Controlling Electrostatic Ignition Hazards during …

1 Controlling Electrostatic Ignition Hazards during Fuel Delivery at Forecourts Graham Hearn1, Ulrich von Pidoll2 & Jeremy Smallwood3 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.

2 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.

3 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.

4 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 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 ).

5 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.

6 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 4500-6000 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 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].

7 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.

8 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. The conductivity of market-place petrol can vary from 5-500 [3]. Experiments have shown that the highest charge generation occurs at a conductivity of 5-50 [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.

9 Entrained air and immiscible impurities could also increase Electrostatic charge generation. According to CLC/TR 50404 [6] typical charge densities of 10 in liquid due to flow in a pipe can be increased tenfold to around 100 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.

10 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 ( valves and other fittings) usually have enough capacitance, to produce incendive sparks when charged by influence of fuel flowing through insulating pipes.