DS 7-76 Prevention and Mitigation of Combustible Dust ... - FM Global

7-76 Combustible Dust Explosion
Page 30 FM Global Property Loss Prevention Data Sheets
Where
L f,max = maximum flame jet length, m
Step 3: For distances R larger than Rmax, calculate the distance-specific blast pressure by the following
equation:
P blast (barg) = (R max/R) 1.5 P blast,max
Note 1: These calculated pressures are very localized near the fireball ejected from a vented explosion.
This is not the same as the overall explosion pressure (P red) created throughout the enclosing room or
building. In a large building, this shock pressure might be the largest pressure that the walls located close
to exploding equipment will feel, but, usually, the pressure on the walls of a room enclosing the vented
equipment depends on the quantity of combustion gases produced by the explosion and vented into the
room. Such pressure is essentially uniform throughout the room, regardless of distance of the walls from the
vented vessel.
Note 2: The equations for predicting pressure effects are valid only for a single explosion vent. For multiple
vents that open in the same direction, calculate P blast,max for each vent, then add the values to estimate
the combined, conservative pressure effect.
3.1.10.2 Estimate the maximum flame jet length in a direction normal to the vent using the following equation:
L f,max = 8 V (1/3)
where
L f,max = maximum flame jet length, m
V = Volume of vented enclosure, m 3
This equation is valid only for ST1 dusts (i.e., K st ≤ 200) with P red ≤ 1 barg (14.5 psig) and P stat ≤ 0.1 barg
(1.5 psig). There are no published correlations for situations outside the stated limits. It is believed that
changes in P stat will change the size of the predicted fireball; however, current estimation methods have not
quantified that effect.
3.1.10.3 Estimate the recoil force from an explosion using the following equations (2.3.3.1.18):
a) For dynamic recoil force, use the following formula:
F R = 119 A v P red
Units: F R (kN), A v (m 2 ), P red (barg)
F R = 1.2 A v P red
Units: F R (lb), A v (in 2 ), P red (psig)
b) For duration of this recoil force, use the following formula (applies only to enclosures without vent ducts):
t f = 10 -4 K st V/(P red A v)
Units: K st (bar m/s), V (m 3 ), P red (barg), A v (m 2 ), 10 -4 (s 2 m -2 ) a constant
c) As an alternative to dynamic force anchoring design, use an equivalent static force (F s) as calculated
using the following formula:
F s = 0.52 F R
Units: kN or lb as in the original equation
3.1.11 Fixed Obstructions near the Face of Explosion Vents (2.3.3.1.12)
A fixed obstruction located too close to an equipment explosion vent opening creates significant resistance
to the free outflow of combustion products from the vented equipment. This is believed to be mainly due
to dust burning after being ejected from the protected equipment. Since the combustion occurs in the
semiconfined area between the vessel and the obstruction, a condition similar to a secondary explosion
occurs. This has significant backpressure effects on the equipment explosion. The state of knowledge
regarding this phenomenon is limited, and it is impossible to quantify the effect. It is also not possible to provide
any guidance on safe distances to nonflat obstructions, due to the complexity of the effects of different
geometries producing partial confinement. The only safe course of action is to locate and orient explosion
vents so they do not point to nearby surfaces.
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