T.R. Consulting, Inc. Monthly Safety Article
As a Potential Ignition Source
At Fueling Facilities
Compiled and Written by: Tony Rieck
T.R. Consulting, Inc.
For a list of archived articles go to www.trconsultinggroup.com/safety/archive.html
The Petroleum Equipment Institute (PEI) has initiated an effort to educate the public regarding the potential ignition hazards associated with static electrical discharges during fueling operations. T.R. Consulting, Inc. is providing this article and the associated links on our website in a cooperative effort to provide information on static ignition hazards at fueling facilities.
PEI’s website contains links to other information on static electricity and a report on refueling fire incidents (both a summary and a detailed version). PEI’s static electricity index page is located at www.pei.org/static/index.htm.
This article will include the following topics:
* Potential hazards due to flammable vapor generation during gasoline refueling
* Potential static electric ignition hazards for petroleum service workers
* What is static electricity
* How static electricity can act as an ignition source
* Minimizing the potential for ignitions due to static electricity
* Handling fuel safely
Potential Hazards Due to Flammable Vapor Generation during Gasoline Refueling
Most of us are aware that gasoline is a flammable liquid that generates vapors that are easily ignited when exposed to an ignition source. But, because we are routinely exposed to gasoline, we don’t think about the flammable nature of gasoline as much as we should or take the ignition risks of gasoline as seriously as we should. Also, most of us do not consider the fact that the same static charge that caused us discomfort as we reached for a door knob or light switch after crossing a carpeted room can be a source of ignition. The way that static electricity accumulates and discharges and how that discharge can act as a potential source of ignition for a flammable atmosphere is not only important for the petroleum service worker to understand, it is important for everyone who refuels their own vehicle to understand.
When we put fuel into the tank of a vehicle or gas can, the vapors within that tank or gas can are displaced by the fuel that is pumped into the tank. In this way, the air space immediately surrounding the nozzle through which the fuel is dispensed into the tank can be within the flammable range for gasoline. Additionally, many motorists “top off” their fuel tanks when refueling. Topping off fuel tanks can lead to retention of a small amount of fuel in the dispensing nozzle and to overfilling of the vehicle fuel tank. A source of ignition in close proximity to gasoline vapors present for these and other reasons at gasoline fueling facilities can result in a fire.
Since flammable vapors are likely to be present during refueling activities, it is important to eliminate sources of ignition from areas where these flammable vapors can accumulate. Potential sources of ignition that can be commonly observed around gasoline fueling facilities include: idling engines, cellular phones and other unprotected electronic equipment, smoking and static electricity.
Potential Static Electric Ignition Hazards for Petroleum Service Workers
Two of the most common static electric ignition hazards that occur during petroleum service work are due to product transfer and ventilation. The accumulation of static charges on personnel is also a potential problem.
When gasoline travels through pipes and hoses, a static charge may be formed. Static electricity results from the separation of like and unlike bodies. When liquid flows, charging occurs because absorbed ions are separated from free ions that are carried into the body of the liquid by turbulence. Metallic and fiberglass piping (those materials common in the petroleum industry) that is in contact with the soil will allow any charge to dissipate into the ground before an amount of energy significant enough to act as an ignition source is formed. The primary containment piping (inner pipe) of secondary containment piping systems can be grounded by wrapping copper wire around the primary pipe and leading the wire through designed orifices in the secondary piping to grounding rods. Transfer of product from one tank to another or to a container can cause a static charge due to the flow of the product through the transfer lines, impingement of the product on the walls of the container to which the product is being transferred and the settling of sludge and rust particles within the receiving tank. These transfer lines are often connected to tank openings that are dielectrically separate from the storage tank. Therefore, even if the tank is well grounded (and some are not), the transfer lines and pump may be subject to the accumulation of a static charge. All transfer lines, pumps and the receiving container must be bonded to a common ground to eliminate the potential for static accumulation and discharge.
One accepted method for making a container safe to work on or in is ventilating the container (i.e. a tank) with air to remove the flammable vapors from within that container. Air traveling through the ventilation device will not itself cause the accumulation of a static charge, but the vapors and particulates in the air will. These vapors and particulates can include dust, dirt, water vapor (humidity) and the petroleum vapors. Therefore, it is safe to assume that static accumulation will result if the air ventilation system in use is not bonded to an appropriate ground.
