Comunicación del inicio del proyecto

3.6.1 General

Matter consists of equal numbers of positive and negative ions and under normal conditions is electrically neutral. This is true of oils, chemical products and other liquids. A redistribution of ions tends to occur when a liquid comes into contact with a solid, giving rise to the formation of an electric double layer where the ions adjacent to a liquid/solid interface are redistributed to produce a layer of ions of the same electrical sign. Close to this layer but slightly further away from the interface a layer of ions of the opposite sign will be formed. Elsewhere the distribution of ions remains unchanged.

When the liquid and solid are separated, as in any pipe line flow, the ions in contact with the solid (pipe) tend to remain attached to the solid and the oil (or other liquid) loses some of its ions, thus becoming electrostatically charged. The separated charges will attempt to recombine, their success depending upon the electrical conductivity of the oil. Black oils, which contain high levels of impurities, have high electrical conductivity and charge recombination is rapid. Water also has high electrical conductivity.

By comparison, most refined products (white oils) contain few impurities and charge recombination in such products is very slow. These products are known as STATIC ACCUMULATOR OILS . The rate of flow through pipes will influence the build-up of charge separation - the faster the flow, the faster the charge separation.

Charge separation will also occur:

• In the filling lines of tanks being loaded with white oils. • When oil mixes with water.

• When air is allowed to "bubble" through white oils and splashing occurs at the surface. • When oil is splashed or sprayed.

• When water droplets in a steam jet (or CO2 particles) leave a nozzle.

• When water passes through tank cleaning machines at high pressure electrostatic charging of fine sprays or mists may result.

3.6.2 Cargo Operations

During cargo operations a potential hazard exists:

• When the cargo being handled is at or above its Flash Point. • When the cargo is loaded into a tank containing flammable vapour.

3.6.3 Electrical Continuity - Bonding and Earthing

Static accumulator oils may be electrostatically charged by pipeline flow. When the charged liquid flows through non-conducting flexible ship/shore connections part of the cargo may be picked up by intermediate metal flanges and other metal parts, which are in contact with the liquid inside the hose.

In order to prevent the accumulation of a dangerous voltage, all metal flanges and parts must be effectively bonded to maintain electrical continuity to earth through the shore or the ship's hull. Generally the bonding of the metal reinforcement in the hoses to their flanges will ensure this.

When ship hoses are to be used for cargo transfer, the Chief Officer is to ensure that electrical bonding is checked prior to commencing operations.

When there is an insulating flange at the shore manifold, intermediate flanges in the ship/shore hose will still be earthed by the continuous path to earth via the ship's manifold and hull.

Investigations have proved that ship/shore bonding wires serve no useful purpose, and they are only to be connected on the insistence of the terminal. Under these circumstances the Chief Officer must ensure that the switch in the bonding circuit is open, before permitting connection to the ship. In addition the bonding wire is not to be connected to the manifold.

Connection to the ship and subsequent disconnection may not take place unless:

• All ullage ports are securely closed and no sampling, sounding or dipping of tanks is in progress.

• No cargo, ballasting, tank cleaning or gas freeing operations are in progress.

3.6.4 Cargo Flow Velocity

Charging of static accumulator oils by pipeline flow will be increased if water is present in the cargo. Charging to an even greater degree can be caused by the subsequent movement of water settling through the cargo in the tank. When the cargo first enters a tank any water on the bottom will be agitated and dispersed into the cargo to re-settle later and cause charging. This can continue long after loading into the tanks has stopped.

To minimise the hazard, flow rates for static accumulator cargoes must be restricted during the initial stages of loading as follows:

• Into Tanks that are not Inerted

The flow velocity through the loading line into each tank is not to exceed 1 metre per second until the cargo has covered the filling inlet and the tank bottom longitudinals, or depth of 0.5 metres above the tank bottom has been reached, whichever is the greater. Thereafter the loading rate may be increased. Loading into tanks, which have not been inerted, is not permitted unless the agreement of the Company has first been obtained

• Into Tanks that are Inerted

Flow rates need not be restricted, however, at the start of discharge a slow pumping rate may be required by the receivers.

3.6.5 Ullaging and Sampling

If the restrictions previously mentioned on flow rates are applicable, metal sampling cans and steel ullage tapes must not be used during loading or for thirty minutes after completion of loading the tank. These restrictions do not apply to the ship's permanent gauging system. In the event of failure of the ship's permanent gauging system M.M.C. gauges are to be used.

The normal practice in Company vessels will be to operate the "closed loading " method by the use of automatic ullaging systems where fitted. On vessels not fitted with automatic ullaging systems, or where the ullaging system is non-operational, the normal practice will be to use M.M.C. gauges.

