Dust explosion sizing comparison

Comparison of experimental dust and gas explosion measurements with published vent sizing correlations

Christopher Bell

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1 Introduction

An explosion may occur if a flammable gas or vapour, or a finely divided combustible dust is dispersed into the atmosphere in the presence of an energy source that has sufficient energy to cause ignition. The flame front will then travel through the flammable gas or dust cloud. If the flammable gas or dust is present within an enclosure, such as an item of process plant equipment i.e. a vessel, the flame propagation will generate pressure, due to expansion of the burned fuel within the enclosure. This may result in the catastrophic equipment failure producing an external explosion when the pressure is released to atmosphere. Hence, it is important that process plant that is a risk of an internal deflagration has a developed basis of safety. The basis of safety may comprise control and/or mitigation measures, and often equipment basis of safety is a combination of both preventative and mitigation measures.

Examples of preventative measures are:-

• Avoidance of flammable atmospheres

• Control of ignition sources

However, an adequate basis of safety may not be achieved by the reliance of the above preventative measures alone. Hence, the preventative measures are often supplemented by additional mitigation measures, which include:

• Explosion containment, where the equipment design pressure, or shock resistant strength, exceed the maximum explosion over-pressure generated based on the fuel and the likely initial conditions;

• Explosion suppression; and

• Explosion venting.

Explosion venting is often relied on within industry as the basis of safety for process plant equipment. The basic premise of explosion venting is to provide a vent of sufficient area that upon opening will release unburnt gas or dust, and products of combustion to escape from the vessel. The size of the vent should be capable of limiting the developed explosion pressure to within the safe limits of the equipment, such that rupture does not occur.

The sizing of the explosion vents is the subject of several industry guides and international standards. However, the various methods are typically correlations based on experimental data, and hence their use outside the published limits of applicability may lead to either impractically large or conversely inadequate vent sizes that comprise the selected equipment basis of safety.

This report aims to review some published experimental explosion data for both gases and dusts and compare the results obtained with several of the currently available vent sizing methods.

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2 Explosion Vent Sizing

2.1 Gases

2.1.1 NFPA 68 1994 Edition

The 1994 edition of the NFPA 68 standard provided the following equation for the estimation of explosion vent area for high strength enclosures (i.e. capable of withstanding greater that 100 mbarg)

dred

cPbv PeaVA stat=

Where V is the vessel volume, m3

e is the base of natural logarithm

Pstat is the vent opening pressure, barg

Pred is the maximum pressure developed during venting within the enclosure, or the reduced explosion pressure, barg

a, b, and c are constants that are dependant on the fuels reactivity, and are shown in the table below:

Table 1: Constants for use in explosion vent sizing [3]

a b c d

Methane 0.105 0.770 1.230 -0.823

Propane 0.148 0.703 0.942 -0.671

Hydrogen 0.279 0.680 0.755 -0.393

Coke Gas 0.150 0.695 1.380 -0.707

This equation was developed based on the explosion nomographs that were published within the standard, with the use of the equation limited to enclosures having a length to diameter ratio of less than 5. For fuels other than those listed in the above table if the fundamental burning velocity is less than 60 cm/sec i.e. 1.3 times that of propane, then the propane constants are used. If the fundamental burning velocity is greater than 60 cm/secs then the hydrogen equation is used.

However, it should be noted that this method is no longer considered appropriate as it does not take sufficient account of the fuels reactivity, for example hydrogen is ten times as reactive as methane yet use of the NFPA 68:1994 edition nomographs, on which the above equation is based, will yield similar results.

2.1.2 NFPA 68: 2007 Edition

Subsequent revisions of the NFPA 68 standard used correlations based on VDI guidelines [10] and [11], which also remains unaltered in the latest 2007 edition of the standard.

For explosion vent sizing of enclosures having a length to diameter ratio of less than 2, the vent size can be estimated from the equation:-

( ) ( ) ( )[ ]{ } ( ) ( )[ ] 32572.03

2582.010 1.0175.00567.0log127.0 VPPVPKA statredredGv −+−= −−

Where KG is the gas deflagration index, = (dP/dt)max V

(1/3)

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(dP/dt)max is the maximum rate of pressure rise obtained from standardised experimental test equipment, bar/s

For enclosures having length to diameter ratios between 2 and 5 an additional vent area should be added to the vent area estimated from the above equation.

750

22

−=∆ D

LKA

AGv

The limits of applicability for the above method are:-

KG ≤ 550 bar.m/sec

Pred ≤ 2 bar and at least 0.05 bar > Pstat

Pstat ≤ 0.5 bar

V ≤ 1000 m³

2.1.3 BS EN 14994:2006

The harmonised European norm standard EN 14994 also utilises the VDI correlation

( ) ( ) ( )[ ]{ } ( ) ( )[ ] 325722.03

25817.010 1.01754.00567.0log1265.0 VPPVPKA statredredGv −+−= −−

However, this standard also provides an alternative simple vent sizing method, which is based on the turbulent Bradley number:-

( ) ( )

( ) ( )25.0

5.25.2

5.25.25.2

8.59.7 :1 if

65.5 : 1 if

t

i

istat

i

red

i

istat

i

red

t

i

istat

i

red

i

istat

i

red

Br

PPP

PP

PPP

PP

Br

PPP

PP

PPP

PP

−=

+

≥

+

=

+

<

+

−

The turbulent Bradley number is subsequently used to solve the following equation:-

Where cui is the speed of sound at initial conditions of explosion, m/s

Ei is the expansion ratio of the combustion products

A is the vent area, m²

Pi is the initial enclosure pressure, bar

Sui is the burning velocity at the initial conditions, m/s

β is an empirical constant = 0.5 for hydrocarbons, and 0.8 for hydrogen

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α is an empirical constant = 1.75 for hydrocarbons and 1 for hydrogen

γu is the ratio of specific heats of the unburned mixture

πv = (Pstat + Pi)/ Pi

π0 = 3.14

πi# is initial pressure expressed in bar i.e. (Pi/ 1, bar)

The quoted limits of applicability for the above simple method are:-

L/D ≤ 3

V ≤ 8000 m³

0.09 < A/V2/3 < 1.23

0 ≤ Pstat ≤ several bar

0 ≤ Pi ≤ 6 bar overpressure

2.2 Dusts

2.2.1 NFPA 68: 1994

The NFPA 68 1994 edition provided two methods for the estimation of dust explosion vent sizes. The Radandt methodology is based on the use of nomographs, with equations provided as an alternative. The Radandt method did not require the use of the dust deflagration index or KST value, (KST = (dP/dt)max V(1/3)), but used the St grouping of the dust instead, which is a classification of the dusts reactivity based on the KST value. The Radandt nomograph equations are:-

