Errata
NFPA 68
Guide for Venting of Deflagrations
2002 Edition
Reference: Various
Errata No.: 68-02-1
The Technical Committee on Explosion Protection Systems notes the following errors in the
2002 edition of NFPA 68, Guide for Venting of Deflagrations.
1. In Table 1.4 Conversion Factors, the English to Metric conversion for KG and KSt should
be changed to read “1 psi–ft/sec = 0.021 bar-m/sec”.
2. Paragraph 3.3.24 definition of Static Activation Pressure should read "….pressure rise less
than 0.1 bar/min = 1.5 psi/min)."
3. Table 4.2.4 for St-1 dusts in the KSt column should read "<200"
4. Revise 4.3.6.2 to read, "….venting for many combustible mists can be based on Equation
6.5, using KG for…"
5. Paragraph 5.3.9.2 should be changed by adding a new reference [113] and changing the
reference in the second sentence of paragraph from [46] to [113] which becomes the Task
Group report “NFPA 68 Impulse Task Force Report to the Committee on Explosion
Protection Systems, September 15, 1999” and also by deleting “recommended by
Reference [46]” from the paragraph.
6. Paragraph 5.6.1 should be changed by inserting the word “hybrid” before “mixtures” in
the second sentence. In the first sentence add “and mists” after gases. In the second
sentence revise “...Chapter 8 addresses gases and dusts in pipes, ducts and elongated
vessels.” (delete “and mixtures”)
7. In paragraph 5.6.14.3 delete reference [112] at the end of the paragraph and move it to the
end of the second sentence; it would follow “... can be used without correction [112].” The
other reference in the 3rd sentence to Lunn (“see Lunn reference”) should also be deleted.
8. Paragraph 5.10.2 the last sentence makes reference to section 7.6 change reference to read
9.7.
9. Paragraph 6.2.1 the word “chapter” should be replaced with the word “section”, the first
sentence should read "Section 6.2 applies to the design of deflagration vents…".
10. Paragraph 6.2.2, no units are indicated for Pred; add units to the end [bar (psi)].
11. Paragraph 6.3.3.4 delete Av from the list of conditions; change “where:” to “where the
following constraints apply:”; the definition of “V” should read V < 1000 m3. The first
sentence of 6.3.3.4.1 should be moved to the list under 6.3.3.4 and changed to read "Initial
pressure before ignition < 0.2 bar."
12. In Paragraph 6.3.3.4.1, after deleting the current first sentence, insert at start of the
paragraph “Equation 6.5 is derived”, the new first sentence should read "Equation 6.5 is
derived from tests made under the following conditions:"
13. Paragraph 6.5.2 should read "Where using equations 6.5 and 6.6 with vent ducting,"
14. Paragraphs 6.5.4.1 and 6.5.4.2, for the definition of P'red, should read "..P'red = a pseudovalue for Pred for use in Equation 6.5 for calculating vent areas for gases when a vent duct
is used [bar (psi)]"
15. Paragraph 7.2.2 should read…"For L/D values of 2 or less…"
16. In 7.2.2.1 add the following limitation:
(4) Pstat < 0.5 bar.
17. Paragraph 7.2.3, Equation 7.2, should be changed by replacing “log” with “log10” and
deleting the definition of Pi, as this parameter is not used in equation 7.2.
18. Revisions for section 7.2.6 as follows:
The committee prepared revised graphs and text to correct errors in the 2002 version of
NFPA 68. The current graphs represent the previous dust methodology. Replace the
published graphs with these new graphs so that results are consistent with the equations.
Instructions for use of these graphs in section 7.2.6 follow:
7.2.6
7.2.6.1
In addition to calculating the vent area using Equations 7.1 and 7.2, the vent area
can be determined by the use of the graphs in Figures 7.2.6(a) through 7.2.6(k),
which are based on Equations 7.1 and 7.2. The restrictions noted for the equations
apply equally to the graphs. The graphs can be used as a primary means for
determining vent area, or they can be used as a backup to verify the vent area
calculated by Equations 7.1 and 7.2.
