ERICO Practical Guide to Electrical Grounding - Manual (Page 105)

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Some specific areas where static electricity is generated
include:
Piping Systems - In piping systems, the generation rate and
the subsequent accumulation of static charges are a function
of the materials, the flow rate, flow velocity, pipe diameter
and pipe length.
Filling Operations - The turbulence experienced in filling
operations caused by high flow rates, splashing or the free-
falling of liquids or powder fines and the need to connect
and disconnect hoses, valves and the like increases the
charge accumulation and the chances of a hazardous
charge.
Filtration - Filters, because of their large surface area, can
generate as much as 200 times the electrostatic charge
generated in the same piping system without filters.
Dispersing Operations - Dispersing operations can be
particularly hazardous in view of the extremely high rate of
charge generation when particulates are present. With
poorly conductive materials, the charge accumulation can
cause hazardous sparking in the mixer, such as to an
exposed agitator bar or to a conductive fill pipe in a ball or
pebble mill. High charge generation rates can also occur
when materials are mixed, thinned, combined or agitated.
Methods of Static Control
In addition to being dependent on the charge generation
rate, charge accumulation is a function of the resistance of
the path by which charges dissipate. Within the material,
the dissipation of static electricity is dependent on the
material’s “conductivity.” Some flammable liquids have a
very low conductivity and tend to accumulate static
charges. Toluene, an example of such a liquid, has a long
history of causing industry fires. Lange’s Handbook lists
conductivity data of some pure liquids. Although the
generation of static electricity cannot be eliminated, its rate
of generation and accumulation can be reduced by the
following procedures:
Piping Systems - The most effective method of reducing
the accumulation of static charges in piping systems is
through the proper pipe sizing to keep flow velocities low
and to keep the flow as laminar as possible. The typical
maximum velocity in piping systems is 15 feet per second.
Table 6-2 lists the flow rates for various pipe sizes for a
velocity of 15 feet per second. Each user must determine
the maximum velocity that can be safely allowed.
Filling Operations - Splash filling and free fall of
flammable liquids should be eliminated to the maximum
extent practical by lowering the fill velocities, by providing
diverters to direct the discharge of material down the side of
the grounded vessel being filled or by submerging fill pipes
below the level in the vessel. Submerging of fill pipes may
not always be practical. In bulk filling operations, the
velocity of the incoming liquid typically should not exceed
3 feet per second until the pipe outlet is covered. The
velocity may then be increased to the 15 feet per second
mentioned previously. Table 6-2 also lists the flow rates for
various pipe sizes for the velocity of 3 feet per second.
Filtration - Experience has shown that the static electricity
hazard may be controlled by installing filters far enough
upstream of the discharge point to provide a 30 second
relaxation time period prior to discharge. The relaxation
time depends upon the conductivity, the liquid velocity and
the type of filter. For example, the 30 second relaxation
time may not be necessary with a highly conductive liquid.
Dispersing Operations - For dispersing operations of
solids into liquids, the conductivity of the liquid should be
raised, if necessary, to above 2000 conductivity units
(C.U.), which is 2 x 10
-5 micromho/cm, before particulates
are added. If possible, polar solvents should be added
before non-polar solvents or particulates are added. Polar
solvents are more conductive than non-polar solvents. In
some instances, proprietary anti-static agents, developed for
use with fuels, can be used as additives to reduce the charge
accumulation. Typically, only a few parts per million of the
additive are required. Tests should be conducted to ensure
that the conductivity additive does not cause formulation
problems. The additive may not be suitable for use in
coatings for food containers. If the liquid conductivity
cannot be raised to the recommended value, the vessel
should be inerted (filled with an inert material). For
dispersing solids into solids, contact with the mixing vessel
or agitator is the usual path to ground. Raising the humidity
Chapter 6: Special Grounding Situations
Flow Rates
Table 6-2
Flow Rates
Schedule 40
Pipe Size Flow Rate Flow Rate
(Diameter (GPM at (GPM at
in Inches) 15 Ft/Sec) 3 Ft/Sec)
1 40 8
1 1/2 95 19
2 157 31
2 1/2 224 45
3 346 69
3 1/2 462 92
4 595 119
Grounding Book 4/14/99 10/5/99 6:02 PM Page 97 (Black plate)