Engineering Practice

The most effective explosion pro-tection strategy is one that pre-vents a blast in the first place.Most plants in the chemical

process industries (CPI), however, arefraught with conditions that weakenthe fabric of solely preventive mea-sures (box, p. 61). Given the cata-strophic consequences that explosionsthreaten to process equipment andnearby personnel, responsive mea-sures are also vital.

A responsive strategy protects theequipment and personnel after a de-flagration* begins. Explosion venting,the most widely used responsive tech-nique in the process industry, hasbeen used for decades to limit or pre-vent overpressure damage caused bydeflagrations in industrial processes.Deflagrations occur most frequentlyin dust collectors, conveyors, eleva-tors, dryers, mills, storage vessels,and entire buildings or rooms.

If designed and sized correctly, ex-plosion vents reduce the maximumpressure (Pmax) of a deflagration to asafe level (Pred) below that which theequipment is designed to resist (Pes)(see Figure 1). The pressure that is de-veloped during a deflagration is re-leased through the vent, and thus, thevessel integrity is not compromised.

The U.S. National Fire ProtectionAssociation (NFPA) publication NFPA68, Guide for Venting of Deflagra-tions, 2002 edition, provides guide-lines for the design and use of explo-

sion venting devices. However, thispopular strategy has been misappliedin many applications, resulting in afalse sense of security and leaving in-dustrial processes still unprotected.

To reconfirm that the installed ex-plosion-protection strategies ade-quately protect the process, facilitiesshould be reviewed in context of NFPAguidelines and other applicable indus-try standards but, more importantly,vis-á-vis industry experience. Considerthe following common applicationswhere explosion venting can be misap-plied, and consult with explosion pro-tection experts to ensure that your pro-tection strategies are adequate.

Equipment located indoorsWhen process equipment is located in-side a building, vent ducts should beused to direct the flame and pressurefrom the enclosure to the outdoors(NFPA 68, Section 5.8.1). A vent ductis ductwork connected to the explo-sion-vent assembly leading to an out-side wall, which provides a path di-recting the flame and pressure to asafe location. The use of a vent ductcan, however, significantly increasesthe pressure (Pred) in the equipmentduring venting.

The old rule of thumb suggestedthat if the discharge, vent-duct lengthwas less than 3 m. (10 ft), the increasein Pred was negligible. But, empiricaldata have shown otherwise; a newequation in Section 7.5 of NFPA 68shows how to calculate the effect of thevent duct at various duct lengths(based on the length-to-diameter ratio

of the duct). Although the new equa-tion produces lower Pred results com-pared to earlier NFPA 68 editions,there still can be significant increasesin Pred with the addition of a vent duct.

Consider, for example, an enclosurewith various lengths of vent duct (Fig-ure 2 and Table 1). A vent duct of only3 m (10 ft) would produce a Pred that ismore than twice that of a vent applica-tion that did not utilize a vent duct. So,a typical rectangular dust collectorthat is rated to resist a Pred of 0.2 barg(3 psig) could actually experience pres-sures that exceed the vessel’s ultimatestrength — and risk rupture — if avent duct were added. In general,larger dust-deflagration index (Kst)values and smaller vessel volumes willproduce the greatest increases in Predwhen vent ducts are used.Alternative protection strategies. Inthe case of equipment located indoors,consider the following alternatives:• Increase the vent relief area to com-

pensate for the effects of the ventduct and achieve an acceptable Pred

• Move the process equipment as closeto an outside wall as possible. Theduct length is directly correlated tothe predicted pressure (Table 1) —the shorter the duct length the lowerthe Pred value. If the vent duct lengthis less than one hydraulic vent diame-ter, there is no additional increase inPred (NFPA 68, Section 5.8.2)

• Use explosion suppression in lieu ofexplosion venting. Explosion sup-pression is a responsive technique ofdetecting and extinguishing the de-flagration during its incipient stage,

To Vent or Not to Vent

Pressure vs. time curve for vented explosions

Pre

ssur

e, b

arg

Time, milliseconds

Pmax

Pmax

Pstat

Pstat

Pred

Vented Explosion CurveVented explosion curve

0

10

0 400

The maximum pressure developed in a contained deflagration of an optimum mixture

The enclosure design pressure sufficient to resist Pred

The maximum pressure developed in a vented enclosure during a vented deflagration

Pressure that activates a vent closure when the pressure is increased slowly

Pes

Pred

Pes

Non-vented explosion curve

Ralph FoilesProcess Protection Inc.

