The principal hazard of static electricity is a spark discharge that could ignite a fire or even cause an explosion. Hazardous products in Class I locations consist of flammable gases, vapors, or liquids. Static electricity along with too much heat can ignite these products.
Dangers in the Petroleum Industry
The petroleum industry’s primary ignition sources are flames, electrostatic sparks, electrical sparks or arcs, and hot surfaces.
Most industrial processes generate static electricity. In hazardous locations – with flammable gases, vapors, or liquids – static electricity may cause a fire or explosion.
Nowadays, there is ample knowledge about ignition risks due to static electricity in hazardous locations. Most of this understanding comes from the experience and research in the chemical and petroleum industries. Research in the field continues providing information about electrostatic phenomena. New processes are examined and standard procedures are developed for hazardous conditions.
A Brief Review of Electrostatics
The fundamental concept of electrostatics is that electrical charge can be accumulated on objects when they are rubbed together and separated quickly.
When the bodies accumulate a sufficient amount of charge, a spark may occur. A typical example is the light shock felt when touching a grounded metal surface, such as a faucet, after walking across a nylon carpet.
The spark might ignite a flammable vapor or gas-air mixture inside the flammable range concentration — the flammable range specifies the percent of vapor in the air necessary for combustion to occur.
Figure 1. Spark discharge between spherical electrodes. Image courtesy of L. G. Britton, 1999.
Glossary of Useful Terms
Before we go any further, let’s solidify the meaning of some important terms pertaining to flammable/combustible materials handled in the petroleum industry.
Hazardous (Classified) locations (NEC): Places where flammable gases, vapors, or liquids, combustible dust, and ignitable fibers and flyings may be present in a concentration sufficient to produce a fire or explosion.
Class I locations (NEC): Places where fire or explosion hazards may exist due to flammable gases, vapors, or liquids.
Vapor: Flammable gases emitted from the surface of flammable or combustible liquids (e.g., gasoline vapors).
Flashpoint: The minimum temperature at which a liquid gives off vapor in sufficient concentration to form an ignitable mixture with the air near the surface of the liquid within a test vessel as specified by an appropriate test procedure and apparatus. For ignition to occur, a flammable or combustible liquid must be at its flashpoint.
Flammable liquids: Those with a flashpoint below 37.8°C.
Combustible liquids: Those with a flashpoint at or above 37.8°C.
Lower flammability limit (LFL): The lowest concentration of flammable vapor or gas in air below which flame propagation does not occur on contact with an ignition source. Below this concentration, the mixture is too “lean” to burn.
Upper flammability limit (UFL): The highest concentration of flammable vapor or gas in the air above which flame propagation does not occur on contact with an ignition source. Above this concentration, the mixture is too “rich” to burn.
Flammable range of a vapor or gas: The span between the LFL and UFL. The flammability limit tests find the concentration limits of fuel and air mixture in the presence of a suitable ignition source.
Static accumulators: Substances able to keep electrostatic charges for long periods of time.
Relaxation: The discharge process.
Relaxation time: The time for the charge to fall to 37% of its initial value.
Conductivity: The capability of a liquid to spread an electrostatic charge. Expressed in picosiemens per meter (pS/m).
Minimum ignition energy (MIE): An ignition source’s minimum energy required to ignite vapors or dust, expressed in millijoules (mJ). The ignitability tests determine the minimum energy or temperature that will ignite the most responsive fuel and air mixture.
Switch loading: The loading of a low volatile product into a tank that contained a high volatile product earlier. This includes instances such as loading diesel, a high static generator, into a tank that previously stored gasoline and flammable vapors from that gasoline remain.
Fire triangle: The three simultaneous conditions required for a fire or explosion to occur: fuel (vapor or gas), oxygen (air), and an ignition source with enough energy (a spark or flame). The ratio of fuel and oxygen needed depends on the combustible gas or vapor.
Figure 2. Fire triangle. Image courtesy of OSHA.
Causes of Static Buildup in Hazardous Locations
Several conditions may generate static electricity in hazardous locations:
- Low conducting liquids flowing through a pipe or hose.
- Turbulence in a tank
- Tank trucks on rubber tires
- High-speed liquid ejection from nozzles
- The impact between solids
- Solid particles lying in liquids
Let’s look at each of these causes in more detail.
