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TANK CLEANING

(Ref: ISGOTT Chapter 9)

Supervision

A responsible officer must supervise all tank cleaning and gas freeing operations.


Preparations

Both before and during tank cleaning and gas freeing operations, the responsible officer should be satisfied that all the appropriate precautions in chapters 2 and 6 of ISGOTT are being observed. All personnel on board should be notified that tank cleaning or gas freeing is about to begin.

If craft are alongside the tanker, their personnel should also be notified and their compliance with all appropriate safety measures should be checked.

Before starting to tank clean or gas free alongside a terminal, the following additional measures should be taken:

  • the precautions in chapter 4 should be observed as appropriate;
  • the appropriate personnel ashore should be consulted to ascertain that conditions on the jetty do not present a hazard and to obtain agreement that operations can start.


Tank cleaning and Gas Freeing Concurrently with Cargo handling

As a general rule tank cleaning and gas freeing should not take place concurrently with cargo handling. If for any reason this is necessary, there should be close consultation with, and agreement by, both the terminal representative and the port authority.


Testing of Tank Cleaning Hoses

All hoses should be tested for electrical continuity in a dry condition prior to use and in no case should the resistance exceed 6 ohms per metre length.


Gas Measuring Equipment

In order to maintain a proper control of the tank atmosphere and to check the effectiveness of gas freeing, a number of gas measuring instruments should be available on the ship.

Depending upon the type of atmosphere being measured, at least two of each of the following portable instruments should be available:
    - with a too lean tank atmosphere:

    • flammable gas indicator capable of measuring gas to the lower flammable limit and with the scale graduated as a percentage of this limit.

    - with an inerted tank atmosphere:

    • gas indicator capable of measuring percentage volume of hydrocarbon gas in an inerted atmosphere.
    • oxygen analyser.

    - with an over rich tank atmosphere:

    • gas indicator capable of measuring hydrocarbon gas concentrations above 15% volume in air.

The instruments to be used for gas measurements should be calibrated and tested in accordance with the manufacturer’s instructions before starting to tank clean or gas free.

Tank atmosphere sampling lines should be, in all respects, suitable for and impervious to, the gases present and should be resistant to the effects of hot wash water.


Tank Atmospheres

Tank atmospheres can be any of the following:
    Inerted: an atmosphere made incapable of burning by the introduction of inert gas and the resultant reduction of the overall oxygen content. For the purposes of this guide the oxygen content of the tank atmosphere should not exceed 8% by volume.

    Too lean: an atmosphere made incapable of burning by the deliberate reduction of the hydrocarbon content to below the lower flammable limit.

    Over rich: an atmosphere which is above the flammable range.

    Undefined: an atmosphere which may be above, below or within the flammable range.

Washing in an Inert Atmosphere

Before each tank is washed, the oxygen level must be determined, both at a point 1 metre below the deck and at the middle level of the ullage space. At neither of these Circularss should it exceed 8% by volume. Where tanks have a complete or partial swash bulkhead, the measurement should be taken from similar levels in each section of the tank. The oxygen content and pressure of the inert gas being delivered during the washing process should be continuously recorded.

If during washing:
  • the oxygen level exceeds 8% by volume, or
  • the pressure of the atmosphere in the tanks is no longer positive, washing must be stopped until satisfactory conditions are restored. If it becomes essential on a product carrier to clean tanks while an inert gas system is inoperative, the inert gas manual should be consulted. On crude oil tankers washing should not be resumed until the inert gas system has been made operative or an alternative source of inert gas provided. (for further information on inert gas failure see ISGOTT section 10.12)

Washing in a too Lean Atmosphere

The following precautions must be observed:
    (a) Before washing, the tank bottom should be flushed with water and stripped. The piping system including cargo pumps, crossovers and discharge lines, should also be flushed with water. The flushing water should be drained to the tank designed or designated to receive slops.

    (b) Before washing, the tank should be ventilated to reduce the gas concentration of the atmosphere to 10% or less of the LFL. Gas tests must be made at various levels and due consideration given to the possible existence of local pockets of flammable gas. Mechanical ventilation and gas testing should continue during washing. Ventilation should, as far as possible, provide a free flow of air from one end of the tank to the other.

    (c) If the tank has a venting system which is common to other tanks, the tank must be isolated to prevent an ingress of gas from other tanks.

    (d) If portable washing machines are used, all hose connections should be made up and tested for electrical continuity before the washing machine is introduced into the tank. Connections should not be broken until after the machine had been removed from the tank. To drain the hose a coupling may be partially opened and then re-tightened before the machine is removed.

    (e) During tank washing regular gas tests must be made at various levels. Consideration should be given to the possible effect of water on the efficiency of the gas measuring equipment. Washing should be discontinued if the gas concentration rises to 50% of the LFL. Washing may be resumed when continued ventilation has reduced the gas concentration to 20% of the LFL and has maintained it at or below that level for a short period.

    (f) The tank should be kept drained during washing. Washing should be stopped to clear any build-up of wash water.

    (g) Recirculated wash water should not be used for tank washing.

    (h) Steam should not be injected into the tank.

    (i) The same precautions relating to sounding and the introduction of other similar equipment should be taken as when washing in an undefined atmosphere.

