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Static electricity: what’s the big deal?
Let’s start with static electricity itself. The clue is in the term.
Static electricity is, essentially, electricity that is static. It is
electricity that is temporarily “stuck” in the same position. It’s
made of the same “stuff” that powers your refrigerator or
lighting, but its characteristics are different to the line power
delivered to your home or place of work.
In the hazardous process industries, more commonly referred
to as the HAZLOC industries, static electricity is generated
virtually all of the time. Various grades of crude oil, refined
petroleum products like LPG, and a host of chemicals fall into a
category of materials that are often referred to as “static
accumulators”. What this term means is that materials in this
category are known to be powerful attractors of electrons from
other materials and resist “letting go” of electrons they come
into contact with. They “accumulate” static charge.
Static grounding protection
for Tank Cars.
The transportation of flammable products by rail, whether that be as a result of the shale driven boom in crude by rail or the transportation
of chemicals from petrochemical manufacturing centres to end user markets, is still one of the most flexible and cost effective methods by
which to move flammable products across the continent of North America in bulk quantities. And while this industry has a host of safety
and environmental regulations to contend with, one area of safety that is often misinterpreted or misunderstood, is the ignition hazard
associated with static electricity and the measures that can be put into practice to control this risk.
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Author Details:
Mike O’Brien, Managing Director for Newson Gale
If you have any questions relating to the topics discussed
in this article, please contact .Newson Gale
www.newson-gale.com
In a typical Lease Automatic Custody Transfer unit (LACT) or
rack loading operation, the static accumulating product is
transferred from, say, a truck, via the LACT unit or from a storage
tank via a rack loading system into a receiving tank car. We can
refer to the equipment involved in the transfer of product
collectively as the product “transfer system”. As the product
makes its way through the transfer system to the customer side
of the transfer, the molecules in the product become
electrostatically charged.
If the tank car is not grounded contact with the charged product
will cause it to become electrified. If this situation is allowed to
exist and persist throughout the transfer operation, it will present
a potentially serious source of ignition in the presence of
flammable atmospheres.
As the tank car builds up electrostatic charges on its surface, the
voltage present on the tank car rises dramatically in a very short
space of time. Because the tank car is at a high voltage, it is
seeking to find ways of discharging this excess potential energy
and the most efficient way of doing this is to discharge the
excess electrons to objects at a lower potential in the form of a
spark. The best object to discharge to is the Earth or an object
with a direct connection to it, i.e. something that is grounded.
This is because the Earth can absorb an infinite amount of
charge due its size and mass. This is what happens during an
electrical storm, where lightning strikes result from huge
potential differences that are present between the Earth and the
layer of storm clouds above its surface. Static electricity is no
different; it’s the same stuff, the only difference being the
amount of electrons being discharged via a static spark or via a
lightning strike.
Energy discharged in static sparks.
Grounded objects that are in close proximity to charged objects
are good targets for electrostatic sparks and permitting the
uncontrolled accumulation of static electricity in a HAZLOC
atmosphere is no different to having an engine’s spark plug
exposed to a potentially flammable atmosphere.
The magnitude of the energy present during the discharge of a
static spark is a product of the capacitance of the tank car and
the voltage present on the tank car at the time the spark is
discharged. The electrostatic voltage that is present on the tank
car is a combination of the charging current generated by the
flow of the liquid, the capacitance of the tank car and the tank
car’s isolation from ground.
Increased flow rates and turbulence can increase the size of the
charging current, but even when safe recommended flow rates
are taken into consideration, if the transfer system is not
grounded, the electrostatic voltage of the tank car can build up
to hazardous levels in less than 20 seconds. Table 1 illustrates
how much energy can be discharged by a spark from a tank car
charged to 20,000 volts.
Page 2 of 6
Static grounding protection
for Tank Cars.