Learn from common misapplications in explosion venting and consider

alternative strategiesFIGURE 1. If designed and sized correctly, explosion vents re-duce the maximum pressure (Pmax) of a deflagration to a safelevel (Pred) that the equipment is designed to resist (Pes)

* Deflagration: propagation of a combustionzone at a velocity that is less than the speed ofsound in the unreacted medium

Reprinted from CHEMICAL ENGINEERING, Month 2004, copyright 2004 by Access Intelligence with all rights reserved.Additional reprints may be ordered by calling Chemical Engineering Reprint Department (212) 621-4958. To subscribe to Chemical Engineering, call (212) 621-4656.

resulting in Pred values similar tothose for explosion venting withoutvent ducts

• Use a flame-arresting device in con-junction with explosion venting. Theflame-arresting device quenches theflame as it exits the explosion vent,and prevents unburned dust fromescaping the process equipment.Typical flame arresting applicationscan provide Pred values similar tothose achieved by explosion ventingwithout vent ducts

No safe place to vent As indicated earlier, explosion ventingreduces the deflagration pressure butdoes not stop the combustion process.As pressure is relieved through the ex-plosion vent, unburned dust is pushedoutside of the enclosure, ahead of theflame front. The discharged flame andhigh-pressure material can injure per-sonnel, cause secondary explosions, orresult in pressure damage to adjacentequipment or buildings. It is impor-tant to take into account the explosionvent location and where the fireballwill be directed. The resulting fireballand external pressure can now bequantitatively estimated by Equa-tions 7.8 and 7.9 of NFPA 68.

The maximum flame distance fromthe explosion vent for a process vesselwith a volume of 10 m3, for instance, iscalculated to be 21.5 m (70 ft). Themaximum external pressure exists ata distance of about one-fifth of thismaximum flame distance. For thisspecific example (Table 2) the maxi-mum pressure at a distance of 14 ftwould be about 0.8 psig; and at 70 ft, itwould be about 0.16 psig. As a point ofreference, glass windows will fail at

approximately 0.15 psig pressure.There goes the neighborhood.Alternative protection strategiesIf finding a safe place to vent provesdifficult, consider the following options:• Use explosion suppression in lieu of

explosion venting. The resultingpressure will be contained in the en-closure and the flame will be extin-guished

• Use a flame-arresting device in con-junction with explosion venting.The flame is quenched and pre-vented from leaving the flame-ar-resting device

Flame propagation In process equipment that is intercon-nected by pipelines, a deflagration thatoriginates in one vessel can propagateto another, even if all the equipment isexplosion vented. This propagation re-sults in pressure piling, increased tur-bulence, and a significant ignitionsource in the adjoining equipment. Fig-ure 3 and Table 3 show the dramaticincrease in explosibility parameterswhen two vessels are interconnectedvia a pipeline. With these elevated con-ditions, the vent area calculated byNFPA 68 equations can be insufficient(NFPA 68, Sections 5.6.7, 7.11).Alternative protection strategiesFor interconnected equipment, con-sider the following options:• Provide an explosion isolation device

between the interconnected equip-ment to prevent the flame propaga-tion (refer to NFPA 69, Chapter 9)

• Provide explosion vents along thelength of the interconnected duct-work, per Chapter 8 of NFPA 68, toreduce pressure and turbulence.This will not prevent the flame frompropagating to the interconnectedvessel. It will merely allow standardvent area calculations to be utilized

Processing toxic materialsThis matter is somewhat obvious. Dueto regulations and environmentally

responsible practices, it is not accept-able to emit toxic materials thatshould be confined to the processequipment. During a vented explo-sion, however, some unburned mater-ial inside the process equipment willbe released into the atmosphere. Alternative protection strategyWhen toxic materials are present, useexplosion suppression in lieu of explo-sion venting. Explosion suppressionmaintains the vessel integrity and con-tains all products within the vessel,eliminating a toxic emission concern.

