Why nitrogen padding ?: 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.

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.

A responsible Officer must supervise the blanketing procedures. The Officer in charge must ensure that all personnel not involved in these operations are restricted from the area. Note that padding via the cargo lines (bubbling through the cargo) is only recommended for low viscose cargoes.

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Following are procedures for 3 different methods of blanketing. Blanketing is to be carried out after gauging and sampling


Blanketing After Loading With Open Venting / Gauging

Blanketing after open venting will normally not create any danger / hazards but the following care must be exercised:

1. In some cases the shore will supply the Nitrogen through the same hose / line through which the cargo was loaded. Then supply pressure must be sufficient to cover some liquid head in the tank plus PV setting. In these cases, this information must be discussed during the preload conference. The discussion should include what precautions the shore will take to make sure the operation is done through a small hose and that the rate will not exceed the capacity of the vent capacity of the tank cargo. Any problems with the shore not being able to comply must be brought to the attention of the management Office before the operation is carried out.


2. Care must be taken that the cargo does not overflow from the tank.

Blanketing After Loading with Controlled Venting / Closed or Restricted Gauging, Without Vapour Return

1. Where possible, fit a pressure gauge to the tank (scale minus 0.5 to plus 1.0 bar). On some vessels the secondary venting system consisting of tank vapour pressure sensor will provide continuous reading of tank vapour pressure.


2. Place the P/V-valve gas freeing cover in open position.


3. For ship supplied Nitrogen, connect N2 supply hose to cargo line at manifold or applicable tank connection.


4. In some cases the shore will supply the Nitrogen through the same hose / line which the cargo loaded. In these cases this information must be discussed during the preload conference. The discussion should include what precautions the shore will take to make sure the operation is done through a small hose and that the rate will not exceed the vent capacity of the cargo tank.
   
   In all cases it is important that the volume of Nitrogen being supplied (size of the line and pressure refer to the guideline charts in the ICS Chemical Tanker Safety Guide is not greater than that which the P/V can handle. Any problems with the shore not being able to comply must be brought to the attention of the Management Office before the operation is carried out.


5. Ensure the cargo line valves to the tank are open.


6. Carefully open the valve controlling the Nitrogen supply.


7. Closely monitor the pressure in the tank during the blanketing.


8. When the required tank atmosphere is reached shut off Nitrogen supply and close the valves.


9. Place the P/V-valve in automatic position and close the gas freeing cover.


10. If a positive pressure is required, slowly re-open Nitrogen supply closely monitoring the pressure.


11. When the required pressure is reached, shut down the Nitrogen supply and close all valves.

Blanketing After Loading with Controlled Venting / Closed or Restricted Gauging, With Vapour Return

1. Where possible, fit a pressure gauge to the tank (scale minus 0.5 to plus 1.0 bar). On some vessels the secondary venting consisting of tank vapour pressure sensor will provide continuous reading of tank vapour pressure.


2. Ensure that the vapour return system remains open throughout the blanketing operation.


3. For ship supplied Nitrogen connect N2 supply hose to cargo line at manifold or applicable tank connection


4. In some cases the shore will supply the Nitrogen through the same hose / line which the cargo loaded. The N2 supply pressure must be sufficient to overcome liquid head in the tank. In these cases this information must be discussed during the preload conference. The discussion should include what precautions the shore will take to make sure the operation is done through a small hose and that the rate will not exceed the vent capacity of the cargo tank.
   
   In all cases it is important that the volume of Nitrogen being supplied (size of the line and pressure refer to the guideline charts in the ICS Chemical Tanker Safety Guide is not greater than that which the P/V can handle. Any problems with the shore not being able to comply must be brought to the attention of the Management Office before the operation is carried out.

• Ensure the appropriate valve(s) to the tank are open
• Carefully open the valve controlling the Nitrogen supply
• Closely monitor the pressure in the tank during the blanketing
• When the required atmosphere is reached, shut off the Nitrogen supply and close the valves including the vapour return system (ship’s valve).
• If a positive pressure is required upon competition of blanketing, reduce the Nitrogen supply pressure.
• Slowly throttle down the vapour return valve
• When the required pressure is reached, shut down the Nitrogen supply and close the valves including the vapour return valve.
• WARNING: Never over pressurise a cargo tank (above the P/V valve setting)

Handling Nitrogen during voyage

When a Nitrogen blanket has been placed on a cargo, it is imperative that the quality of N2 and blanket is maintained according to instructions received.

For certain cargoes, the IBC/BCH codes specify that the blanket has to be maintained under positive pressure. Some shippers may, in addition to any regulatory requirement, set their own requirements for some cargoes to be carried with Nitrogen blanket under positive pressure.

Whenever a ship has been advised of the requirement for a Nitrogen blanket to be maintained under positive pressure, it is very important that the positive pressure is maintained always during the voyage.


Nitrogen blanket with positive pressure

1. Prepare for Automatic feeding of the Nitrogen


2. For ships not equipped with remote pressure readings (computerised), a pressure gauge (scale minus 0.5 to plus 1.0 bar) should be used when taking pressure readings


3. Immediately after placing the blanket after loading, the pressure must be closely monitored for a period to verify that the tank is holding the pressure.


4. If problems are encountered in maintaining the positive pressure, a thorough check must be carried out and any leaks must be rectified.


5. When the tank and line system are verified tight, then the arrangement for automatic feeding of Nitrogen is to be activated / utilised.


6. The positive pressure of the Nitrogen blanket should be maintained 50 mBar always, unless otherwise specified by the Charterer (P/V valves are normally set to lift at 200mBar).


7. At regular intervals, not exceeding 12 hours, take readings of and record the pressure. In the interim period ensure that the “shut off valve” between the tank and the pressure gauge (if remains on between readings) is closed / secured.


8. Record daily the consumption of Nitrogen, pressure & O2 log.

Nitrogen Blanket Without Positive Pressure

1. Check and record the ullage space environment (oxygen content or dew point) immediately after the blanket has been put on after loading
2. If no instruction to maintain O2 or the pressure of N2, then do not to add N2 during voyage.
3. Record daily the pressure and O2 log.

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



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