Handling Petroleum Products &
Static Ignition Hazards
Part I: Static Electricity and Protection
Part II: Lightning, Stray Current and Protection
By Sullivan D. Curran PE, Executive Director, Fiberglass Tank & Pipe Institute
Introduction
Static electricity in one form or another is a phenomenon of nature and often
results in electrostatic discharges that can cause fires and explosions. While
expertise to reduce these hazards is based on research, in addition there is
much industry experience upon which to base safety precautions when handling
petroleum liquids. This is a two-part paper, the first of which addresses a
basic understanding of static electricity and commonly used precautions used in
the operation of vehicle fueling facilities, tank vehicles, storage tanks,
aviation facilities, and miscellaneous hazards. The second part addresses a
basic understanding of lightning and stray currents, and commonly used
protection against such spark promoters, and includes a list of references for
further study.
Published Research
Static ignition research has been sponsored by such organizations as the
American Gas Association, U. S. Department of the Interior, Bureau of Mines, U.
S. Air Force and the petroleum industry. This research is embodied in
recommendations for protection against static ignition in publications and
videos produced by these and other organizations, such as the American Petroleum
Institute and National Fire Protection Association. However, many of these
recommendations are based on years of practical operating experience in the
petroleum industry.
Static Electric Generation
Electricity was first observed as a phenomenon of nature when small sparks were
observed when some materials were rubbed with silk or wool. Later, when
electricity was found to move freely through conductors, the term static
came about to describe electricity that was trapped on a body that was said to
be charged.
A static electrical charge may be either positive (+) or negative (-) and is
manifested when some force has separated the positive electrons from the
negative protons of an atom. Typical forces include flowing, mixing, pouring,
pumping, filtering or agitating materials where there is the forceful separation
of two like or unlike materials. Examples of static generation are common with
operations involving the movement of liquid hydrocarbons, gases contaminated
with particles (e. g., metal scale and rust), liquid particles (e. g., paint
spray, steam) and dust or fibers (e. g., drive belts, conveyors). The static
electric charging rate is increased greatly by increasing the speed of
separation (e. g., flow rate and turbulence), low conductivity materials (e. g.,
hydrocarbon liquids) and surface area of the interface (e. g., pipe or hose
length, and micropore filters).
Static Electric Accumulation
Electrostatic charges typically leak from a charged body because they are under
the attraction of an equal but opposite charge. Thus, most static sparks are
produced only while the generating mechanism is active. However, some refined
petroleum products have insulating qualities and the charges generated during
movement will remain for a short period of time after the product has stopped
moving. This accumulation, rather than dissipation, is influenced by how well
the bodies are insulated with respect to each other. Since air or air/vapor
mixtures are often the insulating body between the opposite charges, both
temperature and humidity are factors in this insulation. Thus, very low or high
temperatures, with resulting low humidity, will increase the accumulation of the
electrostatic charge both while it is being generated and during the normal
relaxation period.
Spark Gap
A spark results from the sudden breakdown of the insulating strength of a
dielectric (e. g., air) that separates two electrodes of different potentials.
This breakdown produces a flow of electricity across the spark gap and is
accompanied by a flash of light, indicating high temperature. For static
electricity to discharge a spark, the voltage across the gap must be above a
certain magnitude. In air, at sea level, the minimum sparking voltage is
approximately 350 volts for the shortest measurable gap. The voltage required
will vary with the dielectric strength of the materials (e. g., air, and vapor)
that fill the gap and with the geometry of the gap.
Flammability of Vapor-Air Mixtures
The flammability of a hydrocarbon vapor-air mixture depends on its vapor
pressure, flash point and temperature. These properties are used to classify
petroleum products whose electrical resistivities are high enough to enable them
to accumulate significant electrostatic charges under certain handling
conditions. Following are the three petroleum product Vapor Pressure
Classifications, including common examples:
- Low – Those with a
closed cup flash point above 100°F (38°C).
- These products do not develop flammable vapors under normal handling
conditions. However, conditions for ignition may exist, if handled at
temperatures above their flash points, are contaminated with higher
vapor-pressure materials, or are transferred into containers where vapors
are at concentrations at or above those necessary to produce a flammable
mixture.
- Examples: #2 Fuel Oil; Kerosene, Diesel, Jet Fuel A (commercial), Motor
Oil, Asphalt, and Safety Solvents
- Intermediate – Those
with a closed-cup flash point below 100°F (38°C).
- These products may create a flammable mixture in the vapor space at
ambient temperatures.
