•
Substances other than oxygen can act as oxidants. NFPA 68 vent calculations apply only where the oxygen
in air is the only oxidant. If chemical oxidants are present as either solid particulate or gas then explosion
suppression and isolation equipment is required as recommended in NFPA 69.
•
Situations can occur in which it is not possible to provide calculated deflagration venting as described in
NFPA 68. Such situations do not justify the exclusion of all venting. The maximum practical amount of
venting should be provided, since some venting should reduce the damage potential. In addition,
consideration should be given to other protection and prevention methods.
•
It is not possible to successfully vent a detonation.
340.3 Explosive Dusts
•
Solid particulates smaller than 420
ц
m (0.017 in.)(capable of passing through a U.S. No. 40 standard sieve)
are classified as dusts. The fineness of a particular dust is characterized by particle size distribution. The
maximum pressure and
Kst
increase with a decrease in the dust particle size.
•
The combustion properties of a dust depend on its chemical and physical characteristics. The use of
published dust flammability data can result in an inadequate vent design if the dust being processed has a
smaller mean particle size than the dust for which data are available, or if other combustion properties of the
dust differ. Particle shape is also a consideration in the deflagration properties of a dust. The flammability
characteristics of a particular dust should be verified by test.
(See NFPA68 Section B-5.)
340.4 Explosion Vents Operation
•
Venting is one means of limiting the pressure generated in an enclosure by a deflagration. By releasing
expanding gases through an opening engineered for the purpose, it is possible to limit the pressure
generated to a reduced maximum pressure,
Pred,
that is below a pressure that can cause unacceptable
damage to the enclosure.
•
An explosion vent is a pressure-relieving device designed to open at a specific pressure to reduce the
maximum pressure rise during a deflagration in an enclosed vessel to an acceptable level.
•
Vent closures shall be directed toward a restricted area, and the vent closure shall not be a missile hazard.
•
Vents are provided on an enclosure to limit pressure development,
Pred,
to a level acceptable to the user
and the authority having jurisdiction. The level of pressure development can be considered acceptable
where no damage to the enclosure is likely, or where some degree of permanent deformation is tolerable.
•
Shields that deflect the gas and pressure waves exiting a vent during a deflagration are not permitted.
•
Vents should function dependably. Closures should not be hindered by deposits of snow, ice, paint,
corrosion, or debris, or by the buildup of deposits on their inside surfaces. Closures should not be bonded to
the enclosure by accumulations of paint. The materials that are used should be chosen to minimize
corrosion. Clear space should be maintained on both sides of a vent to enable operation without restriction
and without impeding a free flow through the vent.
•
Vent closures should be maintained in accordance with Chapter 10 of NFPA 68 and the manufacturers’
recommendations. The occupant of the property in which the deflagration vent closures are located is
responsible for inspecting and maintaining such devices.
•
The vent opening should be free and clear and should not be impeded. If the vent discharges into a
congested area, the pressure inside the vented enclosure increases. A major blast pressure can be caused
by the ignition of unburned gases or dusts outside the enclosure.
•
Appropriate signs should be posted to provide warning as to the location of a vent. Replacement decals are
available from Farr and are listed in the spare parts section of this manual.
•
A vent closure should be inspected and properly maintained in order to ensure dependable operation. In
some cases, ensuring dependable operation can necessitate replacing a vent closure.
340.5 Return of Filtered Air to the Building
•
Recycling of dust collector exhaust to buildings shall be permitted if the system is designed to prevent both
return of dust with an efficiency of 99.9 percent at 10 microns and transmission of energy from a fire or
explosion to the building. Exceptions apply. Consult the NFPA 68. Section 2.
340.6 Location of Vents and Vented Equipment
•
The fireball and blast pressure that are created by the venting process shall not impinge upon unrestricted
personnel pathways.
•
Where an explosion hazard exists, dust collectors shall be located outside of buildings. Exceptions include
isolation and suppression systems as listed in NFPA 654 section 3.1.1
•
Flames and pressure waves that emerge from an enclosure during the venting process can injure personnel,
ignite other combustibles in the vicinity, result in ensuing fires or secondary explosions, and result in
pressure damage to adjacent buildings or equipment
•
Deflagration vents should not be located in positions that allow the vented material to be picked up by air
intakes.
•
The material discharged from an enclosure during the venting of a deflagration should be directed outside to
a safe location. Property damage and injury to personnel due to material ejection during venting can be
minimized or avoided by locating vented equipment outside buildings and away from normally occupied
areas.
(See NFPA 68 SECTION 3-2.3.)
26
Summary of Contents for Gold Series
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