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Andrew W. WongMBA, CFIFireE, CFEI, CFII, CCFI

April 13, 2010

Explosion Venting

for Low & High-Strength

Enclosures

Toronto Chapter

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Applicable Law

2007 Fire Code, Division B, Part 4 “Flammable and Combustible Liquids”

Section 22 of R.R.O. 1990, Reg. 851 as amended “Industrial establishments”

Section 98 to 102 of O. Reg. 67/93 “Health care and residential facilities”

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Changes to NFPA 68 - 2007 From a “guideline” to a “standard”. Need to determine whether the dust in

question is explosive (has a KSt value). Need to commission a hazard analysis of the

proposed dust collection system. Need to maintain extensive documentation. Need to perform annual inspection of the

dust collection system’s explosion venting equipment.

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Low Strength Enclosures

NFPA 68 – 2007Enclosures that are capable of withstanding

reduced pressures, Pred, of not more than 0.1 bar (1.5 psi).

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Enclosure Strength [Pes]

Up to two-thirds the ultimate strength for low-strength enclosures.2/3 x 1.5 psi (215 psf)

= 1 psi (144 psf)0.5 psi (72 psf) safety margin

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Enclosure Strength [Pes]

For high-strength enclosure, the enclosure design pressure sufficient to resist Pred.

Pes Pred

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Deflagration Index

KG (bar-m/sec) for a gas cloud.Propane = 100Toluene = 94Ethyl alcohol = 78Hydrogen = 550Acetylene = 1415

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Maximum Pressure [Pmax]

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

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Reduced Pressure [Pred]

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

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Static Activation Pressure [Pstat]

Pressure that activates a vent closure when the pressure is increased slowly (with a rate of pressure rise less than 0.1 bar/min (0.15 psi/min or 1.6 psf/min)

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Explosion Vents

Explo-vent®

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Pressure Relief Panel

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Reduced-shank machine bolt installed to fail in shear.

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Collapsing washer used with a single-layer panel.

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Collapsing washer installed.

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Collapsing washer in action.

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Explosion Venting

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Fire Rated Blast-Resistant Door

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Ministry of LabourEngineering Data Sheet #4-01

Explosion venting to outdoors at a minimum ratio of 656 cm2/m3

(1 ft2/50 ft3) of room volume.

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Ministry of LabourEngineering Data Sheet #4-01

Explosion venting may be provided by means of open areas, outwardly opening sash or panels with explosion venting catches or clear glass diagonally scored on the outside surface.

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Ministry of LabourEngineering Data Sheet #4-01

The walls of the room shall be made explosion resistant by the use of reinforcing steel in masonry or other means.

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Problem #1

Using the MoL guideline, calculate the size of explosion vent for a dispensing room measuring 30’ x 10’ x 12’ high.

Substance being dispensed is Toluene.

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Answer to Problem #1

Room volume = (30 x 10 x 12) cu.ft. Room volume = 3600 cu.ft. Vent size = (3600 / 50) sq.ft. Vent size = 72 sq.ft.

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Factory Mutual GlobalProperty Loss Prevention Data Sheet #1-44

Use Table 4 for Toluene, & Figure 1 for damage-limiting construction.

Ratio of As to Av should not exceed 7.25. Vent panel weight should be limited to

about 3 lb/ft2. Vent ratio of 1 ft2/15 ft3 of room volume.

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Problem #2 Using the site information given in

Problem #1, determine the following performance criteria using FM guidelines:

A v (Vent size)

P v (Static Vent Opening Pressure)

P r (Resistant Design Pressure)

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Answer to Problem #2

Using situation of Problem #1 Room Volume = 3600 cu.ft. Vent area = (3600 / 15) sq.ft. Vent area = 240 sq.ft.

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Answer to Problem #2 (cont’d)

A s = [2 (30 x 10) + 2 ( 10 x 12) + 2 (30 x 12)] sq.ft.

A s = 1560 sq.ft. A s / A v = 1560 / 240 = 6.5 Using P v = 20 psf (0.14 psi) (Static Vent

Opening Pressure) P r = 125 psf (0.87 psi) (Resistant Design

Pressure)

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NFPA 68

Recommended venting equation for low strength structures (P es < 1.5 psi):

C (A s)---------------

P red 1/2A v =

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NFPA 68

C (A s)---------------

P red 1/2A v =

C = Venting equation constant

P red = Maximum pressure developed in a vented enclosure during a vented deflagration

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Assumptions of Equation

C = 0.17 (for gases with fundamental burning velocity less than 1.3 times that of propane) Propane – 46 cm/sec [1.3 ft/sec]

Masonry wall [8” brick or concrete block, 10’ high] cannot withstand a pressure difference from one side to the other of much more than 0.5 psi.

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Problem #3

Using given conditions in Problem #1, calculate the vent size with the following assumptions:

P red = 0.50 psi C = 0.17

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---------------

P red 1/2A v =

C (A s)

A v = (0.17) (1560) / 0.5 ½

A v = 375 sq.ft.

