committee input no. 21-nfpa 68-2016 [ chapter 7 ]...committee input no. 21-nfpa 68-2016 [ chapter 7...

Committee Input No. 21-NFPA 68-2016 [ Chapter 7 ]

Chapter 7 Venting Deflagrations of Gas Mixtures and Mists

7.1 Introduction.

7.1.1*

This chapter shall apply to the design of deflagration vents for enclosures that contain a flammable gas orcombustible mist and that have an L/D of ≤5.

7.1.1.1

This chapter shall be used in conjunction with the information contained in the rest of this standard.

7.1.1.2

Chapter 6 and 3.3.32.1 shall be reviewed before determining the value of Pred to be used in this chapter.

7.1.2*

The design of a deflagration vent for an enclosure containing a combustible mist shall be based on a valueof Su equal to 0.46 m/s unless a value of Su applicable to the mist of a particular substance is determined

by test.

7.2 Venting by Means of Low Inertia Vent Closures.

7.2.1

When Pred ≤ 0.5 bar, the minimum required vent area, A v0, shall be determined by Equation 7.2.1a and

Equation 7.2.1b:

(7.2.1a)

(7.2.1b)

where:

A v 0 = the vent area calculated from Equation 7.2.1a (m2)

As = the enclosure internal surface area (m2)

Pred = the maximum pressure developed in a vented enclosure during a vented deflagration (bar-g)

Su = fundamental burning velocity of gas-air mixture (m/s)

ρu = mass density of unburned gas-air mixture (kg/m3) = 1.2 for flammable gases with stoichiometricconcentrations less than 5 vol%, and an initial temperature of 20°C

λ = ratio of gas-air mixture burning velocity accounting for turbulence and flame instabilities in venteddeflagration to the fundamental (laminar) burning velocity

Gu = unburned gas-air mixture sonic flow mass flux = 230.1 kg/m2-s for an enclosure initialtemperature of 20°C

Cd = vent flow discharge coefficient

Pmax = the maximum pressure developed in a contained deflagration by ignition of the same gas-airmixture (bar-g)

P 0 = the enclosure pressure prior to ignition (bar-g)

γ b = ratio of specific heats for burned gas-air mixture = 1.1 to 1.2, depending on the gas mixture

National Fire Protection Association Report http://submittals.nfpa.org/TerraViewWeb/ContentFetcher?commentPara...

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7.2.1.1

The C value for flammable gases and vapors with a Pmax value less than 9 bar and a stoichiometric (near

worst case) fuel concentration no greater than about 10 percent shall be permitted to be calculated usingEquation 7.2.1.1:

(7.2.1.1)

7.2.1.2

The value of Pstat shall be less than Pred as specified for the following conditions:

(1) For Pred ≤ 0.1 bar (1.5 psi), Pstat ≤ Pred - 0.024 bar (50 psf).

(2) For Pred > 0.1 bar (1.5 psi), Pstat < 0.75 Pred .

7.2.2

When Pred > 0.5 bar, the minimum required vent area, A v0, shall be determined from Equation 7.2.2a

and Equation 7.2.2b:

(7.2.2a)

(7.2.2b)

where:

Pstat = nominal vent deployment or burst pressure (bar-g)

7.2.2.1

The internal surface area, As , in Equation 7.2.2a shall be determined according to 7.2.5.

7.2.2.2*

The burning velocity, Su , shall be the maximum value for any gas concentration unless a documented

hazard analysis shows that there is not a sufficient amount of gas to develop such a concentration.

7.2.2.3

The value of Cd shall be 0.70 unless the vent occupies an entire wall of the enclosure, in which case a

value of 0.80 shall be permitted to be used.

7.2.3

The value of λ for the gas and particular enclosure shall be determined according to 7.2.6.

7.2.4

The L/D of the enclosure shall be determined according to Section 6.4.

7.2.5* Calculation of Internal Surface Area.

7.2.5.1*

The internal surface area, As , shall include the total area that constitutes the perimeter surfaces of the

enclosure that is being protected.


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7.2.5.1.1

Nonstructural internal partitions that cannot withstand the expected pressure shall not be considered to bepart of the enclosure surface area.

7.2.5.1.2

The enclosure internal surface area, AS , in Equation 7.2.2 includes the roof or ceiling, walls, floor, and

vent area and shall be based on simple geometric figures.

7.2.5.1.3

Surface corrugations and minor deviations from the simplest shapes shall not be taken into account.