Instructions and an example for using the graphs in Figures 7.2.6(a) through
7.2.6(k) follow:
(A) Factor A. Use one of the graphs in Figure 7.2.6(a) or, 7.2.6(b). Plot the line from the
KSt at the bottom up to the Pstat line and then read across to the left to determine Factor
A.
(B) Factor B. Use one of the graphs in Figure 7.2.6(c), Figure 7.2.6(d), Figure 7.2.6(e), or
Figure 7.2.6(f). Plot a line from the volume up to the graph line and then read across
to the left to determine Factor B.
(C) Factor C. Calculate Π, the ratio of Pred to Pmax. Use one of the graphs in Figure
7.2.6(g), Figure 7.2.6(h), or Figure 7.2.6(i). Plot a line from the Π at the bottom up to
the graph line and then read across to the left to determine Factor C.
(D) Factor D. Calculate the parameter, (1/Pred - 1/Pmax). Use one of the graphs in Figure
7.2.6(j) or Figure 7.2.6(k). If using Figure 7.2.6(j), plot the line from the parameter,
(1/Pred - 1/Pmax ) up to the appropriate L/D line and then read across to the left to
determine Factor D. If using Figure 7.2.6(k), plot the line from the L/D ratio up to the
appropriate parametric line and then read across to the left to determine Factor D.
Using the four factors, determine vent size as follows:
Av (m2) = Factor A x Factor B x Factor C x Factor D
(E) Example Problem. Determine the vent size needed to protect an enclosure from a dust
deflagration when the conditions are as follows:
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
Pmax = 10 bar
KSt = 350 bar-m/sec
Pstat = 0.2 bar
Pred = 0.6 bar
V = 25 m3
L/D = 3.0
From Figure 7.2.6(b), Factor A = 0.04
From Figure 7.2.6(d), Factor B = 11
Π = 0.6/10 = 0.06
From Figure 7.2.6(h), Factor C = 4.0
(1/Pred - 1/Pmax) = 1/0.6 - 1/10 = 1.67 - 0.1 = 1.57
From Figure 7.2.6(j), Factor D = 1.6
Av = Factor A x Factor B x Factor C x Factor D
= 0.04 x 11 x 4.0 x 1.6 = 2.8 m2 (Use of Equations 7.1 and 7.2 gives an area
of 2.9 m2. Due to resolution of graphs, the answers may differ slightly.)
19. In Section 7.3 the use of the lower case Pi (π) should be changed to capital Pi (П) to be
consistent in the document.
20. Paragraph 7.3.1, should read.."If Xr < Π…."
21. Paragraph 7.3.3 (I)(1) should read similarly to 7.3.1.."If Xr < Π…."
22. Paragraph 7.4 should be changed. Since publication of the 2002 edition of NFPA 68,
several individuals have noted that equation 7.5 does not provide correct answers. In
reviewing the equation it was noted that there were errors as the equation does not agree with
the FM material initially adopted by the committee. Errors were made when converting
absolute pressure parameters to the gauge-pressure parameters used in NFPA-68. Replace
Peffective in Equation 7.5 with (Peffective + 1) and correct the associated definitions as follows:
Av = (8.535 x 10-5)(1 + 1.75
Where:
Av
Pstat
Pinitial
Peffective
KSt
V
=
=
=
=
=
=
(1 − Πeffective)
Pstat − Pinitial
)KStV0.75 Πeffective
Peffective + 1
Vent area (m 2)
Static burst pressure of the vent (bar)
Enclosure pressure at the moment of ignition (bar)
1/3 Pinitial (bar)
Deflagration index (determined at initially atmospheric pressure) (bar-m/sec)
Enclosure volume (m3)
Πeffective
Pred
P Emax
Pmax
= ( Pred - Peffective)/( P Emax - Peffective)
= Reduced pressure (bar)
= [(Pmax +1) ( Pinitial +1)/(1 bar abs) - 1] maximum pressure of an unvented
deflagration at initially elevated pressure (bar)
= Maximum pressure of an unvented deflagration initially at atmospheric pressure
(bar)
23. Revise Section 7.4 as follows:
7.4 Effects of Initially Elevated Pressure. For enclosures that may contain homogeneous dust-air
mixtures at an elevated pressure (greater than 0.2 bar and less than or equal to 4 bar) prior to
ignition, the ...