    (j) Chemical additives may be employed provided the temperature of the wash water does not exceed 60 degs C
    (k) Wash water may be heated. If the wash water temperature is 60 degs C or less, washing should be discontinued if the gas concentration reaches 50% of the LFL. If the wash water temperature is above 60 degs C, washing should be discontinued if the gas concentration reaches 35% of the LFL.

Control of Washing in an Undefined Atmosphere

In an undefined atmosphere, the vapours in the tank may be the flammable range. The only way to guarantee that an explosion cannot occur during washing in an undefined is to make certain that there can be no source of ignition.

The following precautions must be taken if the risk from static electricity is to be eliminated.:
    (a) No machine may have a throughput greater than 60 m3/hr.

    (b) The total water throughput per cargo tank should be kept as low as practicable and must in no case exceed 180 m3/hr.

    (c) Recirculated wash water must not be used.

    (d) Chemical additives must not be used.

    (e) Wash water may be heated, but must not be above 60 degs C.

    (f) Steam must never be injected into the tank.

    (g) The tank should be kept drained during washing. Washing should be stopped to clear any build-up of wash water.

    (h) All hose connections must be made up and tested for electrical continuity before the washing machine is introduced into the tank. Connections should not be broken until after the machine has been removed from the tank. To drain the hose a coupling may be partially opened and then re-tightened before the machine is removed.

    (i) Sounding and the introduction of other equipment must be done through a sounding pipe if fitted. If a sounding pipe is not fitted, it is essential that any metallic components of the sounding or other equipment are bonded and securely earthed to the ship before introduction into the tank and remain so earthed until removed. This precaution should be observed during washing and for five hours thereafter. If, however, the tank is continuously mechanically ventilated after washing, this period can be reduced to one hour.
    During this period:

    • an interface detector of metallic construction may be used if earthed to the ship be means of a clamp or bolted metal lug.
    • a metal rod may be used on the end of a metal tape which is earthed to the ship.
    • a metal sounding rod suspended on a fibre rope should not be used even if the end at deck level is fastened to the ship because the rope cannot be completely relied upon as an earthing path.
    • equipment made entirely of non-metallic materials may, in general, be used: e.g. a wooden sounding rod may be suspended on a rope without earthing.
    • ropes made of synthetic polymers should not be used for lowering equipment into cargo tanks.
Washing in an Over Rich Atmosphere

The procedures for making a tank atmosphere over rich and thereafter water washing the tank involve special measures intended to prevent the ingress of air. This method of tank washing should only be carried out when authorised by the operator and under the supervision of a person who has received special training in these procedures.
Water washing must not be started, or if in progress must be discontinued and not re-started, if the hydrocarbon content of the tank atmosphere is less than 15% by volume.


Portable Tank Washing Machines and Hoses

The outer casing of portable machines should be of a material which on contact with the internal structure of a cargo tank will not give rise to an incendive spark. Bonding wires should be incorporated within all water hoses. Couplings should be connected to the hose in such a way that effective bonding is ensured between them.
The coupling arrangement for the hose should be such that effective bonding can be established between the tank washing machine, the hoses and the fixed tank cleaning water supply line.
Hoses should be indelibly marked to allow identification. A record should be kept showing the date and the result of electrical continuity testing.
Washing machines should be electrically bonded to the water hose by means of a suitable connection or external bonding wire.
When suspended within a cargo tank, machines should be supported by means of a rope and not by means of the water supply hose.


Free Fall

It is essential to avoid the free fall of water or slops into a tank. The liquid level should always be such that the discharge inlets in the slop tank are covered to a depth of at least one metre to avoid splashing. This is not necessary when the slop and cargo tanks are fully inerted.


Spraying of Water

The spraying of water into a tank containing a substantial quantity of static accumulator oil could result in the generation of static at the liquid surface, either by agitation or by water settling. Tanks which contain a static accumulator oil should always be pumped out before they are washed with water unless the tank is kept in an inert condition. (ISGOTT section 7.4 contains guidance on handling static accumulator cargoes).


Steaming of Tanks Because of the hazard from static electricity, the introduction of steam into cargo tanks should not be permitted where there is risk of the presence of a flammable atmosphere. It should be borne in mind that a non-flammable atmosphere cannot be guaranteed in all cases where steaming might be thought to be useful.
Steaming can produce mist clouds which can be electrostatically charged. The effects and possible hazards from such clouds are similar to those created by water washing, but the introduction of steam can cause very much higher levels of charging than those produced by water washing. The time required to reach maximum charge levels is much less. Furthermore, although a tank may be almost free of hydrocarbon gas at the start of steaming, the heat and disturbance will often release gases and pockets of flammability may build up.


Leaded Gasoline

Whereas shore tanks may contain leaded gasoline for long periods and therefore present a hazard from tetraethyl lead (TEL) and tetramethyl lead (TML), ships’ tanks normally alternate between different products and ballast and thus present very little risk. Ships employed in the regular carriage of leaded gasoline should flush the bottom of the tanks with water after every cargo discharge unless the tank is to be ballasted.
Entry into ships’ tanks used regularly for the carriage of leaded gasoline should not be permitted unless absolutely essential.


Removal of Sludge, Scale and Sediment

Equipment to be used for further tank cleaning operations, such as the removal of solid residues or products in tanks which have been gas freed, should be so designed and constructed, and the construction materials chosen, that no risk of ignition is introduced.


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