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NFPA 68 2007 Overview

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NFPA 68-2002
Guide for Venting of Deflagrations
Updated/Revised to:
NFPA 68-2007
Standard on Explosion Protection
by Deflagration Venting
Two main directives and codes recognized
for the use of explosion protection
National 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.
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
NFPA 68 - 2007 Edition
Guide to Explosion Venting
Applies to the design, location,
installation and maintenance, and use of
devices and systems that vent the
combustion gases and pressure resulting
from a deflagration within an enclosure.
General Organization –
Changes and focus
NFPA 2007
Dusts/Hybrid
Mixtures
Gases/Mists
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
Summary of Major Changes
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•
•
•
Standard
Hazard Analysis and Design Options
Vessel L/D Determination
Revised general correlations for dusts
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Basic equation
Initial pressure
L/D
Turbulence
Panel inertia
Partial volume
Vent ducts
• Inspection and Maintenance
Additional Minor Changes
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•
•
•
•
Dust collector volumes for sizing
Kst Data
Hydraulic diameter
Fireball dimension
FlamQuench building volume
Standard on Explosion Protection
by Deflagration Venting
• Mandatory requirements for the
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–
–
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design,
location,
installation,
maintenance and
use of devices and systems
• More power, authority to the Authority Having
Jurisdiction (OSHA, FM, Company Official)
Hazard Analysis
(part of new Chapter on General Requirements)
• 4.2.3 Hazard Analysis.
• 4.2.3.1 The design basis deflagration hazard
scenario shall be identified and documented.
• 4.2.3.2 A documented risk evaluation
acceptable to the authority having jurisdiction
shall be permitted to be conducted to
determine the level of protection to be
provided.
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.
Explicit Procedure for L/D Calculation
(now specifically defined by NFPA)
•
•
•
•
H = maximum flame travel
Veff based on distance H
Aeff = Veff/H
Dhe = hydraulic diameter
4x[Aeff/p]
H
– where p = perimeter
• Deff = [4xAeff/π]0.5 (round)
• L/D = H/Deff
Additional Examples
Revised General Correlations
• Flow Chart- information input in sequence, as one will now affect
others
• Combination Rule Chart
• New Equations
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–
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–
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Basic equation
Initial pressure
L/D
Turbulence
Panel inertia
Partial volume
Vent ducts
Revised General Correlations For Dusts
NFPA-68 2007 Edition
Determine appropriate input parameters (e.g. Kst, Pmax, Pstat, Pinitial, enclosure
volume and L/D, vent cover area density, etc.
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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.
6.1.2.1 It 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.
.2.2.1 Where the hazard consists of a dust mixture, the vent size shall be based on the KSt
and Pmax of the mixture.
6.2.2.2 Where the dust mixture composition is not certain, the vent size shall be based on
the highest KSt of all components and the highest Pmax of all components
Chapter 11. Inspection and Maintenance
**Critical for Owner/Operators
“Inspected annually - can be increased or decreased
based on documented operating experience”
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 by
an 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.
11.2 Design Parameters and Documentation
Documentation shall include all of the following:
1. Manufacturer’s data sheets and
instruction manuals
2. Design calculations – (VentCalc)
3. General specifications
4. Vent closure specifications
5. End user
inspection/maintenance forms
6. User documentation of
conformity with applicable
standards
7. Vent closure identification
8. Combustible material
properties test report – (Kst
Testing)
9. Copy of vent identification label
10. Process plan view
11. Process elevation view
12. Vent relief (pressure and
fireball) path – (Fireball
dimension)
13. Proximity of personnel to
vent relief path – (Safe Placement
of vents)
14. Mechanical installation details
15. Electrical supervision (if
provided) installation details
16. Vent restraint installation and
design documentation (if
required)
17. Process interlocks (if provided)
18. Event deflagration isolation
requirements (if required)
19. Employee training requirements
Dust Collector Vent Placement
8.7.1(1)
If all vent area is below the
filters;
And
The spacing between bags
is less than the radius;
Then
Calculate the volume
“below the bags only”
–
Dust Collector Vent Placement
– 8.7.1(1)
All vent area is below the
filters;
And
The spacing between bags
is greater than the
radius;
Then
Use entire volume below
the tube sheet minus the
bag volume
Dust Collector Vent Placement –
8.7.1(2)
If the bags extend below the
vent and are modified… .i.e.
the bags are either
completely removed or
shortened so that they do not
extend below the top of the
vent for a distance of one
vent diameter from the vent.
Then
The bags immediately adjacent
to the vent shall be removed
and the remaining bags shall
be restrained from passing
through the vent
Calculate
The entire volume below the tube
sheet minus the bag volume
Full Length Bag
D
Shortened or
Removed Bag
Vent Diameter, D
Bag Restraint for
Full Length Bags
D
Dust Collector Vent Placement
– 8.7.1(3)
If the bottom of vent is at or
above the bottom of the bags;
Then you must
Restrain the row closest to the
vent
And
Calculate the total volume
below the tube sheet (vent size
is larger)
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