1. Cargo tank
Atmosphere

2. A tank is an enclosed space which is sealed and may not have any ventilation.
As a result, there is a accumulation of gases which can be dangerous and harmful to humans.

3. when we speak about Cargo Tank atmospheres ?
What do we
FEAR MOST
when we speak about Cargo Tank atmospheres ?

4. There are Different types of Tankers.
Such as:
Crude Oil Tankers
Product Tankers
Chemical Tankers
LNG
LPG
Bitumen Tankers
Fruit Juice Tankers (stainless steel tanks)
Wine Tankers (stainless steel tanks)
BUT, we are speaking about our vessels :
Crude Oil & Product Tankers

5. Hazards associated with tank atmosphere
Health Hazard
Hazards associated with tank atmosphere
Fire Hazard
Contributing Factors:
Oxygen
Flammable Vapors
External / Internal Spark
Contributing Factors:
Oxygen Deficiency
Hydro carbon Gas
Toxic Gases

6. A potential source of ignition hazard on oil tankers
Pyrophoric Reaction
Or simply said
A potential source of ignition hazard on oil tankers
It is a rapid exothermic oxidation with Incandescence.
What is this ?
In an inerted atmosphere, the Hydrogen Sulphide reacts with Iron Oxide (rust) and forms Iron Sulphide.
When a tank is exposed to air,the Iron Sulphide is oxidized back to Iron Oxide This oxidation is accompanied by generation of considerable heat so that individual particles may be incandescent.
How it is formed ?
Prevention ?
All cargo operation is carried in controlled inerted condition.

7. Contents of a Cargo Oil Tank atmosphere in a Petroleum tanker
Vapor + Gas mixture
Oxygen
Ignition Hazard- Static Electricity

8. Fire hazards can be best explained by a Fire Tetrahedron
It describes the four different factors required for ignition of a fire.
Essentially all four elements must be present at the same time for a fire to occur. Removal of any one of these essential elements will result in the fire being extinguished :
Air (Oxygen)
Heat ( Internal / External Spark)
Fuel (Cargo vapours)
Chain Reaction
The Inert Gas is introduced into a tank to remove the Oxygen side of the tetrahedron by reducing the Oxygen content below required percentage for combustion.

9. Introduction of IG in the tank
Chemical Chain Reaction
In absence of any one side of the original Fire Tetrahedron , the risk of a fire is non-existent.
Inert Gas
NO FIRE
No Fire can take Place even in the presence of Heat or Fuel because there is not enough oxygen to support it
Safe atmosphere in tank

10. The Flammability diagram
Inerted Atmosphere- Oxygen less than 8%
Consider a point representing typical condition of tank atmosphere in a inerted condition
Oxygen-Percentage by volume
Hydrocarbons Gas-Percentage by volume
Flammable Mixture 5 10 15 20 F
What happens when cargo oil tank atmosphere is diluted with air or when air is introduced for gas free condition.
Safe Atmosphere passing well below the critical dilution line
Unsafe atmosphere
Path taken by directly Gas Freeing
Path taken by Purging
Flammable Mixture G
Critical dilution line
Path taken by gas freeing after purging 21
Purged Atmosphere- Hydrocarbons less than 2% by voulme & oxygen-less than 8%

11. Controlling Fire Hazard in Cargo Oil Tank
What is the percentage of oxygen required to sustain combustion ?
More than 11 %
What percentage of oxygen are we required to maintain in the cargo tanks ?
By law less than 8 %.
(Some ports require a vessel to maintain less than 5 %.)
A Cargo tank is considered “Inerted” when the oxygen content in the tank is less than 8 % by volume

12. Health Hazards
Hazards inside the tank
Oxygen deficiency. (Minimum 20.8% required for man entry)
Contact with Body parts
Inhalation and Ingestion
Risk of slip / trip/ fall due to cargo cling age in the access areas of tanks.
Hazards outside the tank
Release of tank atmosphere content during key cargo operations.
Accidental release by negligence or mechanical faults.
Accumulation due to still air conditions.
Beware!!
Hidden risk :The risk of exposure to Health is not only inside a tank but that same atmosphere may also be present on the open deck areas.

13. These includes the Key Cargo Operations such as:
On a oil tanker the cargo oil tank atmosphere changes as per various operations the ship is performing.
These includes the Key Cargo Operations such as:
Discharging
Tank Cleaning
Purging
Gas Freeing
Inerting
Loading
When performing these key cargo operations safely
The most important concern is to have a safe & controlled cargo tank atmosphere.

14. General Policy of Cargo tank atmosphere control
Tankers fitted with an inert gas system should maintain their cargo tanks in a non-flammable condition at all times.
It follows that:
Tanks are kept inerted when they contain cargo /cargo residue
The atmosphere within the tank should make the transition from inert condition to gas-free condition without passing through flammable condition.
In practice before any tank is gas freed, it would be purged with inert gas until the hydrocarbon content of the tank atmosphere is below the critical dilution line.
When a ship is in a gas-free condition before arrival at a loading port, tanks should be re-inerted prior to loading.