TankCar charged to 20,000
volts
Potential Spark Energy
(mJ)
Tank car 1000
Fig. 1 The MIEs of common petroleum products. (Source: NFPA 497)
0.25
0.068
0.24
0.28
0.25
0.2
0.24
0.23
0
0.05
0.1
0.15
0.2
0.25
0.3
propane hydrogen sulfide ethane methane n-butane benzene toluene ethanol
Minimum Ignition Energies (mJ)
M
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E
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y
(
m
J
)
Table 1. Potential energy of sparks from Tank Cars
www.newson-gale.com
When the energy of the sparks discharged by static electricity is
compared with the minimum ignition energies of a wide range of
both raw and refined petroleum sourced products, it’s easy to
see why the tank car and any equipment connected to it, like
flexible hoses and piping, should be bonded and grounded.
As can be seen in Table 1, isolated tank cars can discharge
sparks with a huge amount of energy. At these static spark
energy levels the prevention of electrostatic shocks to workers
is an important safety consideration. Involuntary physiological
reactions caused by electrostatic shocks could lead to trips and
falls and could be particularly hazardous when personnel are
working above ground level.
Of the several factors that contribute to static charging, the one
variable that must definitely be controlled is the grounding of the
tank car. Grounding the tank car ensures that the tank car’s
resistance to the general mass of the earth is maintained at a
level that does not impede the safe transfer of static charges
from the tank car to ground.
Grounding and bonding is not simply achieved by connecting
alligator clips on wires back to the loading rack or LACT unit.
Because of the serious ignition hazard static electricity presents
to a wide range of HAZLOC operations, there are industry
guidelines in place designed to control the risk of fires and
explosions caused by static electricity.
Industry codes of practice related to the static
grounding of tank cars in HAZLOC atmospheres:
Grounding is the act of ensuring the tank car is connected to an
object that has a verified connection to the general mass of the
earth. Article 250 of NFPA 70, the National Electrical Code,
describes these points as “ground electrodes”. Typical ground
electrodes are objects that encompass metal rods buried up to
8 ft. in the ground, pipes in direct and continuous contact with
the earth for more than 20 ft. and building structures, like
loading racks, which should be grounded for electrical fault
protection and lightning protection purposes. Grounding
provides a continuous and uninhibited path for charges
generated during the transfer operation to flow to Earth, via the
object performing the function of a ground electrode.
The National Fire Protection Association and the American
Petroleum Institute each publish codes of practice for
controlling the risks associated with static electricity in
hazardous locations. NFPA 77 “Recommended Practice on
Static Electricity” (2014) and API RP 2003 “Protection
Against Ignitions Arising Out of Static, Lightning, and Stray
Currents” (2008) are publications written by committees of
HAZLOC industry professionals who are recognised experts in
the area of static control for hazardous locations.
Understanding and implementing the recommendations of
these guidance documents will ensure the risk and liability
attached to static hazards is well controlled.
What is clear from the recommendations of NFPA 77 and API RP
2003 (see extracts below) is that 10 ohms in the grounding and
bonding circuit is the maximum resistance recommended for
equipment at risk of electrostatic charging in HAZLOC
atmospheres. While API RP 2003 goes one step further in
recommending 1 ohm or less, if a grounding system with signal
lights is in use, 10 ohms is satisfactory. This is because the
grounding system is continuously monitoring the resistance in
the grounding circuit, so that if it rises above 10 ohms, the
grounding system can signal this potential hazard to the
operator of the loading rack or LACT unit. Another important
recommendation is to use interlocks wherever possible, to
ensure the transfer does not take place if grounding is not
present. By halting the movement of product, the charge
generation source is eliminated thus preventing additional
charging of the tank car. The relevant recommended practice
extracts from both codes of practice are provided below.
NFPA 77, section 7.4.1.3.1, “Charge dissipation” states:
“Where the bonding/grounding system is all metal,
resistance in continuous ground paths typically is less
than 10 ohms…..Greater resistance usually indicates
that the metal path is not continuous, usually because of
loose connections or corrosion”.
NFPA 77, sections 10.1.1 and 10.1.2., “Metal Piping
Systems” state:
“All parts of continuous all-metal piping systems should
have a resistance to ground that does not exceed 10
ohms”.
“Bonding wires might be needed around flexible, swivel,
or sliding joints. Test and experience have shown that
resistance in these joints is normally below 10 ohms,
which is low enough to prevent accumulation of static
charges.”
