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8/16/2019 pbei_20130501_00C2 (1)
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Static electricity
A SK AN E XPERT
John A. Constance The Engineers Collaborative
Get your questions answeredIf you have a question about bulk solids handling or processing, send itto us and we’ll find an expert to provide an answer. Email your questions to editor Terry O’Neill, [email protected].
We need your expertise.If you’re an independent consultant or academic and would like to share your expertise, please contact editor Terry O’Neill, [email protected] information about your experience in the dry bulk solids industry.
QWhat can we do about sta-
tic electricity? Do we need
to ground plastic and metal
ductwork in our pneumatic conveying
lines and dust collection system? Do we
need to ground our dust collector bag
filters?
A To prevent fires and explo-
sions caused by static elec-
tricity, you’ll need to bond
and ground pneumatic conveying lines
and dust control system lines or other
components if the powder and dust being
handled is combustible. Bonding is the
practice of connecting two or more con-
ductive objects (such as metal pipe and
ductwork sections) together by a conduc-
tor so that they are at the same electrical
potential. Grounding is the practice of
bonding one or more conductive objects
to the ground (earth) by a grounding
strap or other grounding device.
One characteristic of combustibledust is its ignitability when dispersed in
air in a sufficient concentration. A mate-
rial’s ignitability is predicted by its mini-
mum ignition energy (MIE), the minimum
amount of released energy that will cause
the material to ignite. You can find MIEs
for different materials listed on various
Internet sites; however, you must test
your specific material to obtain an accu-
rate MIE. Various independent labs con-
duct such tests.
Most combustible dusts have MIEs
over 10 millijoules. These dusts can be
easily ignited by sparks and propagating
brush discharges (sparks generated from
insulating surfaces with ground back-
ing). If a gas or vapor is present in a dustcloud, the dust’s ignition by static elec-
tricity is highly likely.
Propagating brush discharges can
generate ignition energies on the order
of thousands of millijoules. Such a brush
discharge can initiate a dust explosion in
a pneumatic conveying system when
you’re conveying a combustible powder:
• Through nonconductive pipe or
duct.• On a nonconductive surface backed
by a conductive surface (a metal pipe
or duct internally coated with an in-
sulating material).
• In a pipe or duct that has plastic or
glass inspection windows.
Plastic pipe and plastic ductwork
shouldn’t be used with combustible ma-
terials in either pneumatic conveying or
dust control systems. Running a groundwire along the length of plastic pipe or
plastic ductwork, whether on the exte-
rior or the interior, won’t dissipate static
electricity. Instead, use metal pipe and
metal ductwork, with all sections se-
curely bonded and grounded. Pneu-
matic conveying lines typically use
compression couplings with grounding
strips built into the gasketing. Be sure
to regularly check the bonding between
metal pipe and duct sections connected
with these couplings to ensure that the
resistance across the connections is less
than 10 ohms.
Dust collected inside a dust collec-
tor has an electrostatic charge, which
will accumulate on the bag filter sur-
faces. All the dust collector’s conductive
components need to be bonded and
grounded and the metal filter cages need
to be securely bonded to the dust collec-
tor to dissipate static electricity. The sta-
tic charge on all the filters will transfer
to the filter cages. If a conductive filter
cage becomes ungrounded, a spark dis-
charge can occur and ignite the dust
cloud inside the dust collector.
You shouldn’t use conductive bag
filters unless you can prove that the fil-
ters are always bonded to the bonded
cages or grounded dust collector. If a sec-
tion of a conductive filter separates from
itself or if a filter falls off the cage, you
could have a problem. The accumulated
static charge on the filter could discharge
to a conductive section of the dust collec-
tor and ignite combustible dust.
John A. Constance
Principal consultant
The Engineers Collaborative
Canandaigua, NY USA
+1 215 300 9563
fax +1 585 393 4956
www.engcollab.com
As appeared in PBEI | MAY 2013 www.pbeinternational.comCopyright CSC Publishing