Tank Cleaning Guide for Oil and Chemical Tanker

transcript

主题 TopicPage 1 23/4/20

Chemical/oil Tank Cleaning

• Flash point <= 60 degree Celsius

• Annex I or Annex II cargo

Inerted tank

Oxygen content checked: < 8% by vol.

If yes –go ahead

If no-could not carry out tank cleaning.

Oxygen content <8% vol

• If annex I cargo, Please refer to ISGOTT 7.1.6.9

What’s ISGOTT 7.1.6.9 say?

• 7.1.6.9 Tank Washing, including Crude Oil Washing• 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 locations• 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 in the tank 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. (See also Section 7.1.12).

ISGOTT 7.1.12

• 7.1.12 INERT GAS SYSTEM FAILURE• The SOLAS convention requires each ship fitted with an inert gas system

to have a• manual containing detailed guidance on the operation, safety and

maintenance• requirements, and the occupational health hazards relevant to the installed

system. The• manual must include guidance on procedure to be followed in the event of

a fault or failure• of the inert gas system.

Inerted Cargo tank

• If annex II cargo----refer to TSG 7.3.3

7.3.3 TSG

• Precautions when tank washing in an inert atmosphere

Although the atmosphere in a properly inerted tank is incapable of burning, the following precautions should be observed:

• 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.

• The tank should be kept drained during washing. Washing should be stopped to clear any build-up wash water.

When there is a need to maintain an inert atmosphere during tank washing, the following points should be obseved:

• The purity and pressure of the inert gas beig delivered during the washing process should be monitored.

• Before each tank is washed, the oxygen level in the tank should be determined both at point about 1 meter below the deck and at the middle level of the ullage space. At neither location should the oxyge level exceed level exceed 8% by volume.

• If during washing the oxygen level in the inert gas supply exceeds 8% by volume or the pressure of the atmosphere in the tank is no longer positive, washing should be stopped until satisfactory conditions are restored.

Some cargo carried under an inert blanket are not flammable but the atmosphere is inerted for cargo quality reasons. In this case, gradual decay of the inert atmosphere during cleaning is acceptable. In all other cases, if the inert atmosphere is not maintained then the procedures used and precautions taken should be as for an undefined atmosphere.

Non-inerted cargo tank

Oxygen content –not confirmed <8% by vol.

Undefined tank atmosphere

Annex I cargo--- refer to ISGOTT 11.3.3.5

• The only way to guarantee that an explosion cannot occur during washing in an

• undefined atmosphere, is to make certain that there can be no source of ignition.

ISGOTT 11.3.3.5

• 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 60m3 /h.• (b) The total water throughput per cargo tank should be kept as low as practicable and• must in no case exceed 180m3 /h.• (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 60ºC.• (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.

ISGOTT 11.3.3.5

• (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 retightened before the machine is removed.• (i) The introduction of sounding rods and other equipment must be done through a full• depth sounding pipe, if fitted. If a full depth sounding pipe is not fitted, it is essential• that any metallic components of the sounding rod 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 to allow sufficient time for any mist carrying a static charge to dissipate. If• however, the tank is continuously mechanically ventilated after washing, this period• can be reduced to one hour. During this period:

ISGOTT 11.3.3.5

• • An interface detector of metallic construction may be used if earthed to the ship by• means of a clamp or bolted metal lug.• • A metal rod may be used on the end of a metal tape that 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 relied upon to• provide an earthing path.• • Equipment made entirely of non-metallic materials may, in general, be used, for• example, 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.

Annex II cargo--- refer to TSG 7.3.4

Precautions when tank washing in an undefined atmosphere

TSG 7.3.4

Most tank cleaning on chemical tankers is conducted in an undefined atmosphere. In all cases after carrying a flammable cargo, the atmosphere in an empty tank should be treated as flammable. The only way to guarantee that an explosion cannot occur during washing in a undefined atmosphere is to make certain that there can be no source of ignition.

TSG 7.3.4

Good tanker practice will avoid all normal sources but , in addition, the following precautions should be taken if the risk from static electricity is to be eliminated:

TSG 7.3.4

Before washing, the tank bottom should be flushed with water and stripped. The piping system, including cargo pump, crossovers and discharge lines, should also be flushed with water. The flushing water should be drained to the tank designated to receive slops. This operation may not be necessary if the ship is fitted with an efficient stripping system, and the cargo tank and pipelines have been stripped as detailed in the ship’s procedures and Arrangements Manual.

TSG 7.3.4

!!!!! If cargoes are highly water reactive this operation must not carried out.

TSG 7.3.4

When portable washing machines are used, all hose connections should be made p 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.

TSG 7.3.4

Ropes made of synthetic fibers should not be used to support the tank cleaning mac

hines.

TSG 7.3.4

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

• Reticulated wash water should not be used, because it may increase the generation of static electricity.

• Sounding rods and other equipment must be introduced through a sounding pipe reach close to the bottom of the tank and earthed to it. If a sounding pipe is not used the addition precaution in paragraph 7.3.5 should be followed.

• No other material that may create a spark or static electricity should be lowered into the tank.

• Steam should not be injected into the tank.

Carried out tank cleaning

• First step---line and bottom flushing whatever the cargo is Annex I or Annex II

Annex I cargo

• Measure the HC level before tank cleaning.• If HC content < 10 % LEL ,Then carrying tan

k cleaning and continue monitoring the HC level during tank cleaning.

Annex I cargo

If HC level >10% LEL, need ventilation first, when the HC level less than 10% LEL, commence tank cleaning.

• During the tank cleaning, if the HC level > 35% LEL, Stopped tank cleaning and ventilation till the HC level decreased to less than 10% LEL , then tank cleaning restored.

Annex II cargo tank cleaning

Tank washing in undefined atmosphere:During tank washing, Please make sure the following:1. No washing machine have a throughput > 60 M3/H2. No nozzle have a throughput > 17.5 M3/H3. The total water throughput per cargo tank <

110M3/H.

Gas freeing

Before gas freeing, checked/monitored the tank atmosphere, No vapor emission with concentration > 30% LEL /or TLV at deck level as per IBC code.

Tank Cleaning Guide for Oil and Chemical Tanker

Documents