IOSOLIDS HANDLING ANDPROCESSING EQUIPMENT

IO. I INTRODUCTION

This chapter presents potential failure mechanisms for solids handling andprocessing equipment, and suggests design alternatives for reducing the risksassociated with such failures. The types of equipment covered in this chapterinclude:

• Mechanical conveyors• Pneumatic conveying systems• Comminution equipment (mills, grinders, crushers)• Sieving (screening) equipment• Powder blenders (mixers)• Solids feeders (rotary valves, screw feeders, etc.)• Solids enlargement equipment (extruders, briquetters, etc.)• Spray granulators and coaters

This chapter presents only those failure modes that are unique tosolids handling and processing equipment. Some of the generic failure sce-narios pertaining to vessels and solid-fluid separators may also be applicable tosolids handling and processing equipment. Consequently, this chapter shouldbe used in conjunction with Chapter 3, Vessels, and Chapter 9, Solid-FluidSeparators. Unless specifically noted, the failure scenarios apply to more thanone type of solids handling and processing equipment.

10.2 PAST INCIDENTS

Several case histories involving failures in solids handling and processingequipment are presented to reinforce the need for safe design and operatingpractices presented in this chapter.

10.2.1 Silicon Grinder Fire and Explosion

A chemical plant which processed silicon-based chemicals experienced a fireand explosion in a grinder. Raw silicon was received in 1- or 2-inch lumpswhich had to be ground to a 200-mesh powder before being used in chemicalprocesses. The air-conveyed silicon powder discharged from the grinderpassed through a cyclone and then through a bag filter. An explosion and sub-sequent fire occurred in the system. The fire was extinguished within 15 min-utes by a water hose stream. The system had explosion relief, but no sprinklers.

Investigation showed that this incident was caused by hot spot ignitionresulting from grinder parts scraping against the inside of the unit. This mecha-nism was supported by observation of high current draw on the grinder motorbefore the incident. See item 2 in Table 10 for potential design solutions.

Ed. Note: This hazard could have been mitigated by monitoring current-draw andpossibly interlocking current-draw with the motor or a deluge system.

/0.2.2. Blowing Agent Blender Operation Explosion Incident

An explosion occurred in a 3.7 m3 Nautamixer (conical orbiting screw mixer)during the blending of azodicarbonamide (AC) with an aqueous solution ofsalts to produce an AC formulation. During the batch blending cycle, hotwater (8O0C) is circulated through the blender jacket for several hours, and thevacuum in the blender is released by purging with nitrogen.

The explosion caused the mixer vessel to rupture and two large sections ofthe top were torn out completely and struck the floor above. The cone sectionwas thrust downwards into the hopper below. There was extensive damage tothe building, windows were broken up to 90 meters away by the pressurewave, and missiles were projected up to 120 meters away. The four people inthe plant at the time of the explosion were shaken up, but uninjured, whilethere were a few cuts to people in the nearby buildings due to flying glass. TheTNT-equivalence of the blast was estimated at 3.3 kg.

Subsequent experimental testing indicated that the explosion was causedby a decomposition which reached high rates due to a critical degree of con-finement. The initiating source of the decomposition was not positively iden-tified, but it was assumed that the heat was generated by mechanical frictiondue, for example, to the screw rubbing on the vessel wall. Another possibility

is that a small metal item found its way into the vessel and became trappedbetween the screw and the wall (Whitmore et al. 1993). See item 5 in Table 10for potential design solutions.

Ed. Note: A deflagration suppression system might have prevented the explosion.

/0.2.3 Screw Conveyor Explosion

Three employees were killed, and two seriously injured, and a factory buildingcompletely destroyed in an explosion involving skimmed milk powder. Themilk powder was fed into a screw conveyor from a feed hopper and was thencarried to a blender. A deformation occurred in the screw conveyor housing,causing parts of the screw flights to grind against the housing. The grindingproduced sufficient frictional heat and sparks to ignite the dust-air cloud in thefree space of the conveyor. The primary explosion burst the screw conveyorhousing, dispersing a significant amount of additional dust into the air fromthe freshly filled feed hopper. A secondary explosion was then ignited by theflames of the primary explosion (Field 1982). See items 5, 8, and 12 in Table10 for potential design solutions.

