5 Tanker Terms That Can Help You in

Being Safe On Tankers

Written by Capt Rajeev Jassal on August 7, 2016

Did you know a rich flammable mixture can ignite with just 1 Joule of
energy ? If that does not surprise you yet, let me tell you how much
energy we are talking about here.
By just rubbing your hands with force for just one time produces 12
Joules of energy. Surprised ?
When the ships started exporting oil cargoes, people were too afraid to
sail on them. And those who sailed on them had to listen to something
like

You are sailing on a bomb, you never know when it is going to

explode.

And they were absolutely right about that. Back then there were no
procedures and controls to mitigate the risks imposed by carrying
flammable cargoes in bulk.
Fast forward today and we have all the controls to avoid explosions on
tankers.

But irrespective of type of tanker and controls in place, explosions

are always a concern. But what will be a even bigger concern is the
people working on tankers not knowing about these controls and the
basic terms used on tankers.

Well I am sure we all have gone through basic and then advanced
courses on tanker operations. But sometimes all that we are taught
does not seems to sink in.

In this post we will discuss 5 of the basic terms very commonly used
on tankers.

1. Fire Triangle

Ahh !! We all know what is fire triangle ?

But when we talk about safety on tankers, fire triangle has to have
first mention. It is because every thing we are doing for the safety
on tankers is to avoid the three sides of the fire triangle to meet.

Fire needs three things to start and continue burning. Fuel, air (or
Oxygen) and source of ignition. And when three of these meet, Fuel
will catch fire.
On tankers all the three are present in abundance.

The cargo carried on tankers acts as fuel. Air is present on tankers as

it is present elsewhere. And there are few but enough sources of
ignitions which if not controlled can make a tanker explode.

Our only aim is that these three elements of a fire triangle should not
be present at the same location. In fact almost all fire safety related
regulations are based on achieving this.
For example, regulation for inert gas is for reducing the oxygen level in
the tank. Not allowing lighters, match boxes on deck is for removing
the source of ignition.

Requirement of maximum Loading rate of 1 m/s for static cargoes is to

avoid a spark inside a tank (source of ignition).
There are many things we do on tankers to ensure that three elements
of the fire triangle do not meet. Even the deck foam system used for
deck fires uses the smothering effect to extinguish fire, which aims to
remove one element (air) from the fire to extinguish it.
When we follow these guidelines, we succeed. But if we do not then
this there can be explosion and casualties . Only the knowledge of the
atmosphere on and around tanker vessel can help in avoiding these
situation.

2. Flammable limits

Let me ask a question. You are on a tanker and you measure the
atmosphere of cargo tank by a portable gas meter. The reading shows
80% LEL value. Now if there is any spark inside the tank, will the tank
explode ?

If you know the answer, well done. If you do not know or are in doubt
I hope you will get the answer in this post.

As I said fire (or explosion) needs three things to occur. If we leave

aside the spark, for something not to explode, either there should be
insufficient fuel or insufficient oxygen or both.

Lets talk about insufficient fuel. How much less is insufficient ?

A 10 litres Jar containing 1 litre of gasoline will explode if we spark it.

But a 10000 m3 tank containing same 1 litre of same gasoline may not
explode.

This is because It is not the fuel that burn but the vapors. A mixture of
hydrocarbons and air cannot be burned unless its composition is in the
correct range. This range is called flammable range.
Vapors of 1 litres of gasoline in 10 litres jar are enough for ignition but
in 10000 m3 tank, these are too small and will get diluted in the air
inside the tank.

This concentration of hydrocarbons is measured as percentage

of volume.

When we say 10% hydrocarbons by volume  inside the empty tank of

10000 m3 capacity, it means around 1000 m3 of hydrocarbons are
present. Rest 9000 m3 is air.

Now how much percentage of volume of hydrocarbons is insufficient to

burn ? Each gas will have different percentage. Isn’t it ?