For St-1 dusts

Log Av = 0.77957 log V – 0.42945 log Pred – 1.24669

For St-2 dusts

For V= 1 to 10 m³

Log Av = 0.64256 log V – 0.46527 log Pred – 0.99241

For V = 10 to 1000m³

Log Av = 0.74461 log V – 0.50017 log (Pred + 0.18522) – 1.02406

In addition to the Radandt methodology NFPA provided the Simpson nomographs and an equation developed to reproduce values obtained from their use. The Simpson equation is:-

Av = a V2/3 KSTb Pred

c

Where a = 0.000571 e(2 Pstat)

b = 0.978 e (-0.105 Pstat)

c = -0.687 e (0.226 Pstat)

The Radandt method will give different results to those obtained using Simpson's correlation above. However, for all practical purposes they are sufficiently close. If the KST value is known, then the Simpson correlation is preferable to Radandt method.

2.2.2 VDI 3673 Part 1: 1995

The German VDI 3673:1995 [10] standard published the correlation developed by Scholl for cubic enclosures:-

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[ ][ ] 753.05.0max,

569.0max,max

5 1.027.010264.3 VPPPKPxA redstatredST−−− −+=

The Scholl equation is valid for:-

Vessel volumes between 0.1m³ and 10000m³

Static opening pressure, Pstat of between 0.1 and 1 barg

Maximum reduced explosion over-pressure of between 0.1 and 2 barg

Maximum explosion over-pressure, Pmax, of between 5 and 10 barg for a dust with a deflagration index (KST) between 10 bar.m/s and 300 bar.m/s, or a Pmax of 5 to 12 barg for a KST value between 300 bar.m/s and 800 bar.m/s.

For enclosures that were elongated the VDI guideline modified the Scholl equation:-

( )( ) )/log(758.0log305.4 max, DLPAA redL +−=∆ Where the additional vent area is added to that vent area estimated for an enclosure with an L/D ratio of below 2. The use of this equation results in a step change in vent area for vessels with an L/D ratio of greater than 2. The above equations were retained in the 2002 edition of the VDI 3673 guide [11].

2.2.3 NFPA 68:2002

Editions of the NFPA 68 standard after 1994 incorporated the Scholl equation from the VDI guidelines. However, the NFPA 68 guide ceased to use the Scholl equation in the 2002 edition, which published a vent sizing equation that removed the vent sizing step change that was inherent in the use of the Scholl equation for elongated vessels. The NFPA 68 2002 correlation was:-

( )( )

−

+= −

max

max75.05

175.1110535.8

PP

PP

VKPxAred

red

STstatv

For L/D ratios greater than 2 and less than 6 the vent area estimated by the above equation is increased by adding the incremental vent area estimated by:-

−

−=∆ 1log

1156.1

65.0

max D

L

PPAA

redv

2.2.4 NFPA 68:2007

The latest edition of the NFPA 68, which has now changed from a guide to a standard, uses the following equation for dust explosion vent sizing:-

154.11101 max43

544

0, −

+= −

redSTstatv P

PVKPxA

For enclosures with an L/D ratio greater than 2 and less than 6 the vent area is again increased by adding an incremental area estimated by:-

( )

−

−+= 275.0

01 95.0exp26.01 redvv PD

LAA

The limits of the above equation are:-

5 ≤ Pmax ≤ 12 bar

10 bar.m/sec≤ KST ≤ 800 bar.m/sec

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0.1 m³ ≤ V ≤ 10000 m³

Pstat ≤ 0.75 bar

2.2.5 BS EN 14491:2006

The current harmonised European standard EN 14491 retains the use of the Scholl equation used by the VDI 3673 guidelines [10] and [11].

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3 Experimental data

3.1 Gases

The experimental data for gas explosions have been taken from G A Lunn – Venting Gas and Dust Explosions – A review [1].

3.1.1 Methane

The table below summarises methane vented explosion experimental results from Buckland, taken from Table 10 [1]

Table 2: Methane vented explosion test results [1]

Enclosure Volume

Vent coefficient Vent area

Vent opening pressure

Reduced explosion pressure

V K Av P stat P redm3 V^(2/3)/Av m2 barg barg

26.64 8.04 1.11 0.007 0.08326.64 8.04 1.11 0.004 0.05526.64 2.00 4.46 0.017 0.06626.64 2.00 4.46 0.066 0.06226.64 2.00 4.46 0.057 0.10926.64 2.00 4.46 0.063 0.0526.64 4.00 2.23 0.019 0.10926.64 4.00 2.23 0.076 0.10126.64 4.00 2.23 0.079 0.10226.64 2.50 3.57 0.072 0.1126.64 2.50 3.57 0.039 0.1126.64 5.01 1.78 0.115 0.21926.64 5.01 1.78 0.086 0.22126.64 4.00 2.23 0.017 0.09826.64 4.00 2.23 0.09 0.06626.64 4.00 2.23 0.07 0.0726.64 4.00 2.23 0.075 0.13

3.1.2 Propane

The table below summarises propane explosion data results from Bromma, taken from Table 8 [1]

Table 3: Propane vented explosion test results [1]

Enclosure Volume




V K Av P stat Predm3 V^(2/3)/Av m2 barg barg

200 1.11 30.81 0.0549 0.0588200 1.11 30.81 0.0294 0.0333200 1.11 30.81 0.0098 0.0181200 1.11 30.81 0.0289 0.0343200 1.11 30.81 0.0549 0.0588200 1.38 24.78 0.049 0.0637200 1.38 24.78 0.0137 0.0299200 1.38 24.78 0.0196 0.0295200 1.38 24.78 0.0196 0.0348

3.1.3 Pentane

The table below summarises pentane vented explosion experimental results from Harris and Briscoe, taken from Table 4 [1]. Note that the vent opening pressure is 0 barg.

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Table 4: Pentane vented explosion test results [1]

Enclosure Volume




V K Av P stat Pred

m3 V^(2/3)/Av m 2 barg barg

1.7 5.11 0.279 0.14

1.7 9.02 0.158 1.96

1.7 20.20 0.0705 4.83

1.7 75.77 0.0188 5.67

3.2 Dusts

The experimental data for vented dust explosions have been taken from G A Lunn – Venting Gas and Dust Explosions – A review [1] and from the Factory Mutual Global standard 7-76 [2].

