Explosion Design of PowderProcessing Equipment

The chance of explosion is further increased in operationswhere volatile solvents are evaporated from the powderduring drying operations. In all of these cases, the designof powder processing equipment must include an explo-sion-protection strategy. Stress Engineering Services,Inc. (SES) is active in a number of major areas of powder-handling explosion protection, including:

1. Development of specifications elaborating designapproaches for explosionprotection of new and existing (upgraded) systems.

2. Sizing of explosion ventingsystems.

3. Structural analysis ofexisting systems to insure their ability to withstandan explosion and design of retrofits to improvestrength where necessary.

This article describes workperformed on fluid bed dryer(FBD) systems.

FBD systems are typical of powder processing operations used in a variety of industries, includingpharmaceutical and food manufacturing.

Figure 1 shows a schematic of a FBD system, includingthe FBD vessel (machine tower), forced- and induced-draft fans, dust collectors, and associated valves andinstrumentation.

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The processing of organic powders brings with it the possibility of explosion.

Figure 1 Fluid bed dryer showing explosion zones

The explosion design of such systems must include the following:

fi Zonal division of the system into explosion classes

fi Selection of an explosion containment or mitigationstrategy for each zone and each piece of equipmentwithin that zone

fi Confirmation that the pressure rating of each piece of equipment is adequate for the explosion contain-ment/mitigation strategy selected

Figure 1 also shows a typical zonal division. Within eachzone the system designer can select any of the followingstrategies for protecting personnel and equipment froman explosion:

fi Containment

fi Venting

fi Suppression

fi Inerting

Containment seeks to withstand the maximum pressureof an explosion. Venting and suppression strategies seekto reduce the maximum pressure to a level that can bewithstood by weaker equipment. Inerting seeks to elimi-nate the possibility of forming an explosive atmospherewithin the equipment. Pressure versus time curves forthe first three strategies are shown in Figure 2.

Current design practice uses the containment strategy for the protection of the FBD machine tower itself. Themachine tower is typically designed to withstand 11 barg(145 psig) of explosion pressure. This is adequate to con-tain the maximum pressure seen during the explosion ofa pharmaceutical or food powder, as indicated by dataprovided in NFPA 68 and by testing of specific powders.Older systems, however, were typically designed for vent-ed service.

Other dryer systems, such as spray dryers, are often toolarge to make containment of 11 barg feasible and eitherventing or suppression must be used to control possibleexplosions. In these cases, the explosion strength of thevessel must be determined and, if necessary, improvedthrough retrofit to match the available protection scheme.

A typical example of this situation is shown in Figure 3.This figure shows an older FBD designed for ventedexplosion protection. A finite element collapse analysis of this vessel indicated that the support structure for thelower plenum of the vessel would fail due to gross yield-ing and tearing at 1 barg, substantially below the 2 bargspecification for vented service.

Based on this analysis, SES developed a retrofit strategythat would enable the vessel to withstand a 2 barg explo-sion. An important consideration in this retrofit was theintention to redesign the system as an “explosion pres-sure shock resistant design.” This design philosophyaccepts that substantial deformation will occur in theFigure 2 Typical explosion pressure curve

Stress in Fluid Bed Dryer at 1.0 bar Explosion Pressure

Contours of Stress in psi

Failure is Due to Tearing of Support Arms

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Figure 3 Stress in fluid bed dryer at 1.0 bar explosion pressure

event of an explosion, perhaps rendering the equipmentunusable, but aims to protect personnel and other plantcomponents in the event of an explosion. This approachcontrasts with an “explosion pressure resistant design”approach, in which the normal boiler and pressure vesselrules are employed, leaving the equipment undamagedfollowing an explosion.

Given the low probability of explosions, the cost ofstrengthening existing equipment, and the particular difficulties seen in welding 300-series stainless steel thathas been in service for 20-30 years, the use of the explo-sion pressure shock resistant design philosophy has significant attractions.

Ancillary equipment in FBD systems can present signifi-cantly greater challenges for explosion-protection designthan the machine tower. Typical of these challenges is the protection of dust collectors. Figure 4 shows a dustcollector in a FBD system. Newer dust collectors are protected either by venting or suppression and aredesigned accordingly.

Legacy equipment, however, was frequently installed withlittle thought given to explosion protection. This leavessuppression as the only viable means of protecting anexisting dust collector. Unfortunately, many existing dustcollectors cannot withstand the 4-5 psig peak pressureexperienced during a suppressed explosion. Worse yet,

the stitch-welded stiffeners that were frequently used tostrengthen the flat walls of dust collectors do not haveadequate strength and will break loose during an explo-sion. The stress analysis included in Figure 4 shows anexample of this behavior, with the entire wall of the dustcollector yielded and bulging at only 1.3 psig internalpressure.

The only viable solutions to this problem are to: 1) scrapthe old dust collector, or 2) heavily reinforce the wallswith an external cage of structural steel. Reinforcementis frequently selected and can be made to work at a reasonable cost if it is part of an explosion pressureshock resistant design. Proper analysis, or elimination, of stitch-welded joints is critical to the successful designof this type of structural cage.

Explosion protection of powder processing equipmentrequires a careful, multidisciplinary approach whichincludes equipment design, system layout, and explosion-protection strategy implementation. SES’s ability to bringtogether project teams containing experts in systemsdesign, structural analysis, flow and thermal analysis, as well as electrical and civil specialists enables us toprovide a comprehensive answer to the questions raisedby these systems.

Side Wall is Fully Yielded

Contours of Stress in psi

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Dust Collector Roof

Dust Collector

View from Ground

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Figure 4 Stress in Dust collector walls at 1.3 psig explosion pressure

© 2010 Stress Engineering Services, Inc. 428

About Stress Engineering Services

Established in 1972, SES is employee owned. Our staff covers a score of engineering disciplines including mechanical, civil, electrical, metallurgical, materials, water chemistry, theoretical and appliedmechanics. Over 80% of SES engineers hold advanced degrees, most are licensed P.E.’ s and the averageengineer has more than 15 years experience.

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