/0.2.4 Bucket Elevator Explosion

A dust explosion in a sugar refinery caused two injuries and severely damagedthe plant. A number of factors led to the explosion. The factory had been shutdown for a 9-day period and the explosion occurred within two minutes ofrestarting the plant. Before the shutdown, all sugar dust had been removedfrom the pit of the elevator shaft, but during the shutdown sugar had accumu-lated in the pit via a leaking flap valve.

The bucket elevator ran through all 13 stories of the building, collectingsugar from ground level and transferring it to the appropriate processingequipment. On startup, the bucket elevator was under a load for which it wasnot designed. The strain caused a tensioning device to fail, the bucket chainslackened, and the elevator buckets ran out of alignment. The frictional heatproduced by the rubbing metal surfaces was sufficient to ignite the sugar dustsuspension in the elevator shaft (Field 1982). See item 4 in Table 10 for poten-tial design solutions.

10.3 FAILURE SCENARIOS AND DESIGN SOLUTIONS

Table 10 presents information on equipment failure scenarios and associateddesign solutions specific to solids handling and processing equipment. Thetable heading definitions are provided in Chapter 3, section 3.3.

10.4 DISCUSSION

10.4.1 Use of Potential Design Solutions Table

To arrive at the optimal design solution for a given application, use Table 10 inconjunction with the design basis methodology presented in Chapter 2. Useof the design solutions presented in the table should be combined with soundengineering judgment and consideration of all relevant factors.

/0.4.2 General Discussion

Fires and explosions (deflagrations) have the potential to occur in equipmentthat handles and processes combustible powders and bulk solids. These haz-ards can be minimized by the use of appropriate preventive measures, such asthe following:

• Increasing the particle size of the powder raises the minimum ignitionenergy (MIE) and reduces the rate of pressure rise of a dust explosion.

• Using solid additives with large particle size and/or high MIEs.• Using dense-phase pneumatic conveying in lieu of dilute-phase convey-

ing reduces the attrition of the solids conveyed, reduces the static gen-eration per unit mass, and may result in nonflammable mixtures in thetransfer line.

• Using low-speed mills rather than higher-speed ones minimizes dustcloud formation and reduces the potential for high energy metal-to-metal contact.

• Using fluid energy mills in lieu of high-impact mills (e.g., hammermills); nitrogen can be used as the milling gas rather than air, which inmost cases will make the operation inherently safer.

• Using an ionizing spray to dissipate electrostatic charges where possible.

/0.4.3 Special Considerations

Table 10 contains numerous design solutions derived from a variety of sourcesand actual situations. This section contains additional information on selecteddesign solutions. The information is organized and cross-referenced by theOperational Deviation Number in the table.

Dust Deflagration in Pneumatic Conveying Systems (I)Dust deflagrations often occur in end-of-line equipment (e.g., silos, dust col-lectors, cyclones) of pneumatic conveying systems due to electrostatic sparks.The rubbing of particles against particles and the walls of the pneumatic con-veying line generate electrostatic charges on the powder, which are then dis-

charged in the end-of-line equipment, where a dust cloud is often formed, anda dust explosion occurs. A number of preventive and protective measures arecommonly used such as using nitrogen in lieu of air as the conveying gas, usingdense-phase conveying in lieu of dilute-phase conveying to minimize attritionof the powder, providing deflagration venting or suppression systems for theend-of-line equipment, and good grounding and bonding of the pipeline andequipment. Other measures that can be taken involve modification of thesolids being conveyed, such as increasing the particle size (making pellets) orformulating the solids so that they are less friable. Also, it is important to iso-late the pneumatic conveying line from end-of-line equipment by a quick-closing valve or suppressant barrier so that the flame front developed in theend-of-line equipment does not propagate backwards into the equipmentupstream of the conveying system.

Static ignition mechanisms in recovery bins, silos and related equipmentare discussed by Eckhoff 1996. Recommended preventive and protectivepractices are described in BS 5958 1991.

