Handling nitrogen - required safety guides for chemical tankers
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:
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:
Sources of Nitrogen Supply
Nitrogen is generally available from the following sources:
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
Following is maintenance guidance which can be carried out for nitrogen generator. Refer to your onboard maker’s manual for ships specific routine maintenance.
Quantity of Nitrogen Required
The anticipated quantity of nitrogen required for any particular purpose must be carefully evaluated, taking into consideration the following factors.
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.
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 :
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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Inert gas systems