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Insulative Versus
Conductive Underground Piping
By
Steve Fowler and James Pharr
March 1, 2009
 The
underground piping industry has a long running debate about whether
to use conductive or insulative pipes. This debate is a modern issue
brought on by the necessity of changing the old metal piping and
tanks due to corrosion and leakage. Some companies have made efforts
to justify the continued use of plastic insulative piping when conductive
versions are available. The facts are that conductive is better
and fits the best design criteria for fuel systems.
For more than one hundred years, the
petroleum industry used metal piping and tanks. Over time, this
was found to be a concern as corrosion caused environmental problems.
Some time ago, plastic piping was begun to be used to eliminate
this problem. After some initial issues of polymer degradation,
the plastic piping product has proven itself up to the environment
challenge. However, the issue of electrostatic charge generation
and possible problems has been ignored because of the environmental
benefits gained. The need for conductive systems in the transfer
and storage of volatile dielectric liquids did not go away when
the corrosion problems surfaced. The industry chose the best solution
regarding two known hazard - environment over electrostatic hazards.
Historically fuel storage tanks were
constructed of steel and covered with soil. Piping between tanks
and dispensers was metal, again covered with soil. This arrangement
provided continual bonding and grounding (earthing) without intentional
design. When plastic piping was introduced, the industry lost a
very important issue in a well designed fuel transport and storage
system - conductivity, earthing/grounding and bonding. The debate
now being seen in the industry is attempting to make the plastic
piping seem suitable from an electrostatic point of view because
of its benefits from a corrosion standpoint. This debate can make
arguments that appear to be sound. However, if that were the case,
no conductive pipes, nozzles, etc. would be required for dielectric
liquid systems. One could make the same arguments for all piping,
hoses and nozzles in the system.
One stated concern of the most common
design of conductive underground piping is that only the inner layer
is conductive and that it may become an isolated conductor if not
properly installed. Further the concern is that the piping would
have to be continuously checked for continuity and grounding. The
present dispensing system requires a well bonded and grounded hose,
connectors and nozzle. The hose for fuel dispensing is typically
an inner conductive rubber hose with a metal braid that must be
installed properly and checked for continuity. To say that an insulative
hose would be better in this case would violate codes, protocol
and logic. These arguments contradict best practices and stand in
opposition to good design requirements for grounding and bonding
all parts of fuel transport and storage.
One argument for insulative piping
states that there is no chance of ignition in the tank or piping
and if all metal objects related to the piping and fill areas are
grounded, there is no problem from the emanating electric fields.
This ignores the person, operator, truck etc interface.
It also ignores established protocol
for tank and piping design.
In the Report " Static electricity
Field Guide for Marine Tanker and Tank Cleaning Operations"
prepared for the United States Coast Guard by The Volpe National
Transportation System center in June 1996 a major WARNING was
stated: " The Basis of precautions presented in this guide
is that the tank atmosphere is always assumed to be in the flammable
range......" One must never design a system assuming that
the atmosphere is not flammable. This statement is taken from
the "International Safety Guide for Oil Tankers and terminals
(ISGOTT)." by the Oil Companies Marine International Form.
As stated in a paper by Hearn,
Pidoll and Smallwood http://www.esdjournal.com/articles/Smallwood/Controlling
Electrostatic Ignition Hazards final.pdf
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Regarding the transition of the piping
from underground to the fill area - "....Totally buried
insulating plastic pipes usually do not create dangerous discharges
inside and outside of the pipe. However, in an excavated, unburied
or partly buried system extra care must be taken. In chambers
and fill boxes, small sections of the pipe are not buried. The
metal components present in a fill box (e.g. valves and other
fittings) usually have enough capacitance, to produce incendive
sparks when charged by influence of fuel flowing through insulating
pipes."
Regarding of the hazards of fuel
vapors: ".......With this in mind, precautions against
electrostatic discharges from other sources e.g. relating to the
grounding of the vehicle and personnel must be in place."
In the fill area and when interfacing
with the fuel delivery operator, the ungrounded conductor may be
related to the transfer itself or the lack of care in the fill box.
This also ignores the effect of the
plastic piping on the charge generation in the dispensed fuel to
the customer.
In the paper quoted above, the authors
speak of the charge accumulation and generation:
"Charge accumulation
With plastic pipe systems, as with metal pipework, the primary
source of charge generation is due to the flow of fuel through
the pipe, as discussed above. With metal systems the charge on
the metalwork will normally be conducted safely to earth. With
plastic systems, electrostatic charge can accumulate on the pipe
wall and associated ungrounded metallic components, such as the
heating coils in electro-fusion couplings, metal valves and other
metal fittings. This represents the principal difference between
plastic piping systems and earthed metal systems from an electrostatic
point of view."
