Static Electricity 2000 Edition

7.4.5.2 Strainers.
Mesh strainers finer than 150 
µm should be
treated as microfilters. Mesh strainers coarser than 150 
µm
can also generate significant static electric charge when fouled
with accumulated debris. If such coarse strainers are used in
services where debris can be expected to accumulate, then
these strainers should also be treated as microfilters.
7.4.5.3 Polishing Filters.
A polishing filter is sometimes
placed at the end of a delivery line to remove debris. This filter
might be a bag installed on the end of a hose and directly
exposed to the vapor in the tank. Filters used in flammable liq-
uid service should be enclosed in grounded metal housings.
7.4.6 Suspended Material.
Immiscible and marginally solu-
ble liquids and slow-dissolving solids can disperse as droplets
or as an emulsion. (For a discussion of examples of such situations
in mixing operations, see 7.10.1.) Where a nonconductive liquid
contains a dispersed phase, such as water in oil, the continu-
ous phase determines the charge relaxation behavior. Charge
generation is typically greater for such suspensions than that
for a single phase.
7.4.7 Miscellaneous Line Restrictions.
Piping system compo-
nents such as orifice plates, valves, elbows, and tees increase
turbulence and can increase the rate of charge generation.
Brief contact with a plastic component, in particular, can
cause significant charge generation. Suspended material such
as water (see 7.4.6) has also been found to increase this effect.
7.5 Storage Tanks.
7.5.1 General.
Liquid flowing into a tank can carry a static
electric charge that will accumulate in the tank. This charge
can be detected as a potential above the surface of the liquid
in the tank. The maximum surface potential attained depends
not only on the charge density of the incoming liquid but also
on the dimensions of the tank. For commercial tanks of equal
volume, the maximum potential will be greater in tanks hav-
ing smaller cross-sectional areas, because liquid depth
increases faster relative to the rate of charge relaxation.
Smaller potentials will therefore be generated in, for example,
a near-rectangular barge tank than in a cylindrical vertical
tank of the same volume.