Features: - Tanker Operator

TECHNOLOGY - TANK CLEANING
essentially, using a cleaning chemical at a
dilution rate of 10% in water will not
necessarily make it 10 times more effective
than using the same cleaning chemical at a
dilution rate of 1%, and this is extremely
important to consider.
Having looked at the basic make-up of
cleaning chemicals, it should also be
considered that different cleaning chemicals
do contain other active ingredients designed to
make the materials more effective at cleaning
specific residues. Most commonly these
ingredients fall into three main categories:
1) Alkaline based. Primarily for the purpose
of saponification of vegetable oil based
residues.
2) Acid based. Primarily for removing more
stubborn residues that do not readily
dissolve in water, but become more
soluble in acids, for example hardness
salts and other surface contamination that
otherwise render the appearance of the
cargo tank as ‘dull’ or ‘not typical’.
3) Organic solvents for the purpose of
‘cutting’ heavy hydrocarbon residues.
Indeed, many cleaning chemicals that are sold
into the domestic market for various
applications are based on the same
formulations, but that is where the similarity
ends. The method of operation for domestic
cleaning chemicals is completely different to
the cleaning of chemical and product tankers
and this raises a question, relating to the
cleaning chemicals used on board tankers being
‘fit for purpose’ and whether or not any studies
have actually been carried out by the cleaning
chemical manufacturers to determine this.
Domestic cleaning chemicals tend to be
used on floors (horizontal surfaces) and walls
(vertical surfaces), both of which are readily
accessible and can therefore be manually
scrubbed or brushed. Other surfaces or areas
that need cleaning (including clothing) can
usually be soaked in a solution of the cleaning
chemical for any given period of time and it
has to be accepted that soaking is probably the
most effective way of cleaning any material or
surface because there is intimate and
prolonged contact with the cleaning material.
But of course on a tanker, it is just not feasible
to ‘soak’ the inside of a cargo tank with a
cleaning material.
Inefficient method
Furthermore, when one considers the way in
which a cargo tank is cleaned, it soon becomes
apparent that this method is actually quite
inefficient. The aqueous solution of the
cleaning material is sprayed over as many
points of the tank as possible, usually via the
rotating nozzle of a tank cleaning machine,
which really only allows the cleaning
material merely to run down the surface of the
bulkhead, with an extremely short contact time.
Prolonged cleaning will of course increase
the contact time of the cleaning material on
the bulkhead, and this is really the only viable
option for enhancing the efficiency of the
cleaning operation, but when a vessel has a
number of cargo tanks to clean
simultaneously, time is not always available.
Consider one more point, vessels that are
cleaning to a very high purity standard (for
example prior to loading chemical cargoes),
very often have to pass a methanol wall wash
inspection prior to loading. For coated cargo
tanks, the wall wash test is not just a measure
of how clean the surfaces of the tanks are, it is
also a measure of the amount of absorbed/
adsorbed residues that the wall wash solvent
(usually methanol) is able to chemically
extract from the outer layers of the coating.
Organic coatings readily absorb and retain
low carbon chain molecules, for example lower
alcohols, some aromatics, ketones, chlorinated
solvents etc. and it is known that these products
will stay inside the coating, until they are
actively removed, either by tank cleaning
methods or by extraction into a subsequent
cargo. The degree to which they can be
removed is largely influenced by their volatility.
Inorganic (zinc silicate) coatings absorb the
same types of cargoes, but because of the
open/porous nature of the coating surface,
these cargoes are generally not retained.
However, due to their inherent porosity and
their fairly rough surface profile, previous
cargo residues may become absorbed and/or
adsorbed into the coated surface and as a
result some cargo residues may be retained,
including non volatile and ‘oil-like’ cargoes.
These must be actively removed either by tank
cleaning or by extraction into another cargo,
otherwise they could pose a contamination
threat to subsequently loaded sensitive
chemical cargoes.
In other words, if a cleaning chemical
claims to be able to remove previous cargo
residues to a standard where a wall wash
inspection will be accepted, (in the case of
cleaning coated cargo tanks) this actually
means that the cleaning chemical must have
the ability to penetrate inside the coating and
remove traces of previous cargoes that may be
residing there.
This is an extremely bold claim to make,
particularly as the industry is rapidly moving
away from the use of solvents as cleaning
materials for coated cargo tanks (for safety
reasons), even though solvents are still proven
to be the most effective way of removing
absorbed/adsorbed residues from within cargo
tank coatings.
Clearly the challenge is to find surface
active cleaning materials that can clean coated
cargo tanks to a wall wash standard, before
the use of tank cleaning solvents is prohibited,
otherwise owner and charterers of chemical
and product tankers will be facing serious tank
cleaning headaches in the future.
All of this being said, whether the cleaning
chemicals are for cleaning stainless steel or
coated cargo tanks, for general use or for
achieving a high purity standard, they should
still be effective (particularly in view of the
significant volumes that are used on board
tankers today). As noted, there seems to have
been no evaluation of this type in the past,
with the efficiency of most cleaning chemicals
seemingly based on the ability of the chemical
to visually remove previous cargo residues
during the tank cleaning process.
However, as noted with more vessels now
being inspected to a chemically clean standard
(by means of a wall wash inspection), it is
really no longer acceptable for cargo tanks to
be merely visually clean prior to loading many
chemical cargoes and with this in mind, L&I
Maritime (UK) Ltd (LIM) was asked by a
major chemical tanker owner to investigate
this issue in more detail.
Procedure
One of the reasons why monitoring the
efficiency of tank cleaning chemicals does not
appear to have been carried out in the past,
could be because there is no official or
immediately straightforward procedure to do
so. The procedure adopted for this project was
standardised as far as possible in order that the
results generated could be reproduced as long
as the test panels were treated in the same
way. But as noted, the procedure is not an
industry standard.
There are many different causes of cargo tank
contamination and as noted, many different
types of tank cleaning chemicals available to
remove the contamination. However, for the
purpose of this project, one particular
situation/scenario was chosen and that was
cleaning zinc silicate coated test panels after
exposure to Linear Alkyl Benzene (LAB),
which is a known persistent hydrocarbon.
It is known that zinc silicate coatings actively
retain hydrocarbon based, non volatiles and it
also accepted that one of the most common
generic types of tank cleaning chemical is the
‘hydrocarbon remover’ so bearing these two
points in mind, it made sense to base the project
around this particular scenario.
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TANKEROperator November/December 2009