September/October 2017
B ULK D ISTRIBUTOR
Static Protection
Where there’s a spark
Mike O’Brien looks at a recent case of how a spark from an isolated hose ignited a
combustible powder atmosphere
S
tatic electricity is often perceived as an invisible risk. This
case study explains why static electricity provides an
ignition source for serious fires and explosions that could
occur during everyday operations involving the handling and
processing of flammable products.
A company supplying aluminium powder had an order cancelled
when the bulk truck transporter carrying the powder was en route
to a railcar hopper loading station. The truck driver was instructed
to return the aluminium to the plant from where it was
manufactured. As this scenario had never occurred before there was
Improvisation
no standard operating procedure in place to offload the aluminium
from the truck back into the production facility. Shortly after the
operators had worked out how to overcome some practical
challenges for moving the powder back into the plant, an explosion
occurred which propagated throughout the plant.
On return to the plant it was noted that there was no direct
loading point for the finished powder to be injected back into the
production stream directly from the truck. A decision was taken to
convey the powder into the pneumatic transport system entry point
of the plant using the 3ins hoses on the truck.
Unfortunately, the hoses could not reach the entry to the plant’s
pneumatic system so an additional length of hose from the plant
was added to the line of 3ins hoses running from the truck. Both
hose types were constructed of rubber tubing that contained
internal metal helixes which ensured the hose flanges were
electrically bonded together.
Well grounded?
The truck was grounded so the 3ins hoses (assuming they were in
good condition) were also grounded, hence the risk of an
accumulation of electrostatic charge on the truck and hoses was
minimal. One issue that was encountered, however, was that the
plant hose that was used to complete the distance between the
truck’s hoses and the entry to the plant’s pneumatic system was
wider in diameter to the truck’s hoses. This meant that a sealed
connection between the hoses could not be made.
The operators overcame this issue by stuffing rags into the gap
between the hose flanges (Fig 1). This had the effect of electrically
isolating the plant hose from the truck’s hose, potentially impeding
the transfer of electrostatic charges from the plant hose to ground
via the grounded truck. The other end of the hose was assumed to
have been resting on the concrete floor inside the plant. The other
issue was that the density of the air-powder phase coming from the
truck was above the minimum explosive concentration of the
aluminium powder.
The isolated hose was estimated to have a capacitance of 30 pico-
farads and a modest value for the streaming current generated by
the transfer of the powder aluminium was estimated to be in the
region of 10 micro-amps. The resistance to ground of the isolated
hose’s flange, resting on the concrete floor, fell into a range of
between 1010 ohms and 109 ohms. The voltage (V=RI) on the
isolated hose, based on the upper and lower limits of resistance,
was estimated to fall between 100 kilovolts and 10 kilovolts.
The energy, W, from a spark discharge based on the capacitance
and voltage on the isolated hose (W = 0.5 CV2), would have been
in the range of 150 millijoules to 1.5 millijoules. Hence a spark from
the isolated hose would have been likely to have had the capability
to ignite an explosible concentration of the aluminium dust.
As the operators needed to impro