ASTM D4865 Generation and Dissipation of Static Electricity in Petroleum Fuel Systems
5. Background
5.1 Ignition Principles:
5.1.1 For ignition to occur, it is necessary to have an ignition source of sufficient energy and a mixture of fuel and air in the flammable range. The boundaries of the flammable range are defined by the lean and rich limits. Below the lean limit there is not enough hydrocarbon vapor to sustain combustion, whereas above the rich limit there is not enough oxygen. The mixture temperature and pressure and the fuel characteristics, including boiling range and vapor pressure, determine the amount of a given fuel which is vaporized and therefore establish the flammability of the mixture. Normally these limits are measured under equilibrium conditions with the fuel partially or completely vaporized. However, ignitions have occurred below the lean ignition limit when the fuel was in the form of a foam or spray. Also, systems are not normally in equilibrium when there is sufficient fuel flow to generate electrostatic charges. Turbulence in the vapor space can lead to unexpected flammable air-vapor mixtures in localized areas. Equilibrium flammability limits can therefore be used only as rough guidelines of flammability.
5.1.2 The second requirement for ignition is a static discharge of sufficient energy and duration. Discharges occur when the voltage across a gap exceeds the breakdown strength of the fluid or air in the gap. Minimum energy requirements vary widely depending on the nature of the spark, the configuration of the spark gap and electrodes, nature of materials, and other factors. There is no doubt that sparks due to static electricity in petroleum systems can have sufficient energy to ignite flammable mixtures when they occur in the vapor space. Discharges from highly charged fluids are known to penetrate plastic tubing.
5.2 Charge Generation - Whenever a hydrocarbon liquid flows with respect to another surface, a charge is generated in the liquid and an equal but opposite charge is imposed on that surface. This charge is attributed to ionic impurities present in parts per million or parts per billion quantities. At rest the impurities are adsorbed at the interface between the fuel and the container walls, with one part of the ionic material having a strong attachment for the fuel or the container. Under these conditions, there is no net charge on the fuel. However, when the fuel flows, one set of charges is swept along with the fuel while the opposite charges which accumulate along the wall surfaces usually leak to ground. This charge separation results in a rise in voltage in the moving fuel.
5.3 Charge Relaxation - When charged fuel enters a tank, a substantial voltage difference may be produced between the surface of the liquid and the tank walls and this may result in a static discharge. The voltage difference is limited by charge dissipation/relaxation processes which occur both in the pipework downstream of strong charge generating elements and in the tank itself. Relaxation in the pipework reduces the amount of charge that reaches the tank while relaxation in the tank reduces the voltage produced by a given amount of inlet charge. Under most practical loading conditions, the voltage generated by a given inlet charge density is proportional to the relaxation time of the fuel. This relaxation time is inversely proportional to the conductivity and is approximately 20 s when the conductivity is 1 pS/m. The conductivity of hydrocarbon fuels is highly variable as a result of natural product differences, commingling, or the use of additives. Products not containing additives, including diesel fuels, may have conductivities of less than 1 pS/m but many modern additive packages (not just static dissipator additives) provide considerably increased conductivity, possibly up to several hundred pS/m or more. The relaxation time can therefore be anything form a fraction of a second to a number of minutes. It has been found that the reduced relaxation time produced by increasing the conductivity more than compensates for any increase in charge generation that may occur. The highest voltages and electrostatic ignition risks are therefore associated with low conductivities. Unless conductivities are controlled, the possibility of encountering low conductivity product should be allowed for when defining safe loading procedures (3, 4).
6. Practical Problems
6.1 Certain switch loading operations, such as loading of diesel fuel into a truck which previously carried gasoline and still contains vapors or liquid gasoline, are especially dangerous. The combination of a flammable vapor space and charged diesel fuel presents a potential explosion hazard if an electrostatic discharge occurs. Analyses (5) of past tank truck accidents reveal that switch loading or splash filling, or both, account for 80 % of static-initiated explosions. More information on the hazards of flammable atmospheres formed during switch loading will be found in 7.6.
