Controlling Electrostatic Ignition Hazards during Fuel Delivery at Forecourts

Transcription

1 Controlling Electrostatic Ignition Hazards during Fuel Delivery at Forecourts Graham Hearn 1, Ulrich von Pidoll 2 & Jeremy Smallwood 3 1 Wolfson Electrostatics 2 Physikalisch -Technische Bundesanstalt (PTB) 3 Electrostatics Solutions Ltd. The use of electrically insulating synthetic materials, such as plastics, for fuel pipelines and other fuel handling components is now becoming widespread. In the case of buried pipelines in filling station forecourts the use of these materials offers superior corrosion resistance and increased longevity. This in turn reduces the risk of pollution due to fuel leakage. It is well reported that the flow of fuel under certain conditions in both metal and plastic pipes can produce significant levels of electrostatic charge on the fuel. Pipe systems in modern filling stations generally contain both plastic pipe lengths and metal components such as in-line valves and couplings on which electrostatic charge can accumulate. Non-conductive pipework properly installed with grounded metal fittings and capped electrofusion coupler terminals should not create electrostatic ignition hazards. Over the last year or so a small number of fires have occurred around offset fill pipes at various petroleum forecourt refuelling sites in Europe. Initially these were blamed on either static electricity or thermite reaction. A joint investigation by Exxon and Wolfson Electrostatics [1] has shown the most likely cause to be an electrostatic discharge between an electrofusion coupler (EFC) and nearby metal flange. Measurements performed during a repeat of the delivery operation at the site however produced no significant readings of static electricity. Work undertaken independently by the three authors of this article has highlighted a number of factors influencing the levels of electrostatic hazard and these are discussed in this article. The influence on electrostatic ignition hazards due to the introduction of new alcohol-based biofuels such as E85 is considered. This article analyses the conditions necessary for electrostatic ignition and draws important conclusions with regard to the parameters influencing the degree of hazard present. Hazard Analysis There are five general conditions necessary for an electrostatic ignition hazard to be present: 1. Sensitive flammable atmosphere 2. Generation of electrostatic charge 3. Accumulation of charge 4. Electrostatic discharge (ESD) 5. Sufficient discharge energy for ignition If all of the above conditions exist, an ignition hazard will be present, if any of the conditions are removed, the hazard is obviated. As a belt and braces approach attempts are often made to remove more than one of these conditions, however the extent to which any mitigating measures can be applied in practice often involve other considerations which will inevitably include cost and practicality. Is a flammable atmosphere present? Petroleum spirit vapour has a flashpoint of around -43C and is flammable within the range of about 1-6% by volume with air. At most times within a pipe system and