24. Revise 7.5 as follows:
7.5 Effects of Vent Ducts. For cubical vessels and homogeneous dust-air mixtures initially at
atmospheric pressure, the effect of vent ducts...
25. In Equation 7.7 delete “max” from Pred, max. In paragraph 7.5.1.1 change Pred, max to P’red.
Ls = 3.764 x P’red-0.3724
26. Revise Equation 7.6 as follows:
Pred/P’red = 1 + 17.3 [Av/V0.753]1.6 (LD/Dv)
Where:
P'red = a pseudo-value for Pred for use in Equations 7.1 and 7.2 for calculating vent areas
for dusts when a vent duct is used [bar (psi)]
Av = vent area (m2) also equal to duct cross-sectional area
V = vessel volume (m3)
LD = vent duct length (m)
Dv = vent duct equivalent (m) = 2 (Av/π)1/2
27. Delete Section 7.7 and renumber existing Section 7.6 as Section 7.7. Insert the following
chart as Section 7.6 to define the limits of applicability for the models used in the dust
explosion calculations:
Combination Rules and Limitations
For NFPA 68 Dust Models
MODEL
Vent Ducts
Partial Volume
Elevated Initial
Pressure
Panel Inertia
APPLICATION
0.8 ≤ P0 ≤ 1.2 bar abs
Panel density ≤ 2.5 lb/ft2
Allow Partial Volume
1 ≤ L/D ≤ 6
(calculate vent duct effect last)
Allow Vent Duct
Panel density ≤ 2.5 lb/ft2
0.8 ≤ P0 ≤ 1.2 bar abs
1 ≤ L/D ≤ 6
(calculate vent duct effect last)
No Vent Duct
Panel density ≤ 2.5 lb/ft2
0.2 ≤ P0 ≤ 4 bar g
Full Volume Deflagration
1 ≤ L/D ≤ 6
(calculate elevated initial pressure effect last)
0.8 ≤ P0 ≤ 1.2 bar-a
No Vent Duct
2.5 lb/ft2 < Panel density < 41 lb/ft2
Allow Partial Volume
1 ≤ L/D ≤ 6
28. Revise Section 8.1, 3rd sentence as shown: "This chapter applies to pipes, ducts, and
elongated vessels with length-to-diameter ratios of 5 or greater for gases, and 6 or greater for
dusts."
