Static Electricity 2000 Edition

particularly non-polar hydrocarbons, are the result of trace
contaminants that are present in the liquid, sometimes in con-
centrations less than 1 part per million (ppm). Thus, indus-
trial liquids can become more or less conductive by orders of
magnitude, depending on the concentration of contaminants
that result from process, storage, and handling practices.
Conductive liquids that at first could appear to be safe can
present a significant hazard if isolated from ground by an insu-
lating container or if suspended in air. When isolated, essen-
tially all charge on the conductive liquid can be released as an
incendive spark. When suspended as a mist, significant static
electric fields can lead to incendive brush discharge.
7.3.3.1
In the petroleum industry, for tank loading and distri-
bution operations involving petroleum middle distillates, liq-
uids in the semiconductive category are handled as conductive
liquids. Such procedures are possible because regulations pro-
hibit use of nonconductive plastic hoses and tanks and mul-
tiphase mixtures and end-of-line polishing filters are not
involved.
7.3.3.2
In general chemical operations, semiconductive liq-
uids represent a distinct category in which the tendency to
accumulate charge varies greatly with the operation and with
liquid conductivity. These operations can involve multiphase
mixtures, nonconductive tank linings, and microfilters, all of
which promote charge accumulation in equipment.
7.4 Flow in Pipe, Hose, and Tubing.
7.4.1* Metal Piping Systems.
All parts of continuous all-
metal piping systems should have a resistance to ground that
does not exceed 10 ohms. A significantly higher resistance
could indicate poor electrical contact, although this will
depend on the overall system. For flanged couplings, neither
paint on the flange faces nor thin plastic coatings used on nuts
and bolts will normally prevent bonding across the coupling
after proper torque has been applied. Jumper cables and star
washers are not usually needed at flanges. Star washers could
even interfere with proper torquing. Electrical continuity of
the ground path should be confirmed after assembly and peri-
odically thereafter.
Bonding wires might be needed around flexible, swivel, or
sliding joints. Tests and experience have shown that resistance
in these joints is normally below 10 ohms, which is low enough
to prevent accumulation of static charges. However, it is rec-
ommended that the manufacturer’s specifications be checked
or that these joints be inspected, because a few are fabricated
with insulating surfaces. When painted, slip flanges (lap
joints) using nonconductive gaskets can cause loss of continu-
ity in the grounding path. This loss of continuity can be reme-
died by using a conductive gasket, such as a flexible, graphite-
filled, spiral-wound gasket or by installing a jumper wire across
the joint.
It should be ensured that bonding and grounding do not
compromise sections of pipe that are supposed to be isolated.
For example, insulating flanges could have been installed to
Pipe
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Current carried by
liquid emerging
from pipe
Counter current of the
charges flowing in
earth connection
(a) Flow (pipe charging)
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No stirring, no charge
(b) Agitation/stirring
Stirring; charges on liquid
and opposite charges on
solids but not in equal
amounts
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FLAMMABLE AND COMBUSTIBLE LIQUIDS AND THEIR VAPORS
77
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2000 Edition
avoid arcs from stray current or from cathodic protection sys-
tems, which provide a separate ground path.
Figures 7.4.1(a) and (b) provide guidance in estimating
the charge on a nonconductive liquid flowing through a
smooth pipe.