overview of nfpa68 updates april 07
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NFPA 68-2007
Standardon Explosion Protectionby Deflagration Venting
NFPA 68-2002Guide for Venting of Deflagrations
Updated/Revised to:
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ATEX
Atex is the European equivalent to NFPA except it is
the law, not a guideline
Global companies may request compliance to Atex
regardless of location
Two main directives and codes recognizedfor the use of explosion protectionNational Fire Protection Agency (NFPA)
NFPA is an international organization charged with
creating and maintaining minimum standards and
requirements from fire prevention, building codes, and
personnel protective equipment, numbering over 300.
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General OrganizationChanges and focusNFPA 2007
Gases/MistsDusts/Hybrid
Mixtures
Low Strength
L/D < 2-3
High Strength
Compact L/D < 5
Pipes and Ducts
L/D > 5
Compact Enclosures
L/D < 6,8
Pipes and Ducts
L/D > 5
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Standardon Explosion Protectionby Deflagration Venting
Mandatory requirements for the
design,
location,
installation,
maintenance and
use of devices and systems
More power, authority to the Authority HavingJurisdiction (OSHA, FM, Company Official)
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Hazard Analysis(part of new Chapter on General Requirements)
4.2.3 Hazard Analysis.
4.2.3.1The design basis deflagration hazard
scenario shall be identified and documented.
4.2.3.2A documented risk evaluationacceptable to the authority having jurisdiction
shall be permittedto be conducted to
determine the level of protection to be
provided.
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Compliance Options
(New in 2007 Standard)
4.3.2 Performance-Based Design.A
performance-based design shall be in
accordance with Chapter 5 of this standard. 4.3.3 Prescriptive-Based Design.A
prescriptive-based design shall be in
accordance with Chapter 6 through Chapter
11 of this standard.
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Explicit Procedure for L/D Calculation
(now specifically defined by NFPA)
H = maximum flame travel
Veffbased on distance H
Aeff= Veff/H
Dhe= hydraulic diameter4x[Aeff/p]
where p = perimeter
Deff= [4xAeff/]0.5(round)
L/D = H/Deff
H
Additional Examples
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Revised General Correlations
Flow Chart- information input in sequence, as one will now affectothers
Combination Rule Chart
New Equations Basic equation
Initial pressure L/D
Turbulence
Panel inertia
Partial volume Vent ducts
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6.1.2* For dusts, KSt and Pmax shall be determined in approximately spherical calibrated
test vessels of at least 20 L capacity per ASTM E 1226, Standard Test Method for Pressure
and Rate of Pressure Rise for Combustible Dusts.
6.1.2.1It shall be permitted to determine KSt and Pmax per ISO 6184/1, Explosion
Protection Systems - Part 1: Determination of Explosion Indices of Combustible Dusts in Air.
6.1.2.2* It shall be permitted for the owner/user to test the dust with moisture content and
particle size that deviates from the recommended conditions established by the method
described in 6.1.2 or 6.1.2.1(new) provided a documented assessment acceptable to the
authority having jurisdiction has been performed prior to using these KSt and Pmax values
to determine vent sizing.
.2.2.1Where the hazard consists of a dust mixture, the vent size shall be based on the KSt
and Pmaxof the mixture.
6.2.2.2Where the dust mixture composition is not certain, the vent size shall be based on
the highest KStof all components and the highest Pmaxof all components
Determine appropriate input parameters (e.g. Kst, Pmax, Pstat, Pinitial, enclosurevolume and L/D, vent cover area density, etc.
Revised General Correlations For DustsNFPA-68 2007 Edition
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Chapter 11. Inspection and Maintenance
11.2 Design Parameters and Documentation. Data sheets,
installation details, and design calculations shall be developed and
maintained for each vent closure application, suitable for review byan authority having jurisdiction that verifies a vent area is sufficient
to prevent deflagration pressure from exceeding the enclosure
strength and identifies areas exposed to potential overpressure,
event propagation, and fireball effects during venting.
**Critical for Owner/Operators
Inspected annually - can be increased or decreased
based on documented operating experience
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11 2 Design Parameters and Documentation
1. Manufacturers data sheets andinstruction manuals
2. Design calculations(VentCalc)
3. General specifications
4. Vent closure specifications
5. End user
inspection/maintenance forms6. User documentation ofconformity with applicablestandards
7. Vent closure identification
8. Combustible materialproperties test report(Kst
Testing)9. Copy of vent identification label
10.Process plan view
11.Process elevation view12.Vent relief (pressure and
fireball) path(Fireballdimension)
13.Proximity of personnel tovent relief path(Safe Placementof vents)
14.Mechanical installation details15.Electrical supervision (if
provided) installation details
16.Vent restraint installation anddesign documentation (ifrequired)
17.Process interlocks (if provided)
18.Event deflagration isolationrequirements (if required)
19.Employee training requirements
Documentation shall include all of the following:
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Dust Collector Vent Placement8.7.1(1)
All vent area is below thefilters;
And
The spacing between bags
is greater than theradius;
Then
Use entire volume below
the tube sheet minus thebag volume
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Dust Collector Vent Placement8.7.1(2)
Vent Diameter, D
D
D
Full Length Bag
Shortened or
Removed Bag
Bag Restraint for
Full Length Bags
If the bags extend below thevent and are modified .i.e.the bags are either
completely removed orshortened so that they do not
extend below the top of thevent for a distance of one
vent diameter from the vent.
Then
The bags immediately adjacentto the vent shall be removedand the remaining bags shallbe restrained from passing
through the vent
Calculate
The entire volume below the tubesheet minus the bag volume
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Dust Collector Vent Placement8.7.1(3)
If the bottom of vent is at or
above the bottom of the bags;
Then you must
Restrain the row closest to thevent
And
Calculate the total volume
below the tube sheet (vent sizeis larger)