CTM 9.12	Chemical Tanker Manual CTM Use of Nitrogen and Inert Gas	Doc No.: CTM 9.12 Revision: 01 Date: 15 Oct 2024 Issued by: DPA Approved by: MD

1. ATMOSPHERE CONTROL METHODS

Following Atmosphere Control methods are commonly employed:

• Inerting or purging
• Padding (also known as blanketing)
• Drying
• Line clearing (also called pigging and blowing lines) For details See CTM 9.1 A1 Definitions.

2. USE OF NITROGEN (N2)

Nitrogen is commonly used as the inert gas on Chemical tankers for:

Nitrogen is a colourless and odourless gas that can cause oxygen deficiency in confined spaces, and at exhaust openings on deck, during purging of tanks and void spaces.

Nitrogen (N2) is classified as a simple asphyxiate, meaning that it will displace oxygen in high concentrations and create an oxygen deficient (< 21%) atmosphere without any significant physiological effects. Breathing is stimulated and controlled by carbon dioxide (CO2) present in the lungs. As the CO2 level increases, the brain sends a message to increase respiration. When the CO2 level drops, the rate of respiration will also decrease in order to maintain the proper balance.

3. GENERAL GUIDANCE DURING DIFFERENT PHASES OF INERTING

3.1 General

The inerting operation can be divided into six phases:

• Purging before Loading
• Padding of loaded tanks
• Topping up during Loaded Passage.
• Discharging
• Tank Cleaning
• Gas Freeing

3.2 Purging before loading

For most effective and quickest way to achieve purging before loading;

• Enable the displaced air to leave the tank through:
• a purging port, tank cleaning hatch or other opening,
• as remote as possible from the point at which the inert gas enters.
• Loading an inerted tank
• Shut off Nitrogen before loading commences.
• When filling an inerted tank, vent the inert gas that is displaced through the tank high velocity venting system.

3.3 Padding of loaded tanks

Padding may create risk for tank rupture. For details & precautions see CTM 9.4 Section 9 – Tank over pressurization and under pressurization

3.4 Topping up during Loaded Passage (CDI 5.1.6)

• Take precautions to prevent the ingress of air into the tank, through the vacuum side of P/V valve, tank lid etc.
• For above, maintain a slight positive pressure by inert gas in the tank
• Monitor the oxygen content in the tank once a day unless different requirements are issued.



Coution:

• Carry out such testing using a method which does not involve opening the ullage space.


• If the cargo is inerted for quality reasons, then such checks are not necessary.

3.5 Discharge of inerted tanks

• Carefully monitor the tank pressure to avoid vacuum in the tank,
• if so, reduce discharge rate.

3.6 Tank cleaning of inerted tanks

See CTM 9.7 – Section 6 - precautions for tank cleaning & tank atmosphere during cleaning

3.7 Gas Freeing

See CTM 9.7 – Section 8 – gas freeing

4. PRECAUTIONS WHEN HANDLING NITROGEN

The Chief Officer shall arrange for precautions as follows:

• Ensure MSDS sheet applicable for the specific Nitrogen state (liquid or gas) is available.
• Mark the Hatches using placards, of Tanks:
  • being purged using Nitrogen, or
  • which are already in inerted condition to warn that the tanks contain Nitrogen.
• Post Notices near the Gangway when Nitrogen operation is in progress:
  • describing hazards of nitrogen, and
  • PPE to be used
• Post a warning sign at the tank entrance and at the gangway:
  • advising “Nitrogen - Danger of asphyxiation”.
• Agree to the procedure with terminal, for the potentially hazardous supply of nitrogen from shore with respect to:
  • transfer rate, and
  • pressure
• To prevent over-pressurization and the possible rupture a cargo tank:
  • Ensure Adequate capacity and operation of the vent system
  • To allow for the escape of gas which may expand when introduced in the tank from a filing line.
• Ensure if it is possible to do so in a safe manner:
  • For purging, inerting or drying – Keep cargo tank lid open during nitrogen supply from shore.
  • For padding sweeping, pigging or line blowing - use ship’s approved venting system.
• In case of cargoes with vapour return:
  • Ensure the vapour return remains open and
  • Monitor pressure in tank.
• For cargoes without vapour return:
  • Lock the PV valves in open position
  • Closely monitor pressure.
  • Secure the main hatch with strong back but ensure dog legs are not on.

