Objective of tankcleaning is the

next cargoThe relationship between cargo tank coatings and tank cleaning is inextricably entwined.

Failure to understand the impact of either, can lead to prolonged and inefficient tank cleaning or in the worst case, cargo contamination claims*.

What should also be consideredis that the primary role of thecoating is to prevent corrosionof the steel structure of thevessels. The paint manufacturers strive toproduce coatings with the best all-roundcorrosion protection and chemical resistancethat are effective for the trading patterns inwhich the vessels are trading in.

When the vessels move out of these tradingpatterns, for whatever reason; operational orcommercial, the largest impacts are felt. Thisis the time when the greatest understanding ofthe relationship between cargo tank coatingand tank cleaning is required.

It should be noted at this time that thisdiscussion does not look at stainless steel,Teflon, or rubber coatings.

Cargo tank coatingsIt is fair to say that all cargo tank coatings areto some extent, a compromise; the 'perfect'coating does not really exist.

Increasing the pigment volumeconcentration (PVC) in organic coatings canimprove the apparent chemical resistance, butnot for every cargo. Increasing PVC will alsogenerally increase the permeability of thecoating, which allows for a freer movement ofcargoes 'in and out' of the coating. Thisreduction in residency time tends to reduce thecontamination potential for volatile cargoes.However, the increase in permeability mayalso make the coating more sensitive to water,which could be quite disastrous.

One should also consider cross link density,which has a significant impact on both thechemical resistance and the flexibility oforganic coatings. In general, the higher thecross link density, the better the chemicalresistance, but the lower the flexibility.

There appears to be a trend emerginglinking chemical resistance and flexibility andperhaps one characteristic will suffer - or becompromised - at the expense of the other?

When one considers that the chemical

resistance may have to be compromised inorder to produce a durable coating that doesnot crack as soon as the vessel starts tooperate, the specific resistance of chemicaland oil products to the coating becomesextremely important.

This is also the most logical reason whymany coating manufacturers supply a range ofdifferent coatings that can be almost tailoredto meet the exacting requirements of anyspecific trade pattern.

Organic coatingsThere are two types of organic coating incommon use today, phenolic epoxy andstraight (amine cured) epoxy, which areprimarily different in their chemicalresistance; with phenolic epoxies providing amuch higher level of chemical resistancecompared to straight epoxies.

Phenolic epoxies are more expensive thanstraight epoxies because chemical resistancecomes at a premium. As such, phenolic

November/December 2008 TANKEROperator 35

TECHNOLOGY - TANK CLEANING

Our Tank Cleaning Guide helps you find the proper tank cleaning procedure Our Tank Cleaning Supervisors are at your service, anytime, anywhere Our Tank Cleaning Seminars train your sea and shore based staff

Dr. Verwey offers you a full range of services

Tank Cleaning, Our Daily Routine

Chemical Laboratory Dr.A.Verwey Coolhaven 32 - 3024 AC Rotterdam -- Holland PO Box 6003 - 3002 AA Rotterdam -- Holland Phone : +31 10 4761055 Fax: +31 10 4773562

E-mail: sales@drverwey.nl for ordering our Tank Cleaning Guide E-mail: survey@drverwey.nl for Tank Cleaning Assistance E-mail: fred.burgmeijer@drverwey.nl for Training / Seminars

TANKEROperator November/December 200836

TECHNOLOGY - TANK CLEANING

epoxies tend to be utilised far more widely inthe chemical trade as opposed to straightepoxies which tend to be utilised in thevegetable oil, easy chemicals and CPP (AnnexI) trades.

MarineLine is something of an enigmabecause while it is considered as an organiccoating, this is not strictly correct, because ofits make-up; however, for the sake of thisdiscussion, it will be considered as an organicat this time.

MarineLine is neither a phenolic nor astraight epoxy and its chemical resistance issaid to be derived from a very high cross linkdensity, balanced with unique characteristics,which allow flexing of the product after it hasbeen applied and post cured.

Inorganic coatingsIn two words - zinc silicate. This coating typeis quite different from organic coatingsbecause the chemical resistance comes fromthe fact that the fully cured coating isinorganic and the vast majority of liquidcargoes shipped are organic. In other words,the coating and the cargo being carried arechemically opposite and this fact is extremelyimportant.

