Step 2 of 5•6 minutes read
There are three basic stages leading up to a potential electrostatic hazard:
1. Charge separation.
2. Charge accumulation.
3. Electrostatic discharge.
All three of these stages are necessary for an electrostatic ignition of a flammable atmosphere. Electrostatic discharges can occur as a result of accumulations of charge on:
Whenever two dissimilar materials come into contact, charge separation occurs at the interface. The interface may be between two solids, between a solid and a liquid or between two immiscible liquids. At the interface, a charge of one sign (say positive) moves from material A to material B so that materials A and B become respectively negatively and positively charged.
While the materials stay in contact and immobile relative to one another, the charges are extremely close together. The voltage difference between the charges of the opposite sign is then very small, and no hazard exists. However, when the materials move relative to one another, the charges can be separated and the voltage difference increased. The charges can be separated by many processes. For example:
When the charges are separated, a large voltage difference can develop between them. A voltage distribution is also set up throughout the neighboring space and this is known as an electrostatic field. Examples of this are:
If an uncharged conductor is present in an electrostatic field, it has approximately the same voltage as the region it occupies. Furthermore, the field causes a movement of charge within the conductor; a charge of one sign is attracted by the field to one end of the conductor and an equal charge of the opposite sign is left at the opposite end. Charges separated in this way are known as ‘induced charges’ and, as long as they are kept separate by the presence of the field, they are capable of contributing to an electrostatic discharge.
Charges that have been separated attempt to recombine and neutralize each other. This process is known as ‘charge relaxation’. If one, or both, of the separated materials carrying charge, is a very poor electrical conductor, recombination is impeded and the material retains or accumulates the charge upon it. The period of time for which the charge is retained is characterized by the relaxation time of the material, which is related to its conductivity; the lower the conductivity, the greater is the relaxation time.
If a material has comparatively high conductivity, the recombination of charges is very rapid and can counteract the separation process, and consequently little or no static electricity accumulates on the material. Such a highly conductive material can only retain or accumulate charge if it is insulated by means of a poor conductor, and the rate of loss of charge is then dependent upon the relaxation time of this lesser conducting material.
The important factors governing relaxation are therefore the electrical conductivities of the separated materials and of any additional materials that may be interposed between them after their separation.
Refined clean products tend to have very low conductivity, such that the relaxation time is about half a minute. This is not to be confused with the ‘settling time’ for static accumulator oils which is a delay of 30 minutes (settling time) after the completion of loading of each tank before commencing any dipping, ullaging, or sampling with metallic equipment.
Electrostatic discharge occurs when the electrostatic field becomes too strong and the electrical resistance of an insulating material suddenly breaks down. When a breakdown occurs, the gradual flow and charge recombination associated with relaxation is replaced by sudden flow recombination that generates intense local heating (e.g. a spark) that can be a source of ignition if it occurs in a flammable atmosphere. Although all insulating media can be affected by breakdowns and electrostatic discharges, the main concern for tanker operations is the prevention of discharges in air or vapour, so as to avoid sources of ignition.
Electrostatic fields in tanks or compartments are not uniform because of tank shape and the presence of conductive internal protrusions, such as probes and structure. The field strength is enhanced around these protrusions and, consequently, that is where discharges generally occur. A discharge may occur between a protrusion and an insulated conductor or solely between a conductive protrusion and the space in its vicinity, without reaching another object.
For an electrostatic discharge to take place, the two charged materials need to be positioned not more than a specified distance between each other. If two charges are separated by a too large distance, the electrostatic discharge will not take place, one must however remember that a discharge may take place with its adjacent space. The maximum distance for an electrostatic discharge to take place depends upon how strongly the materials are charged. Or in more technical words, how much is the voltage difference between two objects. The higher the voltage difference, the longer the distance may be for an electrostatic discharge to still occur.
Electrostatic discharge is one thing. Electrostatic discharge to produce a spark is another thing. However, this will be discussed further in the next lesson. For an electrostatic discharge to produce a spark, there must be a certain amount of voltage difference between the two charged materials.
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