Dust Deflagration in M/7/s, Grinders, and Other Size Reduction Equipment (2)Size reduction equipment, such as mills, grinders, and the like, create turbu-lent dust clouds due to their operation, which can result in a dust explosion(deflagration) caused by mechanical energy (impact). This hazard can beminimized by using fluid energy mills in place of high-impact mills such ashammer mills. Fluid energy mills use a gas, such as air or nitrogen (an inher-ently safer fluid), to reduce the size of solids. Some types of mills are designedto contain a deflagration; these should be used whenever possible. Care mustbe taken to prevent the entry of tramp metal and other foreign materials intosize reduction equipment. This can be accomplished by installing screens ormagnetic separators upstream of size reduction equipment.

Dust Deflagration and Loss of Containment in Gyratory Screeners (3)Dust explosions (deflagrations) have occurred in gyratory screeners (sieves)because dust clouds are readily formed due to the nature of the operation.Because of its vibratory motion, gyratory screeners are connected to processequipment by flexible sleeves (e.g., rubber socks or boots) as they vibrate. If adeflagration occurs, the flexible sleeves could rupture ejecting a burning dustcloud into the room or building, which then can cause a secondary explosion.To minimize this hazard, several things can be done:

• Install the gyratory screener in a room with an outside wall equippedwith blow-out vent panels.

• Use a rotary screener, which does not vibrate, in lieu of a gyratory screener.• Use nitrogen inerting where feasible.

All metal components, including the screening surfaces, should bebonded and grounded because of the vigorous motion of the powder in thescreeners and the possible generation of static electricity. Considerationshould be given to the use of conductive or anti-static flexible sleeves. Also, fordusts of low MIE, provision of anti-static footwear for operators is recom-mended (Palmer 1973; BS 5958 1991).

Leaky flexible sleeves can result in fugitive emissions from gyratoryscreeners. Leaks can be minimized, or even eliminated, by operating under aslight vacuum, with the screener connected to a dust collector (Palmer 1973).

Overpressure In Racket Elevators and En-masse Conveyors (4)

Bucket elevators and en-masse conveyors contain belts or chains which canloosen and rub against the housing and cause impact sparks or frictional heat-ing, which in turn may cause a dust explosion. Tramp metal that gets into en-masse conveyors can also cause frictional heating which can act as an energysource for an explosion. Sensors for hot material can be installed and inter-locked with a water quench system to extinguish the hot solids. Also, it is veryimportant to prevent the propagation of a dust explosion flame into theupstream and downstream equipment connected to conveying equipment.This can be accomplished by installing material "chokes55 such as rotary valvesor screw feeders at the inlet and outlet sides of conveyors. It has been foundthat material "chokes55 (plugs of powder) quench the flame (Field 1982; Eck-hoff 1996). Quick-closing valves and suppressant barriers can also be used toisolate upstream and downstream equipment from conveyors.

fire Caused by Electrostatic Sparks Igniting Powder on a Belt Conveyor (10)Powders being conveyed on a belt conveyor can be ignited by an electrostaticspark if the powder has a low MIE. The electrostatic spark can often be gener-ated by the belt itself, and the use of belts of anti-static (conductive) materialscan minimize this problem. Electrostatic charges can also be reduced by use ofionized air or inductive neutralizes, such as static combs and tinsel bars(NFPA 77 1993).

10.5 REFERENCES

British Standards Institute BS-5958 1991. Code of Practice for Control of Undesirable Static Electric-ity: Part 1, General Considerations, and Pan 2, Recommendations for Particular IndustrialSituations. London: British Standards Institute.

Eckhoff, R. K. 2nd ed. 1996. Dust Explosions in the Process Industries. Butterworth-Heinemann,Boston.

Field, P. 1982. Dust Explosions. New York: Elsevier Scientific Publishing Company.

NFPA 77 1993. Recommended Practice on Static Electricity. National Fire Protection Association,Quincy, MA.

Palmer, K. N. 1973. Dust Explosions and Fires. London: Chapman and Hall.Whitmore, M. W., Gladwell, J. P. and Rutledge, P. V. 1993. Journal of Loss Prevention in the Proc-

ess Industries. 6:169-175.

Suggested Additional Reading

Grossel, S. S. 1988. Safety Considerations in Conveying Bulk Solids and Powders Journal ofLossPrevention in the Process Industries. 6:62-74.

TABLE 10. FAILURE SCENARIOS FOR SOLIDS HANDLING AND PROCESSING EQUIPMENT

Potential Design Solutions

ProceduralActiveInherently Safer/PassiveFailure ScenariosOperationalDeviationsNo.

