FSS Chapter 15

Inert Gas system

Systems inert cargo tanks to reduce O2 content to less than 8% (Companies go further to reduce it to less than 5%). Maintains positive pressure in tanks. Eliminates need for air to enter tank during cargo operation. It also allows purging tanks of inflammable vapours so that at no time there will be inflammable atmosphere in the tanks (Including when purging with fresh air).

Plant capacity is 125% of maximum rate of discharge. System delivers inert gas with oxygen content of not more than 5% at any required rate of flow.

Construction material of the components is corrosion resistant.

Inert gas is supplied from

  1. Flue gas from boilers
  2. Gas generator
  3. Nitrogen generator.

Combination of above is permitted.

Safety measures

System pressure shall not exceed test pressure of cargo tank. Automatic shutdown of system and components is on predetermined limits. Shutoff arrangements are provided on discharge of each generator. If oxygen limit exceeds 5%, inert gas is vented to atmosphere.

Arrangement is provided for system to stabilize before commencement of cargo discharge.

System components

Non Return devices: At least 2 non-return devices to prevent return of vapour of liquids. First device is deck seal of wet, semi wet or dry type or double block and bleed arrangement. Two shut-off valves in series with venting valve in between may be accepted if they are automatic and have alarm for faulty operation.

Second non-return device is non-return valve or equivalent that can be closed.

Water seal if fitted is supplied by 2 separate pumps (each can supply adequate amount). Audible and visual alarm is provided for low level. Water seal is protected against freezing but overheating is not done. Looping arrangement is provided for supply.

Non-return devices are located in the cargo area on deck.

Inert gas lines

May be branched forward of non-return valves. Stop valves are used to isolate each tank. Valves have locking arrangement. Control system provides operational status of valves.

Means are provided to protect tanks from overpressure or vacuum caused by thermal variation when cargo tanks are isolated.

Pipeline design prevents accumulation of cargo or water in pipelines.

Arrangement is provided for external supply of Inert gas. It is forward of non-return devices and is isolated.

Inert gas piping never passes through accommodation, service and control spaces.

Indicators and alarms:

Operation of IGS is indicated on control panel.

Instrumentation indicates Pressure forward of non-return devices, O2 content. These are placed in cargo control room. Same is displayed on navigating bridge. O2 content meter is also present in Engine control room.

Audible and visual alarms

They indicate following

  1. O2 content more than 5%
  2. Failure of Power supply to indicating devices
  3. Gas pressure less than 100 mm water gauge (Low pressure)
  4. High gas pressure
  5. Failure of power supply to automatic control system

Alarms are placed in Machinery space and cargo control room to be acknowledged by a RESPONIBLE person.

Additional audible alarm or automatic shut-down of cargo pumps is fitted to indicate predetermined low pressure of IG.

Two O2 sensors are fitted in space containing IGS. If level falls below 19%, they trigger off audible and visual alarm inside and outside space to be received by a RESPOSIBLE person.

Manuals are provide for safe operation and maintenance.

Requirements for Flue gas or Inert gas generator systems

TWO Fuel oil pumps are fitted to Inert gas generator. IGG’s are placed outside cargo tank area. Space containing IGG does not have direct access to accommodation, service or control stations. IGG may be in Machinery space. Space has positive space type ventilation system.

Gas regulating valves

They are automatic and located at the forward bulkhead or forward most gas safe space through which IG main passes.

Cooling and scrubbing arrangement

Means are provided to cool and remove solids and sulphur products.

Cooling water is supplied from 2 sources without affecting ship’s normal operation. Filters are provided to minimise water being carried to blower.

Blowers:

At least 2 blowers are fitted. Required capacity is divided between them with minimum of 33% on smaller blower.

Inert Gas isolating valves

For Flue gas, isolating valve is provided between boiler uptake and scrubbing unit. It has open/shut indicators.

Precautions are taken to prevent leakage of Flue gas.

Indicators and alarms

Inert gas temperature on discharge side is continuously monitored.

Audible and visual alarms are provided for

  1. Insufficient fuel supply
  2. Power supply failure to generator
  3. Low water pressure or low water flow rate to scrubber
  4. High level in scrubber unit
  5. High gas temperature
  6. Failure of IG blower
  7. Low water level in water seal unit

Requirements for nitrogen generator systems

These are additional precautions

System has one or more compressors

Feed air system filters free water, particles and traces of oil from compressed air.

Air compressor and Nitrogen generator may be placed in Machinery space or independent compartment having independent extraction ventilation system capable of at least 6 changes per hour.

Compartment does not have direct access to accommodation, service spaces and control station.

Normally Nitrogen tank or buffer tank is placed in a dedicated compartment

Indicators and alarms:

These are additional alarms

Temperature an d pressure at the suction side of the nitrogen generator.

Audible and visual alarms are provided for

  1. Failure of electric heat if fitted
  2. Low feed air pressure form the compressor
  3. High air pressure
  4. High condensate level at automatic drain of water separator