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7.1
Generation and Management of Electrostatic Charge
There is the potential to generate electrostatic charge wherever there is liquid flow. The potential rate of
charging increases as the interfacial area between the liquid and the surfaces increase and the flow velocity
increases.
The level of charge accumulation in a liquid, and therefore the electrostatic hazard potential, is strongly
dependent on its electrical conductivity and dielectric constant (relative permittivity).
3
Conductivity is also a
factor of temperature and viscosity; hence the conductivity of a liquid will be lower when at lower temperatures.
Conductivity can be expressed in terms of picoSiemens per metre (pS/m); for liquids with a dielectric constant
around 2 (e.g. hydrocarbons) these can be classified as:
3, 6
High conductivity (>10,000 pS/m) - any static generated within the liquid can be conducted to the
pipe/filter housing and be dissipated safely via the earthing
Medium conductivity (50-10,000 pS/m) - the rate of charge generation is critical to understand how
charge can dissipate.
Low conductivity (<50pS/m) - liquids are unable to dissipate the static charge. Static build-up can occur,
even if the filter housing is earthed.
Hazardous levels of charge accumulation may occur with all liquids, but special attention is required for low
conductivity liquids. Therefore, for liquids having low conductivity properties (<50pS/m) there exists the
possibility that static charge may be accumulated more quickly than it is dissipated, leading to a potential
discharge. Discharge can lead to various issues from pitting to an explosion.
Table 8 is a summary of high-level precautions that may be effective against ignition hazards with liquid handling
operations; refer to the available guidance documents cited.
3
Parker has adopted a hierarchical approach to
reducing the risk of explosion which involves concentrating on the avoidance of a flammable atmosphere as the
basis of safety.
Precaution
Measure
Earthing and avoidance of isolated
conductors
All conductive parts of the filter housing should be adequately
connected to earth and frequently tested to ensure effective
earthing.
People operating the equipment should be earthed.
Restricting charge generation by
controlling
relevant
process
parameters
Reducing linear flow velocity of the liquid.
Effective residence time for charge relaxation using conductive
pipe or conductive relaxation chamber after filtration.
Prevent the occurrence of flammable
atmospheres
Avoidance of vapour spaces through safe and effective
bleeding of the filter housing.
Inerting vapour spaces after handling volatile liquids i.e. flush
with an inert gas.
Promote charge dissipation by
limiting charge accumulation
Bonding components and people to earth
Increasing the conductivity of the liquids (alternative liquids or
use of additives).
Table 9
7.2
High Charging Equipment – Filters and Filter Housings
Liquid flow through filters can produce significant charge densities; the accumulation of charge density
is related to the pore size of filters used. Microfilters (pore size <30µm) often generate very high levels
of charge, therefore, it is essential that:
a)
There is sufficient residence time between the filter and any downstream tank for the excess charge
to dissipate; refer to cited documents for guidance.
3,4,6
b)
That all conductive parts in the filter housings are bonded together and earthed. For all metal
systems a resistance to earth of less than 25 Ohms is deemed acceptable.
3
c)
Ensure that the filter housing and the relaxation chamber, if used, remain full of liquid during normal
operation in order to prevent a flammable atmosphere.
d)
All Parker filter housings must be earthed; electrical continuity is achieved by the mechanical
connection between conductive parts, where the parts are not coated.
