26 B ULK D ISTRIBUTOR
Road Tanker Earthing
Taking care
of tankers
May/June 2019
vast majority of fl ammable vapours and
combustible dusts can be ignited at these energy
levels.
For sparking to occur in road tanker product
transfer operations, other conductive objects must
come into close proximity with the charged
container of the road tanker.
Examples of conductive ‘objects’ include the fi ll
pipe entering the opening on the top of the
container, fall prevention systems like folding
stairs, and drivers or operators working around
the road tanker. The charges on the road tanker’s
container attract opposite charges to the surface
of the object and rapidly create an electric fi eld
between their respective surfaces.
250
200
150
100
50
0
0
20
40
60
80
100
Charging Time (seconds)
120
Fig 1 Levels of voltage generated on a road tanker, by an electrostatically
charged liquid at approved fl ow rates
T
he loading and unloading of road tankers
with fl ammable and combustible
products, presents one of the most serious
fi re and explosion risks for site operations
within the hazardous process industries.
A study conducted by the American Petroleum
Institute (API) in 1967 identifi ed static discharges
as being responsible for over 60 incidents in road
tanker loading operations and demonstrates just
how long this potential threat has been
acknowledged. The natural presence of static
electricity in product transfer operations,
combined with its associated ignition hazards,
ensures that regulators take static control
precautions for road tankers very seriously.
Powders and liquids with low electrical
conductivities are the prime sources of static
charge generation because their electrical
properties do not easily permit the transfer of
excess charges. Instead, non-conductive and semi-
conductive liquids and powders retain and
accumulate charges after they make contact with
conductive objects.
The most common interface for charging of
non-conductive and semi-conductive product is
contact with metal plant equipment including
pipes, fi lters, pumps, valves, barrels, totes, mixers
and agitators. When the electrostatically charged
liquid (or powder) is deposited into a container
like a barrel, tote, or road tanker charging of the
container will occur if there is nowhere else for
the charges to go. In this situation the charges are
‘static’, accumulate on the surface of the
container and set up a potential difference with
respect to ground.
Over a short time period (less than 20 seconds)
potentials in excess of 50,000 volts can be
induced on a road tanker’s container when it is
being fi lled at normal fl ow rates with a product
that is electrostatically charged. The magnitude of
the voltage induced is directly proportional to the
quantity of charges making contact with the
container.
This voltage represents the ignition source and
the potential energy available for discharge via a
static spark at voltage levels of 50 kV can, for a
typical road tanker, be in excess of 1250 mJ. The
12000
Stored
Mike O’Brien, managing director for Newson Gale, outlines the
hazards of static charge when loading and unloading road tankers
and the precautions users should take
It is the strength of this electric fi eld that causes
the ‘breakdown’ of the air between the container
and the object. When the air is ‘broken down’ a
conductive path for the excess charges to rapidly
discharge themselves is created, leading to a static
spark discharge.
If a combustible atmosphere is present in this
space, ignition of the atmosphere is very probable.
Under ambient conditions an average fi eld strength
of 30 kilo-volts is capable of causing the electrical
breakdown of air over a spark gap of 2 cm.
10000
8000
6000
4000
2000
0
0
10
20
30
40
50
60
Charging Time (seconds)
70
Fig. 2 Potential minimum ignition energies present on road tankers
based on the time period of road tanker fi lling operations
Standards and
recommended practice
As outlined earlier, regulators are extremely
cautious about the ignition hazards presented by
static electricity in road tanker product transfer
operations. Three standards, in particular, provide
clear guidance on what precautions should be
taken. NFPA 77, API RP 2003 and IEC 60079-32
state that grounding of the road tanker should be
the fi rst procedure carried out in the transfer
process. Grounding effectively creates an electrical
circuit that connects the road tanker to the earth
and it is this connection to earth which prevents
static charges accumulating on the road tanker’s
container. The reason the charges can transfer
from the road tanker to earth is because the earth
has an infi nite capacity to absorb and redistribute
static charges, with the positive effect of removing
the ignition source from a potentially combustible
atmosphere.
The electrical resistance of this circuit from the
road tanker to the ‘ground source’ (or ‘grounding
point’) which is in contact with the earth, is a key
performance indicator of the entire grounding
circuit’s capacity to provide a secure and safe
product transfer operation. NFPA 77 and API RP
2003 state the resistance in a healthy metal circuit
should never exceed 10 ohms, therefore the entire
circuit between the truck and grounding point
should be measured and be equal to, or less than,
10 ohms.
If a resistance above 10 ohms is measured this
will indicate problems with parts of the grounding
circuit including the road tanker connection, the
ground point connection or the condition of the
conductor cable.