29. Revise the title caption for Figure 8.6.1 to read “…from exceeding 0.17 bar…”.
30. Edit existing Annex material in A.5.3.9 as follows:
A.5.3.9 The example of the calculation of reaction force, Fr, during venting, is for the
following conditions using Equation 5.3:
(1) Av = 1 m2 = 1550 in.2
(2) Pred = 1 bar = 14.5 psi
(3) Fr = (1.2)(1550)(14.5) = 26,970 lbf
A.5.3.9.2 The example of the calculation of duration of thrust force, t f, resulting from
venting of a dust deflagration, is for the following conditions using Equation 5.4:
(1) KSt = 160 bar-m/sec
(2) V = 20 m3
(3) Pmax = 8 bar
(4) Pred = 0.4 bar
(5) Av = 1.4 m2
(6) t f = (0.0043)(8/0.4)0.5(20/1.4)
(7) t f = 0.27 sec
A.5.3.9.3 The example of the calculation of total impulse, I, resulting from venting of a
dust deflagration is for the following conditions using Equation 5.5:
(1) c = 62
(2) Pred = 0.4 bar
(3) Av = 1.4 m2
(4) tf = 0.27 sec (from A.5.3.9.2)
(5) I = (62)(1.4)(0.4)(0.27)
(6) I = 9.4 kN-s = 9400 N-s
A.5.3.9.4 The example of the calculation of equivalent static force, Fs, resulting from
venting of a dust deflagration is for the following conditions using Equation 5.6:
(1) a = 120
(2) DLF = 2
(3) Av = 1.4 m2
(4) Pred = 0.4 bar
(5) Fs = (120)(2)(1.4)(0.4)
(6) Fs = 134 kN
Note that a dynamic load factor (DLF) of 2 is conservative for most situations.
Experienced users may choose to substitute a value specific to their design. For additional
information on derivation of DLF and for use of the total impulse values, refer to
textbooks on structural dynamics, such as J. M. Biggs, Introduction to Structural
Dynamics.
31. Add asterisk to section 7.3 and add Annex material:
A.7.3 The equations used in this guide have been developed based upon venting initially at
atmospheric pressure without ducts. They have not been evaluated as to the effect of vent ducts
in partial volume applications.
32. In Annex B, Figure B.1 should have the label on the X-axis changed to read “Test vessel
volume (liters)”.
33. In Annex D, change the title to the title in the 1998 edition, “Deflagration Characteristics of
Selected Flammable Gases.”
34. In Annex D, paragraph D.2.2, equation D.2 should have a division line horizontally between
the first and second lines to indicate a quotient of the differences. This is the slope of the
regression line.
B = [KG(propane)-KG(methane)]WB
[KG(propane)-KG(methane)]New
35. Revise Annex F as follows:
F.1 The following procedure can be used to assess the impact of the vent panel mass on Pred.
F.1.1 Introduction. The mass of vent panels is a factor that can limit the effectiveness of the
venting process. To properly assess the influence panel mass contributes, other factors must also
be considered, such as the reactivity of the dust, the enclosure volume and the number, shape,
size and type of deflagration vents utilized. The procedures for determining the effects of vent
panel inertia on deflagration venting are presented in this section. The theoretical development
uses mostly absolute pressures, instead of the gauge pressures used in the remainder of this
document, and new pressure terms are defined. Pressures are used in bar, bar-abs, and Pascalsabs, thus the reader is cautioned to note units of measure directly following each equation.
F.1.2 The reduced deflagration pressure is first calculated using Equation 7.1, based on lowmass vents. Corrections for vessel L/D and Partial Volume can then be added. Next the
correction factors for inertia effects are calculated.
F.1.3 The inertia of the panel can manifest itself in the following two ways:
(1)
As a new factor in the effective vent relief pressure, pvi, higher than the nominal static
value, pv, and
(2)
As a higher reduced pressure, pri, after full vent deployment with respect to the pri0 in the
absence of inertia.
The highest pressure during the vented deflagration can occur either at the point of vent relief or
later after vent deployment. As inertia of the panel effects both pressures, both effects have to be
calculated and the higher value, pvi or pri, used as the reduced pressure produced in the vented
deflagration.
F.1.3.1 The inertia correction is limited to:
(1)vent panel density, σv < 200 kg/m2
(2) nominal static relief pressure, pv < 0.5 bar
F.1.4 Both inertia effects are evaluated using two dimensionless parameters, Σ and Γ. However
one term in the parameters is different, the dust reactivity. In the first case, the deflagration
index, KSt, is used to determine ΣKSt and ΓKSt. In the second case, the effective mixture
reactivity, K, is used to determine ΣK and ΓK.
