Static Electricity 2000 Edition

A.7.5.2.1(c)
See API 2003, Protection Against Ignitions Arising Out
of Static, Lightning, and Stray Currents, for further information.
A.7.7
See API Publication 2219, Safe Operation of Vacuum
Trucks in the Petroleum Service, for general recommendations.
A.7.10.2
See NFPA 69, Standard on Explosion Prevention Systems,
for additional information.
A.7.13.7
See NFPA 326, Standard for the Safeguarding of Tanks
and Containers for Entry, Cleaning, or Repair.
A.7.14
If used for flammable liquid spills, which could involve
a second phase such as spill control granules or debris, wet-dry
vacuum cleaners pose a number of problems, including the
following:
(1) Generation of static electricity
(2) Electrical classification of powered equipment
(3) Chemical compatibility
(4) Industrial hygiene (relative to the exhaust from the
vacuum cleaner)
Commercial machines for Class I, Group D, and Class II,
Groups E, F, and G atmospheres are typically air-operated via
a venturi, so they contain no electrical power. Air supply and
liquid recovery hoses should be conductive and constructed of
semiconductive fabric. Filters are also semiconductive or con-
ductive. The design is such that all parts are continuously
bonded and grounded. Normally ground continuity at pre-
scribed checkpoints is established before each use. Floats or
similar mechanisms are employed to shut off suction once the
recovery tank has reached capacity level. Additional precau-
tions might be needed to avoid overflow via siphoning (if the
recovery hose is completely submerged in liquid) or when
defoaming agents are not used. For flammable liquid spills in
particular, measures should be taken, including training and
personnel grounding, to ensure personnel are not a source of
ignition.
A.8.2.4
The MIE of a dust cloud is determined using a sample
that is representative of the dust in a process. The equipment
and procedures used over the years have been many and var-
ied. Therefore, MIE data might not be comparable from one
data set to the next. Furthermore, the conditions under which
laboratory data are acquired can be different than that of the
process being examined. For these reasons, comparisons of
MIE data are sometimes qualitative rather than quantitative.
Nevertheless, comparisons can be quite useful.
The MIE of a dust decreases with decreasing particle size
and with increasing temperature. The MIE could increase
with increasing moisture content of the dust. The MIE of a
dust varies little with the humidity of the supporting atmo-
sphere, excluding problems with hygroscopic dusts. The fac-
tors affecting MIE should be considered in a hazards analysis
of a process.
A.8.7
Some flexible hoses can be cut to length and put into
service by simply slipping them over a pipe with a hose clamp.
It is important that the spiral wires be in good metal-to-metal
contact with the pipes in order to maintain a proper ground
of the spiral. This contact can be done by stripping the spiral
and bending it under the hose next to the pipe and under the
clamp. In cases where hoses with metal spirals are connected
to plastic pipes, the spirals should be independently
grounded.
The reason for discouraging the use of hose with more
than one spiral is that, if one of the spiral wires is broken in
such a way that it is disconnected from ground, it can become
a source of spark gap ignition.