Petroleum service personnel may act as accumulators of static charges. In the same way that one can accumulate a static charge that is discharged to a door knob or another person, one can accumulate a static charge that will discharge to an oppositely charged conductor on a work site. If that discharge occurs in an atmosphere that is within the flammable limits for the product stored on site and has sufficient ignition energy, a fire or explosion can occur. Work uniforms made from cotton that is treated to reduce its ability to accumulate a static charge and gloves and footwear made from static dissipating materials are common safeguards employed by petroleum workers. In more sensitive environments, special static suits and wrist or boot stats are available.
For more information on bonding and grounding – click here
For more information on static generating processes and control methods common to the petroleum industry – www.fiberglasstanksandpipe.com/static.htm
What Is Static Electricity
Electricity can move freely over the surface of a conductor such as copper, but can flow through or over the surface of non-conductors (insulators) only with great difficulty or not at all. Examples of insulators are air, rubber and glass. When electricity is present on the surface of an insulator and it is prevented from flowing onto another surface (isolated by another non-conductor) or when electricity is present on a conductor that is in contact only with non-conducting materials, it is called static electricity and that material or body is “charged”. This charge can be either negative or positive.
All materials are comprised of atoms. These atoms are composed of electrons, protons and neutrons. The electron is a negatively charged particle and the proton is a positively charged particle (neutrons have no charge). In a neutral or unchanged material, electrons are present in equal numbers and their charges cancel one another. When an event separates some of the electrons from one surface and they are transferred to another surface, the surface from which the electrons were removed has a positive charge and the surface to which the electrons were transferred is negatively charged. An excess or deficiency of as few as 10 electrons for every million atoms will cause a surface to be considered very strongly charged.
Perhaps you remember this phrase from school, “For every force, there is an equal and opposite force.” This is exactly true for static charges as well. Static electricity is not generated it is made free. Somewhere, there is an exact opposite charge. Like charges will repel one another as do the like poles of a magnet and opposite charges will attract one another.
When two conductors are isolated from each other by a dielectric union or dielectric bushing, the ability of the conductor to accumulate a charge is increased. Unlike a spark from an insulating material that will only release a charge from a small area, all of the static energy accumulated on a conducting material can release in a single spark. Thus, releases of static charges from conducting materials can be significant as the ability of a spark to produce ignition is largely governed by its energy, which will be some fraction of the total energy that is stored.
Static Electricity as an Ignition Source
In order for the release of static electricity to pose an ignition hazard, four elements need to be present:
Some mechanisms that can effectively generate static electricity include:
An example of effective isolation would be an above ground storage tank set on a concrete pad. The concrete pad insulates the tank from the ground allowing the tank to accumulate static charges while being ventilated or filled.
Adequate ignition energy simply means that the spark that is produced has to provide enough energy to ignite the fuel in the atmosphere. For example, a match may have enough energy to ignite small twigs, but the ignition energy from a single match will be insufficient to start a large log on fire. Acetylene and hydrogen require very little energy to ignite. The necessary ignition energy fro propane or methane is somewhat higher. Combustible dusts and ignitable fibers require significantly more energy to ignite than do vapors and gases.
The spark must also occur in an atmosphere that is within the flammable range of the fuel present and that contains enough oxygen to support combustion. For example, the flammable range for gasoline is generally accepted to be between 1.4% and 7.8% in air. When oxygen levels are less than 10% (normal oxygen in air is about 20.9%), even gasoline vapor concentrations within the above flammable range cannot be ignited. Therefore, in an atmosphere containing gasoline vapors, the electrical spark can only result in ignition in atmospheres containing more than 10% oxygen and where the concentration of gasoline vapors in the atmosphere is between 1.4% and 7.8% (not taking into account oxygen enriched atmospheres where smaller or greater gasoline vapor concentrations may be ignited).