7.2.5.1.4

Regular geometric deviations, such as saw-toothed roofs, shall be permitted to be “averaged” by addingthe contributed volume to that of the major structure and calculating AS for the basic geometry of the major

structure.

7.2.5.1.5*

The internal surface of any adjoining rooms shall be included.

7.2.5.2

The surface area of equipment and contained structures shall be neglected.

7.2.6* Determination of Turbulent Flame Enhancement Factor, λ.


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7.2.6.1

The baseline value, λ0, of λ shall be calculated from Equations 7.2.6.1a through 7.2.6.1f:

(7.2.6.1a)

(7.2.6.1b)

(7.2.6.1c)

(7.2.6.1d)

(7.2.6.1e)

(7.2.6.1f)

where:



Dhe = the enclosure hydraulic equivalent diameter as determined in Chapter 6 (m)

µ u = the unburned gas-air mixture dynamic velocity = 1.8 × 10-5 kg/m-s for gas concentrations lessthan 5 vol% at ambient temperatures

β 1 = 1.23

β 2 = 0.0487 m/s

Dv = the vent diameter as determined through iterative calculation (m)


au = the unburned gas-air mixture sound speed = 343 m/s for gas concentrations less than 5 vol% atambient temperatures

θ = 0.39

7.2.6.2

The total external surface area, Aobs , of the following equipment and internal structures that can be in the

enclosure shall be estimated:

(1) Piping, tubing, and conduit with diameters greater than ½ in.

(2) Structural columns, beams, and joists

(3) Stairways and railings

(4) Equipment with a characteristic dimension in the range of 2 in. to 20 in. (5.1 cm to 51 cm)

7.2.6.3

When Aobs < 0.4AS , λ1 shall be equal to λ0 as determined in 7.2.6.1.


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7.2.6.4

When Aobs > 0.4AS , λ1 shall be determined as follows:

(7.2.6.4)

7.2.6.5

For L/D values less than 2.5, λ shall be set equal to λ1.

7.2.6.6

For L/D values from 2.5 to 5 and for Pred no higher than 2 bar, λ shall be calculated as follows:

(7.2.6.6)

7.2.6.7

Equations for determining λ shall be subject to the following limitations:

(1) Su < 3 m/s (300 cm/s).

(2) Pmax < 10 bar.

(3) The maximum air velocity in the enclosure prior to ignition is no greater than 5 m/s.

(4) The enclosure is isolated from possible flame jet ignition and pressures caused by a deflagration inan interconnected enclosure.

7.2.6.8

For long pipes or process ducts where L/D is greater than 5, the requirements of Chapter 9 shall be used.

7.2.6.9 Methods to Reduce Flame Enhancement.

7.2.6.9.1

The value of λ shall be permitted to be reduced for gas deflagrations in relatively unobstructed enclosuresby the installation of noncombustible, acoustically absorbing wall linings, provided that large-scale testdata confirm the reduction.

7.2.6.9.2

The tests shall be conducted with the highest anticipated turbulence levels and with the proposed walllining material and thickness.

7.2.7 Partial Volume Effects.

7.2.7.1

When a documented hazard analysis demonstrates that there is insufficient gas in the enclosure to form astoichiometric gas-air mixture occupying the entire enclosure volume, the vent area, A v0, calculated from

Equation 7.2.2a shall be permitted to be reduced as described in 7.2.7.3.

7.2.7.2

A partial volume fill fraction, Xr , shall be calculated as follows:

(7.2.7.2)

where:

Vgas = maximum volume of gas that can be mixed with air in the enclosure

Venc = enclosure volume

xst = stoichiometric volume concentration of gas


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7.2.7.3

If Xr < 1, the minimum required vent area, A v1 shall be calculated from the following equation:

(7.2.7.3)

where:

A v 1 = vent area for partial volume deflagration

A v 0 = vent area for full volume deflagration as determined from Equation 7.2.1a or 7.2.2a

Xr = fill fraction > Π

Π = Pred /Pmax

7.3 Effects of Panel Inertia.

7.3.1*

When the mass of the vent panel ≤ 40 kg/m2, Equation 7.3.2 shall be used to determine if an incrementalincrease in vent area is needed, and Equation 7.3.3 shall be used to determine the value of that increase.

7.3.2

The vent area determined in 7.2.7 shall be adjusted for vent mass when the vent mass exceeds MT as

calculated in Equation 7.3.2:

(7.3.2)

where:

MT = threshold mass (kg/m2)


n = number of panels

V = enclosure volume (>1 m3)

7.3.3

For M > MT , the required vent area, A v2, shall be calculated as follows:

(7.3.3)

where:

M = mass of vent panel (kg/m2)

A v 1 = vent area determined in 7.2.7 (m2)

FSH = vent closure shape factor as defined in Chapter 8

7.4* Effects of Vent Ducts.