The charge that is generated and not
dissipated by the piping is available to be delivered to the customer
at the dispensing point.
 
Over the past decade, reported
incidents involving ignition of gasoline vapors during refueling
activities at retail sales have dramatically increased.
Reference http://www.esdjournal.com/static/refuelfr.htm
http://www.esdjournl.com/static/nfpa_facts.htm
Many factors contribute to this phenomenon,
including the fact that most retail locations are self-service,
meaning that patrons operate the equipment. The Petroleum Equipment
Industry (PEI) indicates a large percentage of the fires result
from discharges when patrons reenter their vehicles then experience
an electrostatic discharge near the refueling port where vapor-air
mixtures are conducive to ignition.
However, This factor does not
explain why some fires occur as patrons are holding the nozzle and/or
refueling containers.
Investigation of factors that lead
to separation of electrical charge has revealed many concerns including
lack of conductive bonding in refueling systems. Modern tanks are
constructed of fiberglass or other non-conductive materials, and
then connected to plastic piping, which is also non-conductive.
At no location between most connections to fill tanks and the dispenser
is any bonding provided. The liquid cannot be grounded or have a
path to conduct its triboelectric generated charges to ground. The
only grounding required is provided through the alternating current
electrical system, and is not intended for reduction of electrostatic
charge accumulations nor is it effective to the charges in the insulative
piping.
It seems that most, if not all, principles
of electrostatic charge management have been abdicated in codes
related only to retail fuel transfer. This seems counter productive,
even dangerous.
An examination of codes related to
flammable liquid transfer reveals that bonding and grounding is
required when transferring even small quantities in industrial settings,
plus is specifically required when transferring large quantities,
however when medium quantities are transferred, little attention
is paid to electrostatic charge separation. Tests conducted by Fowler
and Associates revealed that more than 5,000 volts electrostatic
charge can result from transfer of one gallon of gasoline from a
dispensing pump into a plastic container at 7.5 gallons per minute.
In laboratory conditions, these investigators have documented repeated
ignition of gasoline vapors with as little as 4,000 volts from a
5 gallon container. These tests revealed that electrostatic discharge
fires could be anticipated when transferring fuel in typical retail
arrangements.
Reduction of charge is paramount to
preventing electrostatic discharge. Product flow through non-conductive
piping does not prevent charge separation; however it does preclude
charge dissipation. Conductive piping provides increased opportunity
for dissipation, thus should be generally considered better, however
current products approved for retail facilities do not provide conductivity.
Based on our testing, we believe that conductive piping that provides
opportunity for charge dissipation is preferable to non-conductive
materials alone.
Further, based on testing and experience,
we believe that the intent of the National Fire Protection Association
- NFPA 30 is not met by the current arrangements and materials,
specifically section 6.5.4. Experience from fires occurring at retail
refueling operations indicates this failure.
6.5.4* Static Electricity.
6.5.4.1 All equipment such as tanks, machinery, and piping shall
be designed and operated to prevent electrostatic ignitions.
6.5.4.2 All metallic equipment such as tanks, machinery, and
piping where the potential exists for an ignitable mixture to
be present shall be bonded and grounded.
6.5.4.3 The bond and ground shall be physically applied or shall
be inherently present by the nature of the installation.
6.5.4.4 Any electrically isolated section of metallic piping
or equipment shall be bonded and grounded to prevent hazardous
accumulation of static electricity.
6.5.4.5 All nonmetallic equipment and piping where the potential
exists for an ignitable mixture to be present shall be designed
and operated to prevent electrostatic ignition.
Conclusions and Recommendations
Insulative plastic piping solved a
very real environment issue. The past use of such piping with its
greater electrostatic risk was justified on a cost benefit evaluation.
With suitable conductive piping, this no longer is the case. Now
the risk of electrostatic problems is no longer justified to continue.
The industry should stop justifying the use and begin to practice
good fuel transfer and storage design again with little environmental
impact.
With no method of grounding the charges
generated by insulative tanks and piping and the problem of static
related dangers increases. We recommend that more conductive piping
and fittings be used for the underground systems. In the past before
plastic piping was introduced, we had metal conductive systems.
When the plastic piping came into being, we forgot the basic premises
of safe petroleum transfers. The justification of insulative piping
by the premise that there might be an isolated conductor is counter
to all logic in the petroleum transfer industry. Conductive piping
not only allows for the dissipation of generated charges, it reduces
these charges and provides a Faraday shield for any electric fields
from the charges.
Rule #1: Bond and Ground
Rule #2 No isolated conductors.
The justification for insulative piping
assumes wrongly that there will be no isolated conductors in the
system. This overlooks the personnel and fittings on the insulative
piping to the danger of the industry. It absolutely overlooks the
self service aspect of vehicle and container refueling and possibly
puts the refueling public at greater risk.
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