Features: - Tanker Operator

TECHNOLOGY - TANK CLEANING
How efficient are
cargo tank cleaning
chemicals?
Understanding what is required to clean between different grades of liquid cargo on a
tanker is challenging enough when one considers the various elements that usually have
to be taken into consideration before a tank cleaning operation.*
The four main checklists that should
be considered are:-
1) The chemical and physical
properties of the cargo being
cleaned.
2) The chemical and physical properties of
the next nominated cargo; which will
directly impact on the type, duration and
intensity of the necessary cleaning.
3) The material with which the cargo tank is
lined; stainless steel, organic coating,
inorganic coating, other specialised
coating.
4) The experience of the crew carrying out
the cleaning operation.
But throw into the equation the massive
choice and variety of branded and nonbranded
tank cleaning chemicals and the
degree of complexity moves to another level.
If one considers only the branded cleaning
chemicals, there are literally hundreds to
choose from, all apparently available for the
purpose of cleaning cargo tanks. As one might
expect with such a large number of products
available, there is a considerable price
differential between the various products and
their respective suppliers; with most suppliers
claiming that their products are better than
their competitors.
Cleaning chemicals are usually used when a
vessel is cleaning from a cargo that is oil
based and/or insoluble in water and with this
in mind, they are generally termed as
‘hydrocarbon removers’ or ‘degreasers’. They
are diluted in water and applied to the
contaminated cargo tanks either by means of
recirculation, direct injection into the vessel’s
tank cleaning system or by manual
application, depending on the extent and the
severity of the contamination.
The majority of cleaning chemicals contain
detergents, although perhaps a more accurate
name would be surface active agents, based
What is needed is a cleaning chemical that effectively cleans the bulkhead surfaces –
surface active cleaners, explained L&I Maritime.
on the mechanism that makes them effective
as cleaning agents.
Basically, when we say a cargo tank is
contaminated with a previous cargo, it is
actually the cargo tank bulkheads that we are
talking about. So clearly what is needed is a
cleaning chemical that effectively cleans the
surfaces of the bulkheads, hence the need for
surface active cleaners.
It should also be considered that the internal
surface area of any cargo tank is relatively
small compared to the volume, which means
that only relatively low concentrations of
detergents should be required to effectively
clean any particular cargo tank. However, in
order to maintain recirculation inside a cargo
tank, or optimise the working pressure of a
tank cleaning system, significant volumes of
water need to be used on board vessels and
this can and does have a significant impact on
the amount of cleaning chemicals that are
required for any particular operation.
But now the actual mechanism that makes
detergents effective should be considered. It is
found that the surface active agents in the
cleaning chemical become concentrated at the
oil-water interface, thereby applying the
necessary cleaning action at the place where it
is most required, which in the case of a cargo
tank, is the internal surfaces of the bulkheads.
So even if large volumes of water are required
to ‘carry’ the cleaning chemicals, the amount
of detergent required to effectively clean the
cargo tanks is not directly related to this
volume, it is related to the surface area of the
cargo tanks.
No magic formula
Unfortunately, there is no magic formula that
can be used to indicate the most effective
concentration of cleaning chemical for a cargo
tank (based on its size), but it should be
accepted that the most effective detergents are
‘most effective’ at low concentrations and if a
cargo tank can be cleaned with 50 litres of
cleaning chemical, this will be largely the
same irrespective of the volume of water in
which the cleaning chemical is diluted.
It should be noted that this theory only
applies to surface active cleaners, but
November/December 2009 TANKEROperator 27