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NFPA 68 - Other design criteria

Individual venting panel size should not exceed 33 sq.ft.

Pred should always exceed Pstat by at least 0.35 psi.

Vent closure weight < 2.5 lb/sq.ft.

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Ontario Fire Code, O.Reg. 213/07 made under the Fire Protection and Prevention Act, 1997, as

amended.

High Strength Enclosures

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Fire Code Requirements

Division B, 3.2.1.2.Every machine that produces wood dust,

particles or shavings shall be provided with a blower and exhaust system installed in conformance with NFPA 91 – 2004* edition.

* Any amendments up to and including June1, 2007.

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Combustible Dust Producing Process

Division B, 5.10.1.1.(2)Machinery that produces, agitates or conveys

combustible dusts shall have or be connected to a mechanical exhaust system to the outside atmosphere and dust-tight casings or enclosures.

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Origin of Requirement

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Verification of Data - Pstat

Static activation pressure – pressure that activates a vent closure when the pressure is increased slowly (with a rate of pressure rise less than 0.1 bar/min = 1.5 psi/min) from product specification of explosion panel

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Verification of Data – Pmax & KSt

The deflagration index of the dust cloud. Varies with test samples.

Dust Type Pmax

bar

KSt

bar m/sSawdust 8.9 149

Wood

Chips/dust8.9 144

Wood

Grinding dust9.0 100

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Verification of Data - KSt

Dust Explosion Class - Dust explosion causes a pressure increase in a closed vessel. Dust is classified into dust explosion classes based on the (dp/dt)max in a volume of 1 m3 (the so-called KSt value):

KSt value in bar · m · s -1

St 1> 0 to 200St 2> 200 to 300St 3> 300

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Verification of Data - Pred

Reduced pressure – the maximum pressure developed in a vented enclosure during a vented deflagration.Data from design engineer

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Verification of Volume

Volume calculation shall be in conformance with NFPA 68 – 2007.Design engineer to show details and method

of calculationConfiguration of dust collector affects

calculation

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Alternative Vent Area Methodology for Dusts

Annex H of NFPA 68 – 2007 edition As a primary means for determining vent

area of dust collectors As a backup to verify the vent area

calculation by Equation 8.2.2. of NFPA 68 – 2007 edition.

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Vent size calculation

Av (m2) = Factor A x Factor B x

Factor C x Factor D

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Example Problem

1. Pmax = 10 bar (145 psig)

2. KSt = 350 bar-m/sec

3. Pstat = 0.2 bar (2.9 psig)

4. Pred = 0.6 bar (8.7 psig)

5. V = 25 m3

6. L/D = 3

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Model Answer

From Figure H.2(b), Factor A = 0.041 From Figure H.2(d), Factor B = 11

Pred / Pmax = 0.6/10 = 0.06

From Figure H.2(h), Factor C = 4.0 From Figure H.2(j), Factor D = 1.4

Av = 0.041 x 11 x 4 x 1.4 = 2.5 m2

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Other Considerations

1. Classification of Hazardous Locations2. Explosion Venting and Size of Fireball3. Warning sign on Vent Closures4. Automatic Sprinkler System

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Existing Explosion Venting to the Outdoors Fire Code, Division B, 5.10.1.5.(4) “Existing” is not a defined term

Literal meaning: Prior to June 1, 2007

Acceptable Vent Area

= 0.1 m2 /m3 of collector volume

(1 sq.ft./35 cu.ft.)

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Discharge of Deflagration

safe_venting_area.wmv

(End slideshow to play wmv file)

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Dust Explosion Inside Dust Collector

Flame discharged from explosion ventDiameter of Fire Ball

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Flame Front of the Fire Ball

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where: D = axial distance (front) from the vent (m) K = flame length factor

K = 10 for metal dusts, K = 8 for chemical andagricultural dusts

V = volume of vented enclosure (m3) n = number of evenly distributed vents

Fireball Diameter Calculation for Dust Explosion

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Scenario

Wood dust Volume of vented enclosure = 25 m3

Number of vents = 6

Diameter of fire ball= 8 x (25/6)1/3

= 8 x 1.6 = 12.8m (42 ft)

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where: D = axial distance (front) from the vent (m) 3.1 = formula constant V = volume of vented enclosure (m3) n = number of evenly distributed vents

Maximum individual vent panel should not exceed 33 sq.ft.

Fireball Diameter Calculation for Gas Explosion

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Scenario

Toluene Volume of vented enclosure = 3600 m3

Number of vents = 12 Axial distance of fire ball

= 3.1 x (3600/12)0.402

= 3.1 x 9.9 = 30.7m (100 ft) Hazard zone to the sides = 15m (50 ft)

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Mitigating Exposure Risk

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Warning Sign on Vent Closure

WARNING: Explosion relief device.

or

As per ANSI Z535 Product Safety Signs and Symbols