- Examples: Xylene, Benzene, Toluene, Jet B (commercial), JP-4 (military),
and Stoddard Solvents
- High – Those with a
Reid Vapor pressure above 4.5 psi absolute (31 kilopascals).
- These products, under normal handling temperatures in a closed vapor
space, will rapidly produce a mixture too rich to be flammable. However, in
some areas, a vapor space may pass through the flammable range before
becoming too rich.
- Examples: Motor and Aviation Gasolines, and high vapor pressure Naphthas
Ignition by Static Electricity
As a result, once a means of generating and accumulating an
electrostatic charge exists, it will be a source of ignition under the following
three conditions:
- The accumulated
electrostatic charge is capable of producing an incendiary spark.
- There is a spark gap.
- There is an ignitable
vapor-air mixture in the spark gap.
Control of Ignition Hazards
Reducing Static Generation
Static charge voltage may be prevented from reaching the sparking potential by
reducing the rate of static generation. In the case of petroleum products,
decreasing the activities that produce static can reduce the rate of generation.
Since static is generated whenever two dissimilar materials are in relative
motion to each other, a slowing down of this motion will reduce the rate of
generation. This means reducing agitation by avoiding air or vapor bubbling,
reducing flow velocity, reducing jet and propeller blending, and avoiding free
falling liquid. However, such static control methods may not be commercially
acceptable because of slower production. Thus, reducing or rapidly dissipating
the charge by bonding or grounding is commonly used to reduce
static electricity.
Increasing Static Dissipation
Bonding and Grounding - Sparking between two conducting bodies can be
prevented by means of an electrical bond attached to both bodies. Bonding
prevents the accumulation of a difference in potential across the gap, thus no
charge can accumulate and no spark can occur. The earth may be used as part of
the bonding system. This is known as grounding and is used when a
potentially electrically charged body is insulated from the ground. Thus, the
ground connection bypasses this insulation.
Since the dissipation of the static charge is a function of the liquid’s
conductivity, anti-static additives may be used. These additives do not reduce
static generation, but will permit the charge to dissipate more quickly. They
should be introduced at the distribution beginning point and their effectiveness
may be reduced by passage through clay filters.
Controlling the Environment
Inerting and Ventilation - When static discharge cannot be avoided by
bonding, grounding, reducing static generation, or increasing static
dissipation, ignition can be prevented by excluding ignitable vapor-air mixtures
where sparks may occur. Two commonly used methods are inerting and
mechanical ventilation. Inerting is a method of displacing the air with
an inert gas to make the mixture nonflammable. Mechanical ventilation can be
applied to dilute the ignitable mixture well below the flammable range.
Vehicles, Storage Tanks, and Containers
Following is a discussion of common static electricity problem areas found in
petroleum distribution and fuel handling facilities, and precautions to be
taken.
Highway and Aviation Transport Vehicles
Static from vehicle motion may be generated by the separation of air and dust
particles on the vehicle surface, the separation of the tires from the pavement
and agitation of intermediate vapor pressure products when the tank or
compartment is not full. It has been found that drag chains do not effectively
bond the vehicle to the pavement since paved surfaces are insulated when dry and
bonding is not needed when wet. Thus, static charges may be transported from one
place to another, and a dissimilar electric potential may exist between the
vehicle and loading or unloading facility.
Before tank loading begins, the truck is bonded to the loading facility, which
in turn is grounded. Whether top or bottom loaded, splashing or spraying should
be avoided by limiting the filling velocity to 3 feet per second until the
loading outlet is submerged.
When tank vehicles are unloaded into aboveground storage that may or may not be
adequately grounded (e. g., airplane fueling), the truck is first grounded, then
bonded to the receiving storage and then the nozzle is bonded before refueling
begins. There are special precautions taken with the refueling of airplanes from
tank vehicles since both have been subject to static accumulation due to the air
and tire movement described above. For example, the US Air Force has
discontinued the use of alligator clamps and now uses jack assemblies.
However, unloading into underground fiberglass, interior lined and steel storage
tanks does not present a static ignition hazard, provided that the delivery hose
nozzle is in metallic contact with the tank fill pipe, or tight connections are
used. Experience indicates that the outside of a buried fiberglass, interior
lined or steel tanks is in contact with a conducting medium (i.e., ground), and
accumulated static charges are dissipated.
Tank Cars, Marine Vessels
Generally, tank cars are sufficiently well grounded through the rails, and
bonding of the tank car is not necessary for protection against static
generation. However, there is the possibility of stray currents (see Part II,
Lightning and Stray Currents), and the loading lines should be bonded with the
rails to assure a permanent bond.
