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Electrostatic Hazards – what are they?
There are five general conditions necessary for an electrostatic ignition hazard to be present:
1. Flammable atmosphere
2. Generation of electrostatic charge
3. Accumulation of charge
4. Electrostatic discharge (ESD)
5. Sufficient discharge energy.
Only if ALL of these conditions exist will an ignition hazard be present. If any of these conditions are removed, there is no hazard.
The conditions explained
1. Flammable atmosphere
Many common fuel vapours are flammable in air over a specific range between what is known as their lower explosive limit (LEL) and their upper explosive limit (UEL). The amount of vapour to
air content to create a flammable atmosphere varies between
different fuels. For example, petrol (gasoline) vapours need 1.4% vapour to 7.7% concentration; alcohol based fuel (E-85) vapours need 1.4% to 19% concentration.
2. Generation of electrostatic charge
An electrostatic charge is generated when fuel is pumped along a pipe, the factors influencing the accumulation of a charge being:
• fuel speed
• turbulence
• pipe material
• fuel conductivity
• presence of water in the fuel
• presence of filters and flame arrestors
• temperature
To ignite this flammable atmosphere, you require sufficient electrostatic energy to create a spark, namely 0.25mJ.
In exhaustive laboratory tests, and even when tested at an unrealistically high flow rate, or in systems where filters or flame arrestors are used (which have the effect of speeding up the flow and increasing friction*), UPP non-conductive pipe generates a maximum energy of around 0.10mJ, far less than required to generate a spark.
So, with UPP, there is insufficient energy generated to create
a spark, thus removing one of the conditions for a hazard to
be present.
* See ‘Guidance for the Design, Construction, Modification and Maintenance of Petrol Filling Stations’, published by the APEA and IP.
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3. Accumulation of charge
Any object or surface that can accumulate electrical charges can be described as a capacitor. It is the amount of electrical energy that is stored and then released that influences the degree of ignition hazard present.
UPP pipe, as it is non-conductive, naturally has low capacitance so accumulates little charge and naturally dissipates
to earth. However, any pipe with a high capacitance, such as a
conductive or hybrid conductive (PE) pipe, can store a much higher charge. The circumstances where this becomes hazardous are if the pipe is not efficiently and permanently connected to earth.
UPP pipe does not accumulate electrical charge so, again, the risk of a hazard is removed.
To achieve the same voltage (V), high capacitance pipe contains much higher energy and electrical charge (Q) as demonstrated in the formula C= Q/V , and therefore Q=CxV.
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4. Electrostatic discharges (ESD)
There are two types of electrostatic discharges.
• spark discharges
• brush discharges.
“Spark and brush discharges are very different in nature. Sparks are capable of igniting fuel vapour even at relatively low potentials of a few kilovolts. Brush discharges on the other hand require potentials of several tens of thousands of volts to cause ignition. Although potentials in the tens of kilovolt range are uncommon at forecourts, the lower voltages necessary for spark ignition have been observed during fuel flow. It follows therefore that earth continuity is essential in any system which comprises conductive elements.”
An extract from research by G L Hearn B.Sc C. Eng M.I.E.E of Wolfson Electrostatics Ltd on the electrical/electrostatic performance of conductive and non-conductive polyethylene fuel pipe.
Spark discharges
Spark discharges are responsible for the vast majority of industrial fires and explosions. They occur from conductive surfaces when an electrically isolated conductor becomes electrostatically charged. If this object is approached by another object at a different potential, a spark will occur just prior to contact. The amount of energy (E) in the spark is directly proportional to the capacitance (C) of the conductor.
This can be simply illustrated in the formula below, where V is voltage :
E = ½ CV ²
This shows that when capacitance (C) is high, the energy created is proportionately high.
Brush discharges
Brush discharges occur from charged insulating or non-conductive surfaces. The amount of energy contained in a brush discharge is considerably lower than in a spark discharge because of the low capacitance (C) of non-conductive surfaces.
Brush discharges, therefore, cannot ignite flammable vapours because there is insufficient energy to cause ignition. Plastic surfaces can only store small amounts of electrical energy as they have a small capacitance.
So UPP pipe, being non-conductive, cannot generate enough energy to cause ignition of flammable vapours – removing another condition for hazard to exist.
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