2 storage tank, there is insufficient oxygen to support combustion ie the atmosphere is over-rich. During tanker delivery, a flammable atmosphere may be established around the end of the fill pipe. Similarly flammable petrol vapour concentrations may exist due to fuel spills and within fill-boxes and chambers. This of course applies equally to metal and plastic systems. E85 which is 85% ethanol and 15% petrol has a reported flashpoint of < -20C [3]. It has a wider flammable range than that of petrol of around 2-26% by volume [3]. Due to the fact that the gas phase of E85 contains much more petrol vapour than ethanol vapour, its upper explosion point is strongly dependent on the filling level of the tank (the so called effect of ullage space ). An almost full tank of E85 has a non flammable gas atmosphere above its liquid phase at temperatures higher than 0C, this temperature raises to 18C when the tank has become 99% empty [3,4]. Because of this effect E85 may produce flammable atmospheres in a wider set of circumstances than petrol. Diesel which has a flashpoint in excess of 60C will not produce a flammable atmosphere under normal conditions and is not considered at risk from static electricity at any time during delivery. However mixing diesel with small amounts of petrol, for example when filling diesel into a road tanker containing residual petrol from the last filling, may produce an explosive atmosphere even in a diesel tank. For all fuels the sensitivity to (spark) ignition varies significantly over the flammable range and is easiest to ignite at a concentration roughly midway between the upper and lower flammable limits. At or near this concentration, the ignition energy is very low and vapour ignitions may occur as the result of sparks from charged, ungrounded metal or electrostatic brush discharges from highly-charged insulating surfaces. Charge generation due to fuel flow During fuel delivery it has been estimated that litres is transferred in 10-12minutes. This corresponds to fuel flows of between 7 and 9 litres per second and moderate velocities in the pipe of around 0.3 metres per second. The electrostatic charge that is generated in fuel being pumped along pipes arises from the presence in parts per million (or billion) of ions in the fuel. Positive or negative ions selectively attach themselves to any interfacial surface in contact with the fuel, such as the inner wall of the pipe, due to selective chemical adsorption (and possibly ionic injection from the pipe wall) [2]. As a consequence, the inside surface of the pipe acquires a unipolar charge and ions of the opposite polarity in the fuel are attracted to it. A charged layer then extends from the wall into the fuel of a thickness that increases with decreasing fuel conductivity, the net charge in the pipe being zero when the fuel is at rest. When the fuel flows, the ions in the boundary layer tend to be carried along, while the opposite charge on the wall dissipates to earth at a rate depending primarily on the pipe material's conductivity. This implies that there will be a significant difference between charging behaviour in metal and plastic pipes. Any filters, valves and elbows will generally increase the amount of charge, due to greater interfacial charge separation, higher fuel velocities and increased turbulence. Similarly, the presence of free water in the fuel can also increase the charge concentration, again due to the charge separation arising from the large interfacial area of the emulsified mixtures. Petrol and Diesel are known to produce high levels of charging under certain conditions.

3 The conductivity of market-place petrol can vary from ps.m -1 [3]. Experiments have shown that the highest charge generation occurs at a conductivity of 5-50 ps.m - 1 [5]. Being alcohol-based, the conductivity of E85 is several orders of magnitude higher than traditional fuels and is unlikely to generate hazardous levels of static electricity due to flow through plastic pipes. The presence of filters and flame arrestors in the pipe may increase static charge generation and charging levels are likely to increase if the filter or flame arrestor becomes partially blocked with particulate materials and impurities. Entrained air and immiscible impurities could also increase electrostatic charge generation. According to CLC/TR [6] typical charge densities of 10 µc.m -3 in liquid due to flow in a pipe can be increased tenfold to around 100 µc.m -3 by a blocked strainer. This level of increase would correspond to a similar tenfold increase in voltages induced on ungrounded metal parts such as electrofusion couplers. In addition to the electrostatic charging mechanisms associated with fuel flow, there is also the possibility of electrostatic charge generation by friction with the external pipe wall and other components of the system, such as the walls of plastic chambers and sumps. In such cases, the charge generation mechanism could be frictional contact with a maintenance workers clothing. Charge accumulation With plastic pipe systems, as with metal pipework, the primary source of charge generation is due to the flow of fuel through the pipe, as discussed above. With metal systems the charge on the metalwork will normally be conducted safely to earth. With plastic systems, electrostatic charge can accumulate on the pipe wall and associated ungrounded metallic components, such as the heating coils in electro-fusion couplings, metal valves and other metal fittings. This represents the principal difference between plastic piping systems and earthed metal systems from an electrostatic point of view. Totally buried insulating plastic pipes usually do not create dangerous discharges inside and outside of the pipe. However, in an excavated, unburied or partly buried system extra care must be taken. In chambers and fill boxes, small sections of the pipe are not buried. The metal components present in a fill box (e.g. valves and other fittings) usually have enough capacitance, to produce incendive sparks when charged by influence of fuel flowing through insulating pipes. For this reason, all conductive objects in chambers and fill boxes should be sufficiently earthed and the surface of charged objects should not exceed the limits given in [6]. Furthermore, the electrical connections of electrofusion couplers should be tightly closed using plastic caps [6]. The forecourt ground surface of a filling station is normally made of a dissipative material with a leakage resistance to earth of less than 10 8 Ohms, e.g. concrete [6]. As tyres are usually sufficiently conductive to provide a leakage path to ground, the delivery road tanker parked on the forecourt is normally earthed via its tyres. Faults in the manufacturing process of modern tyres however can occasionally lead to highly insulating tyres. As a consequence, the leakage path to earth of the road tanker cannot be guaranteed [7].