7.4.1*

Where Equations 7.2.6, 7.2.6.4, 7.3.2, and 7.3.3 are used with vent ducting, a lower value shall be used inplace of Pred .

7.4.2


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Duct lengths shorter than 3 m (10 ft) and shorter than four duct hydraulic diameters in length shall betreated using Curve A in Figure 7.4.2. For ducts exceeding either of these limitations, Curve B shall beused.

Figure 7.4.2 Maximum Pressure Developed During Venting of Gas, With and Without Vent Ducts.

7.4.2.1

For vent ducts with lengths of less than 3 m (10 ft) and shorter than four duct hydraulic diameters, thefollowing equation shall be used to determine P′red :

(7.4.2.1)

7.4.2.2

For vent ducts with lengths of 3 m to 6 m (10 ft to 20 ft) or shorter vent ducts longer than four ducthydraulic diameters, the following equation shall be used to determine P′red :

(7.4.2.2)

7.4.3*

Duct lengths shorter than 3 m (10 ft) shall be treated as 3 m (10 ft) in length for calculation purposes.

7.4.3.1

If longer ducts are needed, P′red shall be determined by appropriate tests.

7.4.3.2

Vent ducts and nozzles with total lengths of less than one hydraulic diameter shall not require a correction.

7.4.4

The vented material discharged from an enclosure during a deflagration shall be directed to a safe outsidelocation to avoid injury to personnel and to minimize property damage. (See Section 6.8.)

7.4.5*

If it is necessary to locate enclosures that need deflagration venting inside buildings, vents shall notdischarge within the building.

7.4.5.1*

Vent ducts shall be used to direct vented material from the enclosure to the outdoors.

7.4.6*

A vent duct shall have a cross section at least as great as that of the vent itself.

7.4.7*


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Vent ducts shall be as straight as possible.

7.4.7.1

If bends are unavoidable, they shall be as shallow-angled as practical (that is, they shall have as long aradius as practical).

7.4.8

Where vent ducts vent through the roof of an enclosure, consideration shall be given to climatic conditions.(See Section 6.5.)

7.5 Effects of Initial Elevated Pressure.

7.5.1

For calculations that involve elevated pressure, the procedure in 7.5.1.1 and 7.5.1.2 shall be used.

7.5.1.1*

The value that is used for P 0 shall be chosen to represent the likely maximum pressure at which a

flammable gas mixture can exist at the time of ignition. It shall be permitted to be as low as the normaloperating pressure.

7.5.1.2*

The enclosure shall be located to accommodate the blast wave.

7.6 Vent Design.

See also Sections 6.5 through 6.7.

7.6.1

If an enclosure is subdivided into compartments by walls, partitions, floors, or ceilings, each compartmentthat contains an explosion hazard shall be provided with its own vent.

7.6.2*

Each closure shall be designed and installed to move freely without interference by obstructions such asductwork or piping.

7.6.3*

Guarding shall be provided to prevent personnel from falling against vent closures.

7.6.4*

The vent area for a building shall be distributed as evenly as possible over the building’s skin.

7.6.5

Vent closures shall open dependably.

7.6.5.1

The proper operation of vent closures shall not be hindered by deposits of snow, ice, paint, stickymaterials, or polymers.

7.6.5.2

Operation of vent closures shall not be prevented by corrosion or by objects that obstruct the opening ofthe vent closure, such as piping, air-conditioning ducts, or structural steel.

7.6.5.3

Vent closures shall withstand exposure to the materials and process conditions within the enclosure that isbeing protected.

7.6.5.4

Vent closures shall reliably withstand fluctuating pressure differentials that are below the design releasepressure and shall also withstand any vibration or other mechanical forces to which they can be subjected.

7.6.6

When multiple vents are provided, the vent area shall be distributed symmetrically and evenly on theenclosure external surfaces.


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7.7* Fireball Dimensions.

7.7.1

The hazard zone from a vented gas deflagration shall be calculated by the following equation:

(7.7.1)

where:

D = axial distance (front-centerline) from vent (m)

V = volume of vented enclosure (m3)

n = number of evenly distributed vents

7.7.2

The hazard zone measured radially (to the sides, measured from the centerline of the vent) shall becalculated as 0.5D.