• Manual bonding acrosspotential breaks in continuitysuch as nonconductive rubbersocks

• Deflagration venting of end-of-line equipment

• Deflagration suppression inend-of-line equipment

• Quick-closing isolation valveat inlet to end-of-lineequipment

• Deflagration suppressionbarrier in piping at inlet toend-of-line equipment

• Permanent grounding and bondingvia continuous metal piping

• Use of heavy wall piping andflanges in lieu of tubing andcouplings so that system canwithstand maximum expecteddeflagration pressure

• Use of nitrogen in lieu of air forconveying gas (closed loop system)

• Use dense phase conveying insteadof dilute phase

• Convey solids as pellets instead ofgranules or powder. However,avoid transport of pellets containingeasily ignitable fines fraction.

• Increase particle size

• Use nonfriable solids formulation(avoid fines)

• Use additives with high ignitionenergy

• Use of conductive rubber sleeves(boots and socks) when flexibleconnections are required

Dust deflagrationin end-of-lineequipment (silo,cyclone, dustcollector) dueto electrostaticspark dischargegenerated bypneumaticconveying

Overpressure

(Pneumaticconveyingsystem)

i(T)

• Manual removal of trampmetals and other foreignmaterials

• Manual bonding andgrounding

• Good housekeeping to reducedust

• Frequent routine inspectionand scheduled replacement ofsleeves

• Manual bonding andgrounding

• Good housekeeping to reducedust in building

• Manual grounding andbonding

• Provide inerting• Deflagration venting• Water deluge system in mill• Deflagration suppression in

the mill• Deflagration suppression/

barrier in inlet/outlet piping• Use magnets to remove

tramp metals and otherforeign materials

• Install gyratory screener in aseparate room with blow-outwalls (deflagration vents)

• Operate under vacuum toavoid escape of dusts intobuilding

• Deflagration venting• Deflagration suppression• Provide chokes• Provide negative pressure for

bucket elevators installedinside buildings to minimizedust leakage

• Provide deflagrationsuppression/barrier at feedand discharge points

• Provide hot materialdetection and automaticquench system

• Provide inerting for smallen-masse conveyors

• Permanent grounding of housing

• Equipment design accommodatingmaximum expected pressure

• Use of fluid energy mill with inertgas instead of air

• Use screens to remove tramp metalsand other foreign materials

• Use of nongyratory (rotary) type ofscreener

• Permanent bonding and grounding

• Use of outboard bearings to avoidpotential source of ignition

• Equipment design accommodatingmaximum expected pressure fortubular en-masse conveyors

• Permanent grounding and bonding

• Convey solids as pellets instead ofgranules or powder

• Increase particle size

Dust deflagrationdue to mechanicalenergy or electro-static spark

Dust deflagrationcausing rupture offlexible sleeves andsubsequent secon-dary deflagrationin building

Dust deflagrationdue to impact orfrictional heatingfrom slipping beltsor chains withpossible secondarydeflagration inbuilding

Overpressure

(Mills,Grinders andother sizereductionequipment)

Overpressureand Loss ofContainment(gyratoryscreener)

Overpressure

(bucketelevators and

en-masseconveyors)

2(T)

3(T)

4(T)

Potential Design Solutions

ProceduralActiveInherently Safer/PassiveFailure ScenariosOperationalDeviationsNo.

• Procedures to verify adequatepurging of bottom bearing

• Manual grounding andbonding

• Procedures for periodicinspection and cleaning ofcombustible materials onwalls

• Procedures to process moststable materials first whencampaigning multipleproducts to avoid ignition ofunstable materials

• Manual grounding andbonding

• Manual shutdown on motoroverload

• Manual shutdown ondetection of high pressure

• Provide inerting

• Deflagration venting

• Deflagration suppression

• Provide an overload trip onthe motor driving theorbiting screw

• Provide inerting

• Deflagration venting

• Deflagration suppression

• Deflagration barriers (quick-closing isolation valve orsuppressant) in the pathfrom granulator or coater todownstream equipment (dustcollector, scrubber)