Inadequate surface areaThere are situations in which the vent-ing calculations result in extremelylarge vent-area requirements that areimpractical to apply to the processequipment. Some examples of condi-tions when the relief area requirementmay be impractical are as follows:• The process material has a high Kst

or is a hybrid mixture (dust and gas)• Low Pred values are required to

maintain vessel integrity• Vent panels with a relatively high

density (mass per unit area greaterthan 2.5 lb/ft2) also exhibit rela-tively higher inertia and, thus, re-quire greater vent relief area toachieve equivalent Pred values. Tocompensate for the slower openinginherent to vents with a largermass, the vent area needs to be in-creased comparatively more toachieve the same Pred (Table 4).

In those scenarios where the requiredvent area is impractical, NFPA 68Section 5.6.10 suggests that lesserventing will at least reduce the poten-tial damage; however, other protec-tion and prevention methods mustthen be considered (refer to NFPA 69).Alternative protection strategies

TABLE 1. EFFECT OF VENTDISCHARGE DUCT SIZE ON SAFE

DEFLAGRATION PRESSURE Discharge

duct length, m Pred, barg0 0.2 2 0.4 3 0.5 4 0.6

Calculations based on: Vessel volume = 8 m3;Pstat = 0.1 barg; Kst value = 139 bar-m/s;Pmax = 9 barg; Vent relief area = 0.44 m2

FIGURE 2. As indicated in Table 2,Pred achieved with venting increaseswith an increase in the vent duct dis-charge length (diagram is not to scale)

TABLE 2. EXTERNAL PRESSUREAND DEFLAGRATION

FLAME DISTANCE

Vessel Flame External volume distance pressurem3 m (ft) barg (psig)

5 3.4 11 0.046 0.67

5 17 56 0.009 0.13

10 4.3 14 0.055 0.80

10 21.3 70 0.011 0.16

50 7.3 24 0.083 1.20

50 36.9 121 0.017 0.25

Calculations based on: Pstat = 0.1 barg; Pred= 0.2 barg; Kst = 110 bar-m/s; Pmax = 8.5 barg

• Use explosion suppression in lieu ofexplosion venting. The area re-quired to mount explosion suppres-sion hardware is minimal comparedto the explosion vent area

• Reinforce the process equipment to increase the design strength, which will allow for a smaller explosionvent area

• Use venting devices that are 100%efficiency rated (have a mass perunit exposed-surface area less than2.5 lb/ft2). If a hinged vent closure(door) is used, be sure to obtain thetested venting-efficiency percentage

Obstruction to vent areaThere may be obstructions to the ex-plosion-vent relief, either internally tothe enclosure (such as dust-collector-bag cages) or externally to the unit(such as duct screens, adjacent equip-ment, and walkways), which need tobe considered.

Dust-collector-bag cages or car-tridges can interfere with the ventingprocess. There are essentially twoareas where vents can be located asrecommended by NFPA 68, Section 7.8:• The explosion vents should ideally be

located entirely below the filters. Ifthis is accomplished, then the bag vol-ume can be subtracted from the vol-ume used for relief area calculations.Unfortunately, most dust collector de-signs do not provide a space below thebags where the vents can be located(refer to NFPA 68, Figure 7.8.1(a))

• So, the next best positioning is forthe bottom of the explosion vent tobe no higher than the bottom of thebags as shown in NFPA 68, Figure7.8.1(b)

Another common obstruction arisesfrom terminating the duct with ascreen to prevent the entrance of

birds, animals, or debris. This ob-struction can reduce the venting effi-ciency if the “blockage” (surface areaof the screen) is greater than 15% ofthe explosion-vent area.

