Overview of NFPA68 Updates April 07

8/13/2019 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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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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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)

Overview of NFPA68 Updates April 07

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