DOE-HDBK-1015/2-93
Hazards of Chemicals and Gases
FLAMMABLE AND COMBUSTIBLE LIQUIDS
Rev. 0
CH-05
Page 37
Petroleum liquids are capable of building up electrical charges when they (a) flow through
piping, (b) are agitated in a tank or a container, or (c) are subjected to vigorous mechanical
movement such as spraying or splashing. Proper bonding or grounding of the transfer system
usually dissipates this static charge to ground as it is generated. However, rapid flow rates in
transfer lines can cause very high electrical potentials on the surface of liquids regardless of
vessel grounding. Also, some petroleum liquids are poor conductors of electricity, particularly
the pure, refined products, and even though the transfer system is properly grounded, a static
charge may build up on the surface of the liquid in the receiving container. The charge
accumulates because static electricity cannot flow through the liquid to the grounded metal
container as fast as it is being generated. If this accumulated charge builds up high enough, a
static spark with sufficient energy to ignite a flammable air-vapor mixture can occur when the
liquid level approaches a grounded probe or when a probe is lowered into a tank for sampling
or gaging.
This high static charge is usually controlled by reducing the flow rates, avoiding violent splashing
with side-flow fill lines, and using relaxation time, which allows time for the static charge to
discharge.
When flammable liquids are transferred from one container to another, a means of bonding
should be provided between the two conductive containers prior to pouring, as shown in
Figure 8.
In areas where flammable liquids are stored or used, hose nozzles on steam lines used for
cleaning should be bonded to the surface of the vessel or object being cleaned. Also, there
should be no insulated conductive objects on which the steam could impinge and induce a static
charge accumulation.
Nonconductive materials, such as fabric, rubber, or plastic sheeting, passing through or over rolls
will also create charges of static electricity. Static from these materials, as well as static from
the belts, can be discharged with grounded metal combs or tinsel collectors. Radioactive
substances and static neutralizers using electrical discharges are also employed for this purpose.
Bonding and grounding systems should be checked regularly for electrical continuity. Preferably
before each fill, the exposed part of the bonding and ground system should be inspected for parts
that have deteriorated because of corrosion or that have otherwise been damaged. Many
companies specify that bonds and grounds be constructed of bare-braided flexible wire because
it facilitates inspection and prevents broken wires from being concealed.
Electricity becomes a source of ignition where flammable vapors exist if the proper type of
electrical equipment for these atmospheres either has not been installed or has not been
maintained.