7.8 Deflagration Venting of Enclosures Interconnected with Pipelines.

For interconnected enclosures, explosion isolation or suppression shall be provided in accordance withNFPA 69, Standard on Explosion Prevention Systems, unless a documented risk assessment acceptableto the authority having jurisdiction demonstrates that increased vent area prevents enclosure failure. (SeeA.8.12.2.)

Supplemental Information

File Name Description

CI_21_attachment-_Chapter_7_questions.pdf

Submitter Information Verification

Submitter Full Name: Laura Montville

Organization: [ Not Specified ]

Street Address:

City:

State:

Zip:

Submittal Date: Tue Feb 02 16:05:17 EST 2016

Committee Statement

CommitteeStatement:

The committee intends to reorganize the hierarchy of Chapter 7 to clarify constants and commonequations used in the low pressure and high pressure vent area equations. See attacheddocument. A task group will also develop an example using the gas equations to be located in theannex.

ResponseMessage:


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NFPA 68 VENT CALCULATIONS FOR GAS MIXTURES AND MISTS

I believe that KG is no longer

relevant to this standard.

Resolution: Yes, all references

to KG are irrelevant.

Is chapter 7 intended to apply to

process equipment as well as

rooms and building

compartments? Much of the

wording seems to be directed

towards rooms and building

compartments.

Resolution: Yes, within the

bounds of applicability for the

equations, chapter 7 applies to

both equipment and buildings.

For example, the high pressure

equation is not likely to be

applicable to buildings since Pred

is greater than 0.5 bar (7.2 psi).

§7.2.1 provides the basic

equations for calculating the

vent area when Pred ≤ 0.5 bar.

Provided for info… see below.

§7.2.2 provides the basic

equations for calculating the

vent area when Pred > 0.5 bar.

Provided for info… see below.

§7.2.1.2 provides constraints on

Pstat. Based upon the numbering

scheme (§7.2.1.2 under §7.2.1),

it should be clear that these

constraints apply when Pred ≤

0.5 bar.

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Resolution: Yes, 7.2.1.2 is only

applicable to the low pressure

equation, 7.2.1.

But, there are no constraints on

Pstat following §7.2.2. Are there

really no constraints intended

for Pstat as it is used in §7.2.2 for

Pred > 0.5 bar?

Resolution: No, Chapter 7 does

not apply any constraints to Pstat

for the high pressure case.

However, §6.5.7 and 6.5.8 do

establish generally applicable

constraints on Pstat.

Or is §7.2.1.2 intended to apply

to both the low pressure and the

high pressure equations,

regardless of what the

numbering would imply?

Resolution: No, 7.2.1.2 is only

applicable to the low pressure

case.

§7.2.2.1 refers to §7.2.5 for

instructions for calculating As

for use in equation 7.2.2a.

But, As is also used in equation

7.2.1a. And, no instructions are

provided in the 7.2.1.x sequence

to similarly refer to §7.2.5 to

calculate As for use in equation

7.2.1a.

Is it intended that §7.2.5 be used

to calculate As for use in both

equations 7.2.1a and 7.2.2a?

Resolution: §7.2.5 is intended

to provide instructions for

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calculating As for use in both

equations 7.2.1a and 7.2.2a.

Suggestion: Consider deleting

§7.2.2.1. If you do this, it will

be clearer that §7.2.5 is intended

to apply to both equations 7.2.1a

and 7.2.2a. Leaving §7.2.2.1 in

will prompt the question of why

there is no corresponding

requirement for equation 7.2.1a.

§7.2.2.1 is superfluous.

Similarly, §7.2.2.2 provides

guidance on determining Su. Is

this guidance intended to apply

to both equations 7.2.1a and

7.2.2a?

Common sense would indicate

that it is, but the numbering

scheme would imply that it is

only pertinent to equation

7.2.2a.

Resolution: §7.2.2.2 is intended


determining Su for use in both

equations 7.2.1b and 7.2.2a.

Suggestion: Renumber §7.2.2.2

so that it does not appear to be a

subsidiary requirement to

§7.2.2.

Also, §7.2.2.3 provides

guidance on determining Cd. Is

this guidance intended to apply

to both equations 7.2.1a and

7.2.2a?

Common sense would indicate

that it is, but the numbering

scheme would imply that it is

only pertinent to equation

7.2.2a.

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determining Cd for use in both



so that it does not appear to be a

subsidiary requirement to

§7.2.2.

§7.2.7.1 only mentions using the

partial volume correction with

equation 7.2.2a.