Again one should also consider thepermeability of the coating, because thisproperty does change during the life of zincsilicate coatings. New zinc silicate isextremely porous, and there are some who sayit would be a better coating if it stayed likethis, based on the free movement of cargoes inand out of the coating and little or no retentionof those cargo residues.

However this is not the case. Zinc silicate isquite reactive, which also restricts the type ofproducts that can be carried and uponexposure to water, cargoes and theatmosphere, there is a steady build up of zincsalts, which reduces the permeability of thecoating, at the same time increasing theresistance to organic cargoes.

Zinc salts can cause other problems andthere is tendency to try and remove them byscrubbing or using cleaning materials thatactually dissolve the salts, but as long as thereis sufficient 'free' zinc available in the coating,the salts will return and it is perhaps better todeal with the coating including the salts thanto try and change the characteristics of the coating.

Looking briefly at MarineLine again, itshould be noted that this coating is actuallybased on silicon chemistry, which arguablyputs MarineLine into the inorganic coatingscategory. Bearing this in mind, it should beconsidered that the high level of chemical

resistance that the manufacturers claim fortheir coating is based not only on the highlevel of cross linking, but also on the fact thatthe coating and the cargoes being carried, asnoted for zinc silicate, are chemicallyopposite.

Coatings and cargoesThe nature and chemical resistance of thedifferent coating types determines whichcargoes can or cannot be carried in thosecoatings.

As discussed the vast majority of bulkliquid cargoes are organic in composition andas such it is fair to assume that there is anatural affinity between these cargoes and theorganic coatings. If one then looks at thepermeability of organic coatings it can bequickly understood that aggressive andpenetrating organic solvents are not ideallysuited to organic coatings.

Many phenolic epoxies can actually be usedto carry such cargoes, but there arerestrictions, particularly after the cargoes aredischarged and it is actually noted that somecoating manufacturers are now prohibiting thecarriage of such organic solvents even in thephenolic epoxies.

Straight epoxies can be almost immediatelydestroyed in such solvents and are thusconsidered unsuitable.

But apart from the carriage of aggressiveorganic solvents, most epoxy coatings arequite versatile and suitable for the carriage ofa wide range of cargoes; non aggressiveorganic solvents and derivatives, clean anddirty petroleum products, acidic and alkalinebased products, vegetable oils, waxes.

In terms of tank cleaning, most epoxycoatings are very smooth, which generallyrestricts the amount of clingage of previouscargoes and as such surface tank cleaning

MarineLine is treated as an organic coating.

materials tend to very effective at removingprevious cargo residues. It is also found thatepoxy coatings are quite resistant to extremesof pH, so there are less risks using alkaline oracid based tank cleaning materials certainlycompared with zinc silicate, which is alsoimportant to consider.

The absorption of certain previous cargoresidues into organic coatings is of mostconcern when formulating tank cleaningplans, because removing these residues is noteasy. Unsaturated and aromatic based cargoesare particularly challenging because once theyhave been absorbed into the coating, theseresidues can stay there for a long time, if thecoating is not exposed to conditions thatactively desorb these residues.

It is a popular misconception that if anorganic coated vessel is loaded withintermediate or buffer cargoes after thecarriage of an absorbing cargo, the residues ofthe absorbing cargo will be removed and willno longer pose a threat to a cargo that isparticularly sensitive to these residues. This isnot strictly correct, as it depends completelyupon the chemical nature of the intermediatecargo and has resulted in cargo claims inthe past.

There are really only two ways of removingresidues absorbed into an organic coating.

The first is to raise the temperature of thesteel inside the cargo tank to a level where theresidues are evaporated from the coating. Thisis feasible for low boiling residues, but notpractical for residues with a boiling point inexcess of around +75 deg C.

The second is to load the cargo tank with acargo, usually solvent based, that will extractthe residues without itself becomingcontaminated with the residues.

The tendency is to over-clean organiccoatings because of their noted resistance tomost cleaning materials and this is anotherarea where problems can occur, particularly ifthe vessel is cleaning to a wall wash standard.As the washing temperature increases, thecoatings will start to open and absorbedresidues will be liberated, however it has to beconsidered that the residues may have beenaccumulating for many voyages and while themost volatile residues will be liberated first,the heavier residues, azeotropes and/orreaction products of the previous cargoes, maystay behind in the coating, even afterprolonged hot washing.