Low Conducting Liquids Flowing through a Pipe or Hose
Many petroleum products can become electrically charged when they flow through pipes and hoses – including gasoline, kerosine, fuel oils, jet fuels, diesel, and similar, which are poor electrical conductors. Usually, these static charges do not become unsafe unless the flow velocity is high.
When a liquid flows along a pipe, the pipe’s wall absorbs its ions. This charges both the pipe and the liquid. The charges try to neutralize and stay close, forming an electrical double layer that resembles a capacitor’s plates.
The electrically charged flowing liquid can be considered as an electric current. Although the current is small, about 1 µA, it can generate a voltage high enough to produce a spark.
Static charges increase when pumping the liquid at a high velocity and passing it through filters (a filter can produce from 10 to 200 times more charge), valves, and fittings. Pipe diameter reductions can also increase static charges. Nonconducting piping, such as PVC or fiberglass, and isolated sections of metallic piping also help static charges to build up. Impurities, such as metal oxide, water, and chemicals increase a liquid’s static charge generation properties.
Turbulence in a Tank
Overhead filling of tanks produces friction between the feed pipe and the liquid (most often when there is turbulence). Static charges accumulate on the liquid surface and on the receiving tank’s shell when insulated from the ground. Sparking may arise across the liquid surface, and from the body to the ground or a grounded object – including people.
The likelihood of a spark from the liquid to the tank wall depends on the product’s static charge generation features. With an insulated metal container, the sparking from the liquid surface can be a substantial risk, regardless of the liquid’s conductivity.
Splash filling and fine mist droplet buildup can generate static charges of sufficient magnitude to be a source of ignition.
Tank Trucks on Rubber Tires
Metal tanks resting on a non-conductive base can build up static charges, mainly when they are filled through the dome.
The static charges in tank trucks are produced either by the product being filled or by the tires rubbing against the road. However, the hazard from charging via carbon-loaded road tires is small because the charge bleeds rapidly through the tires to the ground, decreasing the potential to a non-hazardous level in a few seconds.
The main concern is the charging during the liquid transfer because it may spark between the dome and the fill pipe.
Many tank truck events involve switch loading. It is vital to eliminate the remaining flammable vapors before loading low conductivity liquids.
High-Speed Liquid Ejection from Nozzles
Jet nozzles are particularly prone to static charges buildup. High flow velocities increase the spraying inside the tank, fostering static charge generation. Restricting the flow rates in the tank fill line and the discharge nozzles will provide an additional level of protection against excess static electricity.
The Impact between Solids
Impact of solid particles on tank plates – such as sandblasting – and shotcreting.
Solid Particles Lying in Liquids
Sludge and rust fragments settling in tanks.
Humans working in these hazardous locations may be a source of ignition.
Small amounts of water can cause a dangerous buildup of static charges. Also, water washing – and fire-fighting sprays – can generate enough charge to ignite flammable materials.
An ignition source may be a spark initiated by a static discharge from the high-speed fine particle water mist from a high-pressure head. Static charging also occurs when droplets of water settle on the bottom of tanks. Some tank fires have been initiated by static electricity produced during foam application from firemen’s nozzles or remote monitors.
It is vital to keep all petroleum products as free from water as possible.
Figure 3. Examples of charge accumulation. Image courtesy of H. L. Walmsley, 1992.
Reviewing Key Points
The main danger of static electricity in flammable gas, vapor, or air mixtures is a spark discharge that might produce a fire or explosion. A hazardous product can easily ignite when precautions lack or are ignored.
Current evaluations of the ignition hazard due to static electricity are of high quality, based on industrial experience and research.
One way of generating static electricity is by contacting or rubbing two bodies and swiftly separating them afterward.
Static charges in the petroleum industry result from different sources.
A common situation that produces static charges is the flow of liquids through pipes and hoses. The static charges will increase when pumping low conductivity liquids at high velocities through filters. Impurities, such as water, metal oxide, and chemicals, will also increase the static charge generation in liquids.
Splashing and turbulence of a liquid when loading a tank causes additional electrostatic charge generation, and a spark may occur between the pipe and the rising liquid. Extending the line to the bottom of the tank will help to avoid static splashing.
Tank trucks accumulate static charges because they are isolated from the ground by rubber tires.
An ignition could develop when filling a tank that previously held a high volatile liquid (switch loading).
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