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Noxious liquid chemicals discharging safe practice - handling special cargoes

Ballast and heated cargoes

The ballast tanks underneath a cargo tank with heated cargo should not be used for ballast and trimming purposes until the tank has been discharged entirely, otherwise the cargo temperature above the tank floor may drop below the melting point before discharging is completed. This may lead to solidification of the cargo on the tank floor and pump well, which increases the amount of residue as well as increasing cleaning. In addition, solidified or high viscosity cargoes of MARPOL Cat Y will require prewash in this situation.

Discharging cargoes under nitrogen cover

Many cargoes carried under nitrogen cover must be discharged using vapour-return-lines. The tank pressure must be in excess of atmospheric pressure throughout the whole discharge period to prevent ingress of air i.e. oxygen.


Discharge of high viscosity products

Prior to the discharge of highly viscous vegetable oils, agreement must be reached with the terminal as to whether the cargo can be pushed towards the pump well so as to keep the residue in the tank as low as possible.


Discharge of cargoes with high melting points

Prior to discharge of cargoes with high melting points it is advisable to arrange for “Tank Dry” acceptance on a tank by tank basis with the surveyor. Once a tank has been accepted as being dry a small amount of water can be taken in to wash the pump.


Efficient stripping in accordance with P&A Manual

Each tank should be stripped in accordance with the procedure as described in the P&A Manual in order to minimise the residues. It is important that list and trim and time for stripping comply with the vessel P and A manual.

Chemical tanker pipeline system
Fig : Chemical tanker pipeline system


The stripping procedure described in the cargo system instruction manuals are to be followed. On vessels fitted with hydraulic systems and submerged pumps, stripping is achieved with the cargo pumps. Final draining of the tanks is accomplished by careful control of the cargo pump. The pump discharge line is cleared by purging the pump and discharge line with inert gas in the form of nitrogen. Air may be used with certain cargoes of a non volatile nature and which are not static accumulator cargoes. This procedure is described in the pump instruction manual.




Make sure that the cargo residues in the hose connection between ship and shore are not blown back into the ship’s cargo lines.


Before loading arm disconnection

Before disconnecting the loading arm from the manifold, the Master must be sure that the following checks have been carried out:

Prewash according to MARPOL Annex II

Following discharge of Category ‘X’ substances or highly viscous or solidifying Category ‘Y’ substances or breakdown of the vessel pumping or stripping system as described in P and A manual, the tanks previously containing this cargo should be prewashed before the vessel leaves port in accordance with the P&A Manual.

The prewash slops must be disposed of in the port of discharge. A receipt for the disposed slops must be received and appropriate entries must be made in the cargo log and in the cargo record book.

MARPOL Inspector to endorse cargo record book after each prewash. If there is any doubt or ambiguity as to the Pollution Category of the cargo, the Company must be contacted immediately before any operations commence for guidance and further instructions.

In special cases when ship is exempt from prewash and unloading of washings to reception facilities.

At the request of the ship’s master, the Government of the receiving party may exempt the ship from prewash of the tanks and unloading of washings to reception facilities provided that it is satisfied that:
  1. The unloaded tank is to be reloaded with the same substance or another substance compatible
  2. with the previous one and that the tank will not be washed or ballasted prior to reloading; or
  3. The unloaded tank is neither washed nor ballasted at sea. The prewash in accordance with the P & A Manual shall be carried out at another port, provided that it has been confirmed in writing that a reception facility at that port is available and is adequate for such a purpose; or
  4. The cargo residues will be removed by a ventilation procedure approved by the Administration in accordance with the P & A Manual Exemptions referred in this cases must be endorsed by Government surveyor in the cargo record book.




Related Info

Cargo handling equipments for handling noxious liquid substances in bulk

How to prepare a cargo loading / discharge plan ?

Procedure for discharging dangerous chemical cargo at sea - various restrictions

Restriction of radio equipments and other mobile devices in cargo working areas

Importance of environmental care & requirements of MARPOL Annex II for chemical tankers

Cargo discharging safety procedure

Sweeping requirement of cargo residues for animal and vegetable oils upon completion of discharge

What is closed unloading operation ?

Cargo hose disconnection - Personal Safety on Chemical Tankers

How to prepare a cargo loading / discharge plan ?

Technical readiness prior loading operations

Voyage planning and related considerations

Cargo sampling safety precautions

Cargo calculation

Signing a bill of lading and related guideline

Preparation for cargo operation

Preparing a cargo tank atmosphere

Liaison between ship and shore

How to prevent cargo pipeline leakage

Ship shore safety checklist while alongside a terminal


Reference publications

  1. IBC Code / BCH code
  2. International Safety Guide for Oil Tankers and Terminals (ISGOTT)
  3. ICS Chemical Tanker Safety Guide
  4. Ship’s “Procedure and Arrangements Manual” (Approved by Class)
  5. Certificate of Fitness for the Carriage of Dangerous Chemicals in Bulk
  6. Ship’s “VEC System Operational Manual” (Approved by Class)
  7. Ship to Ship Transfer Guide (Petroleum)
  8. Tank Cleaning Manual




Other Info:

Voyage planning and related considerations

Preparation for cargo operation

Preparing a cargo tank atmosphere

Cargo unloading operation safety precautions

Liaison between ship and shore

Cargo line leakage countermeasures

Checklist for handling dangerous liquid chemicals in bulk

Recommended temperature monitoring equipments onboard

Practical example of solving tank cleaning problems

Pre-cleaning /washing of cargo tanks

Risk & hazards of chemical contamination onboard

Cargo compatibility and reactivity of various chemical cargo

Poisoning and required first aid treatment onboard

Chemical tanker safe mooring practice

Determining presence of contaminants in chemical cargo

How to avoid solidification in cargo tanks ?

Cargo segregation requirement for chemical tankers

How to arrange disposal of tank cleaning waste ?

Restrictions on discharge cargo residue into sea

Retention of slops on chemical tankers

Vapour emission control requirement for chemical tankers

Handling self reactive chemicals

Handling of toxic chemical cargoes

Pre-loading meeting safety consideration

How to determine chemical cargo temperatures at different level ?

How to take cargo samples ?

How to avoid solidification in cargo tanks ?

Cargo line clearance requirement for chemical tankers

How to arrange disposal of tank cleaning waste ?

Care of cargo pums - risk of pump overload or underload





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