3.2.1 Aluminium

The table below summarises vented explosion experimental results from Donat for aluminium, taken from Table 32 [1]

Table 4: Aluminium vented explosion test results [1 ]

Vessel Volume Vent area


V Av Pred m3 m2 barg

1 0.4 1.05 0.3 1.05 0.2 1.28 0.1 2.625 0.05 5.72

30 4 0.369 3 0.493

2 0.738

3.2.2 Cork dust

The table below summarises the cork dust vented explosion results from Tonkin and Berlemont , Figure 46 [1].

Table 5: Cork dust vented explosion test results [1 ]

Enclosure Volume Vent coefficient Vent area


V K Av Predm3

V^(2/3)/Av m2 barg1.21 9.46 0.12 0.07

14.19 0.08 0.0918.93 0.06 0.1418.93 0.06 0.1828.39 0.04 0.9

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3.3 Factory Mutual Standard 7-76

The Factory Mutual Global standard 7-76 includes the paper published by Tamanini and Valiulis [2], and includes the experimental test results from a 10m³ vessel containing powders with a deflagration index, KST, of 190 bar.m/s and 290 bar.m/s. The vent opening pressure was 0.2 barg. The test results are detailed below:-

Table 6: FM Global vented explosion test results [2 ]

Enclosure Volume Vent coefficient Vent area



Deflagration index

V K Av P stat Pred KSTm3 V^(2/3)/Av m2 barg barg bar.m/s10 7.25 0.64 0.2 0.45 190

12.21 0.38 0.2 1.4 19016.58 0.28 0.2 2.1 19012.21 0.38 1.86 3 1907.25 0.64 0.2 0.75 290

12.21 0.38 0.2 2.2 29016.58 0.28 0.2 3.6 2907.25 0.64 1.4 1.65 2907.25 0.64 2.5 3 290

3.3.1 Wheat dust

The table below summarises the wheat dust vented explosion results from vented explosions within a 500m³ silo at Boge, Norway, and are taken from Figure 49 [1]. Additional data for vented wheat dust explosions within a 20m³ elongated silo (L/D ratio of 6.25) obtained by Radandt, were taken from Figure 50 [1].

Table 7: Wheat grain dust vented explosion test res ults [1]

Enclosure Volume



V K Av Predm3

V^(2/3)/Av m2 barg500 7.87 8 0.025

7.87 8 0.037.87 8 0.057.87 8 0.067.87 8 0.124.44 14.2 0.0154.44 14.2 0.0312.60 5 0.331.50 2 0.4

20 4.91 1.5 0.36.41 1.15 0.49.82 0.75 0.714.74 0.5 1.124.56 0.3 1.836.84 0.2 1.9

3.3.2 Dextrin

The table below summarises the dextrin dust vented explosion results from Donat, Figure 40 [1].

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Table 8: Dextrin vented explosion test results [1]

Enclosure Volume



V K Av Predm3

V^(2/3)/Av m2 barg30 2.54 3.8 0.1

3.22 3 0.154.83 2 0.36.44 1.5 0.59.65 1 1

12.87 0.75 1.319.31 0.5 232.18 0.3 2.8

1 2.50 0.4 0.13.33 0.3 0.155.00 0.2 0.36.67 0.15 0.4

10.00 0.1 0.62.00 0.5 1.13.33 0.3 2

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4 Comparison of experimental results with vent sizi ng correlations

A spreadsheet was used to calculate and compare the results obtained for the reduced explosion pressures predicted by the various vent sizing correlation. An example output from the spreadsheet is shown in Appendix A.

4.1 Gases

4.1.1 NFPA 68:1994 Calculation:-

For methane and propane, the constants used in the correlation are listed within the standard. For Pentane, table C.1 of the NFPA 68:1994 [3] edition gives the fundamental burning velocity of pentane as 46 cm/s, which is the same as the burning velocity for propane, based on the NFPA 68 quoted data. Therefore, for pentane the vent sizing calculation used the propane constants.

4.1.2 NFPA 68:2007 Calculation:-

To enable vent sizing using this standard it is necessary to know the deflagration index, KG, the vent opening pressure, and the length to diameter ratio. The values for the deflagration index, KG, where obtained from the NFPA 68 Standard 2007 edition table E.1 [6] and are 55, 100, and 104 bar.m/sec for methane, propane, and pentane respectively. For the purpose of the calculation it is assumed that the vent opening pressure is 0.1 barg and that the enclosure has a length to diameter ratio of less than 2. It should be noted that for methane and propane the majority of the test results were obtained using a vent panel that opened at pressures less than 0.1 barg, and the L/D ratio was not stated. For pentane, the vent panel had a negligible opening pressure.

4.1.3 BS EN 14994:2006 Calculation:-

The alternative simple vent sizing method detailed in the above standard requires knowledge of various thermodynamic, and combustion properties of the fuels. This information was obtained from table A.1 of the above standard [8], and is detailed below:-

Table 9:- Thermodynamic data and burning velocity f or some fuel-air mixtures [8]

Ratio of specific heats, γu

Expansion ratio of combustion products, E i

Speed of sound at initial conditions, c ui, m/s

Fundamental burning velocity, S ui, cm/s

Methane 1.39 7.52 353 43

Propane 1.37 7.98 339 45

Pentane 1.36 8.07 335 43

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4.2 Dusts

To enable use of the dust explosion vent sizing correlations the following data was used:-

Table 10: Explosion data for selected combustible d usts [2], [6], and [7]

Deflagration index, K ST, bar.m/sec

Maximum explosion pressure, P max, barg

Aluminium 415 12.4

Cork dust 202 9.6

FM Global dust 1 190 8.5

FM Global dust 2 290 8.5

Wheat dust 112 9.3

Dextrin 106 8.8

The above data was taken from the NFPA 68: 2007 edition Tables E.1 (a) to (e) [6], with the exception of the data from the Factory Mutual Global tests, which were taken from Tamanini and Valiulis [2], and wheat grain dust data which is taken from Table A.1 R K Eckhoff Dust explosions in the process industries, 2nd Edition [7].

Data for the deflagration index and maximum explosion pressure is also available in the G A Lunn Venting Gas and Explosions – A review [1]. However, this data was not used as it was obtained on the Hartmann apparatus, which will not yield similar results to explosion data obtained from the 20 litre sphere or 1m³ iso standard test vessel. The published explosion correlations are based on data not obtained from the Hartmann apparatus, and hence values published by Lunn [1] have not been used.

For the purpose of this report all vent opening pressures, Pstat, were assumed to be 0.1 barg, and the length to diameter ratio is assumed to be less than 2, with the exception of the wheat dust 20m³ enclosure comparison.

