Nitrogen is used on Chemical Tankers as the primary Inert Gas for cargo quality control and / or for safety reasons. The requirements for the use of nitrogen, in respect to cargo handling, are either dictated by the IBC/BCH Codes for tank environment control or when the cargo quality / customers require it.

Prior to loading certain chemical cargoes, it may be necessary to purge the tank(s) with Nitrogen, either to reduce the O2 level or in order to displace vapours from the previous cargo(s). Extensive structural damage can, and has, occurred to cargo tanks due to shore nitrogen being supplied at too high a flow rate or at too great a pressure. Great caution is therefore necessary.

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The following are the general procedures for carrying out this operation, however some differences may exist depending on the number of tanks to be purged onboard and the Terminal and its facilities.

A ship specific Critical Operations checklist is to be created for such an operation on board.

• Purging must be supervised by a responsible officer.



• The purging operation must be discussed with the shore person in charge in respect of the following pressure and volume of the nitrogen supply, communications and emergency shut- down procedures of the Nitrogen supply.



• Ensure that pressure gauges are fitted on the manifold and on the tank system to be purged (if not fitted with computerized pressure system). The manifold gauge should have a scale of 0 to 15 bars. The tank gauge where fitted should have a scale of minus 0.5 to plus 1.0 bar.



• The tank, lines, ship hoses and associated fittings must be set and approved by the Chief Officer.



• Check that all vent lines are free and all P/V-valves are working.



• Place the tank P/V-valve in open position (in some cases where the entire vessel is to be purged it will be advantageous, and use less Nitrogen, to set up a “series” purging system of the vessel and vent at the last tank via a tank manhole opening – unless the atmosphere in the tanks is toxic.



• Ensure that any personnel have vacated all tank(s) and place warning signs at strategic points onboard.



• Secure the tank, leaving at least one tank cleaning unsecured hatch open.



• Close the pump stack valve and / or drop line valve.



• Open drain cocks and cargo valve drain plugs.



• Connect the Nitrogen supply hose to the ship’s manifold line / tank to be purged.



• Ensure correct settings of all valves to have Nitrogen flushed through all “dead ends” in the line system.



• The Officer in charge must ensure that all personnel not involved in this operation are restricted from the area.



• Open the manifold valve at the Nitrogen hose connection and instruct the shore personnel to commence Nitrogen purging at low pressure (2 bars)



• Check that the Nitrogen is flowing from the open drain checks and drain plugs. Observe if any water is present.



• Purge the lines until the Nitrogen is dry at all outlets that are flushed.



• If drains and plugs re fitted without a valve, instruct the terminal / shore personnel to stop purging to fit the plugs.



• Open the pump stack valve and/or drop line valve.



• instruct the terminal to gradually increase the purging pressure to the maximum safe pressure agreed upon.



• Check all line fittings for tightness



• If any leaks or blockages are observed, instruct the terminal to stop purging.



• Release the line pressure and rectify the leak(s) and/or blockages.



• Instruct the terminal to restart the purging slowly, gradually increasing to maximum safe pressure.



• Re-check the line system for leaks.



• If fixed tank cleaning machines are fitted, the units must be purged (flushed0 at the end of line purging, before commencement of tank purging.



• If line system is found tight reduce the line pressure and carefully open the pump stack and drop line valves (if fitted) to commence tank purging. To prevent overpressurization and to conduct efficient purging, at least one tank cleaning hatch must be kept fully open throughout the purging operation.



• Frequently check the pressure gauges fitted at the manifold and on the tank. Ensure the agreed line pressure is not exceeded. Also check that the tank pressure does not exceed the P/V valve setting for your ship.



• To obtain a homogeneous Nitrogen atmosphere (prevent leaving pockets of oxygen) alternate the exhaust opening(s) in a crisscross pattern by always opening the next hatch before closing the present hatch. Frequently measure the tank atmosphere at the various openings for oxygen and/or dew point. Use the Nitrogen log (TNK09).



• When the required tank atmosphere level is reached, instruct the terminal to stop the purging.



• Close the line manifold valve.



• Close the pump stack and/or drop line valve(s)



• Instruct the terminal to release the pressure on the Nitrogen hose.



• Open the bleeder valve on the manifold to ensure that the intact pressure has been released from the Nitrogen hose.



• Secure the tank for loading



• WARNING: Never over-pressurize a cargo tank (above the P/V valve setting)

Reference Publications :

• IBC/BCH Code
• IMO – Inert Gas Systems
• Ship’s “Inert Gas Systems Instruction Manual” (Approved by Class)
• Ship’s “Procedure and Arrangements Manual” (Approved by Class)
• International Safety Guide for Oil Tankers and Terminals (ISGOTT)
• ICS Chemical Tanker safety Guide
• SOLAS

Related Info:

1. Purging requirement of chemical tank(s) with Nitrogen - safe method
   Prior to loading certain chemical cargoes, it may be necessary to purge the tank(s) with Nitrogen, either to reduce the O2 level or in order to displace vapours from the previous cargo(s).


2. Nitrogen blanketing - safety method
   Nitrogen Blanketing or Padding is used to establish a positive pressure on a tank which has already been filled with product in order to prevent the ingress of air or water as the tank cools and thus prevent a possible dangerous reaction / damage between the cargo and water/air.


3. Supply of Nitrogen from shore - safety guideline
   The nitrogen may be supplied at high pressure (up to 10 bar) and at a high flow rate. Agreement on the procedure for handling the nitrogen is paramount, and should be part of the pre-loading checklist between ship and shore, with emphasis on a clear understanding of the transfer rate and pressure


4. Failure prevention and maintenance guideline for inert gas systems onboard
   Continuous availability of inert gas and its correct use is highly important for chemical tankers. To ensure its availability it must be operated, tested, and maintained at regular intervals, and in accordance with the manufacturer’s instructions and the vessel’s Planned Maintenance System.


5. Gas freeing safety precautions for chemical tankers
   A space is considered as “gas free” when the concentration of flammable gases in its atmosphere is less than 0% LEL, the concentration of toxic gases (including IG components) is less than the TLV and the Oxygen concentration is not less than 20.8%.


6. Poisoning and required first aid treatment onboard
   The poison is a very toxic substance which when absorbed into the human body by ingestion, skin absorption, or inhalation produces a serious or fatal effect. Poison may enter the human body orally, by inhalation, or by skin contact. After being absorbed by the body it may affect certain organs or give a general poisonous effect. Lately the cancerogene effects of some industrial chemicals have been noticed. This has led to significant reductions of hereto accepted TLV- values in many countries.


7. How to test a tank environment prior entry ?
   Entry into an enclosed space that is not in normal daily use, great care should be taken to create and maintain safe working conditions, even if the duration of the work is to be short. Many fatalities in enclosed spaces have resulted from entering such spaces without proper supervision or adherence to agreed procedures. In almost every case the fatality would have been avoided if the simple guidance in this section had been followed.

More Info:

Isolation of cargo tanks and piping systems

Ship shore cargo connection safe method

Venting of cargo tanks safety procedure

Ship to ship transfer operation

Ship to ship transfer guideline using VEC systems

Gas freeing safety precautions for chemical tankers

Poisoning and required first aid treatment onboard

How to test a tank environment prior entry ?

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