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As already explained in the previous lessons, petroleum products, such as clean oils (distillates), frequently fall into the non-conductive (static accumulator cargo) category with a conductivity typically below 10 pS/m. Chemical solvents and highly refined fuels can have conductivities of less than 1 pS/m. It is prudent to assume that the surface of a non-conducting liquid (static accumulator) may be charged and at a high potential during and immediately after loading.
Three classifications of filters may be used in tanker operations, as follows:
These do not generate a significant amount of charge, and require no additional precautions provided that they are kept clean.
These can generate a significant amount of charge and therefore require sufficient time for the charge to relax before the liquid reaches the tank. It is essential that the liquid spends a minimum of 30 seconds (residence time) in the piping downstream of the filter. Flow velocity should be controlled to ensure that this residence time requirement is met.
To allow sufficient time for the charge to relax, the residence time after microfine filters must be a minimum of 100 seconds before the product enters the tank. Flow velocity should be adjusted accordingly.
If cargo or ballast is loaded from the top in such a way of free fall into the tank (loading overall), the cargo (or ballast) will splash. This produces a mist of electrically charged droplets in the ullage space of the tank, as well as an increase in the petroleum gas concentration in the tank. To avoid the explosion, ISGOTT does not allow loading on top for static accumulator cargoes. Volatile petroleum, or non-volatile petroleum having a temperature higher than its flashpoint minus 10ºC, should never be loaded over the top into a non-gas-free tank. There may be specific port or terminal regulations relating to loading over the top. However, non-volatile petroleum having a temperature lower than its flashpoint minus 10ºC may be loaded over the top in the following circumstances:
The free end of the hose should be lashed inside the tank coaming to prevent movement. Ballast or slops must not be loaded or transferred over the top into a tank that contains a flammable gas mixture.
A metal probe, remote from any other tank structure but near a highly charged liquid surface, will have a strong electrostatic field at the probe tip. Protrusions of this type may be associated with equipment mounted from the top of a tank, such as fixed washing machines or high-level alarms. During the loading of static accumulator oils, this strong electrostatic field may cause electrostatic discharges to the approaching liquid surface.
Metal probes of the type described above can be avoided by installing the equipment adjacent to a wall or other tank structure to reduce the electrostatic field at the probe tip. Alternatively, a support can be added running from the lower end of the probe down to the tank structure below, so that the rising liquid meets the support at earth potential rather than the insulated tip of a probe. Another possible solution in some cases is to construct the probe-like device entirely of a non-conductive material. These measures are not necessary if the vessel is limited to crude or black oil service or if the tanks are inerted.
The spraying of water into tanks, for instance during water washing, gives rise to electrostatically charged mist. This mist is uniformly spread throughout the tank being washed.
The electrostatic levels vary widely from tank to tank, both in magnitude and in sign. When washing is started in a dirty tank, the charge in the mist is initially negative, reaches a maximum negative value, then goes back through zero and finally rises towards a positive equilibrium value. It has been found that, among the many variables affecting the level and polarity of charging, the characteristics of the wash water and the degree of cleanliness of the tank have the most significant influence. The electrostatic charging characteristics of the water are altered by recirculation or by the addition of tank cleaning chemicals, either of which may cause very high electrostatic potentials in the mist. Potentials are higher in large tanks than in small ones. The size and number of washing machines in a tank affect the rate of change of charge but they have little effect on the final equilibrium value.
The charged mist droplets created in the tank during washing give rise to an electrostatic field, which is characterised by a distribution of potential (voltage) throughout the tank space. The walls and structure are at earth (zero) potential and the space potential increases with distance from these surfaces and is highest at points furthest from them.
The field strength, or voltage gradient, in the space is greatest near the tank walls and structure, more especially where there are protrusions into the tank. If the field strength is high enough, electric breakdown occurs into the space, giving rise to a corona. Because protrusions cause concentrations of field strength, a corona occurs preferentially from such points. A corona injects a charge of the opposite sign into the mist and is believed to be one of the main processes limiting the amount of charge in the mist to an equilibrium value. The corona discharges produced during tank washing are not strong enough to ignite the hydrocarbon gas/air mixtures that may be present.
Under certain circumstances, discharges with sufficient energy to ignite hydrocarbon gas/air mixtures can occur from unearthed conducting objects already within, or introduced into, a tank filled with charged mist. Examples of such unearthed conductors are a metal sounding rod suspended on a rope or a piece of metal falling through the tank space.
An unearthed conductor within a tank can acquire a high potential, primarily by induction, when it comes near an earthed object or structure, particularly if the latter is in the form of a protrusion. The unearthed conductor may then discharge to earth giving rise to a spark capable of igniting a flammable hydrocarbon gas/air mixture.
The processes by which unearthed conductors give rise to ignitions in a mist are fairly complex, and a number of conditions must be satisfied simultaneously before ignition can occur.
These conditions include the size of the object, its trajectory, the electrostatic level in the tank, and the geometrical configuration where the discharge takes place. As well as solid unearthed conducting objects, an isolated slug of water produced by the washing process may similarly act as a spark promoter and cause ignition.
Experiments have shown that high capacity, single nozzle, fixed washing machines can produce water slugs, which, owing to their size, trajectory and duration before breaking up, may satisfy the criteria for producing incendive discharges. However, there is no evidence of water slugs capable of producing incendive discharges being produced by portable types of the washing machine. This can be explained by the fact that if the jet is initially fine, the length of slugs that are produced are relatively small so that they have a small capacitance and do not readily produce incendive discharges.
Following extensive experimental investigations and using the results of long-term experience, the tanker industry has drawn up the tank washing guidelines set out in ISGOTT chapter 12.3. These guidelines are aimed at preventing excessive charge generation in mists and at controlling the introduction of unearthed conducting objects when there is charged mist in the tank.
Charged mists very similar to those produced during tank washing occur from time to time in partly ballasted holds of OBOs. Due to the design of these ships, there may be violent mist-generating impacts of the ballast against the sides of the hold when the ship rolls in even a moderate sea. The impacts also give rise to free-flying slugs of water in the tank, so that if the atmosphere of the tank is flammable all the elements for ignition are present. The most effective counter-measure is to have tanks either empty or fully pressed up so that the violent wave motion in the tank cannot take place.
People who are insulated from earth by their footwear or the surface on which they are standing can become electrostatically charged. This charge can arise from physical separation of insulating materials caused, for instance, by walking on a very dry insulating surface (separation between the soles of the shoes and the surface) or by removing a garment.
Experience over a very long period indicates that electrostatic discharges caused by clothing and footwear do not, however, present a significant hazard in the oil industry. This is especially true in a marine environment where surfaces rapidly become contaminated by deposits of salt and moisture that reduce electrical resistances, particularly at high humidity.
An increasing number of items manufactured from synthetic materials are being offered for use onboard ships. It is important that those responsible for their provision to tankers should be satisfied that, if they are to be used in flammable atmospheres, they will not introduce electrostatic hazards. These include items such as polypropylene ropes.
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