4.3 Results

The graphs below show the experimental test results for the reduced explosion pressure within particular test equipment equipped with a defined vent area. In addition, the graphs show the results of the various vent sizing correlations, using information related to the enclosure, and data either obtained from referenced texts or assumed as detailed above.

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4.3.1 Gases

Figure 1: Methane gas explosion vent sizing results

Comparison of methane experimental data with publis hed

vent sizing methods (vessel volume = 26.64m 3, Pstat below 0.1 barg)

0.01

0.1

1

10

100

0.1 1 10

Vent area, A v, m2

Red

uced

pre

ssur

e, P

red,

bar

NFPA 68:2002 prEN14994 prEN14994 alt Table 10 Page 46 Buckland NFPA 68:1994

Figure 2: Propane gas explosion vent sizing results

Comparison of propane experimental data with publis hed vent

sizing methods (vessel volume= 200m 3, Pstat under 0.1 barg)

0.01

0.1

1

10

100

0.1 1 10 100

Vent area, Av , m2

Red

uced

exp

losi

on p

ress

ure,

Pre

d, b

ar

NFPA 68:2002 prEN14994 prEN14994 alt

Table 8 Page 42 Bromma NFPA 68:1994

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Figure 3: Pentane gas explosion vent sizing results

Comparison of experimental data for Pentane with pu blished

vent sizing methods (vessel volume 1.7m 3)

0.01

0.1

1

10

100

0.01 0.1 1 10

Vent area, A v, m2

Red

uced

exp

losi

on p

ress

ure,

Pre

d, b

ar

NFPA 68:2002 prEN14994 prEN14994 alt Table 4 Page 33 Harris & Briscoe NFPA 68:1994

4.3.2 Dusts

Figure 4: Aluminium dust explosion vent sizing resu lts

Comparison of experimental cork dust explosion with

published vent sizing methods (vessel volume 1.21m 3)

0.001

0.01

0.1

1

10

100

0.01 0.1 1

Vent area, A v, m2

Red

uced

exp

losi

on p

ress

ure,

Pre

d,

bar

Simpson Radandt Scholl NFPA 68:2002 NFPA 68:2007 Exp. Data Fig 46 Lunn

- 16 -

Figure 5: Cork dust explosion vent sizing results

Comparison with aluminium experimental data with

published vent sizing methods (vessel volume = 1m 3)

0.01

0.1

1

10

100

0.01 0.1 1

Vent area, A v, m2

Red

uced

exp

losi

on p

ress

ure,

Pre

d,

bar

Simpson Scholl

NFPA 68:2002 NFPA 68:2007

Aluminium Dust Table 32 Page 122

Figure 6: FM Global vented explosion test data and comparative vent sizing results

Comparison of FM Global test data from FM std 7-76 with published vent sizing methods (vessel volume 10m 3, Kst=190 bar.m/s, P stat = 0.2 barg)

0.001

0.01

0.1

1

10

100

0.01 0.1 1 10

Vent area, A v, m2

Red

uced

exp

losi

on p

ress

ure,

Pre

d,

bar

Simpson Radandt Scholl NFPA 68:2002 NFPA 68:2007 FM Global Std 7-76

- 17 -

Figure 7: FM Global vented explosion test data and comparative vent sizing results

Comparison of FM Global test data from FM std 7-76 with published vent sizing methods (vessel volume 10m 3, Kst=290 bar.m/s, P stat = 0.2 barg)

0.001

0.01

0.1

1

10

100

0.01 0.1 1 10

Vent area, A v, m2

Red

uced

exp

losi

on p

ress

ure,

Pre

d,

bar

Simpson Radandt Scholl NFPA 68:2002 NFPA 68:2007 FM Global Std 7-76

Figure 8: Wheat dust vent sizing results in an elon gated 20m 3 silo

Comparison of wheat dust experimental data with ven t sizing

methods (vessel volume 20m 3 and L/D = 6.25)

0.001

0.01

0.1

1

10

100

0.1 1 10

Vent area, A v, m2

Red

uced

exp

losi

on p

ress

ure,

Pre

d,

bar

Simpson Radandt Scholl NFPA 68:2002 NFPA 68:2007 Wheat grain dust Elongated

- 18 -

Figure 9: Wheat dust vented explosion sizing result s

Comparison of wheat dust experimental data with ven t sizing

methods (vessel volume 500m 3)

0.001

0.01

0.1

1

10

100

1 10 100

Vent area, A v, m2

Red

uced

exp

losi

on p

ress

ure,

Pre

d,

bar

Simpson Radandt Scholl NFPA 68:2002 NFPA 68:2007 Wheat grain dust

Figure 10: Dextrin vented explosion and vent sizing results

Comparison of dextrin experimental data with publis hed vent sizing

methods (vessel volume = 1m 3)

0.001

0.01

0.1

1

10

100

0.01 0.1 1

Vent area, A v , m2

Red

uced

exp

losi

on p

ress

ure,

Pre

d, b

ar

Simpson Radandt Scholl

NFPA 68:2002 NFPA 68:2007 Dextrin Figure 40 Page 117

- 19 -

Figure 11: Dextrin vented explosion test and vent s izing results

Comparison of dextrin experimental data with publis hed

vent sizing methods (vessel volume = 30m 3)

0.001

0.01

0.1

1

10

100

0.1 1 10

Vent area, A v , m 2

Red

uced

exp

losi

on p

ress

ure,

Pre

d, b

ar

Simpson Radandt Scholl

NFPA 68:2002 NFPA 68:2007 Dextrin Figure 40 Page 117

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5 Conclusions

For methane it is apparent that all the vent sizing correlations will provide sufficient area to adequately an internal deflagration. The current NFPA 68 and European standard utilise the same correlation and offer an improvement in accuracy when compared to the NFPA 68 1994 methodology. The current European standard ‘simple’ calculation will significantly over-estimate the required vent area for a given enclosure configuration. This situation is also apparent when considering the vent sizing results obtained for the propane vented explosion data.

For pentane, the European standard simple calculation yields vent sizing results lower than those obtained from NFPA 68 and the preferred method in EN 14994. However, the pentane vented explosion test results are enveloped by the NFPA and EN standard methods, whilst half of the test data points lie outside of the published correlations limits of applicability, i.e. reduced explosion pressure greater than 2 barg. Hence, based on the test results assessed, the current industry standards (NFPA and BS EN 14994) will over-estimate the required vent area that limits the reduced explosion over-pressure to within acceptable limits. Therefore, the current gas vent sizing correlation offers a margin of safety when estimating required vent areas.

