ASTM D4865 Generation and Dissipation of Static Electricity in Petroleum Fuel Systems
7. Possible Approaches to Electrostatic Charge Alleviation
7.1 A number of approaches to alleviate electrostatic charging problems are described in Refs (1, 2, 7, 8, 9). These approaches try to reduce or eliminate charge generation or accumulation, eliminate the possibility of spark formation, or change ullage space composition out of the flammable range. Summaries of a number of such techniques follow. Greater detail will be found in the cited references. (Warning - None of the following approaches eliminates the need for proper bonding and grounding, which is necessary to prevent voltage differences from developing on the system (piping, receiving tank, and so forth) or on unbonded objects within a tank or compartment. For proper bonding and grounding procedures, consult Ref (2) and BS 5958 (Part 2).)
7.2 Line Velocity Reductions - Although earlier practice was to keep velocities below 5 to 7.5 m/s, later work has shown that other factors such as the volume of the tank being filled, the fill pipe diameter, the fuel conductivity, and the mode of filling (top or bottom loading) need to be considered. Current thinking is to generally keep velocities below 7 m/s and, in addition, to impose further restrictions as applicable depending on the factors previously listed. For detailed recommendations the reader is referred to Refs (1, 10) and BS 5958 (Part 2). The reduction of flow rate through a filter may not reduce charge density significantly but it will reduce current flow and will increase residence time downstream of the filter.
7.2.1 In systems where switch loading might occur, valves, meters, pumps, and other fittings may result in flow restrictions which give significantly higher velocity past these surfaces than estimated for a system's riser arm and hosing. It is suspected that the higher velocities in these fittings might increase electrostatic charging and they should be located as far as practical upstream of inlets to vessels.
7.3 Relaxation Time:
7.3.1 Even at the lowest conductivities, where the risk of static discharge is greatest, the charges produced by pipe flow are normally safely dissipated within the receiving tank if the velocity limits mentioned in 7.2 are adhered to - this is the reason for choosing these limits. At higher fuel conductivities (see 8.1.1), the reduction in relaxation time more than compensates for any increased charge generation that might occur; consequently, the voltages generated inside tank compartments are lower.
7.3.2 During tank truck loading or storage tank filling, high charge densities caused by filters or similar flow obstructions should be relaxed back to normal pipe charging levels by providing at least 30-s residence time downstream of the filter before the product reaches a loading arm or fill pipe. For products with conductivities less than 2 pS/m (or where the actual or possible minimum conductivity at field temperature conditions is unknown) longer residence time may be required (1).
7.3.2.1 The residence time for aircraft fueling has been substantially less than 30 s. Residence times as low as 3 to 7 s after a system containing water absorbing media monitors and 5 to 21 s after filter coalescer elements have been reported (11). These have been found by experience to be satisfactory for the particular conditions encountered in existing aircraft fueling systems (see 6.7). Care should be taken in fueling aircraft so that new designs or materials in the ground handling systems do not markedly change charging tendency or residence times compared with those known to be safe at that site.
7.3.3 Charge relaxation may also be required under circumstances where flow has stopped but a charge has been created before flow stoppage. Thus, relaxation time is required in the Particulate Contaminant Test for Aviation Fuel (Test Method D2276), where electrical charging has been caused by the membrane filter used in the test. A 1-min wait is therefore recommended for charge relaxation before disassembling the housing which holds the test capsule. The same waiting period is appropriate for the case of the plastic sample container mentioned in 6.10. Test Method D5452 contains laboratory filtration procedures which have been modified to reduce electrostatic hazards. A much longer waiting time, possibly up to 30 min, is recommended before sampling large storage or ships' tanks (1). This is based on measurements taken in large tanks which have shown a slower decay of field strength than would be expected by normal charge relaxation. The slow decay may be due to further charging by the settling of charged particles of water, dirt, or other materials.
7.4 Elimination of Splash Loading - When trucks are top-loaded with overhead lines, that is, drop tubes, these lines should reach to the bottom of the compartment to avoid dropping the product with subsequent splashing. When bottom or pressure loading is used, the fuel inlet should be baffled to avoid spraying fuel all over the compartment during initial filling. Electrostatic risks are greatly reduced by using a loading velocity of less than 1 m/s until the fill pipe inlet is completely covered. This practice is applicable also to storage tank filling and ship loading where it minimizes the disturbance of water bottoms and sediment.
7.5 Elimination of Unbonded Charge Collectors:
7.5.1 Unbonded, loose objects in a compartment or tank are a major hazard and must be eliminated by periodic compartment inspection to ensure proper cleanliness. Care should also be taken not to design in unbonded charge collectors such as wire bundle clamps or fittings on fuel hoses.
7.5.2 A gaging tape or sample container can be a charge collector if lowered into the fuel before charges have relaxed to a safe level. At least 1 min should elapse between the stoppage of flow and the lowering of any object into a small compartment (1). A much longer waiting time, possibly up to 30 min, is recommended for large storage or ships' tanks (see 7.3.3).
7.6 Elimination of Flammable Vapors in Ullage Spaces - When compartments or tanks are consistently used for either high- or low-vapor pressure products, the ullage space in these compartments is either too rich or too lean to be flammable. However, when switch loading from high- to low-vapor pressure products occurs, the ullage space frequently ends up in the flammable range before becoming either too rich or too lean. Such switch loading has therefore resulted in many loading fires or explosions and the practice is best eliminated. Where this is not feasible or where intermediate vapor pressure products such as crude oil or Jet B are handled, either ullage oxygen content should be reduced to render the compartment nonflammable or filling rates should be restricted to prevent the occurrence of hazardous potentials (see 7.2). Large crude-carrying tankers routinely use filtered flue gas inerting, but nitrogen or carbon dioxide can be employed for the same purpose in smaller systems. However, system operation should be checked to ensure the nonflammability of the ullage space. It may also be necessary to establish that the resultant dissolved gases do not cause operating problems later through product contamination or evolution at reduced pressures.
7.7 Use of Low-Charging Filters - Depending on the filter material, different filters with the same filtration performance may charge petroleum products to radically different electrical levels. To identify the problem and to select low-charging filters, a procedure for determining the charging level of aviation fuel filter-coalescers and separator elements has been developed (12).
NOTE 1 - However, tests on a single fuel/material combination may not be definitive in determining the maximum charging potential of that material. The use of low-charging filters cannot substitute for the other precautions listed earlier.
7.8 Use of Additives - The use of additives to control the effects of electrostatic charging is covered in Section 8.