Is it truly the intent that the

partial volume correction will

not be used with equation

7.2.1a?



applying the partial volume

correction to both equations

7.2.1a and 7.2.2a.


to clarify that the partial volume

correction is applicable to both


§7.3 addresses vent panel

inertia.

But, equation 7.3.3 shows the

inertia adjustment applied to

Av1, which is the vent area after

application of the partial volume

correction.

Depending upon your answer to

the question immediately above,

is it truly the intent that the vent

panel inertia correction will not

be used with equation 7.2.1a?

Resolution: §7.3 is intended to

provide instructions for

applying the vent panel inertia

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correction to both equations

7.2.1a and 7.2.2a.

Suggestion: Clarification of the

applicability of §7.2.7.1 (above)

may remove any ambiguity as to

the applicability of §7.3.

The definition of terms after

equation 7.3.3 states that FSH is

defined in Chapter 8. The

information on FSH in Chapter 8

seems inconsistent with the

information in Annex G for

another term defined as “shape

factor”… denoted cs.

Are these the same concepts?

Resolution: FSH and cs are not

intended to refer to the same

parameter.

Suggestion: Clarify this

inconsistency in nomenclature.

§7.4.1 states that a lower value

for Pred is to be substituted into

equations 7.2.6, 7.2.6.4, 7.3.2

and 7.3.3.

By equation 7.2.6, I am

assuming that you mean

7.2.6.1e. Is that the intent?

Resolution: Yes.

Note that Pred does not appear in

equation 7.2.6.4.

Resolution: This is an error in

the equation references.

Also, Pred appears in equations

7.2.1a and 7.2.2a. Is the

substitution intended to be made

in these equations also?

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Resolution: This is an error in

the equation references.

Is it the intent that the vent duct

correction can be applied to the

low pressure (equation 7.2.1a)

case?

Resolution: Yes. The

substitution of the adjusted

value of P’red is intended to be

applied to any equation in which

Pred appears. This applies to

both the low pressure and the

high pressure cases.

Suggestion: Correct the

equation references in §7.4.1.

As to the substituted value for

Pred, is it the intent that the value

for P’red determined in 7.4.2 be

substituted for Pred in each

relevant equation?

Resolution: Yes. Substituting

P’red (which is lower than Pred)

in the equations ensures that the

pressure actually experienced

when venting through ducts

does not exceed the allowable

Pred.

Suggestion: An example

calculation in the annex would

be helpful to demonstrate the

intent here.

It appears that the equations in

§7.4.2.1 and §7.4.2.2 are the

equations for the curves Figure

7.4.2. Is that correct?

Resolution: Yes.

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With regard to reflecting the

effects of higher pressure, what

are the “procedures” mentioned

in §7.5.1? I don’t see anything

that looks like a procedure in

§7.5.1.1 or §7.5.1.2.

Is the intent that you just

substitute the appropriate value

of Po in the various equations,

without constraints (apart from

the limit on Pred of 0.5 bar in

equation 7.2.1a)?

Resolution: Yes. There is no

limit on the value of Po that can

be used in equation 7.2.2a. As

noted above, the values of Po

that can be used in 7.2.1a are

limited to the extent that Pred

must not exceed 0.5 bar.

Suggestion: reword §7.5.1 to

eliminate the word “procedure”

and simply state the intent, as

outlined above.

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Committee Input No. 23-NFPA 68-2016 [ Section No. 7.2.1 [Excluding any Sub-Sections]

]

When Pred ≤ 0.5 bar, the minimum required vent area, A v0, shall be determined by Equation 7.2.1a and

Equation 7.2.1b:

(7.2.1a)

(7.2.1b)

where:

A v 0 = the vent area calculated from Equation 7.2.1a (m2)

As = the enclosure internal surface area (m2)




λ = ratio of gas-air mixture burning velocity accounting for turbulence and flame instabilities in venteddeflagration to the fundamental (laminar) burning velocity

Gu = unburned gas-air mixture sonic flow mass flux = 230.1 kg/m 2 -s for an enclosure initialtemperature of 20°C

Cd = vent flow discharge coefficient

Pmax = the maximum pressure developed in a contained deflagration by ignition of the same gas-airmixture (bar-g)

P 0 = the enclosure pressure prior to ignition (bar-g)

γ b = ratio of specific heats for burned gas-air mixture = 1.1 to 1.2, depending on the gas mixture




Street Address:

City:

State:

Zip:


Committee Statement

CommitteeStatement:

By providing values for the constants in these equations, it was not the intent of the committee torequire these constants to be used in all cases if more accurate information is available for thematerials involved. The typical values currently provided will still be permitted and applicable tomost applications.


