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Static electricity

 A SK AN   E XPERT 

 John A. Constance The Engineers Collaborative

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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

 jac@engcollab.com

www.engcollab.com

 As appeared in PBEI | MAY  2013   www.pbeinternational.comCopyright CSC Publishing