Solvents may also be used to clean thecoating, because they can actively penetrateinside the coating and remove the moststubborn residues that remain after cleaningwith water/detergents. It should always be

considered that not all of the residues will beremoved in one go and wall wash samplesmay fail, even after cleaning with a solvent.

What should also be noted after cleaningorganic coatings at high temperature or with asolvent is that the coating will be softened fora period of time after the cleaning has beencompleted. During this time the coating willcool down and harden and it is very commonto see wall wash results improve over a periodof time, without any additional cleaning.

The most important consideration whencleaning an organic coating is the quality ofthe next nominated cargo and theunderstanding of the nature of the residuesthat may be absorbed in the coating fromprevious voyages. If the next nominated cargois particularly sensitive to aromatic residuesand the previous cargo was a medium boilingpoint aromatic solvent, it is unlikely that anyamount of tank cleaning will removesufficient of the previous cargo to preventcontamination of the next cargo.

This challenge has been made more difficultby the ever increasing demands on the qualityspecification of shipped cargoes - not justchemical but also fuel based - and also theanalytical capabilities of the laboratories whoanalyse and certify the quality of thesecargoes. In the past, parts per million levels ofcontamination were considered to be quitestrict, but now many contaminants aremeasured in parts per billion levels and assuch, it is now far more common for cargoesto be rejected, even though vessels arecleaning longer and harder than they ever didin the past.

In order to put this into perspective; One part per million is equal to one second

in 11 days. One part per billion is equal to one second

in 31 years.This is not strictly a reflection on the qualityof the crews on board the vessels, it is purelythe fact that the receivers are able to analysethe cargoes to far more stringent levels and infact, one of the most successful tank cleaningoperations today prior to the carriage of themost sensitive cargoes, does not involve anytank cleaning at all, it is more preventativemanagement and involves loading

intermediate lower grade cargoes that removeresidues known to be a threat to the highergrade cargoes.

In complete contrast, zinc silicate is totallydifferent.

First, the number of cargoes that areacceptable for loading in zinc silicate isconsiderably smaller, primarily because of thenature of the zinc, which reacts with productsthat have anything but an almost neutral pH.

A pH range of 5 - 9 is normal for most zincsilicates so this immediately rules out allalkaline and acid based products and alsovegetable oils that have a significant fatty acid content.

It is found that most fuels are neutral pH, soclean and dirty petroleum products do notpose any problems, but where zinc silicatereally comes into its own is in the carriage ofaggressive organic solvents, because the zincsilicate is chemically opposite and thuscompletely unreactive to any neutral organicproduct, solvent or otherwise.

Tank cleaning zinc silicate is alsocompletely different to cleaning organiccoatings because while zinc silicate absorbsorganic solvents, it does not retain them.Perhaps more significantly though, unlike thesurface of organic coatings the surface of zincsilicate is far from smooth; in fact in manycases it is extremely rough to the touch.

The latter point creates very challengingtank cleaning issues because it is found thatnon volatile cargo residues are readilyadsorbed on to the surface of the coating andalso absorbed into the matrix of the coating.

So when cleaning from oils it is extremely commontobefacedwithagreasysurfacethatis potentially very difficult to clean. Thisproblem is made more challenging for tworeasons:1) Surface active cleaning materials

(detergents) are exactly that; surface cleaners; and it is known that previous cargo residues are trapped within the coating matrix.

2) The most effective solutions for cleaning oil based residues usually employ ingredients containing caustic or meta-silicate, which have a pH in the region of 12 or 13 and are thus prohibited for use on zinc silicate.

November/December 2008 TANKEROperator 37

TECHNOLOGY - TANK CLEANING

It is fair to say that all cargo tank coatings are to some extent, a compromise;

the perfect coating does not really exist.

With the introduction of thesecond version of TMSA earlierthis year, Videotel, has added toits range of training titles with'Tank Cleaning Practice'.The programme describes different types of

tank cleaning, from a quick wash betweencompatible products, to the complex washingprocedures required between incompatibleproducts. It also looks at how decisions aremade about which type of cleaning process touse.

Captain Milind Karkhanis, vice president,Videotel Training Services, said: "Tankcleaning is one of the most important aspectsof cargo transportation. Our programmeoutlines the key processes and procedures thatare prerequisites for safe and effective tankcleaning, emphasising the importance of crewfamiliarisation and training."

