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Insulative Versus
Conductive Underground Piping
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 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.
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.
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.
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 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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