2010 sfpe cpd on explosion venting
TRANSCRIPT
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Andrew W. WongMBA, CFIFireE, CFEI, CFII, CCFI
April 13, 2010
Explosion Venting
for Low & High-Strength
Enclosures
Toronto Chapter
204/12/23
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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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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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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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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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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Dust Explosion Inside Dust Collector
Flame discharged from explosion ventDiameter of 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)