F.1.5 The deflagration index, KSt, of a dust is basically the maximum rate of pressure rise
generated in a confined deflagration. The effective mixture reactivity is a parameter based on
KSt, but which contains two corrections to account for the effects of the deflagration vent relief
pressure and the volume of the protected enclosure. The vent relief pressure correction is the
following:
⎡
⎛ ∆p ⎞⎤
(F.1) K St ,v = K St ⎢1 + 1.75⎜⎜ v ⎟⎟⎥
⎝ p0 ⎠ ⎦
⎣
where:
KSt,v = deflagration index with vent relief pressure correction
KSt = deflagration index (bar-m/sec)
∆pv = vent relief pressure (bar) = Pstat
p0 = initial pressure (bar abs)
The volume correction for Equation F.1 is the following:
0.11
⎡ V ⎤
(F.2) K = K St ,v ⎢
3
⎣10m ⎥⎦
where:
K = volume correction to deflagration index
V = enclosure volume (m3)
This volume correction is applied only where the enclosure volume is greater than 10 m3,
otherwise K = KSt,v.
F.1.6 The shape factor for the vent(s) is:
For square panels, cs = 1.
For circular panels, cs = 0.886.
For rectangular panels, apply the following equation:
(F.3) c s =
1+ α
2 α
where:
α = the ratio of the rectangle's smaller side to its longer side
F.1.7 Calculate ΣKSt and ΣK using Equations F.4 and F.5.
(F.4) Σ K St
σv
⎡ K St ⎤
= 1
⎢
⎥
⎛⎜ n 2 ⎞⎟(c )⎛⎜α 12 ⎞⎟( p )⎛⎜V 13 ⎞⎟ ⎣ ∆pm ⎦
⎝
⎠ s ⎝ cd ⎠ o ⎝
⎠
σv
⎡ K ⎤
(F.5) Σ K = 1
⎢
⎥
1
1
⎛⎜ n 2 ⎞⎟(c )⎛⎜α 2 ⎞⎟( p )⎛⎜V 3 ⎞⎟ ⎣ ∆pm ⎦
s
cd
o
⎝
⎠ ⎝
⎠
⎝
⎠
ΣKSt , ΣK = dimensionless parameters
σv = vent panel density (kg/m2)
n = number of equal-sized panels
cs = shape factor
5
5
2
2
αcd = constant = 232.5 m/sec
po = initial pressure (Pascal absolute, N/m2)
V = enclosure volume (m3)
KSt = deflagration index (bar-m/sec)
K = effective mixture reactivity (bar-m/sec)
∆pm = unvented pressure rise (bar) = pm-p0
F.1.7.1 For hinged vent closures, increase the value of vent panel density, σv, by 33 percent.
F.1.8 Calculate ΓKSt and ΓK.using Equations F.6 and F.7.
⎛ A
(F.6) ΓK St = α cd ⎜⎜ 2v
⎝V 3
⎛ A
(F.7) ΓK = α cd ⎜⎜ 2v
⎝V 3
⎞⎛ ∆p m ⎞
⎟⎜
⎟
⎟⎜ K ⎟
⎠⎝ St ⎠
⎞⎛ ∆pm ⎞
⎟⎜
⎟⎝ K ⎟⎠
⎠
where:
ΓKSt, ΓK = dimensionless parameters
Av = vent area (m2)
F.1.9 Calculate the Pressure function, f(Pv), using Equations F.8 and F.9.
(F.8) Pv =
pv − p0
p m − p0
(F.9) f (Pv ) = (1000 Pv )
0.5
where:
Pv = pressure ratio
pv = vent panel static relief pressure (bar abs)
p0 = initial pressure (bar abs)
pm = unvented deflagration pressure (bar abs)
F.1.10 Calculate the panel inertia parameter, η, using Equation F.10.
