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:
 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:
 Oxygen
 Oxygen Deficiency
 Flammable Vapors
 Hydro carbon Gas
 External / Internal Spark
 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
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
Hydrocarbons Gas-Percentage by volume
15
What happens when
cargo oil tank
SafeUnsafe
Atmosphere
atmosphere is diluted
passing well
atmosphere
with air or when air is
below the critical
introduced for gas
dilution line
free condition.
Flammable
Mixture
F
10
5
G
5
Purged AtmosphereHydrocarbons less than
2% by voulme & oxygenless than 8%
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




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.
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.
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.
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
Shore Hose / Arm
Deck Discharge
line
Tank atmosphere is
controlled with use
of IG
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 line
Suction bell
mouth
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:
 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
Inert gas or
fresh air
PV valve gas free
cover kept open
for venting.
Dilution method
Completion of gas
replacement
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.
Inert gas or
fresh air
Displacement method
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
Gas free cover on
PV Valve for
venting from top
Inert gas or
fresh air
Inlet of gas
into tank
from bottom
Completion of gas
replacement
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
Loading Operation
Shore Hose / Arm
Manifold
Deck Cargo line
Mast
Riser
IG line
From pump room
Pressure
Vacuum
valve
Venting done through the
Mast Riser connected to IG
line
Drop line
Increase of level,
& reduction in
ullage space.
Suction line
Ullage
space
Cargo
the
tankin
tank
Increase of
pressure,as
Cargo
loading is done in
Oil Tank
closed ullaging
system to control
the safe
atmosphere
in
Bell mouth
tank
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.)
Portable Gas Detectors on board
Oxygen meter
Multi
gas
Monitor
Hydro carbon detector
Gas Detector Tubes
Thank you
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Venting system for a cargo oil tank