For dust explosion vent sizing the results obtained show a greater degree of variation when compared to actual test results, than the gas explosion venting correlations. For aluminium, the published maximum explosion pressure, Pmax, is outside the limits of applicability of both the Scholl and NFPA correlations. However, it is the Scholl equation that provides better results as the degree of under-estimation of the required vent area is less than that of the NFPA 68 equations. For cork dust, all the correlations significantly over-estimate the required vent area. However, the current correlations provide an improvement in vent sizing when compared to the previous Simpson and Radandt equations.

For the dusts used in the FM Global tests, the Scholl equation correlates very well, whereas the current NFPA standard consistently under-estimates the required vent area. For the elongated enclosure with vented wheat dust explosions both the elongated correlations published in the harmonised European standard and the current NFPA 68 standard provide good agreement. For the larger volume vented explosion of 500m3 all the correlations over-estimate the required vent area. This over-estimate is considered to be attributable to the reduced degree of turbulence likely to be present within the large silo, when compared to smaller enclosure volumes. The degree of turbulence would be reflected in a reduced deflagration index, or KST value. However, in the standard laboratory equipment (20litre sphere) for measuring KST, there is a high degree of turbulence, and hence the test yields a higher deflagration index than that which would be obtained if there was a reduced degree of turbulence within the enclosure.

The results obtained for the dextrin vented explosions show poor correlation with the test data. However, without exact explosion property data for the dextrin i.e. KST and Pmax, or enclosure and vent information it is difficult to attribute the reasons for the poor correlation. For this reason, it is important that when vent sizing, that the vent is calculated using actual dust explosion test parameters. This is because the deflagration index will vary with various factors such as particle size, and moisture content, which may be altered by the actual processing being undertaken e.g. attrition of particles due to pneumatic conveying. Hence, the reliance on published explosion data for KST and other parameters is not recommended.

- 21 -

However, it is evident from the above graphs that the current NFPA 68 correlations will yield a smaller vent size that that obtained from the Scholl equation adopted by the EU standard. From the above graphs, it is evident that the NFPA 68 standard correlation may not be conservative when compared to actual vented explosion results. Hence, it is considered that the Scholl equation represents a more appropriate correlation on which to base equipment safety.

References

1. Venting Gas and Dust Explosions – A review, G A Lunn

2. FM Global Property Loss Prevention Data Sheets – Prevention and mitigation of

combustible dust explosions and fire 7-76 May 2006

3. NFPA 68 Guide for venting of deflagrations 1994 edition



6. NFPA 68 Standard on explosion protection by deflagration venting 2007 edition

7. Dust Explosions in the Process Industries, R K Eckhoff, 2nd Edition

8. BS EN 14994:2006 Gas explosion venting protective systems

9. BS EN 14491:2006 Dust explosion venting protective systems

10. VDI 3673 Part 1:July 1995 Pressure venting of dust explosions

11. VDI 3673 Part 1:November 2002 Pressure venting of dust explosions

Appendix A

Typical spreadsheet output for gas and dust vent sizing

- 24 -

REF REV1 Gas2 KG Parameter K G 55 bar.m/s3 Maximum Pressure Pmax 7.1 barg4 Vessel L/D ratio L/D 1.005 Vessel Volume V 26.64 m³6 Vessel Surface Area As 53.52 m²7 Initial Pressure Pinit 0 barg8 Vent open pressure Pstat 0.1 barg9 Reduced pressure Pred 0.35 barg OR Av 0.178396 m²101112 m² valid barg Valid13 2.700 OK 37.23 ERR14 2.700 ERR 42.04 ERR15 2.685 OK 36.96 ERR16 3.166 ERR17 3.572 OK 4.74 OK18 3.528 13.12192021 Limits: lower upper2223 KG Parameter KG 55 bar.m/s KG 0 550 bar.m/s OK24 Maximum Pressure Pmax 7.1 barg Pmax 5 10 barg OK25 Vessel L/D ratio L/D 1 L/D 0 2 OK26 Vessel Volume V 26.63981 m³ V 0.1 1000 m³ OK27 Vent open pressure Pstat 0.1 barg Pstat 0.1 0.5 barg OK28 Pred 0.15 2 barg OK2930 Reduced pressure Pred 0.35 barg Av 2.700 m²3132 OR3334 Reduced pressure Pred 37.22826 barg 0.000 Av 0.179 m² ERR35 Note:- alter Pred to obtain desired Av3637 Limits: lower upper3839 KG Parameter KG 55 bar.m/s KG 0 550 bar.m/s OK40 Maximum Pressure Pmax 7.1 barg Pmax 5 10 barg OK41 Vessel L/D ratio L/D 1 L/D 2 5 ERR42 Vessel Volume V 26.63981 m³ V 0.1 1000 m³ OK43 Vent open pressure Pstat 0.1 barg Pstat 0.1 1 barg OK44 Pred 0.15 2 barg OK4546 Reduced pressure Pred 0.35 barg Av 2.700 m²47 delta A 0.000 m²4849 OR Av 2.700 m²5051 Reduced pressure Pred 42.0418 barg 0.000 Av + dA 0.179 m² ERR52 Note:- alter Pred to obtain desired Av53545556 KG Parameter KG 55 bar.m/s KG 0 550 bar.m/s OK57 Maximum Pressure Pmax 7.1 barg Pmax 5 10 barg OK58 Vessel L/D ratio L/D 1 L/D 0 10 OK59 Vessel Volume V 26.63981 m³ V 0.1 1000 m³ OK60 Vent open pressure Pstat 0.1 barg Pstat 0.1 1 barg OK61 Pred 0.15 0.1 barg ERR626364 C 0.03565 Av 3.166 m² Using NFPA 68 2002 constants6667686970717273747576 KG Parameter KG 55 bar.m/s KG 0 550 bar.m/s OK77 Maximum Pressure Pmax 7.1 barg Pmax 5 10 barg OK78 Vessel L/D ratio L/D 1 L/D 0 3 OK79 Vessel Volume V 26.63981 m³ V 0.1 1000 m³ OK80 Vent open pressure Pstat 0.1 barg Pstat 0.1 0.5 barg OK81 Pred 0.15 2 barg OK8283 Reduced pressure Pred 0.35 barg Av 2.685 m²8485 OR8687 Reduced pressure Pred 36.95866 barg 0.000176 Av 0.179 m² ERR88 Note:- alter Pred to obtain desired Av Pred adj 36.959 barg for high initial P89

9091929394

Methane

0.045Gases with S u<1.3Su

propane

prEN 14994 Gas Explosion Venting Protective systems 2004 (Same method as NFPA 68 2002)

0.013Methane 0.035

Fuel C (bar½) mixtures in air only

Anhydrous NH 3

Approved byChecked byPrepared byDate

NFPA 68:2002 Low Strength Enclosures

ELONGATED NFPA 68 2007 Edition)

Revision A B C D E F

Fuel Characteristic Constant for Venting Equation

pr14994:2004 AlternativeNFPA 68:1994

NFPA 68: 2007 (same as 2002, and 1998 editions)

NFPA 68:2007NFPA 68:2007 ELONGATED

pr14994:2004NFPA 68 Low Strength

INPUT DATA

Gas Explosion CalculationCalculation Sheet

RESULTSVent Area (Av) Red Pressure

Table 6.2.2 from NFPA 68:2002

No dimesional limit to

the size. However,

panels should be

evenly spaced for

elongated enclosures

ie for L/D>3

Note that data on

which this correlation

is based gives a

maximum L/D of 2.