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It is the intention to add equations for these parameters to the annex at the Second Draft meeting.

The committee intends to:

Remove the default values for rho_u, G_u, and gamma_b under 7.2.1, leaving the definitions of theterms.

Add new paragraphs:

7.2.1.1* It shall be permitted to assume rho_u = 1.2 for flammable gases with stoichiometric

concentrations less than 5 vol% and an initial temperature of 20oC

A.7.2.1.1 Parameters for gases and gas mixtures at various initial pressures and temperatures canbe

estimated using thermodynamic tools such as GASEQ and CHETAH.

7.2.1.2 It shall be permitted to assume G_u = 230.1 kg/m2-s for an enclosure initial temperature of

20oC

7.2.1.3 It shall be permitted to assume gamma_b = 1.25 [this value is not as conservative as theoriginal 1.1

– 1.2 range and will be discussed]

ResponseMessage:


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Committee Input No. 24-NFPA 68-2016 [ Section No. 7.2.6.1 ]

7.2.6.1

The baseline value, λ0, of λ shall be calculated from Equations 7.2.6.1a through 7.2.6.1f:

(7.2.6.1a)

(7.2.6.1b)

(7.2.6.1c)

(7.2.6.1d)

(7.2.6.1e)

(7.2.6.1f)

where:

ρ u

= mass density of unburned gas-air mixture (kg/m 3 ) = 1.2 for flammable gases with stoichiometricconcentrations less than 5 vol%, and an initial temperature of 20°C


Dhe = the enclosure hydraulic equivalent diameter as determined in Chapter 6 (m)

µ u = the unburned gas-air mixture dynamic velocity = 1.8 × 10 -5 kg/m-s for gas concentrations lessthan 5 vol% at ambient temperatures

β 1 = 1.23

β 2 = 0.0487 m/s

Dv = the vent diameter as determined through iterative calculation (m)


au = the unburned gas-air mixture sound speed = 343 m/s for gas concentrations less than 5 vol% atambient temperatures

θ = 0.39




Street Address:


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City:

State:

Zip:


Committee Statement

CommitteeStatement:

By providing values for the constants in these equations, it was not the intent of the committee torequire these constants to be used in all cases if more accurate information is available for thematerials involved. The typical values currently provided will still be permitted and applicable tomost applications.

It is the intention to add equations for these parameters to the annex at the Second Draft meeting.

The committee intends to:

Remove the definition for rho_u under 7.2.6.1 because it is used already in 7.2.1.

Remove the default values for mu_u, and a_u under 7.2.6.1, leaving the definitions of the terms.

Add new paragraphs:

7.2.6.1.2 It shall be permitted to assume mu_u = 1.8E-05 kg/m-s for gas concentrations less than 5vol%

at ambient concentrations. (See also A.7.2.1.1)

7.2.6.1.3 It shall be permitted to assume a_u = 343 m/s for gas concentrations less than 5 vol% at

ambient conditions (See also A.7.2.1.1)

ResponseMessage:


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7.2.6.6

For L/D values from 2.5 to 5 and for P red no higher than 2 bar , λ shall be calculated as follows:

(7.2.6.6)



Organization: National Fire Protection Assoc

Street Address:

City:

State:

Zip:

Submittal Date: Wed Feb 24 16:16:07 EST 2016

Committee Statement

CommitteeStatement:

The committee recognizes that the current Pred limitation results in a significantly reduced rangeof applicability for the overall gas equations. The committee will review the limits of the L/Dcorrection associated with Pred at the Second Draft meeting.

ResponseMessage:


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Committee Input No. 12-NFPA 68-2016 [ Section No. 7.7.1 ]

7.7.1

The hazard zone from a vented gas deflagration shall be calculated by the following equation:

(7.7.1)

where:

D = axial distance (front-centerline) from vent (m)

V = volume of vented enclosure (m3)

n = number of evenly distributed vents




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Committee Statement

CommitteeStatement:

The committee seeks Public Comments to compare gas flame jets to fireballs, and better definehazard zone or use an alternate term (fireball or flame length). (See also A.7.7)

The fireball from a vented gas or dust deflagration presents a hazard to personnel who may be inthe vicinity. People caught in the flame itself will be at obvious risk from burns, but those who areoutside the flame area can be at risk from thermal radiation effects. The heat flux produced by thefireball, the exposure time, and the distance from the fireball are important variables to determine thehazard. The thermal dose is a function of the fireball, heat flux and the exposure time, that is:

Dose = t((I)4/3 Where t is the exposure time (seconds)

I is the heat flux, W/m2

The thermal radiation dose (hazard) from gaseous fireballs is relatively less than from dust fireballs.One reason for this difference is the sustained burning characteristics of dusts. The length of the

projected flame from a vented gas deflagration can be calculated by the following equation:

D = 3.1 V0.402 Where: D = axial distance (front-centerline) from vent

V = volume of vented enclosure

The width of the projected flame measured from the centerline of the vent can be calculated as 1/2D.