Even equipment adjacent to or nearthe protected equipment can some-times affect the performance of the ex-plosion vent. The explosion ventshould be able to fully open withoutcontacting handrails, walkways, orother equipment.Alternative protection strategiesWhen obstructions to the vent areaare potentially of concern, considerone of the following:• Mount the explosion vent on the ves-

sel in a location ensuring that thevent is not internally or externallyobstructed

• Use explosion suppression in lieu ofexplosion venting if unobstructedventing is not an option

When explosion venting is applied cor-rectly, it is one of the best responsivestrategies to economically and safely

protect your personnel, facility, andcompany profitability. Be safe — theright way. ■

Edited by Rebekkah Marshall

TABLE 4. EFFECT OF VENT MASS ON SAFE DEFLAGRATION

PRESSURE [3]Vent Vent Reducedmass response pressure

time (Pred)

lb/ft2 kg/m2 m-sec m-bar

0.073 0.356 14.5 156

0.68 3.32 31.0 199

2.29 11.17 42.6 235

4.26 20.79 54.0 314

Calculations based on: Vessel volume = 2.6m3; Pstat = 0.1 barg; 5% Propane; Vent relief area = 6 ft2

AuthorRalph Foiles is the presidentof Process Protection Inc.(16134 W 80th St., Lenexa,KS 66219; Phone: 913-481-2587; Fax: 800-760-3987;Email: rfoiles@processprotection.net; Web: www.processprotection.net). Ralph holds aB.S. in chemical engineeringfrom the University ofKansas. He has over 15 yearsexperience in the explosion

protection industry, specifically involving haz-ard evaluation, solution design, code complianceand training seminars. Past positions includeproduct manager, sales manager, and businessdevelopment manager. He is an AIChE andNFPA member. He also is a frequent presenterat industrial conferences.

TABLE 3. EFFECTS OFINTERCONNECTED VESSELS ON

EXPLOSION SEVERITY [2]Vessel Pmax, dp/dt, volume, m3 barg bar/s

Separate vessels:

1 7.4 55

5 7.4 32

Interconnected vessels:

1 23 10,000

5 9.7 645

FIGURE 3. As indicated in Table 3, if vessels are interconnected (whether vented ornot) during a deflagration the flame and pressure, effects will be exaggerated in bothvessels, resulting in inappropriately sized explosion vents

PREVENTING EXPLOSIONS IN CPI FACILITIES Industrial processes will experience an explosion if all four of the following conditions exist:1. The suspended-fuel concentration is within the flammable range — below the upper

explosive limit (UEL) and above the lower explosive limit (LEL)2. The oxidant (usually air) concentration is sufficient to support combustion3. An ignition source is present4. The immediate environment is contained within an enclosure, such as process equip-

ment or a building Preventive strategies seek to eliminate at least one of these conditions. Unfortunately,

conditions 1 (fuel), 2 (oxidant), and 4 (enclosure) are inherently present in many CPIprocesses during normal operations. The primary preventative strategy comes down toeliminating potential contact with ignition sources.

Ultimately, it is impossible to guarantee that all ignition sources have been identified andprevented. A study [1] of dust explosions over a ten-year period identified the ignitionsources to include over a dozen different types including these: mechanical sparks, ma-chinery friction, welding, flames, electrical, spontaneous ignition, and electrostatic dis-charges. ❒.

Effects of interconnected vessels on explosion severity

5 m3

Pmax: 23 barg Pmax: 9.7 barg

1 m3

References1. FM Global, Property Loss Prevention Data

Sheet 7-76.2. Bartknecht, W., “Dust Explosions: Course, pre-

vention protection,” Springer-Verlag, 1989.3. NFPA 68, Guide for Venting of Deflagrations

Table 5.6.14.1.