15. Venting system for a cargo oil tank
Pressure side
Gas outlet
Gas Free Cover
Vaccum side
Primary Venting System
Mast Riser
Secondary Venting System
High Velocity Pressure Vacuum valve

16. Discharging Operation
Shore Hose / Arm
Tank atmosphere is controlled with use of IG
Manifold
Deck Discharge line
Pressure Vacuum valve
From Inert Gas System
Pump room
Empty cargo tank
Ullage space
This increase in ullage space is filled with Inert gas, so that there is no vacuum and no ingress of air in tank
Discharge line
Drop of level & increase of ullage space.
Cargo in the tank
Cargo Oil Tank
Suction bell mouth
Cargo oil Pump
Suction line

17. Tank Cleaning Operation
Manifold
Deck Discharge line
Pressure Vacuum valve
From Inert Gas System
IG introduced to maintain safe atmosphere
Generation of static electricity due water droplets
Pump room
Discharge line
Unsafe Atmosphere
Wash water in slop tank
Cargo Oil Tank
Cargo oil Pump
Suction line
Main Suction bell mouth
Stripping line to Stripping pump or Eductor
Stripping Suction bell mouth

18. Purging (For reducing the hydrocarbon /H2S content )
After tank cleaning there are 3 operations which involve replacement of gas
in cargo tanks.
Namely:
Purging (For reducing the hydrocarbon /H2S content )
Gas-freeing (For man-entry).
Inerting (For preparation before arrival load Port)
And there are 2 principal means of Gas Replacement for above operations, these are :
Dilution , which is a mixing process
Displacement, which is layering process

19. Dilution method Completion of gas replacement Important!!!
Inert gas or fresh air
Dilution method
PV valve gas free cover kept open for venting.
Dilution theory assumes Incoming inert gas / air, mixes with the original gas mixture in tank to form a homogenous mixture throughout the tank,
In result concentration of original gas decreases gradually.
Completion of gas replacement
Important!!!
For complete replacement the entry velocity of incoming gas should be high enough for the jet to reach the bottom of tank.

20. Displacement method Completion of gas replacement Important!!!
Inert gas or fresh air
Purge Pipe kept open for venting.
For ideal replacement a stable horizontal interface should be formed between the lighter gas entering at the top of tank and heavier gas being displayed from the bottom of tank through some suitable pipping arrangment.
In result the tank atmosphere is changed gradually.
Completion of gas replacement
Important!!!
For complete replacement a relativley low entry velocity of gas is required. In practice more than one volume change is necessary

21. Several arrangements are put in use on board tanker, when using dilution or displacement method.
One feature which is common to all is that the inlet and outlet points should be so located that efficient gas replacement can take place through out the tank.
There are three principal arrangement:
Inlet Point Outlet point Principle
Top Top Dilution
Bottom Top Dilution
Top Bottom Displacement or dilution

22. Completion of gas replacement by displacement
Displacement /Dilution method-using bottom cargo lines from one tank to other tank
Inert gas or fresh air from top
PV valve gas free cover kept open for venting from top
PV Valve on auto mode.
Completion of gas replacement by displacement
Completion of gas replacement by dilution
Cargo Bottom line

23. Dilution method-using bottom cargo lines from IG line to tank
Gas free cover on PV Valve for venting from top
Inert gas or fresh air
Completion of gas replacement
Inlet of gas into tank from bottom

24. Gas Freeing blowers Used for Gas freeing purposes
Continuous ventilation during the duration of man entry in cargo oil tank
Butterworth holes for cargo oil tank
Main deck
Ventilation ducts

25. Loading Operation
Bell mouth
Suction line
Cargo in the tank
Ullage space
Drop line
Deck Cargo line
Cargo Oil Tank
Manifold
Shore Hose / Arm
Pressure Vacuum valve
From pump room
IG line
Mast Riser
Venting done through the Mast Riser connected to IG line
Increase of level, & reduction in ullage space.
Increase of tank pressure,as loading is done in closed ullaging system to control the safe atmosphere in tank

26. Portable Gas Detectors on board
For a safe and effective control of tank atmosphere, it is important to know and understand the tank content at all stages of operations.
There a various equipments available on board for the detecting the contents in a cargo oil tank.
Some of these are:
Portable Oxygen Meter (To check oxygen content)
Explosimeter or Combustible Gas Indicator (To measure hydrocarbon content in a gas free condition)
Tankscope (To measure hydrocarbon content in a inerted condition)
Multi-Gas meters (Basically used to check oxygen, hdrocarbon and any one toxic gas for a gas free tank).
Gas detector tubes (To measure other toxic gases such as; ammonia, benzene, carbon monoxifr,mercaptans.)

27. Portable Gas Detectors on board
Oxygen meter
Hydro carbon detector
Multi gas Monitor
Gas Detector Tubes

28. Thank you