredder area.

The shredder is, in a sense, also a fan. There is

an uncertainty as to the flow rate and direction under various conditions of operating and loading.

Consequently, the dribble chute is used as a recircu­

lating duct to minimize flue gas requirements.

PRESSURE - BARS

o 0 I 2 20 3 4 5 6 1

< 8 m Z 9 of 10 »" D m 12 » 13 I �14 R115

16

40

60

< 80 m Z 100 of » 120 D m » 140 I

'11 160 of •

I\) 180

0 • -

11 18 19 20 21 22

23

200 • 220

240

260

• 0 0 ... -

,

PRESSURE - PSI

� � � � � -r- '"

l{

�

FIG. 1

221

0 '"

I

G

'"

0 0 • '" .,.

,

.. ... en -

;;;r

0 0 -• • • ... <D 0

- '" <D ... O 0

- '" '" .,. • • • .

... 0 0 0 , , I ,

gj 8 uo � 0

�I � til

�

It is important to minimize flue gas require­ments, not only to control duct and fan size and horsepower, but also to avoid excessive shredder temperature. Maximum shredder housing tempera­ture limitations of 140 F (60 C) have been sug­gested. At this point, we are contemplating 325 F (163 C) flue gas for inerting. Temperature limita­tions of explosion suppression systems must be considered if they are also to be "employed in con­junction with inerting. Some drying will occur with flue gas inerting, but again, caution should be con­sidered to avoid too much drying. With a high vola­tile fuel like shredded MSW, extreme drying could magnify the possible explosion problems down­stream and in storage.

As with venting, the discharge point of the flue gas must be carefully considered. Although it is un­likely that this stream will be carrying its own igni­tion energy, it will become flammable when ex-

100 30 90

215 80 70

20 eo

50 15

40

� � ..... � ......

� .....

i'. I""t'oo �

posed to additional oxygen. Consequently, release to the atmosphere must be at a safe point, and cleaning will undoubtedly be required. For one application, the discharge will be through the stand­by oil burners which will release the material into the furnace where controlled ignition will occur when there are flammables in the mixture.

The application and effectiveness of seals at the shredder infeed and outfeed constitute the biggest uncertainties needing further study in the implemen­tation of this approach to explosion management.

EXPLOSION SUPPRESSION

Commercial explosives result in detonations as opposed to the term deflagration as applied to the sudden potentially destructive pressure rise result­ing from ignition of an appropriate air/fuel mix­ture. There is very limited ability to manage de-

OVERPRESSURE VS DISTANCa

..... 1. 1111 L. TN T .au I VAL. NT BHR.DD.R FAILUR. AT BD P8 I

.10 30 lEt ••

I-1-7 • 0

� I' "" � W. • 115 IL

I I

.4 • U U Z5 Z C C l- I-• • -2 -Q Q

• 7

•

!I

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3

2 ('oj

ci ('oj ci

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� "

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PRESSURE - PSI 0

I I • I III 0 If) 0 d N

PRESSURE - BARS

FIG.2

222

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IN

o ,.;

r-..

�IC ., T

o •

o on

,

\ \ \ A.�L .CT.C:

o cD

q Q

o �

tonations at this point in time whereas all of the

previously mentioned techniques have successfully been applied to the management of deflagrations.

In addition, the velocity of a deflagration flame front appears to be instantaneous, but, in fact, is slow enough to permit a teclmique called suppres-

•

SlOn.

Effective suppression requires early detection of

pressure rise or other phenomena, very rapid activa-

tion of the suppression system, and high velocity

release of a suppression or inerting agent. Generally,

the suppressant is confined under relatively high

pressure by a rupture disc which is blown by a small

explosive charge similar to a dynamite cap when triggered by the detection system. Among the suppressants which have been effectively employed

in various operations are water, steam, ammonium phosphate and sodium bicarbonate powders and

BOIL.R N-"

I.O.OUTL

B OILERS

...---->0,--- N - a ... N·:I

(280 �) 6000 CFM 325 ° F

(162 °C)

I. O. OUTLETS

(280 �) 6000 CFM 325 ° F

(162°C)

(280 � ) 6000 CFM 3250 F

(162°C)

'------...1....----,-------,-------'--4 1-_____ J CHECK VALVE �.�.J-

( 140 � ) 3000 CFM 1 1/2 H.P. �

(I.I K .W. ) �g COOLING (IF REQUIRED)-::

(35 �) 750 CFM

HIGH TEMP. TRIP -

(175 �) 3750 C.�F:M_�..,.� I H. P.