Marine vessel loading and unloading does not require bonding cables between the
vessel and the shore. This is unique since the hull of the vessel is inherently
grounded by virtue of its contact with water. Thus, accumulation of static
charges on the hull is prevented. Instead, an effort is made to electrically
separate the loading and unloading lines from the shore piping by inserting an
insulating flange between the vessel piping manifold and the shore piping
manifold.
Aboveground Steel and Fiberglass Storage Tanks
There is product movement during filling that can develop a static charge
between the liquid surface and tank shell, or metallic fittings, in a
non-metallic tank (e. g., manhole). To minimize the risk: avoid splash filling,
limit the velocity of the incoming stream, avoid ungrounded objects in the tank
(e. g., gauge floats), don’t introduce entrained air with product flow, and
allow a minimum relaxation time of 30 minutes for the charge to bleed off before
opening.
Internal floating-roof tanks require some form of bonding between the floating
roof and the tank roof. Open floating-roof tanks require bonding shunts between
the floating roof and the tank wall. While these shunts are required for
lightning protection (See Part II, Lightning and Stray Currents), they also
provide protection from electrostatic charges caused by the product’s movement.
Note: The addition of grounding systems (e. g., grounding rods) will not reduce
the hazard associated with electrostatic charges in the liquid.
Portable Drums and Cans
Drums and container filling-line operations on conducting conveyors should not
require additional protection against static accumulation. However, relaxation
time should be provided downstream of any micron-type filters.
Single metal containers should be filled with metal spouts that are held in
contact with the container or a funnel throughout the filling operation to
prevent static accumulation and discharge. However, when transferring into or
out of open top (i.e., not spout equipped) containers, the filling stream is
broken and splashing occurs. In these operations (e. g., filling an open pail
from a drum), a bonding wire should be used to connect the two containers.
Plastic containers are not conductive to a metal filling spout or funnel and can
accumulate a static charge on the liquid surface. This may cause a discharge to
the spout as the liquid level rises. When large plastic containers are filled, a
grounding rod (i. e., connected to a bonding wire) should be inserted to the
bottom of the container before filling. A recent survey documented 27 gasoline
fires involving the filling of both metal and plastic containers on a plastic
truck bed or carpeted car trunk. However, small plastic containers (e. g., one
gallon) are less of a problem if the filling velocity is slow and the container
is placed on the ground surface.
Vehicle Fueling
Experience indicates that fuel dispensing does not require bonding for fueling
from a service station type dispenser at rates below 25 gpm. However, faster
fueling of large equipment (e. g., aircraft) requires bonding the hose nozzle to
the receiving equipment with a bond wire and clip.
Piping Systems
Small Diameter Piping
Static electricity accumulation is most likely to be a problem in pipes
conveying non-polar fluids at high velocities. Typical small diameter
underground fiberglass and steel piping (e. g., 2 to 6 inches) for motor fuel
refueling is not considered a discharge hazard. However, large diameter piping
that is located in general industrial service, where electrical charge build-up
is possible (e. g., aviation installations) is a potential hazard.
Large Diameter Piping
Jet fuel movement in large pipelines has been the subject of U.S. Air Force
studies on static electricity. Both buried steel and fiberglass piping were
found to build up static electricity at about the same rate with fluid flows up
to 15 ft/sec. The study also found that the charge was conducted along the layer
of fluid next to the inside pipe wall and was drained off non-metallic
piping when the fluid came in contact with metal valves or fittings.
Although test data is limited, 10-to12 ft/sec is considered to be the maximum
velocity for non-metallic piping handling jet fuel, and metal valves or
fittings should be properly grounded.
Another method of discharging static electricity from non-metallic piping
is by wrapping a copper wire around the pipe in a helix and attaching it to a
grounding rod at approximately 500-foot intervals.
In the case of double-wall pipe, static electricity is discharged from
the primary pipe by wrapping a copper wire around the primary pipe in a helix
and grounding it by passing it through a threaded outlet saddle on the secondary
pipe.
Non-metallic fiberglass pipe and fittings are available with a grounding
wire entrained in the resin and meets MIL-P-29206A for jet fuels and petroleum
liquids.
Filters and Relaxation Chambers
When a fluid is pumped through a pipe, the magnitude of the electrostatic charge
generated will increase as the velocity increases. When this liquid is
transferred into a smaller pipe, the liquid velocity will increase as will the
static charge. When a filter is placed in the pipe, the static charge generation
increases by a factor of 10 to 100. However, there is no danger from this
excessive charge as long as the liquid is kept in the pipe (i.e., not
discharged). Thus, before the liquid is discharged at least 30 seconds
relaxation time should be provided in the piping system by means of a relaxation
chamber between the filter and the point of discharge.