Reference to Bartnecht, pg. 573-574


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ResponseMessage:

Public Input No. 16-NFPA 68-2016 [Section No. 7.7.1]


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8.2.6.2

The vent area shall be adjusted for vent mass where the vent mass exceeds MT as calculated in Equation

8.2.6.2:

(8.2.6.2)

where:

MT = threshold mass (kg/m2)

Pred = reduced pressure after deflagration venting (bar)

n = number of panels

V = volume (m3)




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Committee Statement

CommitteeStatement:

It has been noted that this equation does not sufficiently bound the weight of the vent panel. Theequation was based on test volumes of 25m3 and less. Very large volumes results in vent panelsthat could be very thick. However, 8.2.6.1 limits the mass of vent panel to 40 kg/m2. For vent panelsgreater than 40 kg/m2, then Annex G is prescribed, with mass up to 120 kg/m2. At the Second Draftmeeting, the committee plans to re-visit the inertia effect and tether requirements.

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Committee Input No. 33-NFPA 68-2016 [ Chapter D ]

Annex D Deflagration Characteristics of Select Flammable Gases

This annex is not a part of the requirements of this NFPA document but is included for informationalpurposes only.


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D.1 General.


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The values of fundamental burning velocity given in Table D.1(a) are based on NACA Report 1300 [82].For the purpose of this guide, a reference value of 46 cm/s for the fundamental burning velocity of propanehas been used. The compilation given in Perry’s Chemical Engineers’ Handbook [83] is based on thesame data (NACA Report 1300) but uses a different reference value of 39 cm/s for the fundamentalburning velocity of propane. The reason for using the higher reference value (46 cm/s) is to obtain closeragreement with more recently published data as presented in Table D.1(b) .

Table D.1(a) Fundamental Burning Velocities of Selected Gases and Vapors

Gas

Fundamental

BurningVelocity

(cm/s) Gas

Fundamental

BurningVelocity

(cm/s)

Acetone 54 Ethyl acetate 38

Acetylene 166* Ethylene oxide 108

Acrolein 66 Ethylenimine 46

Acrylonitrile 50 Gasoline (100-octane) 40

Allene (propadiene) 87 n-Heptane 46

Benzene 48 Hexadecane 44

n-butyl- 37 1,5-Hexadiene 52

tertbutyl- 39 n-Hexane 46

1,2-dimethyl- 37 1-Hexene 50

1,2,4-trimethyl- 39 1-Hexyne 57

1,2-Butadiene (methylallene) 68 3-Hexyne 53

1,3-Butadiene 64 HFC-23 (Difluoromethane) 6.7

2,3-dimethyl- 52 HFC-143 (1,1,2-Trifluoroethane) 13.1

2-methyl- 55 HFC-143a (1,1,1-Trifluoroethane) 7.1

n-Butane 45 HFC-152a (1,1-Difluoroethane) 23.6

2-cyclopropyl- 47 Hydrogen 312 *

2,2-dimethyl- 42 Isopropyl alcohol 41

2,3-dimethyl- 43 Isopropylamine 31

2-methyl- 43 Jet fuel, grade JP-1 (average) 40

2,2,3-trimethyl- 42 Jet fuel, grade JP-4 (average) 41

Butanone 42 Methane 40*

1-Butene 51 diphenyl- 35

2-cyclopropyl- 50 Methyl alcohol 56

2,3-dimethyl- 46 1,2-Pentadiene (ethylallene) 61

2-ethyl- 46 cis-1,3-Pentadiene 55

2-methyl- 46 trans-1,3-Pentadiene (piperylene) 54

3-methyl- 49 2-methyl-(cis or trans) 46

2,3-dimethyl-2-butene 44 1,4-Pentadiene 55

2-Buten 1-yne (vinylacetylene) 89 2,3-Pentadiene 60

1-Butyne 68 n-Pentane 46

3,3-dimethyl- 56 2,2-dimethyl- 41

2-Butyne 61 2,3-dimethyl- 43

Carbon disulfide 58 2,4-dimethyl- 42

Carbon monoxide 46 2-methyl- 43

Cyclobutane 67 3-methyl- 43


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Gas

Fundamental

BurningVelocity

(cm/s) Gas

Fundamental

BurningVelocity

(cm/s)