(.75 KW )

-112 SIG

CHECK VALVE -''''

• CHECK VALVE

(140 � ) 3000 CFM

'-- I 1/2 H. P. '18 (1.1 K.W. ) C '-COOLING (IF REQUIRED)

- 1/2 PSI G

(35 � ) 750 CFM

--·HIGH TEMP. TRIP

(175 �) 3750 CFM

........... - 1 H.P. (.75 KW )

,t" ... --- CHECK VALVE

...--------r---....J'----------..1..----H � -----,

(354 � ) 7500 CFM

(354 �) 7500 CFM

(354 �) 7500 CFM

I I I I

,I \ I I I

BOILER N-" BOIL.R N- II \NINOBOX \NINOBOX

BOIL.R N- :I FUTUR. N- 4 \NINOBOX

FANS TO PRO VIOE PERMISSIVE FOR BHR. OO.R

OAMPERS TO BE INT.RL OCKEO \NITH BOIL.R

FIG. 3 SHREDDER INERTING SCHEMATIC

223

60000 ICFM 21540 �

40000 VFM 1890 �

PANTOGRAPH TYP. ..AL

STACK

-

FIG.4 SHREDDER INERTING SCHEMATIC ALTERNATE

224

1.0. FAN

Halon 1301 and 2402. The burning and pressure rise rates of only

hydrogen and aluminum dust are outside the range of effective suppression with current technology.

Although one authority (Bartknecht) has found water or steam to be more effective than Halon as a suppressant in several applications, at least one leading supplier of suppression systems will not apply their system to refuse shredders unless they are used in conjunction with Halon 1301 as the suppressant. Halon 1301 currently costs $5.00/1b ($11.00/kg) with approximately 240 Ib (109 kg) consumed in each activation. To this must be added the cost of replacing the rupture discs and any ser­vice required which can bring the cost of each in­cident to a reported cost of $2400.00 to $4000.00. ConSidering the probability of four controlled inci­dents for every one incident which would be ob­served under controlled conditions, a 1000 tons per day (907 t/day) installation could expect one discharge every 20 days on the average, or 18 per year. On the surface, an expenditure of possibly $80,000.00/year to prevent $25,000.00 + damage every 1.2 years does not appear to be a good trade­off; but the risk of injury to personnel, more ex­tensive damage, and downtime must also be con­sidered.

In some instances, the supplier of explosion suppression equipment requires that it be serviced exclusively with his representatives' personnel. Ser­vice is guaranteed to be available within 24 hr of notice. The benefits of adequately trained service personnel and the costs are not really consequen­tial in view of the additional protection provided. However, the loss of equipment availability can be a serious matter. So far as we are aware, the only instances of explosions on systems properly pro­tected with explosion suppressing systems have occurred when the operators elected to resume shredding prior to recharging the suppressors. Con­sequently, we believe it important to pursue systems which can and will be rapidly recharged by the nor­mal plant personnal and/or improve service arrange­ments to result in no more than a few hours down time.

Normal maintenance of the equipment is also of vital importance. Instances have been reported where plugging of the sensors prevented proper actuation of the systems. Newer systems use air or flue gas to purge sensor ports. With the latter, temperatures must be kept within limits specified by the sensor manufacturer.

The use of explosion suppression equipment

can buy additional assurance, but it cannot buy insurance.

WATER SPRAYS

Continuous water sprays have been applied to shredders in a few cases with apparently consider­able success in reducing the frequency and severity of explosions. In addition to the potential nuisance problems, this method is not desirable where fuel or energy recovery is contemplated. Accordingly, no further discussion will be given to this method at this time.

EXPLOSIVE DETECTORS

Detectors are available for flammable vapors and can be applied to shredder hoods for intermittent water or steam spray or pre-activation of vents or suppression systems. Work is also progressing on explosion detectors and it is possible that there may be limited beneficial applications for such devices in the future to detect commercial explosives.

CON CLUSIONS

Solid waste shredders inherently have explosion potential and data on explosion experience is in­creasingly being accumulated and correlated. Many of the earlier installations appear to have been built with minimal regard for this potential. The basic mechanisms applicable to shredders for explosion management are venting, explosion suppression, and possibly inerting. Design of structures and equipment in the vicinity should consider pro­visions for minimizing damage from explosions and projectiles. An increasing amount of informa­tion is becoming available to the designer to assist in alerting him to the problems and options; how­ever, much remains to be discovered or developed. New designs should increasingly incorporate ex­plosion prevention/management provisions to re­duce the severity and frequency of explosions.

REFERENCES

[11 "Solid Waste Shredders, A Survey," NCRR Bul­

letin, Fall 1974 , Vol. IV, No.4 .

