Now the cargoes that are not good conductor of electricity would be able to hold the charge for longer period of time. These cargoes possess the hazard of static electricity.
These cargoes are called static accumulators.
ISGOTT has assigned a number to define static accumulator cargoes. As per ISGOTT, these are the cargoes that have conductivity of less than 50 picoSiemens/metre (pS/m).
ii) Free fall in the tank
If a cargo or ballast is loaded from the top in such a way of free fall into the tank, the cargo (or ballast) will splash. This produces a mist of electrically charged droplets in the ullage space of the tank.
To avoid the explosion, ISGOTT does not allow loading on top for static accumulator cargoes.
iii) Water Mist
Like free fall in the tank, water jet from the tank cleaning machines during tank cleaning also generate mist of electrically charged droplets.
iv) Inert gas
Inert gas can carry the electrically charged small particle with it. These particles can be carried into the tank along with the inert gas in to the tank.
v) Other sources
There can be number of other sources on board for static charges generation. And it is not possible to check if static charge exists or not. The best possible way is that whenever in doubt, assume static charge exists.
Precaution against static electricity
Whatever the reason of static electricity generation is, the more important thing is not to allow the explosion.
Even if the electrostatic discharge results in a spark generation, two more things need to be present for the explosion to occur
Flammable mixture which can be ignited
air to support the combustion
If the tanks are in inert condition, there are no specific precautions required for static electricity. This is because there is no oxygen inside the tank to support combustion.
Let us say tanks are not in inert condition and vessel is loading the static accumulator cargo. The flammable mixture and oxygen would be present inside the tank.
To avoid explosions due to static electricity, we need to take steps to
minimize the charge separation and charge accumulation
Avoid the electrostatic discharge
Let us see how we can achieve that
Minimizing the charge separation and charge accumulation
I have already described how charge separation takes place. By friction between two dissimilar materials or by mixing two immiscible liquids among many. If we understand the reason for static charge generation, we would understand the steps required to minimize these.
Linear velocity restriction
When a static accumulator cargo flows through the pipeline, the pipeline lose some of its electrons. The cargo gains these electrons and becomes negatively charged.
This cargo when enters the empty tank, it splashes. This again increases the static charge generation.
To avoid this static charge generation, we need to reduce the friction between the cargo and the pipeline. Also we need to reduce the splashing of the cargo in the tank. This can only be achieved by reducing the rate of flow.
ISGOTT requires that for loading static accumulator cargoes, we must restrict the linear velocity to
1 m/s until the cargo is loaded to a level where there is no splashing during loading. This is generally possible when filling pipes and all other structures at the tank bottom has been submerged to twice the filling pipe diameter.
After all the splashing has stopped, linear velocity can be increased to a maximum 7 m/s.
Calculating the maximum loading rate as per these linear velocities is not that difficult. Here is the calculation for loading rate through 10-inch pipe with linear velocity of 1 m/s.
The linear velocity of loading static cargo
The maximum loading rate for other sizes of pipeline diameter can be calculated in a similar way rates-for-1-m-per-second-static-electricity
When loading static accumulator cargoes, we must not increase the loading rate as per these linear velocities.
Avoiding free fall of the cargo in the tank
Free fall of the cargo from top of the tank causes splashing and thus static charge generation. So far as possible, we should not allow the free fall of the cargo into the tank.
In case of static accumulator cargoes, it is a must to load only through the bottom line unless the tanks are in inert condition.
Linear velocity restriction
When a static accumulator cargo flows through the pipeline, the pipeline loose some of its electrons. The cargo gains these electrons and becomes negatively charged.
This cargo when enters the empty tank, it splashes. This again increases the static charge generation.
To avoid this static charge generation, we need to reduce the friction between the cargo and the pipeline. Also, we need to reduce the splashing of the cargo in the tank. This can only be achieved by reducing the rate of flow.
30 ноя 2021