4 Electrostatic discharges There are essentially two types of electrostatic discharges (ESD) that can occur during fuel delivery: spark and brush discharges. With regard to ignition hazard the characteristics of these two discharges are very different. (a) (b) Figure 1 (a) Spark discharge between a charged metal object and a grounded metal electrode (b) brush discharge from a grounded metal electrode to a highly charged dielectric sheet. Figure 1 (a) shows a spark between two metal objects at different potential. Figure 1 (b) shows a brush discharge between a highly charged dielectric and an approaching earthed electrode. Figure 2 compares the discharge waveforms for the two. The spark discharge current in (a) looks like a damped sine wave whereas the brush discharge current (b) is a single broad pulse. It is important when comparing the waveforms in figure 2 to note that the y-axis for (a) has been compressed and that the spark current is 20 times greater. Generally sparks produce a higher current density than brush discharges resulting in higher temperatures and are therefore more likely to cause ignition. (a) (b)

5 Figure 2 (a) Waveform of the current and its integral of a spark discharge between a charged pulse capacitor (93,3 pf at 7 kv) and a grounded metal electrode (b) same for a brush discharge from a grounded metal electrode to a highly charged dielectric sheet. Ignition of fuel vapour Spark discharges from charged, isolated metal components constitute a strong ignition hazard if they have sufficient energy. Metal components in a fill-box are generally earthed. Any unearthed items such as EFC heating coils and clips can only store small amounts of electrical energy and need a high potential to have sufficient energy to ignite an optimum fuel-air mix. Even if these conditions are met, a nearby ground is required to provide the spark gap. To constitute an ignition source a brush discharge will require not only a highly charged electrically-insulating surface but a grounded metal electrode to initiate the discharge. Figure 1 (b) illustrates how the energy of a brush discharge is concentrated close to the ground electrode and it is this region of the discharge most capable of providing the source of ignition. A few millimetres away from the ground, the discharge becomes diffuse and non-incendive. Brush-like discharges inside pipes have been recorded, especially for fluorinated pipes, but they are normally not incendive because of the over-rich or too-lean atmosphere inside pipes. These discharges may be hazardous if an optimised fuel-air concentration were to exist [7]. Flame arrester Measurement wire Possible site of spark? Grounded metal flange Electrofusion coupler Figure 3. All the conditions necessary for ignition? Flammable atmosphere due to residual vapour in the fill box; charge generation due to fuel flow; accumulation on the heating coil of an electrofusion coupler; spark to nearby grounded metal flange. Summary of conclusions 1. Static electricity has been blamed for fires that have occurred around offset fill pipes during tanker delivery. Non-conductive pipework properly installed (grounded metal fittings and capped electrofusion coupler terminals) should not create electrostatic ignition hazards.