[2 1 Zalosh, R. G., Wiener, S. A., and Buckley, J. L.,

"Assessment of E xplosion Hazards in Refuse Shredders,"

prepared for ERDA, April 1976.

[31 Zalosh, R. G., "Explosion Protection in Refuse

Shredding," presented at the Fifth National Congress on

Waste Management Technology and Energy and Resource

Recovery, December 1976.

225

(4 ) Bartknecht, W. , "The Course of Gas and Dust

Explosions and Their Control," from Loss Prevention and Safety Promotion in the Process Industry, C. H. Bushman,

ed. 1974.

(5) Catalog No. 73-1, Fike Metal Products Corp.

(6) Bulletin No. MC-215B, Fenwal Inc.

Key Words

Explosion

Refuse

Shredding

226

Discussion by

William D. Robinson

Hammermills, Inc.

Trumbull, Connecticut

Referring to subheadings in the paper:

Experience with Explosions

It is commendable that the author has had the perspicacity to examine the vast experience in the auto shredding industry regarding explosions.

Solid waste processing plant designers might also benefit from investigation of the integrated shredding system design and operating experience in that industry which has closely related require­ments for air handling, dust and emissions control, reliability, maintenance and operator protection.

Explosion Relief

As the paper states, there have been no known serious downstream secondary explosions, bag­house fires, etc. to date in solid waste RDF/mater­ials recovery installations.

This is because there are few, if any, plants in continuous operation anywhere near design capacity.

The precautions of prolonged shakedown-retro­fit status likely account for this.

To this discussor, the design shortcomings of most existing plants are notable.

Also, it is apparent that most if not all serious episodes of shredder explosions have occurred in topfeed mills with the usual hood superstructure configuration.

This leads to a conclusion consonant with the author's observation:

"The greater the volume of the secondary space, the less the resulting pressure. On the other hand, if the entire volume con­tributes flammable dust, then the problem is severely aggravated. " The latter factor is more critical and of &reater

concern to this observer than any higher volume­lower pressure advantage or the recompression phenomenon.

Despite disagreement amongst investigators over pressure relief (venting) design criteria, there is a consensus that adequate pressure relief is para­mount throughout the system as the author indi­cates. Within limits of structural deficiencies and/or excessive leakage, "more is better" might be axiomatic.

227

Continuous purging with an induced air stream can be effective, but present designs are mostly inadequate with regard to volumes and velocity.

likewise, the penchant for recirculating increas­ingly concentrated dust laden air can be a danger­ous practice.

Water Spray

The author's comment is entirely valid, but I would not dismiss the concept summarily.

It appears that a highly atomized mist or fog (say 300 psi header press) can be an effective sup­pressant without perhaps adding excessive moisture to an RDF product (approx. 1 percent for 50 toni hr @2-3 gpm). Further investigation is certainly warranted.

Inerting

With combustion flue gas as the medium, care­ful gas temperature control is imperative, - not only to prevent an excessive drying-fire possibility as the author states, but also because:

• High temperature (over 400 F) can significant­ly increase shredder wear parts maintenance.

• A dew point-HC1, corrosion possibility may exist in the shredder-process circuit if 100 percent firing of RDF with a high plastic con­tent.

DISCUSSOR'S CONCLUSION

The author is to be commended for discussing a sensitive and vital subject forthrightly. He is in the fortuitous position of investigating critical refuse shredder phenomena with a following oppor­tunity to put his ideas into practice.

AUTHOR'S REPLY

To William D. Robinson

Continuous purging would result in dilution, one of the available techniques which could con­ceivably holq the concentration of flammables be­low limits of flammability. It would be reasonable to expect this to have considerable effect at least on the smaller volumes of liquids received by the shredder. On the other hand, duct work, clean-up systems, etc. would contribute to the contained volume. Additional attention would be necessary to proper relief from such or the seriousness of

major incidents could well be aggravated. It is agreed that water spray shQWS promise and

possibly may not be incompatible with shredding for fuel processes.

For inerting to be compatible with temperature and other practical considerations, the flue gas quantity must be limited. This requires effective curtaining of the shredder. One important aspect for this to be effective is to use the dribble chute as a recirculation duct in order that the flue gas

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quantity does not have to be so large as to over­corrie the inherent windage characteristics of the shredder. This was a necessary consideration for the feasibility of incorporating inerting in the Akron project even for test purposes. Limiting the required flue gas quantities to the minimum nec­essary for inerting also can make chemical treat­ment practical in the event that corrosion problems should be experienced.