Nfpa 68 2007
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NFPA 68-2002. NFPA 68-2007. Guide for Venting of Deflagrations. Updated/Revised to:. Standard on Explosion Protection by Deflagration Venting. Two main directives and codes recognized for the use of explosion protection. ATEX

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NFPA 68-2007

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Nfpa 68 2007

NFPA 68-2002

NFPA 68-2007

Guide for Venting of Deflagrations

Updated/Revised to:

Standard on Explosion Protection by Deflagration Venting


Nfpa 68 2007

Two main directives and codes recognized for the use of explosion protection

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

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.


Nfpa 68 2007 edition guide to explosion venting

NFPA 68 - 2007 EditionGuide 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

General Organization – Changes and focus


Summary of major changes

Summary of Major Changes

  • Standard

  • Hazard Analysis and Design Options

  • Vessel L/D Determination

  • Revised general correlations for dusts

    • Basic equation

    • Initial pressure

    • L/D

    • Turbulence

    • Panel inertia

    • Partial volume

    • Vent ducts

  • Inspection and Maintenance


Additional minor changes

Additional Minor Changes

  • Dust collector volumes for sizing

  • Kst Data

  • Hydraulic diameter

  • Fireball dimension

  • FlamQuench building volume


Standard on explosion protection by deflagration venting

Standard on Explosion Protection by Deflagration Venting

  • Mandatory requirements for the

    • 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

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

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

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]

    • where p = perimeter

  • Deff = [4xAeff/π]0.5 (round)

  • L/D = H/Deff

H

Additional Examples


Revised general correlations

Revised General Correlations

  • Flow Chart- information input in sequence, as one will now affect others

  • Combination Rule Chart

  • New Equations

    • Basic equation

    • Initial pressure

    • L/D

    • Turbulence

    • Panel inertia

    • Partial volume

    • Vent ducts


Nfpa 68 2007 edition

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.

  • 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

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

Manufacturer’s data sheets and instruction manuals

Design calculations – (VentCalc)

General specifications

Vent closure specifications

End user inspection/maintenance forms

User documentation of conformity with applicable standards

Vent closure identification

Combustible material properties test report – (Kst Testing)

Copy of vent identification label

Process plan view

Process elevation view

Vent relief (pressure and fireball) path – (Fireball dimension)

Proximity of personnel to vent relief path – (Safe Placement of vents)

Mechanical installation details

Electrical supervision (if provided) installation details

Vent restraint installation and design documentation (if required)

Process interlocks (if provided)

Event deflagration isolation requirements (if required)

Employee training requirements

11.2 Design Parameters and Documentation

Documentation shall include all of the following:


Dust collector vent placement 8 7 1 1

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 11

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

Vent Diameter, D

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

Bag Restraint for Full Length Bags

D


Dust collector vent placement 8 7 1 3

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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