4.1 Safety aspects

By using or handling nitrogen in gas form, the chief officer shall consider the following safety aspects and implement relevant mitigating actions:

• Always use approved testing equipment in the suspect areas.
• Nitrogen is handled under pressure. To prevent structural damage to cargo tanks, always:
  • have a pressure gauge including a "shut off valve" fitted, and
  • monitor the pressure when adding/feeding nitrogen into a closed tank;
• Wear suitable PPE.
• Control the pressure and flow rate of incoming nitrogen.



Note:

• Use of a small hose or reducer prior to manifold will restrict the flow rate. However, pressure must be controlled and monitored by shore as agreed and cross monitored by ship.


• Frequently check the pressure gauges fitted at the manifold and on the tank.
• Monitor that the pressure in the tank does not rise.



Coution:

• A slight overpressure (<50 mbar) is to be expected but the pressure should never be allowed to rise to the P/V valve setting.




Note:

• Set tank pressure alarm - 10% higher than the set P/V valve operating pressure.


• To obtain a homogenous nitrogen atmosphere (preventing air pockets):
  • alternate the exhaust opening(s) in a criss-cross pattern.
  • Always open the next hatch before closing the existing open hatch.
  • Frequently measure the tank atmosphere at the various openings for oxygen and/or dew point.
  • When the required tank atmosphere oxygen level is reached and verified, follow the agreed shutdown procedure.
• Before blanketing, fit a suitably scaled pressure gauge to the cargo tank.

5. HAZARD IDENTIFICATION AND PREVENTIVE MEASURES FOR NITROGEN OPERATIONS

5.1 Hazard identification

Some of the hazards of nitrogen include:

• There is NO warning when an atmosphere is oxygen deficient as a result of nitrogen displacement.
• Any inhalation of nitrogen can cause a loss of consciousness and possible death. Even a small inhalation of nitrogen can lead to fatality (see also MSC.1/1401).
• Inhalation of N2 is possible even when standing in the open.
• Over pressurisation of cargo tanks (By uncontrolled/excessive volumes of nitrogen).

5.2 Best practice recommendations

• Oxygen levels must be maintained at or above the minimum level required by oxygen-dependent inhibitors used with certain self-reactive chemical cargoes (examples: styrene and acrylonitrile).
• Cargo tank gas freeing should only be carried out using the approved venting system on the vessel.

6. NITROGEN OPERATIONS

6.1 Safety precautions when handling nitrogen

Following safety precautions should be considered:

• Areas restricted only to the personnel who are directly involved in the operation. Personnel working in these areas should wear a personal gas detection meter.
• If a space may be oxygen-deficient, all tank openings (besides those used as part of the gas freeing operation) should be kept closed until the space has been cleaned, ventilated, and tested to ensure it is gas-free.
• When nitrogen is provided from shore, the pretransfer conference between the vessel and shore personnel must include a discussion addressing the operational and safety hazards as well as maximum pressures and flow rates.

6.2 Entry into enclosed spaces containing Nitrogen

In addition to above, please see: IMO MSC.1/1401. Guidelines on tank entry for tankers using nitrogen as an inerting medium.

6.3 Purging/Inerting operations

Clean and empty cargo tanks may be inerted with nitrogen prior to loading cargo. It should be ensured that there is sufficient venting capacity to avoid over-pressuring the cargo tank or space.

6.4 Padding operations

Vessels may receive nitrogen after loading in order to reduce the oxygen content in the ullage space of the cargo tank or to apply a positive pressure to prevent the ingress of moisture or oxygen during the voyage. Numerous tank over-pressurisation incidents have occurred during padding operations.

• Preferable that flow should be controlled through a small diameter line fitted with a ball valve, enabling the vessel to control the flow of nitrogen and not through main cargo/vapour line.
• In lieu of above, shore might provide a pressure control system fitted at shore inlet line and the ship can manage the pressure and flow coming on board the vessel.
• In all cases, the delivery rate of nitrogen (m3/hr) should not exceed the operational limits of the tank venting system at any time.

Introducing N2 into the void space over top (Sweeping)

This method is:

• Safest since the change of over pressurization and spills are minimal.
• Takes a long time, and is
• Not suitable for more sensitive cargoes as it is likely to leave air pockets in the void space.
• Only possible if the tank has got a connection for a nitrogen hose, or through vapour return line.

