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.

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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.



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:

1. Cargo line leakage countermeasures
   There are many reason that may lead to cargo line failure on board chemical tanker. Galvanic corrosion in the cargo and stripping pipelines may cause several leakage. One of the sources of such corrosion in pipelines is variation in corrosion resistance at adjacent points in the piping.


2. Checklist for handling dangerous liquid chemicals in bulk
   Is information available giving the necessary data for the safe handling of the cargo and, where applicable, is a manufacturer's inhibition certificate available? Information on the product to be handled should be available on board the ship and ashore before and during the operation.


3. Recommended temperature monitoring equipments onboard
   Temperature sensors are fitted so that the temperature of the cargo can be monitored, especially where required by the IBC Code. It is important to know the cargo temperature in order to be able to calculate the weight of cargo on board, and because tanks or their coatings often have a maximum temperature limit. Many cargoes are temperature sensitive, and can be damaged by overheating or if permitted to solidify. Sensors may also be fitted to monitor the temperatures of the structure around the cargo system.


4. Practical example of solving tank cleaning problems
   Tank cleaning is essential on-a chemical tanker, but it must be recognised as a potentially hazardous operation, and rigorous precautions should be observed throughout the process. Together with gas freeing, it is probably the most hazardous operation routinely undertaken on a chemical tanker.
   Pre-cleaning /washing of cargo tanks


5. Pre-cleaning /washing of cargo tanks
   Washing between different grades of cargo is the most common reason for tank cleaning. In most cargo sequences on chemical tankers, this cleaning may consist of no more than a simple hot or cold seawater wash. A simple water wash will disperse many types of chemicals and has been found effective between clean petroleum products such as gas oil and kerosene.


6. Final cleaning of cargo tanks prior loading
   Method of final cleaning to be used depends on both previous cargo and cargo to be loaded. As a general rule the tanks and piping shall be completely drained of water or residues before loading. The bottom of the tanks may have to be dried up with rags.


7. Tank cleaning and posoning hazards
   Certain substances affect the tissues locally as an irritant (cashew nut shell oil) or cause grave damage to the eyes, skin or mucous membranes (e g strong acids and caustic). Other substances may be absorbed by contact to the skin without local effects (e g nitrobenzene, aniline).


8. Testing of tanks and cargoes
   Most common tests and checks for oil and chemical cargoes include testing tank walls for cleanliness. Testing is normally carried out by independent surveyors who, according to local practice or a written agreement in the charter party, are accepted by shipper, receiver and owner.


9. Practical tank cleaning methods for various noxious liquid cargo
   Tanks that may have contained monomer or drying oils should first be cleaned with sufficient cold water quantities to avoid polymerization of cargo residues. In some cases, it is necessary to employ tank cleaning chemicals, but their use is generally limited as it may be difficult to dispose of slops.


10. Special tank cleaning method
    If a special method involving cleaning agents is to be used, it may create an additional hazard for the crew. Shipboard procedures should ensure that personnel are familiar with, and protected from, the health hazards associated with such a method. The cleaning agents may be added to the wash water or used alone. The cleaning procedures adopted should not entail the need for personnel to enter the tank.


11. Determining proper tank cleaning by acid wash method
    The acid wash method is used if there is any suspicion that a cargo of aromatics may have been contaminated by a previous oil cargo. The method is also used as a check that a tank is sufficiently cleaned before loading aromatics.


12. Supervision of all tank cleaning and gas freeing operations
    Tank cleaning is essential on-a chemical tanker, but it must be recognised as a potentially hazardous operation, and rigorous precautions should be observed throughout the process. Together with gas freeing, it is probably the most hazardous operation routinely undertaken on a chemical tanker.


13. Disposal of tank washings, slops and dirty ballast - safe method
    During normal operations of a chemical carrier, the main need to dispose of chemical residues, slops or water contaminated with cargo will arise during or immediately after tank cleaning. Final disposal of slops or washwater should be in accordance with the ship's P&A Manual. Tank washings and slops may be retained on board in a slop tank, or discharged ashore or into barges.


14. PV valves -function and maintenance requirements
    Pressure/Vacuum valves are designed to provide protection of all cargo tanks against over/under pressure and provide for the flow of small volumes of tank atmosphere resulting from temperature variations in the cargo tank(s) and should operate in advance of the pressure/vacuum breaker, where IG system is in use....


15. Deck seal, tank non return valves and tank gauging requirements
    On vessels fitted with an inert gas system it is a requirement to maintain a positive seal between the cargo tanks and the inert gas generation plant this is usually accomplished by the use of a non - return valve and a Deck Water Seal...