For high initial Pressure (up to 3

barg) adjust Pred guess until

adjusted Pred is within required

limits

Calculate

- 25 -

REF REV1 Pred 4.735262 Limits2 Dimensionless Reduced Pressure πred 0.35 4.735262 Lower Upper3 Dimensionless Static Pressure πv 1.1 Av/V^(2/3) 0.400513 0.09 1.23 OK4 Dimensionless pressure complex 0.275795 3.731318 Pinit 0 0 6 OK5 Turbulent Bradley Number Br t 3.346454 0.26686 Pstat 0.1 0 3 OK6 Specific heat ratio for unburned mixture k ui 1.39 V 26.63981 0 8000 OK7 Expansion ratio of combustion products E i 7.52 L/D 1 0 3 OK8 Speed of sound at initial conditions c ui 353 m/s9 Burning velocity at initial conditions s ui 43 cm/s10 Empirical constant α 1.7511 Empirical constant β 0.51213 Vent area Av 3.572488 0.178396 m²1415 RHS of Transcendental equation 0.175811 0.0140216 LHS of transcendental equation 0.176634 0.01307117 Error 0.0008 -0.0009181920212223242526272829303132333435363738394041424344454647484950515253545556575859 Vessel Volume V 26.63981 m³60 Vent open pressure Pstat 0.1 barg61 Reduced pressure Pred 0.35 barg a b c d62 Burning velocity at initial conditions s ui 43 cm/s Methane 0.105 0.77 1.23 -0.82363 Constant a 0.105 Propane 0.148 0.703 0.942 -0.67164 Constant b 0.77 Hydrogen 0.279 0.68 0.755 -0.39365 Constant c 1.23 Coke gas 0.15 0.695 1.38 -0.70766 Constant d -0.8236768 Av 3.528 m²697071 Reduced pressure Pred 13.11597 barg 0.000 Av 0.179 m²72737475

7677787980

Alternative Vent Sizing pr14994:2004 Annex A Method

Calculation SheetGas Explosion Calculation

NFPA 68:1994

If burning velocity is greater than 60 cm/s i.e. greater than 1.3 x that of propane, then hydrogen constants are used. Otherwise propane data is used.

- 26 -

REF REV1 Combustible Dust2 Dust Explosion Class St 23 Kst Parameter Kst 290 bar.m/s4 Maximum Pressure Pmax 8.5 barg5 Vessel L/D ratio L/D 1.006 Vessel Volume V 10.00 m³7 Vessel Surface Area As 27.85 m²8 Duct Length L D 0.00 m9 Vent open pressure Pstat 0.2 barg

10 Reduced pressure Pred 0.866 barg OR Av 1 m²11121314 m² valid m² valid barg Valid15 1.000 OK 2.537 OK 0.866 OK16 0.475 ERR - - 0.175 ERR17 0.659 OK 1.149 OK 0.406445 OK18 0.659 ERR 1.149 ERR 0.406445 ERR19 0.558 OK 0.648 OK 0.289662 OK20 0.571 OK 0.303356 OK21 0.659 OK 0.701 OK 0.406445 OK22 0.898 ERR - -23 1.287 ERR - -2425 Limits: lower upper26 Dust Explosion Class St 2 St 1 3 OK27 Kst Parameter Kst 290 bar.m/s Kst 10 600 bar.m/s OK28 Maximum Pressure Pmax 8.5 barg Pmax 0 10 barg OK29 Vessel L/D ratio L/D 1 L/D 0 5 OK30 Vessel Volume V 9.999517 m³ V 1 1000 m³ OK31 Vent open pressure Pstat 0.2 barg Pstat 0.1 0.5 barg OK32 Pred 0.3 2 barg OK3334 Reduced pressure Pred 0.866 barg Av 1.000 m² OK35 a 0.0008518336 OR b 0.9576761537 c -0.7187648838 Vent Area Av 1 m² Pred 0.866 barg OK39 EFFECT OF DUCT40 Limits: lower upper41 Duct Length LD 0.00 m LD 0 6 m OK4243 Reduced pressure P'red 0.2371986 barg Av 2.537 m² OK44 Note : Simpson equation published in NFPA 68 1994 ed section 7-1.1.14546 Limits: lower upper47 Dust Explosion Class St 2 St 1 2 OK48 Rate pressure rise Kst 290 bar.m/s Kst 0 300 bar.m/s OK49 Maximum Pressure Pmax 8.5 barg Pmax 0 9 barg OK50 Vessel L/D ratio L/D 1 L/D 0 5 OK51 Vessel Volume V 9.999517 m³ V 1 1000 m³ OK52 Vent open pressure Pstat 0.2 barg Pstat 0.1 0.1 barg ERR53 Pred 0.25 2 barg OK5455 Reduced pressure Pred 0.866 barg Av 0.475 m² OK56 St 1 0.3628257 OR St 2 0.475345859 Vent Area Av 1 m² Pred 0.175 barg ERR60 St 1 0.0817061 St 2 0.1751162 Note : Radandt equation published in NFPA 68 1994 ed section 7-2.3.16364 Limits: lower upper65 Dust Explosion Class St 2 St 1 3 OK66 Rate pressure rise Kst 290 bar.m/s Kst 10 800 bar.m/s OK67 Maximum Pressure Pmax 8.5 barg Pmax 5 10 barg OK68 Vessel L/D ratio L/D 1 L/D 0 2 OK69 Vessel Volume V 9.999517 m³ V 0.1 10000 m³ OK70 Vent open pressure Pstat 0.2 barg Pstat 0.1 1 barg OK71 Pred 0.1 2 barg OK7273 Reduced pressure Pred 0.866 barg Av 0.659 m²7475 OR7677 Reduced pressure Pred 0.406445 barg 0.000 Av 1.000 m² OK78 Note:- alter Pred to obtain desired Av79 EFFECT OF DUCT80 Limits: lower upper81 Duct Length LD 0.00 m LD 0 6 m OK8283 Reduced pressure P'red 0.3161994 barg Av 1.149 m²84 Note: Scholl equation published in NFPA 68 1998 ed section 7-2.2

