Plentiful of tank explosions which happens in the tanks during operation and dry docking of ships are blamed to be because of Static electricity.
It would be savory on our part that we once more discuss the issue of importance in our own way of learning that is by cutting the “crap” and putting the point straight.
So here are we:
- All matter are made of an equal number of positive and negative ions and under normal conditions is electrically neutral.
Electrostatic charging occurs when these ions are separated. The separated charges will attempt to recombine (which produces spark), their success depending upon the electrical conductivity of the product.
LIKELY OCCASIONS WHEN CHARGE SEPARATION WILL OCCUR
(i) In the filling lines of tanks being loaded with white oils.
(ii) When oil mixes with water.
(iii) When oil bubbles through white oils, and splashing occurs at the surface.
(iv) When oil or slops are splashed or sprayed (i.e. loading overall).
(v) When water droplets in a steam jet (or CO2 particles) leave a nozzle.
(vi) There is a possibility that high pressure tank cleaning machines may
produce electrostatic charging of fine sprays or mists.
(vii) There is a chance that electrostatic charging may occur in slack slop
Holds in O.B.O.'s, where a free surface exists.
BONDING & EARTHING
In order to prevent the accumulation of a dangerous voltage on the flanges of ship/short hoses, all metal flanges and parts must be bonded efficiently.
Generally the bonding of the metal reinforcement in hoses provides a continuous path to earth via the manifold and hull.
The Chief Officer with his team does check all cargo hoses for correct bonding, prior to starting cargo operations.
- Prior to using portable tank cleaning machines, the electrical continuity of tank cleaning hoses to be checked, as well.
- Ship/shore bonding wires should only be connected if the Terminal insists. The officers should ensure that switch in the bonding wire circuit is open, before the wire is connected to the ship.
This is never to be done:-
(i) When cargo is being handled.
(ii) During Ballasting or Gas freeing.
ULLAGING & SAMPLING
- Whenever possible Ships should operate the "closed loading" method,using automatic ullaging systems.
- Ships not fitted with the latter should use wooden floats with non conducting ullage tapes. Wooden dip sticks should be used for topping off.
- Metal sounding tapes should only be used for bunker and ballast tanks.
- Water droplets issuing at high velocity in a jet of steam may become charged by contact with jet nozzles and produce a charged mist.
- Steam should never be introduced into a tank that contains flammable vapour. If it is necessary when cleaning tanks to use steam its injection velocity must be kept as low as possible.
- Steam must never be injected into a tank containing flammable mixture with a tank washing machine or other conductor (whether earthed or unearthed) suspended in it and such equipment must never be lowered into a tank containing flammable mixture and steam. Such conductors may accumulate charge from the steam cloud.
PORTABLE TANK WASHING MACHINES
- These are normally bonded to the ships structure and furthermore, when in operation the washing water provides a path for an electrostatic charge to dissipate.
- However, these machines may sometimes become insulated objects and a potentially hazardous situation may exist in the two following cases:-
(i) Where bonding wire is defective.
(ii) Where the hose is disconnected from the hydrant before lifting the
machine out of the tank as is sometimes done in order to drain water
from the hose.
- A coat of paint in good condition is sufficient to insulate the house flange from the steel of the deck. Under these conditions, when the machine is lifted the incentive spark may jump to the lip of the tank cleaning hole, either from the machine or from the securing rope.
- Tank cleaning hoses are therefore to remain connected to their hydrants until the machines are out of the tank. Draining of the hose can be done by loosening the coupling at the hydrant carefully, to let air in, and by
tightening the coupling again.
- When a sounding rod is lowered into a tank filled with charged mist, a high static charge can be fed to the operator via the wet suspension rope, if the operator is insulated by the deck paint or his footwear.
Such a charge accumulation has been shown to be sufficient to cause sparking when the operator, the sounding rod, or rope, come into contact with the ship's structure.
- This, of course, does not apply if the rod is lowered into a sounding pipe extending to the bottom of the tank.
- On no account therefore, are sounding rods should be lowered into a tank during cleaning, or for one hour after completion of washing, if the tank is being blown, or for five hours if the tank is not being blown.
STATIC ELECTRICITY AND INERT GAS
- Measurements carried out on board a number of tankers have shown that scrubbed boiler flue gas used in tankers to inert the cargo tanks may carry an appreciable electric charge.
- In the tests charge densities and space voltages were found to vary appreciably and it was difficult to relate them to specific conditions of boilers and/or the scrubber systems.
- There are strong indications that the origin of the electric charge is the combustion process and that electric charge resides on the fine particles in the flue gas which are not removed in the scrubber.
- In one shipboard test space voltages of atleast 50 Kv were observed inside a VLCC tank due to inert gas admitted during an apparently normal cargo discharge.
- This figure should be compared with levels in the range of 20 Kv found during the tank washing investigations.
- Because of a very high potential that may be carried in inert gas particulates it is not wise to assume that corona discharges from introduced conducting equipment will be non-incendive.
- In normal operation, the inert gas system will be used whenever cargo is handled and therefore an explosive mixture will never exist inside the tanks.
- However, in the case of a failure of the inert gas system and the introduction of air, potential hazards due to static electricity may arise.
- In view of this potential hazard the problem has been investigated by the ICB Tanker Accident Working Group and the following procedures for safe operations are recommended:-
(a) If the inert gas plant breaks down during discharge and air enters the
tank, no dipping, ullaging, sampling or other equipment should be
introduced into the tank for 30 minutes after the cessation of the injection
of inert gas. After 30 minutes equipment may be introduced provided that
all metallic components are securely earthed.
(b) During the re-inerting of a tank following a breakdown and repair of the
inert gas system, no dipping, ullaging, sampling or other equipment
should be inserted until it has been established that the tank is inert. This
should be done by monitoring the efflux gas from the tank being inerted,
when it is known that the efflux gas is fully representative of the gas
condition throughout the tank. However, if it is necessary to introduce a
gas sampling system into the tank for this purpose, there should be a
wait of 30 minutes following the cessation of inert gas injection before
insertion of the sampling system. Metallic components of the sampling
system should be securely earthed:
(c) During the initial inerting of a non-gas free tank, the same precautions
should be taken as when re-inerting after breakdown and repair of the
inert gas system.
(d) The earthing of introduced metallic conducting equipment as
recommended above should be maintained until 5 hours have elapsed
from the cessation of inert gas injection.
Static electricity for the seafarers happens to be a sort of mysterious source of fire which is something coming out, as if “from inside the ship’s hull”.
We seafarers are not scientists but we work with cutting edge technologies and are fed with outlandish “fundas” of science, which we gobble without even raising our eyebrows as if we are born with that knowledge.
Nevertheless “Once a seaman, always a seaman”.
Note: (please also input your comments also to correct the mistakes or to add the missing points, thanks)