77
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2000 Edition
7.9 Marine Vessel and Barge Cargo Tanks.
Marine vessel and
barge cargo tanks are beyond the scope of this recommended
practice. The recommendations given in the International Safety
Guide for Oil Tankers and Terminals (ISGOTT) should be fol-
lowed.
7.10 Process Vessels.
7.10.1 Means of Static Electric Charge Accumulation.
Accumulation of static electricity in process vessels occurs by
the same methods as described in Section 7.5 for tanks. Where
a conductive and a nonconductive liquid are to be blended,
the conductive liquid should be added to the vessel first, if pos-
sible, so that the conductivity of the mixture is as high as pos-
sible throughout the mixing process. Re-entry of recirculation
loops should be designed to minimize splashing and surface
disruption, for example, by use of subsurface jets that do not
break the liquid’s surface.
7.10.2* Procedures for Transfer to Tanks.
When introduc-
ing two or more nonconductive liquids to a blending tank, the
less dense liquid should be loaded first to avoid a surface layer
comprising the lighter, more highly charged component.
Splash recirculation should normally be done only if the vessel
is inerted or vapor enriched.
7.10.3 Agitation.
Agitators should be covered with sufficient
depth of liquid before being operated, to minimize splashing,
or should be operated at reduced speed until sufficient depth
has been achieved. In cases where hazardous charge accumu-
lation cannot be avoided using the measures discussed in this
section, the vessel can be inerted.
7.10.4 Vessels with Nonconductive Linings.
The accumula-
tion of static electric charge can result in pinhole damage to
equipment such as enamel- or glass-lined reactors. Since static
electric discharges often occur at the liquid interface as liquid
drains from the wetted wall, a vapor ignition hazard could also
exist. In some cases, it is possible to specify static dissipative
coatings for the vessel or agitator. It is recommended that con-
ductive vessels and appurtenances be bonded and grounded.
In some cases, inerting might be necessary.
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