Page 3 of 6
Static grounding protection
for Tank Cars.
www.newson-gale.com Page 4 of 6
Static grounding protection
for Tank Cars.
API RP 2003, section 4.3.2 “Tank Car Loading” states the
following:
“Many tank cars are equipped with nonconductive
bearings and nonconductive wear pads located between
the rail car and chassis. As a result, the resistance from
the tank car compartment to ground through the rails
may not be low enough to prevent the accumulation of an
electrostatic charge on the tank car body. Therefore,
bonding of the tank car body to the fill system piping is
necessary for protection against static accumulation.”
NFPA 77, section 12.4.2 “Railroad Tank Cars” reflects the
guidance in section 4.3.2 of API RP 2003 but provides the
additional recommendation:
“In general, the precautions for railroad tank cars are
similar to those for tank vehicles specified in Section
12.2.”
NFPA 77, section 12.2 “Summary of Precautions for Loading
Tank Vehicles” states:
“Bonding and Grounding. Tank trucks should be bonded
to the fill system, and all bonding and grounding should
be in place prior to starting operations. Ground
indicators, often interlocked with the filling system,
frequently are used to ensure bonding is in place.”
API RP 2003, section 4.2.2 “Bonding and Grounding” states
the following:
“The entire bond circuit, including clamps and
connectors, should be included in the continuity test.
Bond circuit resistance should typically be
1 ohm or less. Resistances less than 10 ohms may
function satisfactorily, but test results showing
resistances over 1 ohm may be a “warning sign” to
prompt further testing or physical examination to ensure
that there are no incipient bonding discontinuities (such
as damaged wire, loose connection or paint under a
bonding screw). Bond or ground indication instruments
are available for installation at truck loading racks to
monitor the bond connection. These systems can be
operated in conjunction with signal lights or can be
electrically interlocked with the control circuits to prevent
the loading pumps from being started when a good bond
is not present.”
In summary the precautions recommended for grounding tank
cars should be similar to the recommendations for grounding
tank trucks.
Specifying a Static Grounding System for your
operations.
One of the main problems with static electricity is that it is not
something the operators can see, smell or hear. This
characteristic of static electricity can, unfortunately, promote an
attitude of “it can’t happen to me” or “it doesn’t exist” amongst
personnel operating LACT units and loading rack systems. A
grounding system that combines a simple visual “GO / NO GO”
communication via a traffic light model of indication with
interlock control capability is the most effective means of
controlling the risk of ignitions caused by static electricity during
tank car product transfer operations. Interlocking the transfer
system with the grounding system is probably the ultimate layer
of protection equipment specifiers and designers can take to
ensure the tank car is grounded.
To demonstrate compliance with NFPA 77 and
API RP 2003 source grounding systems that:
1. Monitor the grounding circuit to 10 ohms or less.
2. Provide operators with a visual reference that indicate a “GO
/ NO GO” action via RED and GREEN indicators.
3. Provide dry contacts to interlock the grounding system with
the LACT unit / loading rack pumps or PLCs.
4. Display the full range of HAZLOC approvals with the mark of
a recognised NRTL.
Point 4 is of particular importance to the HAZLOC professional
tasked with protecting workers and the business from fires
caused by static electricity. When a grounding system is to be
specified it is important to anticipate what gases and vapors
could be present at the grounding system’s installation.
Grounding systems that display Class I, Div.1 approvals for gas
groups A, B, C, D are systems that can be mounted in any
hazardous location.
www.newson-gale.com Page 5 of 6
Static grounding protection
for Tank Cars.
Be careful to differentiate where the XP controller can be
installed and where the ground monitoring circuit, delivered by
the clamp, is approved for use. Specifiers can easily
misinterpret HAZLOC Certificates of Compliance (CoCs) and
think that because the ground monitoring circuit is certified as
being suitable for all gas groups, the same applies to the XP
controller. Very often, this is not the case as the XP controller can
only be mounted in lower gas group atmospheres, e.g. groups
C and D.