When all of the above conditions are met, and the differential in electrical energy becomes great enough or an electrically charged body is brought close to a grounded or oppositely charged conductor (creating a spark gap), an arcing occurs and the (once) static charge becomes a source of ignition.
Minimizing the Potential for Ignitions Due to Static Electricity
Bonding/grounding is an effective method for reducing the potential for accumulation of electrostatic charges and, thus, the potential for the discharge of static charges that could result in a fire or explosion.
Bonding is the use of a conductive link (such as a wire) to connect one conductor to another. This equalizes the electrical charge between the two conductors by providing a conductive pathway. Since the charge between the two conductors is equal, any additional charge applied to the bonded components will distribute over the entire bonded system. Since the electrical charge will follow the path of least resistance (the conductive link), there is little likelihood of a static discharge across an insulated pathway (spark) that could result in ignition.
Grounding is a specific type of bond that provides a conductive link from a conductive object to the ground. Because a bonded system can eventually build a significant charge that can be passed to a transient conductor (i.e. tools and personnel) or which will eventually be capable of bridging larger gaps between the bonded system and conductors outside the bonded system, bonded systems should always include a ground. In this way, rather than accumulate on the bonded system, charges dissipate by flowing to the ground.
Other methods for controlling static discharges include the use of static suits, static guard treatments for work uniforms, and boots and gloves made from static dissipating materials.
Handling Fuel Safely
The following are some basic safety procedures when refueling:
* Never use matches or lighters or smoke while refueling
* Do not leave your vehicle running during refueling
* Do not “top off” your fuel tank
* Do not get back in your vehicle while refueling
* Keep electrical equipment away from areas where flammable vapors are likely to be present.
* Never place fuel into unapproved containers
* Always place gas cans in contact with the ground and assure that the dispenser nozzle is in contact with the container
Matches, lighters and smoking are potential sources of ignition that should never be allowed around a flammable fuel source. The law requires that smoking not be permitted within 25 feet of a no smoking sign. Idling vehicles can also be a source of ignition.
“Topping off” your tank is a potential hazard due to the possibility of causing a small fuel spill. Even a very small amount of spilled gasoline presents a very real fire hazard. When fueling from locations equipped with stage II vapor recovery, even larger spills may occur when the nozzle is removed. “Topping off” of fuel tanks can lead to retention of small amounts of fuel within the nozzle (the nozzle will drip fuel upon removal from the dispenser). The retained fuel can vaporize, creating a potentially flammable atmosphere at the dispenser. In such instances, an electrostatic discharge when reaching for the nozzle could result in ignition of the vapors present at the dispenser.
It is never a good idea to reenter one’s vehicle while refueling. The rubbing of clothing against the vehicle seat is a potential source of electrostatic accumulation. Flammable vapors displaced from the vehicle fuel tank during refueling can cause the atmosphere immediately adjacent to the nozzle to be within the flammable range. Under such circumstances, a person wearing non-conductive footwear could cause an electrostatic discharge when reaching for the nozzle. This electrostatic discharge could result in ignition. Because the car and the nozzle are a bonded system due to the contact of the nozzle to the stem of the gas tank, it is important to equalize the electrostatic charge of one’s body to that of the bonded system prior to touching the nozzle. In other words, if you cannot avoid getting back in your vehicle during refueling, touch a metal component of the vehicle prior to reaching for the nozzle.
Not all containers are made from materials that are static dissipating (materials that will allow an electrical charge to flow off of them). Approved gas cans are made from static dissipating materials. The electrostatic energy that can be available due to the flow of the gasoline through the dispensing hose and the impingement of the gasoline on the walls of the container is a potential source of ignition. By keeping the nozzle in contact with an approved container, an electrical bond is created between the nozzle and the container. By placing the container on the ground, the bonded system is also bonded to the ground. Thus, any electrostatic energy flows off the bonded system to the ground rather than accumulating and forming a potential source of ignition for the vapors displaced from the container during filling.
T.R. Consulting, Inc. publishes monthly safety articles. Previous articles can be accessed at http://www.trconsultinggroup.com/safety/archive.html. On-line and traditional HAZWOPER and Confined Space training courses are also available. Visit our safety homepage – click here.