3.6.6 Loading Overall (Splash filling) (free Fall Loading)

If the cargo has been electrically charged by flow through a shore pipeline, loading overall or splash filling will deliver it to the liquid surface in the tank where the hazard from electrostatic charging, in the presence of flammable vapours, is most likely to occur. For these reasons, therefore loading or

transferring of flammable cargoes– by “loading overall” is not permitted - on the Company’s ships.

3.6.7 Pipeline and Hose Clearing

Compressed air must not be used for clearing pipelines or ship/shore hoses back into a ship's tank if the cargo is a static accumulator and the tank contains flammable vapour.

3.6.8 Steaming Operations

Water droplets issuing at high velocity in a steam jet may become charged by contact with jet nozzles and produce a charged mist.

Steam must NEVER be introduced into a tank, which contains flammable vapour.

If, as a result of hot washing, a tank contains a mixture of steam and flammable vapour, a tank washing machine or other conductor (whether earthed or unearthed) must NEVER be lowered into it. Such conductors may accumulate charge from the steam cloud.

3.6.9 Tank Washing with Water

The injection of water under high pressure through tank washing machines gives rise to the formation of an electrostatically charged mist in the tanks or holds being cleaned. The degree of electrostatic charging varies in direct proportion to the pressure and to the total throughput of water.

It has been found that higher electrical potentials are developed by single nozzle machines whereas twin nozzle machines, which rotate at a higher speed, develop opposite potentials at each nozzle thus producing rapidly changing potentials within the space which do not reach the high levels generated by single nozzle machines.

Impingement of water jets on the tank structure causes disintegration of the jet, which gives rise to the formation of charge water droplets. The coarser drops all assume the same electrical sign while the finer mist takes the opposite sign. Excess water dripping from protrusions will form elongated droplets with pointed ends at, which may occur, a small "corona" type discharge, which could create a hazard under certain circumstances.

A higher risk exists from a slug of water which, after descending through a charged mist, it becomes electrically charged, and on approaching an earthed probe may create an incendive spark between the slug and the probe.

3.6.10 Portable Pumps

The use of portable pumps (such as Wilden or Graco types) can also create a risk of build up of static electricity. Such pumps, when used in hazardous environment, must therefore be properly grounded.

This is normally done by means of a “grounding clamp”. This consists of a wire fitted with a ring- eye, which is attached to one of the bolts on the pump. The other end of the wire has a clamp arrangement, which is to be secured to the nearest/best grounding point. After the ring eye has been mounted properly on the pump the connection must be tested with an Ohm meter to verify that there is a full electrical connection between the clamp and pump. (i.e zero resitanace.)

All vessels must ensure that a suitable arrangement such as the above exists before using portable pumps in a hazardous area.

3.6.11 Tanker Explosions

In the course of investigations into electrostatics, certain new factors have emerged which are relevant to the safety of tank cleaning operations, not only in very large tankers but in tankers of all tonnage’s. It has been demonstrated that electrostatic charging to the water mist, present in the tanks, exists under any cleaning condition, and that when washing operations are stopped the level of charge decreases only slowly in the tank, and can remain present for a long time, especially in the absence of ventilation. The following factors therefore need to be considered:-

• In studying "charge concentration mechanisms", it has been found that insulated objects are not normally present in ship's tanks, but the use of sounding rods might present such a hazard.

• When a sounding rod is lowered into a tank filled with charged mist, a high static charge can be fed into 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 incendive sparking when the operator, the sounding rod, or rope, comes 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.

• 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. However, these machines may sometimes become insulated objects and a potentially hazardous situation may exist in at least the two following cases;-

Where the bonding wire is defective.

Where the hose is disconnected from the hydrant before lifting the machine out of the tank. Such disconnection, prior to lifting, is commonly done in order to drain water from the hose. A coat of paint in good condition is sufficient to insulate the hose flange from the steel of the deck. Under these conditions, when the machine is lifted an incendive spark may jump to the lip of the tank cleaning hole, either from the machine or from the securing rope or from the operator who guides the machine through the hole.

• The following precautions are required to prevent the foregoing hazards:

Not to use sounding rods through any deck opening other than the sounding pipe, either during tank cleaning, or for one hour after cessation of washing if the tank is being blown, or five hours if the tank is not being blown.

To carry out checks on the electrical continuity of bonding wires on the tank cleaning hoses before each use.

To keep the hoses connected to the hydrants until the machines are out of the tank. The draining of the hose can be done by loosening the coupling of the hose carefully to let the air in and by tightening the coupling again.

It is emphasised that while the likelihood of all the factors necessary to lead to an incendive spark being present at any one time is remote, the fact that neglect of the foregoing precautions might lead to an explosion remains a possibility.