• Provide emergency reliefdevice

• Provide overload trip onmotor

• Provide pressure measure-ment at die with interlockshutdown on high pressure

• Equipment design accommodatingmaximum expected pressure

• Permanent grounding and bonding

• Increase particle size

• Permanent grounding and bonding

• Equipment design accommodatingmaximum expected pressure

• Eliminate use of flammable solvents(e.g., aqueous solvents)

• Use high flash point solvents

Dust deflagrationdue to electrostaticspark discharge orfrictional heating(orbiting screw orribbon rubbingagainst vessel wall)

Deflagrationand/or firescaused by use offlammable orcombustiblesolvents

Blockage of die

Overpressure(orbitingscrew powderblender, fluidbed blender,or ribbonblender)

Overpressure(spraygranulatorsand coaters)

Overpressure

(extruder)

5

6

7

• Provide a temperature sensorin the conveyor trough/barrelwith an alarm alerting theoperator to activate delugesystem or deluge steam

• Manual removal of trampferrous metals

• Operator activation ofsprinklers or water spray

• Manual shutdown ondetection of low speed

• Operator activation ofsprinklers or water spray

• Provide an overload trip onthe motor driving the screw

• Provide a temperature sensorin the conveyor trough/barrelautomatically tripping themotor and/or activating awater deluge system orsnuffing steam

• Use magnets to removetramp ferrous metals

• Provide automatic sprinklersor water spray protectioninterlocked to shutdown thebelt drive on sprinkler waterflow initiation

• Provide belt velocitydetection interlocked toshutdown on low speed

• Provide automatic sprinklersor water spray protectioninterlocked to shutdown thebelt drive on sprinkler waterflow initiation

• Provide ionizing blower toeliminate static charge

• Use other type of conveyor (e.g.,vibratory conveyor)

• Use screens to remove trampmaterials

• Provide "fire retardant" belts

• Use other type of conveyor (e.g.,vibratory type)

• Use sealed roller bearings tominimize ingress of solids

• Provide belts of anti-static material

• Increase minimum ignition energy

• Provide passive static eliminationdevice (e.g., tinsel bar)

Fire caused byjamming ofconveyed materialand frictionalheating

Fire caused byoverheating due toa jammed idlerroller, or if the beltjams, as a result ofdrive rollers con-tinuing to run

Fire caused byelectrostatic sparksigniting powderon the belt

HighTemperature(screwconveyorsor extruders)

HighTemperature

(beltconveyors)

HighTemperature(beltconveyors)

8

9

10(T)

Potential Design Solutions

ProceduralActiveInherently Safer/PassiveFailure ScenariosOperationalDeviationsNo.

• Provide a temperature sensorin the valve body with analarm alerting the operator totrip motor and activatequench

• Ensure dust collector bagsand cages are properlysecured

• Provide temperature sensorsin the trough with an alarmalerting the operator to tripmotor and activate quench

• Provide a temperature sensorin the extruder barrel (body)with an alarm to alert theoperator to take action

• Periodic contaminationtesting of area

• Provide an overload trip onthe motor driving the rotaryvalve

• Provide a temperature sensorin the valve bodyautomatically tripping themotor and/or admittingquench water into the valve

• Provide an overload trip onthe motor driving the screw

• Provide temperature sensors(multipoint or line type) inthe trough automaticallytripping the motor and/oradmitting quench water tothe conveyor trough

• Provide an overload trip onthe motor driving theextruder screw

• Provide a temperature sensorin the extruder barrel (body)automatically tripping themotor

• Provide negative pressureventilation to contain andcapture any emissions

• Design dust collector bag cages andfilters to be properly secured toavoid falling into rotary valve

• Provide robust bar screen at rotaryvalve inlet

• Provide outboard bearings toprevent failure due to solidscontamination

• Use different type of conveyor(e.g., vibratory conveyor)

• Provide "dust-tight" design

• Use other type of conveyor (e.g.,en-masse conveyor)

Fire caused byjamming andfrictional heating

Fire caused byshaft misalignmentresulting infrictional heatingdue to the shaftrubbing againstthe trough

Fire caused byjamming andfrictional heating

Emission ofcombustibleand/or toxic duststo the atmosphereor building

HighTemperature(rotaryvalves)

HighTemperature(screwconveyors)

HighTemperature(extruders)

Loss ofContainment(bucket ele-vators, screwconveyors)

11

12

13

14