16. Loading / stress computer
    This instrument is provided to supplement the stability booklet for the vessel. It allows the Officer responsible, to carry out the various complex calculations required to ensure that the ship is not overstressed or damaged during the carriage of the nominated cargoes..


17. Various cargo handling safety equipments carriage requirements
    It is essential on chemical tankers that everyone knows his ship's safety equipments thoroughly prior handling noxious chemical cargo. Also the master/chief officer must assume responsibility for this.


18. Vapour emission control requirement for chemical tankers
    Vessels fitted with a VEC system must have an independent overfill alarm providing audible and visual warning. These are to be tested at the tank to ensure their proper operation prior to commencing loading, unless the system is provided with an electronic self-testing capability. Fixed gauging systems must be maintained in a fully operational condition at all times. .....


19. Draegar Chemical detector tubes use and reading correction guideline
    These instruments, often referred to as Draeger tubes, normally function by drawing a sample of the atmosphere to be tested through a proprietary chemical reagent in a glass tube. The detecting reagent becomes progressively discoloured if a contaminant vapour is present in the sample. The length of the discoloration stain gives a measure of the concentration of the chemical vapour which can be read from the graduated scale printed on the tube. Detector tubes give an accurate indication of chemical vapour concentration, whatever the oxygen content of the mixture


20. Requirements of various grade chemical cargo heating
    : The voyage orders will contain heating information, if heating is required. As a rule the final heating instructions are given by the Shipper in writing to the Master / Chief Officer in the port of loading. If those written instructions are not given, the master should request them and issue a Letter of Protest if they are not received at departure. In the latter case the management office should be immediately informed.


21. Recommended temperature monitoring equipments onboard
    :Temperature sensors are fitted so that the temperature of the cargo can be monitored, especially where required by the IBC Code. It is important to know the cargo temperature in order to be able to calculate the weight of cargo on board, and because tanks or their coatings often have a maximum temperature limit. Many cargoes are temperature sensitive, and can be damaged by overheating or if permitted to solidify. Sensors may also be fitted to monitor the temperatures of the structure around the cargo system.


22. Cargo instruments
    :In order to maintain a proper control of the tank atmosphere and to check the effectiveness of gas freeing, especially prior to tank entry, several different gas measuring instruments need to be available for use. Which one to use will depend upon the type of atmosphere being measured.


23. Liquid level gauges
    :The accuracy required of chemical carrier level gauges is high because of the nature and value of the cargo. To limit personnel exposure to chemicals or their vapours while cargo is being handled, or during carriage at sea, the IBC Code specifies three methods of gauging the level of a liquid in a tank - open, restricted or closed


24. Overflow control
    :Certain cargoes require the designated tank to be fitted with a separate high level alarm to give warning before the tank becomes full. The alarm may be activated by either a float operating a switch device, a capacitive pressure transmitter, or an ultrasonic or radioactive source. The activation point is usually pre-set at 95% of tank capacity.


25. Oxygen analysers
    :Oxygen analysers are normally used to determine the oxygen level in the atmosphere of an enclosed space: for instance, to check that a cargo tank can be considered fully inerted, or whether a compartment is safe for entry.
    
    Vapour detection
    :Ships carrying toxic or flammable products (or both) should be equipped with at least two instruments that are designed and calibrated for testing the gases of the products carried. If the instruments are not capable of testing for both toxic concentrations and flammable concentrations, then separate sets of instruments should be provided.


26. Alarm circuit
    :An important feature of many modern measurement and control instruments is the ability to signal a particular situation. This can be a main operational alarm that gives an indication of a pre-set situation such as liquid level in a tank, or a malfunction alarm indicating a failure within a sensor's own operating mechanism. The designs and purposes of alarm and shutdown circuits vary widely, and their operating system may be pneumatic, hydraulic, electrical or electronic. Safe operation of plant and systems depends on the correct operation of these circuits and a knowledgeable reaction to them.


27. Venting of cargo tanks safety procedure
    The cargo tank venting system should be set for the type of operation to be performed. Cargo vapour displaced from tanks during loading or ballasting should be vented through the installed venting system to atmosphere, except when return of the vapour to shore is required. The cargo or ballast loading rate should not exceed a rate of vapour flow within the capacity of the installed system. .....

Other info pages

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



Main Info pages!

Home page ||| Chemical hazards ||| Cargo planning & Stowage ||| Cargo loading ||| Cargo documents ||| Safe stability ||| Cargo care ||| Preparation for unloading ||| Inert gas systems |||Gas freeing ||| Nitrogen handling ||| Chemical handling Safe practice |||Handling equipments ||| Cargo & Ballast pumps ||| Cargo tanks |||Tank cleaning |||Special cargoes |||Spills emergencies |||Fire protection



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