®Newson Gale recommends the Earth-Rite PLUS for bonding
tank cars to LACT units and loading rack superstructures. Along
with demonstrating the full range of Class I, Div. 1 approvals for
all gas and liquid vapor groups, it also ensures there is a 10
ohm, or less, connection between the tank car and the product
transfer system. This provides equipment specifiers with the
abi l i ty to demonstrate ful l compliance with the
recommendations of NFPA 77 and API RP 2003. By simply
®connecting the grounding clamp to the tank car the Earth-Rite
PLUS automatically verifies if the tank car is connected to the
LACT unit or loading rack by delivering an Intrinsically Safe
monitoring circuit to the system’s Factory Mutual approved
grounding clamp. The Factory Mutual approved stainless steel
heavy duty grounding clamp ensures that a strong initial
connection, via a pair of tungsten carbide teeth, is made to the
tank car, and is then maintained for the duration of the product
transfer operation, resisting movement caused by vibration or
accidental dislodging.
Unlike standard grounding systems that rely on the their non-
monitored electrical ground connection to dissipate the static
®charges generated by the transfer, the Earth-Rite PLUS
ensures that its dedicated static grounding connection to the
LACT unit or loading rack is always monitored, via the static
ground connections G1 and G2 (ref. Fig. 2). This ensures there
is a monitored connection directly between the tank car and the
LACT unit or loading rack. This is an important feature as we are
depending on the LACT unit / loading rack’s verified ground
connection to dissipate static charges from the tank car to the
general mass of earth.
Loading Rack
Superstructure
NOTE: Loading Rack Superstructure
assumed to be grounded.
Newson
Gale
G1 G2
2 dry contacts (N/O or
N/C) to interlock with the
loading rack or LACT
unit’s pump.
10 ohms or less between tank car
and loading rack / LACT unit.
True Earth : the target destination for static
2 tungsten carbide teeth
on FM approved heavy
duty clamp.
®Fig. 2 Typical Earth-Rite PLUS tank car loading rack installation. The tungsten carbide teeth on the grounding clamp are the
contact points for the signal that verifies if the tank car is bonded to the loading rack (or LACT unit) to 10 ohms or less.
Static grounding protection
for Tank Cars.
®When the Earth-Rite PLUS verifies the tank car is bonded to
the LACT unit or loading rack superstructure, a cluster of
attention grabbing green LEDs pulse continuously to inform
the operator that the system is actively monitoring the integrity
of the ground loop.
A pair of dry contacts can be interlocked with the power
delivered to the pump or PLCs to halt the product transfer
®operation if the Earth-Rite PLUS detects a resistance of more
than 10 ohms in the ground loop between the tank car and the
product transfer system.
®The Earth-Rite PLUS can be powered off a 110 or 230 volt AC
source or 24 volt / 12 volt DC source and is cCSAus approved
Class I, Div.1 for gas groups A, B, C, D and all combustible dust
and fibre groups.
Codes of Practice:
NFPA 70, the National Electrical Code.
NFPA 77, Recommended Practice on Static Electricity.
API RP 2003, Protection Against Ignitions Arising Out of Static,
Lightning, and Stray Currents.
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®Fig. 3 Earth-Rite PLUS ground status indicators
pulse continuously when grounding is in place.
www.newson-gale.com
United Kingdom
Newson Gale Ltd
Omega House
Private Road 8
Colwick, Nottingham
NG4 2JX, UK
+44 (0)115 940 7500
[email protected]
Deutschland
IEP Technologies GmbH
Kaiserswerther Str. 85C
40878 Ratingen
Germany
+49 (0)2102 5889 0
[email protected]
South East Asia
Newson Gale S.E.A. Pte Ltd
136 Joo Seng Road, #03-01
Singapore
368360
+65 6704 9461
[email protected]
United States
IEP Technologies, LLC
417-1 South Street
Marlborough, MA 01752
USA
+1 732 961 7610
[email protected]
Static grounding protection for Tank Cars
Static electricity: what’s the big deal? Let’s start with static electricity itself. The clue is in the term. Static electricity is, essentially, electricity that is static. It is electricity that is temporarily “stuck” in the same position. It’s made of the same “stuff” that powers your refrigerator or lighting, but its characteristics are different to the line power delivered to your home or place of work.
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