For example, for propane this concentration is 2.3% and

for pentane this concentration is 1.5%.
If the concentration of propane in a tank is 1.5%, and we spark it, it
will not burn. That is because the concentration is below minimum
required to burn the mixture of air and propane.

This minimum concentration of hydrocarbons required in the air for it

to be able to ignite is called lower flammable (or explosion) limit.

Want the text book definition of lower flammable (or explosion) limit ?
Here it is..

Lower flammable (or explosion) limit is the concentration of

hydrocarbons below which there is insufficient hydrocarbon to
support and propagate combustion

Now let us talk about the insufficient oxygen.

When oil is placed in a container, it has a tendency to vaporise.
Different liquids (and different oils) have different tendency to
vaporise. The amount of vapors emitted also depends upon the
surrounding temperature.

The concentration of these vapors dilutes the concentration of other

gases in the air including oxygen.

At some point, these vapors may dilutes the air to a level when there
is insufficient oxygen to ignite these vapors.

This concentration of hydrocarbons is called upper flammable (or

explosion) limit. The text book definition of UFL is

Upper flammable limit is the concentration of hydrocarbons

above which there is insufficient air (or oxygen) to support the
combustion.
Again let us take example of propane. Propane has UFL of 9.5% by
volume. So if the concentration of propane is 11%, we can say that
there will not be enough air to support combustion and this mixture
will not ignite.

Now lets say you measure the concentration of butane in a tank and
found it to be 2.0% by volume. Are you in flammable range ?

Ahh !!

You need to check what is the lower explosive limit of Butane. Right ?
And so do we need to know the LEL of each cargo we load.  That
sounds like crazy. But wait, how about this ?

What if we say for any cargo or flammable liquid,

LEL by percentage of volume = 100% LEL. And then we get an
equipment to measure percentage of LEL.

So, for propane  2.3% Volume = 100% LEL

Now if this equipment shows a reading of 70% LEL, it means it is 30%

below the lower explosion limit.

Returning back to our question, we now understand that when we say

100% LEL, we have just touched the lower explosive limit. Anything
below 100% LEL, we are not in the flammable limit.

When the concentration is above 100% LEL, we need to measure the

concentration in percentage of volume.
Now have you ever come across this flowchart ? This flow chart is from
ISGOTT.

This is the flowchart for tank cleaning on oil tankers in non-inert

atmosphere. If you notice the flowchart tries to maintain the
hydrocarbon concentration between 10% to 35% LEL. The whole idea
is not to allow the tank atmosphere reach in the flammable range
(100% LEL).

3. Flash Point

Flammable liquids has a tendency to vaporise. Some of the the vapors

also turns back into the liquid and these finally attain an equilibrium
state. At this equilibrium state, these flammable liquid may have
a mixture of hydrocarbon and air that is between LFL and UFL
(flammable range).

But not all the flammable liquids will be in flammable range at this
equilibrium. This depends upon the tendency of the flammable liquid to
vaporise.

It won’t be hard to believe that with more temperature, the liquid will
vaporise more. And at some temperature it will release sufficient
vapors to form a flammable mixture.
 Flash point is the minimum temperature at which the liquid
emits sufficient vapors to form flammable mixture with the air.

Flash point is calculated by heating a liquid slowly and then a spark is

applied to the surface of the liquid. The temperature at which surface
of the liquid catches fire indicates the presence of flammable mixture.
This temperature will be the Flash point of that liquid. This method of
determining the flash point is called close cup method.
Now why knowledge of flash point is important ? If the flash point of a
liquid is less than the ambient temperature, it just needs a spark
(source of ignition) to explode. We need to have more stringent
controls for these cargoes.
This is also the criteria for determining the cargoes that would need
stringent controls. Industry has set the ambient temperature criteria
as 60 C as a safety margin.

Liquids with the flash point of less than 60 C are called volatile liquids.
And the one with flash point above 60 C are called non-valatile liquids.

