Chemical Tankers / Day 1 / Day 1 Topic 6 Static Electricity
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6.5 Elimination of static electricity
Sampling or manual ullaging.
The normal practice in Company vessels will be operate the “closed loading” method by the use of automatic ullaging systems where fitted. In the event of the failure of the ship’s fixed gauging systems, portable restricted type gauges (eg: UTI, MMC) may be used, bearing in mind the nature of the cargo.
6.5 Elimination of static electricity
Sampling or manual ullaging.
Lowering of non-conducting and intermediate conducting materials may be acceptable in some circumstances, for example plastic sample bottle holders can be lowered safely with natural fibre rope only.
Natural fibre rope should be used because synthetic rope generates significant static charge when sliding rapidly through an operator’s gloved hands.
6.5 Elimination of static electricity
Sampling or manual ullaging.
Never use ropes or tapes made of synthetic materials for lowering into cargo tanks containing a static accumulative cargoes at any time.
6.5 Elimination of static electricity
Sounding pipes.
Operations carried out through sounding pipes are permissible at any time, because it is not possible for any significant charge to accumulate on the surface of the liquid within a correctly designed and installed sounding pipe. A sounding pipe is defined as a conducting pipe which extends the full depth of the tank and which is effectively bonded and earthed to the tank structure at its extremities.
The electrostatic field strength within a metal sounding pipe is always low due to the small volume and to shielding from the rest of the tank.
6.5 Elimination of static electricity
Pipeline and Hose clearing.
Compressed air must never be used for clearing the pipelines or ship/ shore hoses back into a ship’s tank if the cargo is a static accumulator and the tank contains flammable vapour.
6.5 Elimination of static electricity
Steaming operations.
Steam jet from a nozzle generates a mist of charged water droplets.
Therefore, steam should never be injected into a tank that may contain a flammable atmosphere.
If, as a result of hot washing with fixed machines, a tank contains a mixture of steam and flammable vapour, a tank washing machine or other conductor (earthed or unearthed) must NEVER be lowered into it. Such conductors may accumulate charge from the steam cloud.
6.5 Elimination of static electricity
Portable pumps.
The use of portable pumps (such as Wilden or Graco) can also create a risk of static electricity build up. Such pumps, when used in flammable environment, must therefore be properly grounded.
This is normally done by means of “grounding clamp”. This consists of a wire fitted with a ring-eye. First end attached to one of the bolts on the pump and second end should be secured to the nearest grounding point.
After grounding the connections must be tested with an Ohm meter to
verify zero resistance.
6.5 Elimination of static electricity
Tank cleaning equipment.
Fixed and portable tank cleaning machines are normally bonded to the ship’s structure and furthermore, when in operation the washing water provides a path for the electrostatic charge to dissipate.
When portable washing machines are used, all hose connections should be made up before the washing machine is introduced into the tank. Connections should not be broken until after the machine has been removed from the tank. However, to allow draining of a hose, a coupling may be partially opened and then re-tightened before the machine is removed.
6.5 Elimination of static electricity
Tank cleaning equipment.
All tank cleaning hoses supplied for tank washing machines and portable gas freeing fans should be tested for electrical continuity every 3 month or before major tank cleaning operations, whichever is earlier.
Tank cleaning hoses resistance should not exceed:
o 6 Ohms per meter length;
6.5 Elimination of static electricity
Tank cleaning equipment.
For portable tank ventilators:
o at 500 Volts on Megger, max reading should be 0.5 Meg Ohm