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.

The main reasons for the use of Nitrogen onboard Chemical Tankers is to replace the air atmosphere of the cargo tanks / lines an adjacent space in order to:

• Prevent Fire / explosion hazard
• Prevent an unwanted reaction between cargo/air
• Prevent moisture pick-up by cargo(s)

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Safety Precautions when handling Nitrogen

Any cargo tank which contains nitrogen must have a warning tag secured to the tank lid. The Chief Officer is to control the tag system and tags can only be removed under the Chief Officer’s authority.

Nitrogen can cause oxygen deficiency in confined spaces and at exhaust openings on deck during purging of tanks and void spaces.

To safeguard against accidents caused by oxygen deficiency, always use approved equipment and comply with safe working procedures of the vessel. Nitrogen is usually handled under pressure and therefore in order to prevent accidents only approved equipment must be used.

When filling tanks and other spaces with Nitrogen, extreme care must be taken to prevent over- pressurization. (Ref ICS Chemical Tank Safety Guide Chapter 5)

Uses of Nitrogen

The main uses of Nitrogen onboard are:

• Purging of lines and tanks and adjacent spaces before loading as per IBC/BCH Code Requirements
• Blanketing of cargoes after loading
• Maintenance of nitrogen blanket during the voyage
• Nitrogen displacement during the discharge
• Inerting of void spaces, surrounding cargo tanks

Sources of Nitrogen Supply

Nitrogen is generally available from the following sources:

• Liquid Nitrogen storage / supply plant (normally 99.9 percent purity)
• Onboard Nitrogen generator plant (normally 98 – 99 percent purity)
• Portable Nitrogen cylinders (commercially or ultra high purity)

When Nitrogen is delivered to the ship, a certificate specifying the purity must be requested from the supplier and retained onboard.

Onboard Nitrogen generator safety check

It is expected that the N2 generator onboard is kept in working condition at all times. Normally generators fitted on board vessels have both 99.9% and 95% mode. It can be used for padding highly reactive chemical cargos as well as a substitute for a conventional inert gas system for loading and discharging any Annex I cargoes.

Calibration of some analyzers could be complicated therefore it is imperative that concerned staff should fully familiarize themselves upon joining the vessel as mentioned in the working manual. Regular operation of the system will detect any malfunctioning at an early stage so that we have sufficient time to rectify the problem.

To keep the system in the state of readiness it should be tried out every month without fail in both 99.9% and 95% mode.

Following checks to be carried out regularly as given in the operating manual

• The oxygen analyzer should be calibrated regularly to ensure that it gives the correct reading. The sample line should be clean and free of moisture accumulation.
• The trying out should not be limited to only running the system. The nitrogen produced should be delivered to deck. This can be done by opening any flange on the Nitrogen branch line on deck.
  
  This will ensure smooth operation of the pneumatically operated deck valves as well as pressure control valve and main supply valve in engine room. This practice will minimize the chances of the valves getting seized due to long period of in operation and will also put the entire electronic control system into test.
• Manual operation of the valves should be looked into so that the valves can be operated manually if required.
• Concerned staff should read the manual and do the relevant maintenance as required related to the compressors, filters and auto drain valves.

Following is maintenance guidance which can be carried out for nitrogen generator. Refer to your onboard maker’s manual for ships specific routine maintenance.

1. Check alarm oil level, drain valve, belt, oil leakage during operation of compressor.
2. Change oil of compressor as per running hour or once every year. Check operation of drain valve overhaul as required.
3. Check filter differential is maintained within acceptable limit, when exceeding 0.07Mpa change element.
4. Calibrate oxygen analyzer before every operation. Pass instrument air to check alarm during operation.
5. Check operation alarms once every 3 months by simulating the alarm condition. Take out heater out of shell once every year. Replace charcoal bed once every 5 years.

Quantity of Nitrogen Required

The anticipated quantity of nitrogen required for any particular purpose must be carefully evaluated, taking into consideration the following factors.

• The number of tanks that require maintenance of nitrogen blanket
• Duration of the voyage of which Nitrogen blanket has to be maintained.
• The production / storage capacity of nitrogen generator (if installed)
• Any interim ports in which stocking up may be possible
• The expected weather conditions during the voyage.

Nitrogen Displacement during discharge

Cargoes that require specially controlled atmosphere, according to the IBC/BCH code, must be discharged with either vapour return from the receiving tank or by N2 displacement. Particular care must be taken when a part discharge is to take place in order to ensure that Nitrogen is available to top up the system.

When vapour return cannot be provided for, the following must be adhered to and the Nitrogen required for this operation must be supplied from shore (terminal). This operation must be supervised by a responsible Officer.


Procedures

• Where possible, ensure a suitable pressure gauge is placed on the tank (scale minus 0.5 to plus bar). (On some vessels secondary venting pressure system. Has remote vapour pressure reading)



• Disconnect the Automatic Nitrogen Feeding System, if used during the voyage.



• Connect Nitrogen supply hose with valve to the appropriate connection on the tank or vent line system.



• Before starting the cargo pump, slowly open the Nitrogen supply and allow pressure to gently build up to the lifting pressure of the P/V-valve to ensure proper operation of the same. Closely monitor that the pressure does not exceed the setting of the P/V valve.



• When proper operation of the P/V-valve is verified, 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 of approximately 70 mBar.



• Closely monitor the tank pressure during the entire discharge operation to ensure that balance is always maintained between the pumping rate and Nitrogen supply always.



• Upon completion of the discharge, ensure that the controlled tank environment is maintained throughout the cleaning operation, if so required.

Related Info:

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). ....

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. .....

Supply of Nitrogen from shore - safety guideline
It is a frequent practice at chemical loading ports to control the atmosphere in cargo tanks with nitrogen supplied from shore, for the purpose of drying a tank and its associated piping system, purging a tank before loading the cargo or padding cargo in a tank. The nitrogen may be supplied at high pressure (up to 10 bar) and at a high flow rate......

Failure prevention and maintenance guideline for inert gas systems onboard
All inert gas safety systems must be checked prior to every operation commencing to ensure that correct operation is possible when required. These tests are described in SOLAS, the Inert Gas Operations and Equipment Manual and the manufacturer’s instructions, and must also include the following:- .....

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%. Comply with IBC/BCH code and ICS Chemical Tanker safety Guide requirement for gas freeing for chemical cargoes. (Ref IBC code CH.8) ....






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:

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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