Introducing N2 into the tank by bubbling through the cargo

This method leads to an increased risk of:

• Spillage, and
• Over pressurization.

• Carry out a risk assessment before following this procedure.



Note:

• Spills can occur due to the bubbles of nitrogen pushing the cargo through openings at tank deck level.


• Do not open tank hatches in such cases.
• Carry out venting through PV Valves only.



Note:

• If this method has to be used for high viscosity cargoes, make a separate risk assessment for the specific circumstances.
  In some cases, the shore clears the shoreline with nitrogen and continues the flow of nitrogen for somewhat longer in order to create a blanket.


• Discuss this arrangement in the preload meeting.

6.5 Using nitrogen during discharge

During the discharge of cargo, nitrogen may be used to avoid ingress of air or moisture as the cargo is being discharged. This nitrogen may be supplied by the vessel or from shore.

Precautions must be in place to avoid under-pressure and over pressurisation of the cargo tank during the discharge. This could include the proper setting of pressure/vacuum alarms, and monitoring of pressure, and other similar precautions.

When vapour return cannot be provided for, the chief officer shall adhere to the following:

• Place a suitable pressure gauge on the tank (scale minus 0.5 to plus 1.0 bar).
• Connect Nitrogen supply hose with valve to the appropriate connection.
• Before starting the cargo pump:
  • slowly open the nitrogen supply and
  • allow pressure to gently build up pressure to 100-150 mb.
• Start the cargo pump and slowly increase the speed simultaneously as adjusting the nitrogen supply to balance the pumping rate.
• At full pumping rate, adjust the nitrogen supply to maintain an over-pressure at least 50 Mb.
• Upon completion of the discharge, ensure that the controlled tank environment is maintained.

6.6 Tank cleaning under nitrogen

6.6.1 Tank washing under nitrogen using fixed tank cleaning machines

All tank hatches or other openings should be closed during this operation, and the P/V valves should be at the appropriate setting. The oxygen level in the tank should be regularly monitored to ensure it is within the appropriate range.

6.6.2 Tank washing under nitrogen using portable tank cleaning machines

If a vessel is to carry toxic or flammable cargoes, it is preferable that permanent installation of fixed machines be used or conduct tank cleaning as defined in the Tanker Safety Guide Chemicals for undefined atmospheres.

7. VESSEL WITHOUT A NITROGEN GENERATOR

Vessels without a nitrogen generator may still receive nitrogen from other sources. As such, these vessels should observe the relevant items listed in Section 4 above.

8. VESSEL WITH A NITROGEN GENERATOR

When the nitrogen generator is used, all safety aspects of section 4 of this paper should be taken into consideration. Appropriate alarms to warn of any depletion of oxygen content should be fitted to spaces used for the bulk storage or production of nitrogen, as well as other spaces with nitrogen lines running through them.

9. MISCELLANEOUS OPERATION WITH NITROGEN

9.1 Pigging and shoreline clearing operations

Pigging and shoreline clearing operations can lead to a high volume of compressed gas that can suddenly be released into the cargo tank (vessel or shore), quickly overwhelming the venting arrangement and leading to an over-pressurized tank. This method must be closely monitored to avoid this situation.

9.2 Sampling and gauging operations

When a vessel is operating under nitrogen (even when not required under the IBC Code), closed sampling and gauging devices should be preferably used.

10. LOSS OF NITROGEN OR INERT GAS

• Loss of Inert gas (flue gas or nitrogen) pressure can be due to:

• Leakage in the system, or
• Breakdown of generator, or
• Stoppage of shore supply because of some shore-controlled factor.

• In such case:

• Stop the operation (purging, sweeping or discharging) immediately with agreement from shore.
• Secure all the valves.
• Continuously monitor tank pressure.

• Any failure of the Inerting system shall be reported to the office and the charterers in accordance with the Charter Party.

• The operation shall only be resumed once the reason for loss of nitrogen is rectified.

11. REFERENCED FORMS AND CHECKLISTS

• IMO DSC 15/10
• MSC.1/Circ.1401
• SOLAS Chapter 3
• ISGOTT Chapter 21.2.5
• IBC Code 8.5
• Tanker Safety Guide – Chemicals 6.5.7, 7.3.3, 7.7.1
• CSB US Chemical Safety Board 25th June 2003
• Safety Bulletin – U.S. Chemical Safety and Hazard Investigation Board – No. 2003-10-B – June 2003