Static Electricity 2000 Edition

7.5.2 Conductive Fixed-Roof Storage Tanks.
Charge accumu-
lation in the liquid in a tank can lead to static electric discharge
between the liquid surface and the tank shell, roof supports, or
tank appurtenances. The charge generation rate is influenced
by turbulence in the liquid and by the settling of particulate
matter, such as water droplets, iron scale, and sediment.
7.5.2.1 Precautions.
If the vapor space in the tank is likely to
contain an ignitible mixture (e.g., in cases where intermediate
vapor pressure products or low vapor pressure products con-
taminated with high vapor pressure liquids are stored), or
where switch loading is practiced, the following protective
measures should be taken:
(a) Splash filling and upward spraying should be avoided.
The fill pipe should discharge near the bottom of the tank,
with minimum agitation of water and sediment on the tank
bottom.
(b) If possible, the inlet flow velocity should be limited
during the initial stage of tank filling to reduce agitation and
turbulence. The flow velocity of the incoming liquid should
be no greater than 1 m/sec until the fill pipe is submerged
either two pipe diameters or 0.6 m, whichever is less. Since too
low a velocity can result in settling out of water at low points
in the piping, the inlet flow velocity should be kept as close to
1 m/sec as possible during this period. Otherwise, subsequent
re-entrainment of water or other contaminants when the
velocity is increased could significantly increase the product’s
charging tendency.
(c)*For storage tanks greater than 50 m
3
containing liquids
that are either nonconductive or whose conductivity is
unknown, the inlet flow velocity can be increased to 7 m/sec
after the fill pipe is submerged. Where operating experience,
such as in the petroleum industry, has shown that the practice
is acceptable, the inlet flow velocity can be increased above 7
m/sec, but in no case greater than 10 m/sec. See Figures
7.4.1(a) and (b) for determining flowrate.
(d) If the liquid is nonconductive and contains a dis-
persed phase, such as entrained water droplets, the inlet flow
velocity should be restricted to 1 m/sec throughout the fill-
ing operation.
(e) A 30-second minimum residence time should be pro-
vided for liquid to flow between upstream microfilter screens
and the tank (see 7.4.5).
(f) Tanks should be inspected for ungrounded conductive
objects, such as loose gauge floats and sample cans. If these
objects are floating on the liquid surface, they can promote
sparks.
(g) Lines should not be blown out with air or other gases
if the liquid is a Class I liquid or is handled at or above its flash
point. Introducing substantial amounts of air or other gas into
a tank through the liquid can create a hazard due to charge
generation, misting of the liquid, and formation of an
ignitible atmosphere.