8586878889

FM Global dust 2

NFPA 68:2007

Approved byChecked byPrepared by

Revision A B C D E F

NFPA 68:2002

NFPA 68:1998 Low Strength

Date

BS EN 14491:2006

Modified Swift eqnSIMPSON (VDI 3673:1979 & NFPA 68 1994 Section 7-1.1 .1)

RADANDT (NFPA 68:1988)

SCHOLL (VDI 3673:1995 & NFPA 68 1998 Edition)

Simpson (VDI 3673:1979)Radandt (NFPA 68:1988)

Scholl (VDI 3673:1995)Elongated (VDI 3673:1995)

RESULTS

Red PressureVent Area (Av) with

ductVent Area (Av)

INPUT DATA

Dust Explosion CalculationCalculation Sheet

Pmax upper limit is

11 bara for St1 and 2

13 bara for St 3

ref Dust Explosion Prevention and

Protection Part 1 page 73

This is a correlation to the Radandt nomographs, which are dependent only on the St group, and not the Kst parameter. This method will give different results to those obtained using Simpson's correlation above. However, for all practical purposes they are sufficiently close. If the Kst value is known, then Simpson is preferable to Radandt.

Straight duct of

maximum length 6m

Equations are derived from the

Figure 5-4(b) in NFPA 68 1994

page 68-18

Upper limit assumed to be 6m based

on subsequent NFPA 68 issues

Calculate Pred

- 27 -

REF REV12 Limits: lower upper3 Dust Explosion Class St 2 St 1 3 OK4 Rate pressure rise Kst 290 bar.m/s Kst 10 800 bar.m/s OK5 Maximum Pressure Pmax 8.5 barg Pmax 5 10 barg OK6 Vessel L/D ratio L/D 1 L/D 2 6 ERR7 Vessel Volume V 9.999517 m³ V 0.1 10000 m³ OK8 Vent open pressure Pstat 0.2 barg Pstat 0.1 1 barg OK9 Pred 0.1 1.5 barg OK1011 Reduced pressure Pred 0.866 barg Av 0.659 m²12 delta A 0.000 m²1314 OR Av 0.659 m²1516 Reduced pressure Pred 0.406445 barg 0.000 Av + dA 1.000 m² OK17 Note:- alter Pred to obtain desired Av1819 EFFECT OF DUCT20 Limits: lower upper21 Duct Length LD 0.00 m LD 0 6 m OK2223 Reduced pressure P'red 0.3161994 barg Av 1.149 m²24 delta A 0.000 m²2526 OR Av 1.149 m²27 Note: Elongated Scholl equation published in NFPA 6 8 1998 ed section 7-2.3, for homgenous dust clouds only2829 Limits: lower upper30 Dust Explosion Class St 2 St 1 3 OK31 Rate pressure rise Kst 290 bar.m/s Kst 10 800 bar.m/s OK32 Maximum Pressure Pmax 8.5 barg Pmax 5 12 barg OK33 Vessel L/D ratio L/D 1 L/D 0 6 OK34 Vessel Volume V 9.999517 m³ V 0.1 10000 m³ OK35 Vent open pressure Pstat 0.2 barg Pstat 0.1 1 barg OK36 Pred 0.1 2 barg OK3738 Reduced pressure Pred 0.866 barg Av 0.558 m²39 delta A 0.000 m²4041 OR Av 0.558 m²4243 Reduced pressure Pred 0.2896624 barg 0.000 Av + dA 1.000 m² OK44 Note:- alter Pred to obtain desired Av45 EFFECT OF DUCT46 Limits: lower upper47 Duct Length LD 0.00 m LD 0 6 m OK4849 Reduced pressure P'red 0.6591674 barg Av 0.648 m²50 delta A 0.000 m²5152 OR Av 0.648 m²535455 Limits: lower upper56 Dust Explosion Class St 2 St 1 3 OK57 Rate pressure rise Kst 290 bar.m/s Kst 10 800 bar.m/s OK58 Maximum Pressure Pmax 8.5 barg Pmax 5 12 barg OK59 Vessel L/D ratio L/D 1 L/D 0 6 OK60 Vessel Volume V 9.999517 m³ V 0.1 10000 m³ OK61 Vent open pressure Pstat 0.2 barg Pstat 0 0.75 barg OK6263 Reduced pressure Pred 0.866 barg Av 0.571 m²64 delta A 0.000 m²6566 OR Av 0.571 m²6768 Reduced pressure Pred 0.3033561 barg 0.000 Av + dA 1.000 m²6970 EFFECT OF DUCT71 1 Limits: lower upper72 Duct Length LD 0.00 m LD 0 6 m E173 E274 Reduced pressure P'red 0.659167 barg Av 0.648 m²75 delta A #NUM! m²7677 OR Av 0.648 m²787980 Limits: lower upper81 Dust Explosion Class St 2 St 1 3 OK82 Rate pressure rise Kst 290 bar.m/s Kst 10 800 bar.m/s OK83 Maximum Pressure Pmax 8.5 barg Pmax 5 10 barg OK84 Vessel L/D ratio L/D 1 L/D 0 6 OK85 Vessel Volume V 9.999517 m³ V 0.1 10000 m³ OK86 Vent open pressure Pstat 0.2 barg Pstat 0.1 1 barg OK87 Pred 0.1 2 barg OK8889 Reduced pressure Pred 0.866 barg Av 0.659 m²90 OR9192 Reduced pressure Pred 0.406445 barg 0.000176 Av 1.000 m²9394 EFFECT OF DUCT95 Max duct length that needs to be considered Ls 5.0020351 m96 1 0.0648919697 Pred max with duct and an L/D ratio = 1 P'red max 1.24 barg Av for L/D=1 0.087 m²98 Pred max without duct and an L/D ratio = 1 Pred max 0.7727633 barg99 Pred max for a L/D ratio = 6 P'red max 0.866 barg Av for L/D=6 1.227 m²100 Pred max 0.8438026 barg101102 Pred max for a L/D ratio between 1 and 6 Pred max 0.7727633 barg Av for 1 ≤L/D ≤ 6 0.701 m²

103104105106107

NFPA 68:2007

ELONGATED (VDI 3673:1995 & NFPA 68 1998 Edition)

BS EN 14491:2006

Checked by

NFPA 68:2002

Approved by

Prepared by

D E FDateRevision A B C


This equation is sensitive to Pred.

