(176a) Dust Explosion Hazards: Housekeeping Quantified

DUST-EXPLOSION HAZARDS: HOUSEKEEPING
QUANTIFIED

Richard W. Prugh, PE, CSP

Principal Process Safety Engineer

Chilworth Technology, Inc., a Dekra
Insight Company

609-799-4449; Richard.prugh@dekra.com

Abstract

Limiting
the accumulation of combustible dusts in the workplace is critically important
in the prevention of primary and secondary dust explosions.  Guidance on
accumulation limits began in 1997 by the NFPA in the form of a layer-thickness
criterion.  As stated in NFPA 654, “the hazard area shall include areas where
dust accumulations exceed 1/32 inch (0.8 mm).”  In the 2000 issue, this
criterion was modified to allow for differences in bulk density of the dust,
with the 1/32-inch criterion applying to a bulk density of 75 pounds per cubic
foot.  For lower bulk densities, the dust-layer thickness could be greater.

The
layer-thickness criterion was used in NFPA 499 to determine appropriate
electrical classifications of dusty areas.  In the 2008 issue, a layer
thickness of 1/8 inch was considered sufficiently hazardous to warrant a Class
II, Division 2 requirement.

In
the 2013 issue of NFPA 654, two additional methods for quantitatively
determining whether or not a hazardous amount of dust had accumulated in the
workplace.  Mass Methods A and B were introduced, with examples of the use of
these methods in Annex D of this standard.

In
the 2016 issue of NFPA 652, the term Threshold Housekeeping Dust Accumulation
was introduced, with reference to other NFPA standards for the quantity
limitations.

This
paper compiles the current methods for determining the amounts of dust that
would be hazardous as fuel for flash fire or secondary explosion.  Also
presented is guidance for housekeeping to minimize the explosion hazards and to
provide an environment where general-purpose electrical equipment could be
used, in an “unclassified” area.