4. Flammability Diagram

Anyone who is or have been on tankers would have seen this diagram
called “flammability diagram”.

The principle used in the flammability diagram is simple. It emphasizes

on the fact that at no point we should be in flammable range during a
critical operation. It also shows us a way to do it.
The principle used in the flammability diagram is simple. It emphasizes
on the fact that at no point we should be in flammable range during a
critical operation. It also shows us a way to do it.

Let us say our cargo tank is at point F (10% flammable cargo vapors,
oxygen reduced by inert gas). We wish to gas free the tank to arrive at
point A where there are no flammable vapors and oxygen content is
21%.

If we start putting air inside the tank, the decrease in concentration of

flammable vapor and increase in oxygen level will follow a line FA. This
passes through flammable range which we need to avoid.

It is thus required to first reduce the concentration of flammable

vapors by inert gas to a point H . After this when we introduce air
inside the tank, the dilution of flammable mixture will follow the line
HA which ensures that we are never in the flammable range during this
process.

Knowledge of flammability diagram reminds us how important it is to

be outside the flammability range.

5. Exposure limits

While we cannot deny that flammability is the top concern for anyone
involved with the tanker operations. But no one can deny this too that
human exposure to toxic gases is will be a close second if not top
concern.

It is the fact that many of he cargoes carried on all tankers contain

deadly gases like benzene and  H2S. Even WHO has recognised the
exposure to benzene as a major public health concern. This makes it
of utmost importance that a person taking first step on a tanker is
aware of presence and exposure limits of these gases.
To know how best to protect ourselves from the health hazards of
these gases, we must know the terms related to exposure limits.
i) Threshold limit value (TLV)
This is also sometimes known as Permissible exposure limit (PEL).
However the term PEL is not used on tankers these days. This is
because the word “permissible” gives the feeling of allowed.
Though this is safer limit but we need to aim for minimum exposure
(and not the permissible exposure) of toxic vapors. Threshold seem to
be a better word for that.

TLV is the maximum concentration of toxic vapors that a

person can be subject to each day without any adverse health
effect.

Though there are three types of TLVs , persons working on tankers

should know at least two of these.

TLV-TWA (Threshhold limit value – Time weighted average)

 This is concentration of toxic vapours that a person can be subject
to continuously for 8 hours in a day without any adverse health effect.
TLV-TWA takes into account the normal working schedule of 8 hours
per day and 40 hours per week.

TLV-STEL (Threshold limit value- Short term exposure limit)

TLV-STEL is the maximum concentration of toxic vapours that a person
can be subjected to for a short duration of 15 minutes without any
adverse health hazard.

Since STEL is for shorter duration than TWA, it will always be more
than TWA value.  But does that mean we can expose a person to the
value of STEL for 15 minutes, take a break for 5 minutes and then
again expose him for 15 minutes. A time duration need to be set. Right
?

So if a person is exposed to toxic vapors concentration above TWA but

below STEL,

 The person can be exposed to maximum 15 minutes

 The minimum duration between subsequent exposure should be 60

minutes

 Exposure cannot be more than 4 times in one day

ii) Odour threshold value

This is the minimum concentration of toxic vapors at which a normal
human being can sense the smell (odour) of the gas.

Why it is important to know the odour threshold value of a toxic

gas ? If odour threshold value is more  than TWA or STEL, you may get
exposed to the toxic gas before you can even smell the presence of the
toxic vapours.
Everyone working on tankers must be aware of the exposure limits of
the vapours released by the cargo onboard. And they should not enter
any compartment which has the vapour concentration in excess of
these exposure limits.

Conclusion

Every vessel type have some must know terms. And like all the
vessels, tankers too have some of these terms which any one
associated with tankers must know before they board a tanker. On
tankers these terms are related to the flammability and exposure limits
of the toxic vapours.

Knowledge of these terms helps the seafarers in identifying the risks.

And once you know the risks, it just needs you to follow standard
practices to mitigate these risks.