ethyl- 53 2,2,4-trimethyl- 41

isopropyl- 46 1-Pentene 50

methyl- 52 2-methyl- 47

Methylene 61 4-methyl- 48

Cyclohexane 46 cis-2-Pentene 51

methyl- 44 1-Pentene 63

Cyclopentadiene 46 4-methyl- 53

Cyclopentane 44 2-Pentyne 61

methyl- 42 4-methyl- 54

Cyclopropane 56 Propane 46*

cis-1,2-dimethyl- 55 2-cyclopropyl- 50

trans-1,2-dimethyl- 55 1-deutero- 40

ethyl- 56 1-deutero-2-methyl- 40

methyl- 58 2-deutero-2-methyl- 40

1,1,2-trimethyl- 52 2,2-dimethyl- 39

trans-Decalin(decahydronaphthalene)

36 2-methyl- 41

n-Decane 43 2-cyclopropyl 53

1-Decene 44 2-methyl- 44

Diethyl ether 47 Propionaldehyde 58

Dimethyl ether 54Propylene oxide(1,2-epoxypropane)

82

Ethane 47 1-Propyne 82

Ethene (ethylene) 80* Spiropentane 71

Tetrahydropyran 48

Tetralin (tetrahydronaphthalene) 39

Toluene (methylbenzene) 41

*Gases that have been critically examined in [84] or [85] with regard to fundamental burning velocity. TableD.1(b) compares selected values from these references with those in this table.

Table D.1(b) Comparison of Fundamental Burning Velocities for Selected Gases, Fundamental BurningVelocity (cm/s)

Gas Table D.1(a)

Andrews and

Bradley [84] France and Pritchard [85]

(in air)In air In oxygen

Acetylene 166 158 1140 —

Ethylene 80 79 — 0

Hydrogen 312 310 1400 347

Methane 40 45 450 43

Propane 46 — — 46


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D.2 Pmax Values.

Table D.2 provides Pmax values for several gases. The values were determined by tests in a 5 L

(0.005 m3) sphere with ignition by an electric spark of approximately 10 J energy. Where the fuels hadsufficient vapor pressure, the tests were done at room temperature. Where the fuels did not havesufficiently high vapor pressure, the tests were done at elevated temperature, and the test results werethen extrapolated to room temperature. The source of the test data is the laboratory of Dr. W. Bartknecht,Ciba Geigy Co., Basel, Switzerland (private communication).

Table D.2 Flammability Properties of Gases 5 L (0.005 m3) Sphere; E = 10 J, normal conditions [101]

Flammable Material

Pmax

(bar-g)

Acetophenonea 7.6

Acetylene 10.6

Ammoniab 5.4

β-Naphtholc 4.4

Butane 8.0

Carbon disulfide 6.4

Diethyl ether 8.1

Dimethyl formamidea 8.4

Dimethyl sulfoxidea 7.3

Ethanea 7.8

Ethyl alcohol 7.0

Ethyl benzenea 7.4

Hydrogen 6.8

Hydrogen sulfide 7.4

Isopropanola 7.8

Methane 7.1

Methanola 7.5

Methylene chloride 5.0

Methyl nitrite 11.4

Neopentane 7.8

Octanola 6.7

Octyl chloridea 8.0

Pentanea 7.8

Propane 7.9

South African crude oil 6.8–7.6

Toluenea 7.8

aMeasured at elevated temperatures and extrapolated to 25°C (77°F) at normal conditions.

bE = 100 J–200 J.

c200°C (392°F).




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Organization: National Fire Protection Assoc

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Submittal Date: Fri Mar 18 16:12:41 EDT 2016

Committee Statement

CommitteeStatement:

Based on research by Zhao et al. (“The initial temperature and N2 dilution effect on the laminarflame speed of propane/air”) that supports lowering the burning velocity of propane to 39 cm/s, thecommittee will review burning velocity data in Annex D at the Second Draft meeting and considermodifications to the burning velocities that are experimental proportionalities to propane.

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committee input no. 21-nfpa 68-2016 [ chapter 7 ]...committee input no. 21-nfpa 68-2016 [ chapter 7...

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