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

Combustible Dust Explosion 7-76
FM Global Property Loss Prevention Data Sheets Page 13
2.3.3.1.13 Where a weather protection cover (‘‘rain hat’’) is provided over the end of an explosion vent duct,
estimate the effect on the venting efficiency as follows:
a) If there is a distance of at least one explosion vent diameter between the end of the duct and the weather
cover, there is no effect on efficiency.
b) If the distance is between 1 ⁄4 and 1 explosion vent diameter, consider the weather cover as a 90° bend
in the vented flow when calculating the effect of the vent duct.
c) A distance of less than 1 ⁄4 explosion vent diameter away from the vent duct is unacceptable unless it
is designed to fly off in the event of an explosion.
For non-circular ducts, use the hydraulic diameter for these calculations (2.3.3.1.12).
2.3.3.1.14 Do not provide explosion venting if the dust or its combustion products are poisonous, radioactive,
corrosive to nearby equipment or structures, or for any other reason should not be released from an otherwise
closed system. Use alternatives to venting, such as explosion hazard elimination, explosion suppression,
explosion containment, or high vacuum operation.
2.3.3.1.15 Where a vessel requiring explosion venting contains significant obstructions to the free flow of
gases, provide several distributed vents at different places on the vessel rather than one large vent of the
same area (3.1.12).
2.3.3.1.16 Where a vessel contains both suspended combustible dust and ignitable liquid vapors or flammable
gases (a hybrid mixture), the reactivity of the mixture will require larger vent areas than the combustible dust
on its own.
a) The presence of a flammable vapor or gas can be ignored if the concentration is at or below 5% of
its Lower Explosive Limit (LEL).
b) Where the gas concentration exceeds 5% of its LEL, determine the reactivity of the mixture by testing.
2.3.3.1.17 The required explosion venting area for a vessel with a dust explosion hazard operating at
pressures exceeding 0.1 barg (1.5 psig) requires special consideration and needs to be carefully analyzed.
FM Global DustCalc software can handle initial pressure up to 4 barg (58 psig) for full-volume explosions.
a) Set the vent-relief pressure, P stat above the normal maximum operating pressure by at least 0.1 barg
(1.5 psig) (3.1.13).
b) Obtain vent sizing criteria from experts familiar with high initial pressure venting.
2.3.3.1.18 Recoil forces from the venting of an explosion can dislodge even large vessels that are not properly
anchored. Implement one of the following to control recoil forces:
a) Provide equal-sized vents at opposite sides of the vessel.
b) Calculate the magnitude and duration of the dynamic recoil force (or an equivalent static force) from
a vent, and provide anchorage to resist those forces (3.1.10.3).
2.3.3.2 Isolation
Avoid multiple pieces of interconnected equipment containing a dust explosion hazard. Properly protected
equipment can fail when an explosion propagates from another piece of equipment. Isolation systems can
prevent that condition (3.1.14).
2.3.3.2.1 Provide explosion isolation on all connections of vessels (or vessel groups) designed to contain
the explosion pressure (explosion-resistant design).
2.3.3.2.2 Provide explosion isolation on all connections between vessels (or vessel groups) individually
protected by explosion venting (or other mitigation method) but subject to unacceptable property damage
or business interruption by a propagating event.
2.3.3.3 Isolation Systems
This section includes acceptable forms of explosion isolation and the features needed to ensure the systems
serve as effective flame barriers.
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