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Cargo line clearance safe procedure for chemical tankers

Why line clearance ? : The lines used for cargo operations must be cleared and the pressure released to prevent solidification, polymerisation, reaction, pitting and danger to personnel connecting and disconnecting hoses or taking samples.

The final onboard cargo line and systems clearance after the hoses / chicksans have been removed and the cargo systems secured is the responsibility of the vessel.

well maintained Cargo pipelines
Fig: Well maintained Cargo pipelines

Line clearance should be carried out immediately after any of the following, unless safety or pollution concerns dictate otherwise:
  1. On the completion of loading (each line system).
  2. After any internal transfer of cargo (tank to tank).
  3. After cargo recirculation operations via the manifold or any part of the line being used.
  4. After a part discharge from any of the tank(s).
  5. On completion of discharge each line system must be cleared according to Regulations (MARPOL).
  6. As a result of prolonged stops in the cargo handling operations which may cause line solidification due to high melting point or freezing weather condition.

In order to prevent blockages from freezing it is important that lines, drain cocks / plugs are cleared from such cargoes as Phenol, Caustics, Acetic Acid, Benzene, Paraxylene, Cresols, Vegoils etc in cold weather.

All cargo manifold lines to be checked by knocking to ensure they are clear prior vessel arriving port. All line clearance operations must be carefully planned to ensure safe and proper execution. Critical Operation Checklists should be created and utilised to assist.

Special care must be taken to ensure that cargo tank and line systems are not over-pressurised or overfilled during such operations After clearing the cargo lines, the respective valves to be kept closed.

In freezing weather the vacuum formed in the lines may suck the cargo from the cargo tanks and causing solidification of pipe lines. Hence it is imperative that the cargo pipe lines are checked daily.

Chemical tanker at sea
Fig: Chemical tanker at sea

Clearing the Line from the Terminal / Barge or Coaster

Normally the Terminal / Barge / Coaster will clear their lines into the loading vessel’s tanks. If line clearance into the vessel’s tanks is not possible the line should be cleared into a Slop Tank (bearing in mind possible reactions with other chemicals / materials which may already be in the tank) or other suitable containers.

Normally the cargo lines are steamed on the outside of pipe line with live steam and drained into the tank via drop and delivery lines of respective tanks by blowing with air/nitrogen , as appropriate, simultaneously .

Deepwell Pump Line Clearance

Before putting pressure on the cargo line systems, the lines should be completely secured with all valves closed and all flanges bolted. When the cargo line system is being pressurised the pump stack valve or drop line valve should be carefully controlled, the valve should be opened and closed repeatedly until the line is clear of all cargo (an indication of this can be obtained by listening for the flow of cargo passing the valve). On completion of the line clearance any residual line pressure must be released into the drip tray or other suitable container in a controlled manner wearing suitable PPE.

General Line Clearance Procedures

This procedure is to be followed whenever line clearance is being carried out by ship or shore.

  • The operation will be supervised by a responsible officer and the procedure clearly agreed with the terminal representative. A record of this agreement is to be recorded in the Port Log and Ship/Shore Checklist.

    All involved in line clearance should wear appropriate PPE.

  • The vessel’s list / trim may complicate the line clearance procedure and special care must be taken under such circumstances to ensure that lines and systems are clear of residues.

  • Ensure that the tank into which the line(s) are to be cleared has sufficient ullage to prevent overflow.

    Ensure a crew member remains at the vessel’s manifold valve for controlling pressure. Tank vent to be kept open to prevent over tank pressurization. During line clearance monitor tank vapour pressure continuously to avoid over pressurization.

  • On completion of clearing the hoses / chicksans systematically close all valves starting at the manifold.

  • Ensure that no pressure remains in the ship’s lines by closing the pump stack or drop line valve(s) last.

  • Ensure no pressure remains in the hoses / chicksans by opening the drain cock outboard of the manifold valve.

  • If further line clearance is required (solidifying cargoes, freezing conditions or low freezing point cargoes) first disconnect the hoses / chicksans, the necessity for further clearance can be ascertained by knocking the pipeline with a suitable instrument, paying particular attention to low points in the system.

  • If the cargo has been loaded via individual manifolds and lines then each system must be cleared into its individual tanks. The off shore manifold section shall also be cleared.

  • Prepare for the displacement by placing blind flanges with suitable connections (steam, air, Nitrogen) on each side of the manifold.

  • Ensure that the medium chosen for the line displacement is suitable for the particular cargo

  • Connect the hoses and supply the selected medium from both sides of the manifold simultaneously.

  • Ensure that the tank / system is not over pressurised and that there is sufficient ullage space in the receiving tank to prevent an overflow, by ensuring that tank vent is open during line clearance and tank vapour pressure is being continuously monitored.

  • Open the pump stack or drop line valves.

  • Slowly open the valves on both sides of the manifold controlling the selected cleaning medium and checking the tightness of all connections.

  • When clearing each line system, open and close the pump stack or drop line valve repeatedly until the flow of liquid is no longer detected passing the valve.

  • When the line is determined as clear, firstly close the supply valve of the purging medium then close the pump stack or drop line valve.

  • Ensure that “dead ends” of pipeline systems and vent systems are cleared also.

  • On completion of each line clearing operation ensure that all valves, flanges and dust caps are properly sealed and secured.

    Precautions to Be Observed During Line Clearance Operations

  • Clear instructions must be issued with regards to the Procedures and Safety issues to be observed during clearing operations.

  • The plan of line clearing must be systematically followed in order to be effective.

  • Good communications between all parties involved in the line clearing operations are of paramount importance.

  • The appropriate PPE must be worn by all personnel involved in the cleaning operations; any personnel not involved in the operation must be kept clear of the area.

  • The choice of clearance medium (steam / air / Nitrogen) must be considered carefully as the choice of the wrong medium may affect both the safety of the vessel and the cargo quality

    - Compressed Air: may contain water/oil vapour / droplets; static electricity hazards

    - Steaming, condensed water will introduce / increase water content of the cargo;

    static electricity hazards ; heat may have an adverse effect on some cargoes and tank/pipeline coatings.

    - Nitrogen: this is the most commonly used medium for clearing lines, however consideration must be given to both the purity of the Nitrogen used and also its use for line clearance when Oxygen dependent inhibitors have been added to the cargo.

  • When applying steam to the inside of pipelines always make sure that the condensate and melted product have an escape route.

  • Risk Assessments must be carried out for Line Clearance operations and maintained on file. Although the line clearance procedure should, if correctly and methodically carried out after each cargo operation, remove all liquid from the cargo lines, the following checks must be carried out in ample time prior to arrival at the discharge port:

  • Check the entire cargo lines including the manifold crossovers for possible blockages by tapping with a suitable instrument.

  • Check that all cargo valves are moveable and not “frozen”.

  • Check that vent systems are free to operate and are not blocked.

  • Ensure that manifolds are free of liquid by opening the drain valves and loosening the flanges (these must be secured after the checks are completed).

  • When dealing with cargoes which freeze at ambient temperatures it is important to check and prevent the cargo lines and valves from becoming blocked or “frozen”. Pay particular attention to PV lines also and continuously monitor vapour pressure in each tank particularly when tanks are heated. During heavy weather freezing temperature, it is possible for PV and IG lines to be filled with solidified cargo blocking these lines.

  • Related 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

    Handling various grade liquid chemicals during loading

    How to prepare a cargo loading or discharge program ?

    How to avoid solidification in cargo tanks ?

    Cargo segregation requirement for chemical tankers

    How to take cargo samples ?

    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

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