6 2. If static electricity is the cause of these fires it is almost certain to be the result of sparks from ungrounded metal fittings or electrofusion couplers igniting residual fuel vapour in the box outside the pipes. If air is present in the pipe, brush discharges may present an ignition hazard. Brush discharges from a buried plastic pipe surface are unlikely to produce sufficient current density in the discharge to cause ignition. 3. Incidents of fire have also been blamed on thermite reactions between aluminium fittings and corroded steel but like static electricity this ignition source has not been ratified. 4. Within a normally-functioning pipe system (plastic or metal) there is insufficient oxygen present to support or propagate combustion. Furthermore. air does not enter the inlet pipe of a fill pipe in modern European installations. The installation is under pressure (35mbar) causing vapour to come out of the fill after disconnection rather than air going in. This will tend to leave an explosive atmosphere just outside the pipe rather than inside. 5. The alcohol-based fuels E50 E100 have a much higher electrical conductivity than petrol and Diesel and will not generate hazardous static potentials in pipe systems assuming they are in contact with an earth point somewhere in the tank system. 6. Flame arresters similar to those in place at the site of the incidents in Hungary [1] (Figure 3) may increase the level of charge generation during fuel flow and may therefore increase the ignition risk. Fire prevention The precise cause of the recent flash fires during fuel delivery cannot be considered certain on the basis of the available evidence. However some recommendations of preventative action can be considered. The ignitions were probably caused by an electrostatic discharge external to the fill pipes. Fuel or vapour may well have been present due to spillage or vapour release. It was found that some vapour leakage could occur from the Stage 1 and fill connections on opening, and through a connection that was not vapour tight. If the possibility of fuel or vapour in the vicinity of the fill pipes can be excluded then the risk of fire from this cause may be eliminated. Reliable exclusion of fuel vapour would be perhaps the best way of removing the risk of ignition outside the pipe. There appears to be little if any risk of ignition inside the pipe if it is either buried or air intake is avoided. It has been suggested that filling the box with sand to above the pipes and sealing the top layer against fuel ingress. This may be one way of successfully removing the possibility of a flammable mixture near the pipes. This procedure would have added benefit in effectively grounding electrofusion couplers in contact with the sand, in all but the driest conditions. Furthermore, if by some chance vapour did accumulate and a spark could ignite it, the sand would prevent the propagation of flame. In any location where a flammable atmosphere can occur, it is important to ground all conductive objects of significant size. Electrofusion couplers may be significant in this context. Additionally it is important that a long term reliable earth connection to these items is achieved.

7 Another approach is to electrically insulate the heating coil terminals in the coupler by some means to prevent electrostatic discharge. This is likely to be effective if a reliable seal is obtained for example by a tight fitting cap or peg. Figure 4 is a photograph of a properly installed underground fill pipe, with grounded metal connectors and capped electrofusion couplers. In this situation, steps 3 and 4 of the five hazard conditions (charge accumulation and ESD) are removed. Ground wire Capped terminals Figure 4. Properly installed underground fill pipe with grounded fitting and caps fitted to electrofusion coupler terminals. With alcohol based fuels E50-E100 it is postulated that fuel flow will not result in significant electrostatic potentials. However with existing installations the presence of a sensitive flammable atmosphere may still exist. Furthermore due to their wider flammable range and different ignition properties, these fuels may be somewhat easier to ignite. With this in mind, precautions against electrostatic discharges from other sources e.g. relating to the grounding of the vehicle and personnel must be in place. References [1] Wolfson Electrostatics report R436/JMS for Exxon Mobil dated 1 February 2006 [2] Feleci, N. J., (1984). J. Electrostatics, 15, [3] Brandes, E., PTB Braunschweig, section 3.41, Safety data of E50, E60, E85 and E>85, unpublished datasheet dated [4] Vaivads, R. H., Bardon, M. F. Rao, V. K. and Battista, V., Flammability Tests of Alcohol/Gasoline Vapours, SAE Technical paper , [5] von Pidoll, U., Krämer, H. and Bothe, H., Avoidance of electrostatic hazards during refuelling of motorcars, J. of Electrostatics 40&41 (1997), [6] CENELEC Technical report TR50404, Code of practice for the avoidance of hazards due to static electricity, [7] von Pidoll, U., Electrostatic ignition hazards in motor cars occurrence, detection and prevention. Proceedings ESA/IEEE Joint Meeting on Electrostatics, , University of Arkansas, Little Rock,