In making these programmes, Videotel hasbeen working with INTERTANKO and otherleading training establishments and operatorsto guarantee that these products support theINTERTANKO TOTS (Tanker OfficerTraining Standards) scheme. TOTS providesthe tanker industry with a standard thatensures tanker officer competence throughtraining, both on board and onshore.

Videotel's tanker training programmes areavailable on video and DVD and are suitablefor use in training sessions on board ship andare accompanied by detailed support booklets.Computer-based training (CBT) interactiveversions are also produced for independentstudy, wherever the trainee may be.

TANKEROperator November/December 200838

TECHNOLOGY - TANK CLEANING

One other point to note at this time is theeffect of fuel generated inert gas in zincsilicate coated cargo tanks, prior to loadingand during the discharge of flammable cleanpetroleum products. In short, inert gas isacidic and with prolonged contact to thecoating it is quite common to observeyellow/brown dust on the surface of the cargotanks, which is most likely a reaction productof the zinc silicate and the acidic inert gas.

This powder massively increases the surfacearea of the coating and significantly increasesthe potential of previous cargoes becomingadsorbed to the surface and trapped justunderneath the surface layer. The only way toremove this problem is manual scrubbing,which is time consuming and limited in itseffectiveness.

The last coating type is MarineLineTheoretically, this coating utilises the

benefits of organic coatings and zinc silicatecoatings in one system.

In the first case, the coating is very smoothand as a consequence the majority of cargoeswill run off the surface without causing aretention problem. High melting orparticularly viscous products will of coursecling to the surface, but temperature alone isgenerally sufficient to tackle this issue.

The next consideration is the chemicalcomposition of MarineLine. It is accepted thatthe product is treated as an organic coating,

because during application, the product ismixed with various organic solvents in orderto expedite application. However, as noted,once these solvents are removed during dryingand post curing, the backbone of theMarineLine coating is silicon; not carbon.

If MarineLine is then considered to be aninorganic coating, it would be expected to beparticularly resistant to organic solvents; andindeed this is found.

Without the pH limitations of zinc silicate,it would also be expected that the number ofproducts suitable for carriage in MarineLine ismuch higher compared to zinc silicate andagain this is true. The manufacturers state thatthe product is actually suitable for acidcarriage, which is a bold statement, when oneconsiders that acid transportation has been thealmost exclusive domain of stainless steel formany years.

The potential of a smooth surfaced coatingwith good resistance to penetrating organicsolvents and not restricted by pH isimmediately obvious in several trade patternsand the author is currently investigating thispotential further.

In summary Understanding how cargoes behave and/orreact with the various coating types - alwaysbearing in mind that any cargo absorbed intoor adsorbed onto the surface of that coating

will behave differently when it reacts with adifferent cargo - is essential if an efficient andeconomical tank cleaning procedure is to becarried out.

How the previous cargo residues areexpected to be presented - absorbed,adsorbed, retained in a surface profile - iscritical to the correct choice of tank cleaningchemical and the duration of each cleaningcycle. And of course the resistance of thecargo tank coating to the cleaning chemical isalso extremely important, not only toeffectively remove the previous cargoresidues, but also to prevent short and longterm damage to the coated surface.

Finally the quality of the next loaded cargoshould never be overlooked, particularly if itis known that a coated surface is contaminatedwith a previous cargo residue. The objectiveof any tank cleaning procedure is to clean to acondition where the next cargo can be loadedwithout risk of contamination. By carefulunderstanding and appropriate monitoring ofeach tank cleaning step, this objective isreadily attainable.

*This article is the second in aseries written by Guy Johnson,BSc (Hons) MRSC CChem CSci,director L&I Maritime (UK).

TO

NAVADANs head Frank R. Tschicaja

Videotel adds Tank Cleaning to its titles

Effective 1st December CaptThomas Marvig Rasmussen hasbeen appointed as salesmanager for NAVADAN APS.With his background as master andoperations manager in companies such as Norient Product Pool and Weco-Shipping/Dannebrog Rederi in Denmark, he has gained substantial knowhow of themarkets' requirements for chemicals andthe chemical tank cleaning solutions that are offered on a worldwide basis by the company.

NAVADAN appoints sales manager