6.2 Microfilters and filter-separators are prolific generators of electrostatic charges. The type of ionic impurity in the product as well as the type of surface determine the magnitude and polarity of separated charges that are swept away in the flowing stream. Many additives in fuel increase the level of charge generation upon filtration, although in the case of static dissipator additives this is more than compensated by enhanced charge dissipation. Most common filter media such as fiberglass, paper, and cloth as well as solid adsorbents are potent charge generators. When carrying out operations such as meter proving that involve the use of temporary or mobile equipment, care should be taken not to introduce filters without adequate residence time (6).
6.3 Flow rate is an important parameter in charge generation because the delivery of more fuel per second delivers more charge per second (that is, a larger electrical current). This results in higher surface voltages. Also, an increased flow velocity frequently generates more charge per unit volume of fuel.
6.4 Certain types of pumps, such as centrifugal or vane pumps, can be prolific charge generators due to high exit velocities at impellers.
6.5 Splash filling of a storage tank or tank trunk represents another mode of charge generation. Spraying of droplets causes charges to separate, leading to the development of both charged mist and foam as well as charge accumulation in the liquid. If the drop tube in a fill line fails to extend to the bottom of a receiving vessel or below the liquid level, splashing will result.
6.6 Conductive objects exposed to charged fuel become charge accumulators if unbonded to the receiving vessel. In cases where an incentive discharge has taken place, an unbonded charge collector is likely to have been present because a charged hydrocarbon surface by itself makes a poor electrode. A high potential is needed form hydrocarbon surfaces to develop a spark with sufficient energy for ignition, but a conductive object (such as a metal can or insulated fitting) in contact with a hydrocarbon at lower potential can more readily carry accumulated charge to the sparking point and provide an incendiary spark at much lower potential. Conductive objects are not always metal. A piece of ice can act as a charge collector and a surface pool of free water can accumulate a high surface charge. Objects dropped into a tank such as pencils, flashlights, or sample thief parts are a source of dangerous accumulators.
6.7 Fueling aircraft, where the fuel is highly charged following the necessary fine filtration, can create a difficult electrostatic situation. Hose and manifold residence time is usually too short to provide a significant amount of charge relaxation. However, accidents due to electrostatic ignitions have been rare compared to truck loading explosions primarily because aircraft fuel is usually bottom-loaded, aircraft have smaller compartments, and aircraft fuel tanks contain protrusions which tend to encourage low-intensity corona rather than the more incendiary spark discharges. The nonflammability of Jet A or A-1 at most fueling temperatures as well as the use of conductivity-improving additives are other alleviating factors.
6.7.1 While fueling aircraft, bonding between the aircraft and the fueler is required to prevent a voltage differential from developing between them. Grounding is not required (see NFPA Standard No. 407). Grounding does not provide any additional benefit in a properly bonded system during fueling operations (5).
6.8 Filling a large storage tank or tanker compartment can lead to charge generation even when splash loading is avoided. The movement of air bubbles or water droplets through the bulk fuel as the tank contents settle is a charge generation mechanism and can cause a high charge level to accumulate in a low-conductivity fuel. Charge generation by settling can persist for many minutes after filling ceases (see 7.5.2).
6.9 Filling an empty filter-separator vessel can create an electrostatic hazard if liquid is not introduced slowly. Fuel filling an empty vessel at high rates will cause charges separated on the elements to develop high voltages and discharge through the vapor space which contains air. In virtually all such cases, filter elements exhibit burn marks due to low-order combustion of fuel foam. Explosions which have ruptured the vessel have occurred when flammable mists or vapors were present. Residence time is extremely short and even if the fuel contains conductivity improver additive, the raised conductivity may be insufficient to reduce potentials by enough to avoid static discharges.
6.10 Sampling a low-conductivity fluid into a plastic container poses a special problem because it is obviously impossible to bond the filling line to the plastic. Pouring from or shaking a plastic receptacle containing low-conductivity fuel will also cause charges to separate.
6.11 Coatings which are normally applied on steel surfaces for corrosion protection do not affect the electrical behavior of charged fuels; thus, coated tanks and pipes act similarly to bare metal.
