Cargo tank
Atmosphere
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.
What do we
FEAR MOST
when we speak
about Cargo
Tank
atmospheres ?
There are Different
types of Tankers.
Such as:
ü
ü
ü
ü
ü
ü
ü
ü
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
Hazards associated with tank
atmosphere
Fire Hazard
Health Hazard
Contributing Factors:
Contributing Factors:
ü Oxygen
ü Oxygen Deficiency
ü Flammable Vapors
ü Hydro carbon Gas
ü External / Internal Spark
ü Toxic Gases
Pyrophoric Reaction
Or simply said
A potential source of ignition hazard on oil tankers
What is this ?
How it is formed ?
Prevention ?
It is a rapid exothermic oxidation with
Incandescence.
• 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.
All cargo operation is carried in controlled
inerted condition.
Contents of a Cargo Oil Tank atmosphere
in a Petroleum tanker
Vapor + Gas
mixture
Oxygen
Ignition
Hazard- Static
Electricity
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 :
1.
2.
3.
4.
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.
Introduction of IG in the tank
In absence of any
one side of the
original Fire
Tetrahedron , the
risk of a fire is nonexistent.
Chemical
Chain
Reaction
NO FIRE
Inert Gas
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
The Flammability diagram
Inerted AtmosphereOxygen less than 8%
Consider a point representing
typical condition of tank
atmosphere in a inerted
condition
y Purging
10
F
Path taken b
Hydrocarbons Gas-Percentage by volume
15
5
G
What happens when
by
cargo
oil tank
dir
SafeUnsafe
Atmosphere
ec
tly
Ga
atmosphere
is diluted
sF
passing well
ree
atmosphere
ing
with air or when air is
below the critical
introduced for gas
dilution line
free condition.
Flammable
Critical
dilution
Mixture
line
Pa
th
Path taken b
tak
en
y gas freeing
5
Purged AtmosphereHydrocarbons less than
2% by voulme & oxygenless than 8%
after purging
10
Oxygen-Percentage by volume
15
20
21
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
Health Hazards
Hazards inside the tank
ü
ü
ü
ü
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
ü
ü
ü
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.
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:
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.
General Policy of Cargo tank atmosphere control
Tankers fitted with an inert gas system should maintain their cargo tanks in a nonflammable condition at all times.
It follows that:
ü Tanks are kept inerted when they contain cargo /cargo residue
ü The atmosphere within the tank should make the transition from inert condition to gas
-free condition without passing through flammable condition.
ü 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.
ü When a ship is in a gas-free condition before arrival at a loading port, tanks should be
re-inerted prior to loading.
Venting system for a cargo oil tank
Pressure side
Gas outlet
Gas Free Cover
Vaccum side
Primary Venting System
Secondary Venting System
Mast Riser
High Velocity Pressure Vacuum
valve
Discharging Operation
Deck Discharge
line
Tank atmosphere is
controlled with use of
IG
Shore Hose / Arm
Manifold
Pressure
Vacuum
valve
From Inert Gas System
Pump room
Discharge line
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
Drop of level & increase
of ullage space.
Cargo
in the
tank
Cargo Oil Tank
Cargo oil Pump
Suction bell mouth
Suction line
Tank Cleaning Operation
Manifold
Deck Discharge
line
Pressure
Vacuum
valve
From Inert Gas System
Discharge line
Pump
room
IG introduced to
maintain safe
atmosphere
Unsafe
Cargo Atmosphere
Oil Tank
Wash water in
slop tank
Cargo oil Pump
Suction line
Main Suction
bell mouth
Generation
of static
electricity
due water
droplets
Stripping line to Stripping
pump or Eductor
Stripping Suction bell
mouth
After tank cleaning there are 3 operations which involve replacement of gas
in cargo tanks.
Namely:
üPurging (For reducing the hydrocarbon /H2S content )
üGas-freeing (For man-entry).
üInerting (For preparation before arrival load Port)
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
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.
Important!!!
For complete replacement the entry velocity of incoming
gas should be high enough for the jet to reach the bottom of
tank.
Displacement method
Inert gas or fresh
air
Purge Pipe
kept open for
venting.
Completion of gas
replacement
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.
Important!!!
For complete replacement a relativley low entry velocity of
gas is required. In practice more than one volume change
is necessary
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
Displacement /Dilution method-using
bottom cargo lines from one tank to
other tank
Inert gas or
fresh air from
top
PV Valve on auto
mode.
Completion of gas
replacement by
displacement
PV valve gas free
cover kept open
for venting from
top
Completion of gas
replacement by
dilution
Cargo Bottom line
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
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
Loading Operation
Shore Hose / Arm
Manifold
Deck Cargo line
From pump room
Pressure
Vacuum
valve
Ullage
space
Drop line
Cargo
in the
tank
Cargo
Oil Tank
Bell mouth
Suction line
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:
ü
ü
Portable Oxygen Meter (To check oxygen content)
ü
ü
Tankscope (To measure hydrocarbon content in a inerted condition)
ü
Gas detector tubes (To measure other toxic gases such as; ammonia,
benzene, carbon monoxifr,mercaptans.)
Explosimeter or Combustible Gas Indicator (To measure hydrocarbon
content in a gas free condition)
Multi-Gas meters (Basically used to check oxygen, hdrocarbon and any
one toxic gas for a gas free tank).
Portable Gas Detectors on board
Oxygen meter
Hydro carbon detector
Multi gas
Monitor
Gas Detector Tubes
Thank you