Special Situations
Switch Loading
Potential ignition conditions can exist when low-pressure product is loaded into
a vessel that contains a flammable vapor from previous use at or above the lower
flammable limit. The most common example is the loading of diesel fuel into a
tank transport that previously contained gasoline. However, similar conditions
can develop when product lines are flushed, manifold valves leak, and during
vacuum truck operations. Static generation will be reduced by filling at the
lowest possible rate until agitation is minimized or blanketing the liquid
surface with an inert gas.
Sampling, Gauging, and High-Level Devices
Both conductive probes and insulating conductive floats can cause sparking at
surface potentials much lower than those required for sparking from the free oil
surface to the vessel or the vessel’s internal supports. It has been found that
there is a slower than normal decay of field strength (i. e., due to relaxation)
in large storage or ship’s tanks thus, 30 minutes delay should be observed
before hand gauging or sampling. In smaller vessels, (e. g., tank trucks, tank
cars), a one-minute delay time should be sufficient to allow for dissipation of
the static charge.
Purging and Cleaning Tanks and Vessels
Purging involves removing a fuel vapor from an enclosed space and completely
replacing it with air or inert gas. The purging operation can involve static
electricity generation if steam jets, or CO2 jets are discharged into a
flammable vapor-air mixture. Both steam and CO2 can generate static charges on
the nozzle and should be avoided.
Vacuum trucks are often used to remove hydrocarbon liquids from vessels that are
being cleaned. Ignitions may occur unless suction hoses and conductive pipe
wands have electrical continuity.
The refilling of empty vessels when returned to service, should begin at the
lowest flow rate to avoid the incoming stream from breaking the liquid surface.
And, in the case of floating roofs, the flow should be reduced until the roof is
floating off its support- legs.
Part II: Lightning and Stray Currents
Lightning
Lightning is nature’s greatest manifestation of static electricity. Falling
raindrops develop a static charge by breaking and separating and bringing one
charge to earth and leaving a separate charge in the cloud. Thus, electrical
storms involve the relatively slow movement of these heavily charged clouds,
which set up an electrostatic field over a large area of the earth’s surface
below the cloud. The charge on the earth’s surface includes tanks, equipment and
other objects. As the cloud passes through the atmosphere, the opposite ground
surface charge follows the cloud. In this scenario, lightning (i.e.,
electrostatic discharges) may impact facilities or equipment located on the
earth’s surface in the following manner:
Direct-stroke Lightning - At some point when the gap between the cloud
and an object on the earth’s surface narrows, there is a direct lightning
stroke. When this happens, a heavy ground current flows toward the impact point
where facilities or equipment are in the path of a high lightning-caused
current. Direct stroke lightning generates high temperatures that can severely
damage objects in its path and ignite flammable materials.
Indirect Lightning Currents – The abrupt change in the electrical field
caused by direct-stroke lightning neutralizes the static charge almost
instantaneously and collapses the field. These abrupt changes can induce
secondary sparking at equipment that is relatively remote from the direct
stroke.
Cloud-to-Cloud Lightning – The static charge developed by the breaking
and separating of raindrops brings one charge to earth and leaves a separate
charge in the cloud. As a result, adjacent clouds and the ground charge beneath
each cloud may have opposite charges. When the gap between the charged clouds
narrows or accumulates a heavy charge, lightning will occur between these
clouds. While this neutralizes the charged clouds, the earth bound charges will
also neutralize by way of a passage of current through the conductor with the
lowest path of resistance (e. g., ground, and pipeline). These abrupt changes
can induced charge-causing sparks, and usually occur when an insulated metallic
body is present.
Protection
Nature has a way of preventing most lightning-caused electrical discharge
damage, since cloud formation is often accompanied by high humidity. While high
humidity does not prevent the generation of static electricity, it does help
bond earth surface bodies to the ground and dissipates the electrical charge.
However, the accepted method of artificial protection against damage from
direct-stroke or indirect stroke lightning is to dissipate the charge with a
minimum of damage. Metallic structures that are in direct contact with the
ground or bonded to the ground (e. g., underground piping) are sufficiently well
grounded to provide safe dissipation of lightning strokes. However, artificial
grounding (e. g., ground rods) does not provide adequate dissipation and damage
may result. Non-metallic structures may be protected from direct-stroke
lightning by means of properly designed lightning rods, conducting masts or
overhead wires.
The accepted method of protection against damage from induced ground currents is
to bond structures and equipment to each other and provide a low resistance path
for the ground current.