For low values of Pred the additional

area is relatively large.

For Pred values of 1.5 bar and above

the dAv equation should not be used,

and only use the eqn for Av.

This equation is sensitive to Pred.

For low values of Pred the additional

area is relatively large.

For Pred values of 1.5 bar and above

the dAv equation should not be used,

and only use the eqn for Av.

Vent pipes with a length of L>Ls have no

additional effect upon the pressure

increase, as flow reaches sonic velocity

NOT VALID FOR METAL DUSTS

Pmax upper limit is

10 barg for St1 and 2

12 barg for St 3

ref WinmVent Handbook April 2001

page 42

Calculate for L/D =1

- 28 -

REF REV12 Limits: lower upper3 Dust Explosion Class St 2 St 1 3 OK4 Rate pressure rise Kst 290 bar.m/s Kst 10 600 bar.m/s OK5 Maximum Pressure Pmax 8.5 barg Pmax 5 12 barg OK6 Vessel L/D ratio L/D 1 L/D 0 6 OK7 Vessel Volume V 9.999517 m³ V 0.1 10000 m³ OK8 Vent open pressure Pstat 0.2 barg Pstat 0.1 1 barg OK9 Pred 0.1 0.2 barg ERR

101112 Av 0.898 m² Using NFPA 68 1998 constants1314 C 0.043 bar½

15 Av 1.287 m² Using Lunn data for C1617181920 C (psi ½) C (bar½) 2122 10 0.005 0.00123 20 0.01 0.00324 30 0.015 0.00425 40 0.021 0.00626 50 0.027 0.00727 75 0.041 0.01128 100 0.055 0.01429 150 0.084 0.02230 200 0.105 0.02831 250 0.127 0.03332 300 0.163 0.04333 400 0.21 0.05534 500 0.248 0.06535 600 0.3 0.07936373839 Limits: lower upper For flame length equations40 Dust Explosion Class St 2 St 1 2 OK41 Rate pressure rise Kst 290 bar.m/s Kst 0 300 bar.m/s OK42 Maximum Pressure Pmax 8.5 barg Pmax 0 10 barg OK43 Vessel L/D ratio L/D 1 L/D 0 2 OK44 Vessel Volume V 9.999517 m³ V 0.1 10000 m³ OK45 Vent open pressure Pstat 0.2 barg Pstat 0.1 0.2 barg OK46 Reduced explosion pressure Pred 0.406445 Pred 0.1 2 barg OK47 Vent area Av 1 m²48 Method of evaluating Pred Elongated49 Pressure venting orientation Horizontal50 For Horizontal pressure venting Limits: lower upper For VDI 3673 Pmax,a and Pr (at distance) equations51 Flame Length L F 21.544 m St 1 1 ERR52 For Vertical pressure venting Kst 0 200 bar.m/s ERR53 Flame Length L F 17.235 m Pmax 0 9 barg OK54 Flame Width W F 6.034 m L/D 0 2 OK55 Maximum external peak overpressure Pmax,a 0.123 barg V 0 250 m³ OK56 Distance to peak external overpressure R S 5.386 m Pstat 0 0.1 barg ERR57 Distance to peak external overpressure RS 4.309 m Pred 0.1 1 barg OK5859 barg psig barg psig Barg psig Limits: lower upper For EU CREDIT FORMULAS60 5.386 0.123 1.784 4.3088 0.123 1.784 1 0.690243 10.011 St 1 1 ERR61 6 0.105 1.518 5 0.106 1.538 2 0.345121 5.006 Kst 10 200 bar.m/s ERR62 7 0.083 1.204 6 0.088 1.282 3 0.230081 3.337 Pmax 5 10 barg OK63 8 0.068 0.986 7 0.076 1.098 4 0.172561 2.503 L/D 0 6 OK64 9 0.057 0.826 8 0.066 0.961 5 0.138049 2.002 V 0 1000 m³ OK65 10 0.049 0.705 9 0.059 0.854 6 0.11504 1.669 Pstat 0.1 0.2 barg OK66 11 0.042 0.611 10 0.053 0.769 8 0.08628 1.251 Pred 0.1 2 barg OK67 12 0.037 0.537 11 0.048 0.699 10 0.069024 1.00168 13 0.033 0.476 12 0.044 0.641 12 0.05752 0.834 Distance to struc./obstacle 15.36 m69 14 0.029 0.426 13 0.041 0.591 14 0.049303 0.715 Maximum pressure at r obs 0.069028 barg70 15 0.026 0.384 14 0.038 0.549 16 0.04314 0.626 Lateral flame spread 6.378725 m71 16 0.024 0.349 15 0.035 0.513 18 0.038347 0.55672 17 0.022 0.318 16 0.033 0.481 20 0.034512 0.50173 18 0.020 0.292 17 0.031 0.452 22 0.031375 0.45574 19 0.019 0.269 18 0.029 0.427 24 0.02876 0.41775 21.544 0.015 0.223 21.544 0.025 0.357 26 0.026548 0.38576

7778798081

Distance,m

Distance,m

Distance,m

Pressure HattwigPressure EU CREDITPressure VDI 2002

mixtures in air only



Fuel

St-2 dustsSt-3 dusts

NFPA 68:1998 LOW STRENGTH ENCLOSURE (SWIFT EQUATION )


Checked by

FLAME PROPAGATION - VDI 3673 Part 1 2002

Approved by

Prepared byDateRevision A B C D E F

0.0260.03

0.051

Fuel C (bar½) mixtures in air only

St-1 dusts

Table 4 from Venting Gas and Dust Explosions 2nd Edition GA Lunn

Table 4-3.1 from NFPA 68:1998

Taken from the EU CREDIT project. The equation

is only valid if Kst<= 200 bar.m/s

EU Credit report formula:-

For venting directed vertically

Rs = 0.25 LF ,

For venting directed horizontally

Rs = 0.2 LF

Hattwig method uses equation:-Pblast = Pred C1 C2 / rlog C 1 = -0.26/Av + 0.49VDI and CREDIT eqns

Psmax = 0.2 Predmax A0.1 V0.18

HATTWIG FOR COMPARISON ONLY

Venting towards an obstructionMethod uses EU CREDIT project equations only and not VDI for estimation of distance to peak external overpressure, Rs.Pr,obs = 2 (Rs / robs ) Ps,max

For CURRENT VDI Guidelines

2002 use Scholl ONLY

VDI 3673 Part 1 2002

Max external peak

pressure occurs at a

distance,

Rs = 0.25 LF

Documents

Dust explosion sizing comparison