(F.10) η =
⎤
⎛ mgσ v ⎞
2 1 ⎡
⎟⎟ f (Pv )⎥
−
⎢ Max{1, f (Pv )} + 3.2⎜⎜
3 60 ⎣
⎝ pv − p0 ⎠
⎦
where:
η = panel inertia parameter
m = vent gravity coefficient, assisting or slowing vent opening as defined in Table F.1.10.
g = gravitational acceleration (m/sec2)
pv = vent panel static relief pressure (Pascal absolute, N/m2)
p0 = initial pressure (Pascal absolute, N/m2)
F.1.11 The new effective vent relief pressure with inertia can be determined as follows:
⎡Σ
K
(F.11) pvi = pv + 0.21⎢ 1St
⎢Γ 2
⎣ K St
η
⎤
⎥ ∆pm
⎥
⎦
where:
pvi = effective vent relief pressure with inertia (bar abs)
pv = vent panel static relief pressure, pstat + 1 (bar abs)
∆pm = unvented pressure rise (bar) = pm-p0
F.1.12 The new reduced pressure after full vent deployment can be determined as follows,
depending on the value of ΓK:
(F.12) For ΓK < 1;
pri = pr 0 + ( pm − p0 )(Σ K )
For 1 < ΓK < 3;
pri = pr 0 + ( pm − p0 )(Σ K )
For ΓK > 3;
pri = pr 0
3
5
3
5
(0.26ΓK )
(0.26)(ΓK − 3)(0.25 − 0.75ΓK )
where:
pri = the reduced pressure developed with inertia (bar abs)
pr0 = the reduced pressure developed with low-mass vents, Pred +1 (bar abs)
pm = unvented deflagration pressure, Pmax + 1 (bar abs)
p0 = initial pressure (bar abs)
F.1.13 Compare the results obtained in Equations F.11 and F.12. The larger of the two results,
pvi or pri, represents the new maximum reduced deflagration pressure (in bar abs) due to the vent
panel inertia effect. The value of pvi or pri must be converted to gauge pressure as Pred to iterate
Equation 7.1. If the calculated pressure exceeds the enclosure strength, the user should repeat the
calculation with a larger vent area.
Revise Table F.10 (existing F.1.4.1)
Panel Characteristics
Horizontal panel, on top of the vessel
Other orientations
Value of m
1
0
36. Revise Equation F.1 by deleting “delta pv “ and substituting “pv – po ”. Define pv and p0 in
bar-abs for this equation.
37. In Equation F.4 the definition of delta pm should be changed by adding to the parenthesis “pm
- po “.
38. In Equation F.5 (revised F.3), the minus sign (“-“) should be changed to a plus sign (“+”).
39. In Equation F.10 (revised F.12) for “ΓK > 3” it should read “pri = pro”. The remainder of the
line should be deleted.
40. In Appendix F replace “σ” with “σv” where it appears in Appendix F.
41. In example F.2 there are several places where the text mixes upper and lower case for
symbols making it inconsistent with the text. The committee agreed to change the text to all
lower case.
42. In Example F.2, on page 68-53 Equation F.3 (new F.11) is solved as follows:
pv = pstat +1 = 1.05 and the substitution into Eq. F.3 is as follows:
⎡Σ
K
pvi = pv + 0.21⎢ 1 St
⎢Γ 2
⎣ K St
η
⎤
⎥ ∆pm
⎥
⎦
= 1.05 +
⎡Σ
K
0.21⎢ 1St
⎢Γ 2
⎣ K St
η
⎤
⎥ ∆pm
⎥
⎦
= 1.098
43. In Example F.2 on page 68-54, the substitution of values in equation F.10 should read as pri
= 1.076 + (9 – 1)(0.0125)3/5 (0.26)(1.85 – 3)[(-0.75)(1.85) + 0.25] = 1.272 bar abs.
44. Change the text for this same example in the following paragraph to read “…the new
pressure due to panel inertia is 1.272 bar abs.”
Issue Date: September 17, 2004
Copyright © 2004 all Rights Reserved
NATIONAL FIRE PROTECTION ASSOCIATION