Aboveground Tank Protection
Ground mounted metallic fixed roof and horizontal tanks are bonded to the ground
and will safely dissipate direct-stroke lightning. However, these tanks are
known to ignite when flammable vapors are venting through roof openings (e. g.,
gauge hatches) or vents not adequately equipped with back-flash devices such as
pressure-vacuum vent valves. Non-metallic tanks must be protected against
direct-stroke lightning by lightning rods or other means.
Open floating-roof tank “rim fires” occur when there is a direct-stroke
lightning or when an induced charge is released by clouds discharging to the
ground in the tank vicinity. Most of these fires occur above the seal and are
extinguished with hand foam or dry-chemical extinguishers. The most common
method of protection is to install metallic straps (shunts) on the circumference
of the roof, between the floating roof and the metallic shoe that slides on the
inside of the shell. These shunts will permit the charge to drain off without
igniting the vapor in the seal area.
Internal floating-roof tanks are typically covered with conductive roofs that
will act as “lightning rods.” However, the floating roof still requires bonding
to the shell for protection against electrostatic charges due to product flow.
Introduction to Stray Currents
The term stray current applies to any electrical current flowing in paths
other than those deliberately provided for it. Such paths include the earth,
pipelines, and other metallic structures in contact with the earth.
Stay currents can accidentally result from faults in electrical power circuits,
cathodic protection systems or galvanic currents resulting from the corrosion of
buried metallic objects. While stray current voltages are typically not high
enough to spark across an air gap, intermittent charges can result in a spark
that would ignite a flammable mixture, if present.
Protection Against Stray Currents
Pipelines – Where stray currents are known or suspected in a pipeline,
arcing at points of separation (e. g., valves, and spools) is reduced by
connecting a bond wire of reasonably low electrical resistance.
Spur Tracks – Tank cars loading or unloading spots on spur tracks are
typically served by a pipeline located alongside the rails. Stray currents may
flow in the pipelines or in the rails. Thus, the pipeline and rail should be
permanently bonded with low electrical resistance material.
Wharf Lines – The resistance of the vessel’s hull to ground (water) is
very low and the connecting and disconnecting of wharf piping may produce
sparks. Insulating flanges in the pipe manifold proved the best assurance
against sparking at the point of connection and disconnection of the hose.
Cathodic Protection Systems – Generally, an engineering study is required
to locate and size bonding when cathodic protection systems are employed to
protect a facility against corrosion. For example, the option of de-energizing
an impressed current system does not immediately remove the potential and render
it safe, since the polarized metal structure will persist for a period of time.
References
Following is a list of relevant published references and videos for those
wishing to explore the subject of static electricity further:
- Protection Against
Ignitions Arising out of Static, Lightning, and Stray Currents, American
Petroleum Institute (API) RP 2003
- Precautions Against
Electrostatic Ignition During Loading of Tank Truck Motor Vehicles, API
Publication 1003
- Safe Operation of Vacuum
Trucks in Petroleum Service, API Publication 2219
- Safe Entry and Cleaning
of Petroleum Tanks, API Publication 2015
- Cleaning of Mobile Tanks
in Flammable or Combustible Liquid Service, API Publication 2013
- Ignition Hazards Involved
in Abrasive Blasting of Atmospheric Hydrocarbon Tanks in Service, API
Publication 2027
- Test Method for
Electrical Conductivity of Liquid Hydrocarbons by Precision Meter, ASTM
D 4308
- Static Electricity,
National Fire Protection Association, (NFPA) NFPA RP 77
- Lightning Protection Code,
NFPA 78
- International Safety
Guide for Oil Tankers and Terminals, OCIMF
- Aviation Ground Operation,
Safety Handbook, National Safety Council
- Electrostatic Charging
Test for Aviation Fuel Filters, Coordinating Research Council, Inc.
- Survey of Portable
Container Fires 1990 through 1995, Petroleum Equipment Institute
- Aboveground Storage Tank
Guide, Thompson Publishing, June 1996
- Bondstrand Series 7000
Anti-Static Fiberglass Pipe and Fittings, Ameron, Fiberglass Pipe Group
- Engineering and Design
Guide, Smith Fiberglass Products Inc.
- Flammability
Characteristics of Combustible Gases and Vapors, U.S. Bureau of Mines,
Bulletin 627
- Video – Minimizing Static
Electricity, U.S. Bureau of Mines, A Kennedy Production
- Video – Great Balls of
Fire, U.S. Air Force
- Video – Minimizing Static
Electric Hazards, Gulf Publishing Company, Houston, TX
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