This document discusses cargo tank atmospheres on oil and chemical tankers. It notes that cargo tanks are enclosed spaces that can accumulate dangerous gases if not properly ventilated. The main hazards associated with tank atmospheres are fires and health issues caused by oxygen depletion, toxic gases, or flammable vapors. Inert gas systems are used to control tank atmospheres and remove oxygen, preventing fires. Key operations like loading, discharging, and tank cleaning require maintaining a safe inert atmosphere through controlled introduction of inert gas to dilute tank contents. Atmosphere monitoring equipment is used to check oxygen, hydrocarbon, and toxic gas levels during operations.

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

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

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

6. Or simply said
A potential source of ignition hazard on oil tankers
Pyrophoric Reaction
What is this ? It is a rapid exothermic oxidation with
Incandescence.
How it is formed ?
• 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.
Prevention ? All cargo operation is carried in controlled
inerted condition.

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

8. Fire hazards can be best explained by
a Fire Tetrahedron
It describes the four different factors
required for ignition of a fire.
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.
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 :
1. Air (Oxygen)
2. Heat ( Internal / External Spark)
3. Fuel (Cargo vapours)
4. Chain Reaction

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

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

11. 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
Controlling Fire Hazard in Cargo
Oil Tank

12. Health Hazards
Hazards inside the tank
ü Oxygen deficiency. (Minimum 20.8% required for man entry)
ü Contact with Body parts
ü Inhalation and Ingestion
ü Risk of slip / trip/ fall due to cargo cling age in the access areas of
tanks.
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.
Hazards outside the tank
ü Release of tank atmosphere content during key cargo operations.
ü Accidental release by negligence or mechanical faults.
ü Accumulation due to still air conditions.
Beware!!

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

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:
üTanks are kept inerted when they contain cargo /cargo residue
üThe atmosphere within the tank should make the transition from inert condition to gas
-free condition without passing through flammable condition.
üIn 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.
üWhen 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
Secondary Venting System
High Velocity Pressure Vacuum
valve
Primary Venting System
Mast Riser
Pressure side
Vaccum side
Gas Free Cover
Gas outlet

16. Cargo oil Pump
Suction bell mouth
Suction line
Cargo
in the
tank
Ullage
space
Discharge line
Deck Discharge
line
Pump room
Cargo Oil Tank
Manifold
Shore Hose / Arm
Pressure
Vacuum
valve
From Inert Gas System
Drop of level & increase
of 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
Discharging Operation
Tank atmosphere is
controlled with use of
IG
Empty cargo tank

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

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

19. Dilution method
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.
For complete replacement the entry velocity of incoming
gas should be high enough for the jet to reach the bottom of
tank.
Important!!!

20. Displacement method
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.
For complete replacement a relativley low entry velocity of
gas is required. In practice more than one volume change
is necessary
Important!!!
Inert gas or fresh
air
Purge Pipe
kept open for
venting.
Completion of gas
replacement

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
1. Top Top Dilution
2. Bottom Top Dilution
3. Top Bottom Displacement
or dilution

22. 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
Cargo Bottom line
Completion of gas
replacement by
dilution

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

24. Gas Freeing blowers
Used for
ü Gas freeing purposes
ü 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

26. 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:
ü Portable Oxygen Meter (To check oxygen content)
ü Explosimeter or Combustible Gas Indicator (To measure hydrocarbon
content in a gas free condition)
ü Tankscope (To measure hydrocarbon content in a inerted condition)
ü Multi-Gas meters (Basically used to check oxygen, hdrocarbon and any
one toxic gas for a gas free tank).
ü Gas detector tubes (To measure other toxic gases such as; ammonia,
benzene, carbon monoxifr,mercaptans.)
Portable Gas Detectors on board

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

28. Thank you