Wednesday 20 April 2011

Marine Cargo Tank Cleaning & Tank Coatings

Tank Cleaning is one of the least understood activities on Tankers and the complexities are galore. However, one of the most neglected subjects that are never addressed, or understood to say the least is the result that one expects to achieve, vis-à-vis Tank coatings.

In the past, quite a few cargo related claims have arisen on board our vessels and following have been identified as the problem areas:
1)    Inadequate Tank Cleaning – not using correct temperature of water
2)    Inadequate or no cleaning of cargo pipelines / cargo pumps and valves
3)    Inadequate sampling and / or commencement of loading into more than 1 tank prior ensuring the cargo quality by sampling.
4)    Failure at 1st Foot samples.
5)    Off-hire owing to Tank Failures for loading by Surveyors
The below Guidance is provided to ensure adequate tank cleaning / operational precautions to minimize cargo claims due to off-spec cargoes and or tank failure incidents on board.

The tanker industry is used to using various Tank Cleaning guides, be it Miracle, Dr. Vervey’s or for Oil cargoes, Shell Tank Cleaning guide, it is very important to understand that most of these guides do not take into account the type of Tank Coatings. To make it simpler to understand, most of the Acidic cargoes, tank cleaning is advised based on the assumption that it will be loaded in Stainless Steel Tanks, depending upon pH. Further most of the oil cargoes are based on the assumption that they would be loaded into Epoxy coated tanks, without considering the type of epoxy used. Same for bases like Caustic Soda.

However, it is very important to understand that a tank cleaning should be such that it serves and addresses two purposes –
1.     Provides the required results, eg. White Wash Standard or Wall Wash Standards
2.     Maintain the corrosion resistant properties of the Cargo tank coatings.

Firstly, when we talk of cleaning standards, let’s discuss what involves in various standards.

WHITE WASH STANDARDS

The following points are considered when deciding if a tank has reached a water white standard -

v  NO trace of previous cargo in the tank or line system, including pumps, valves etc..
v  NO odor.
v  NO suspendables ( visual particles ).
v  NO discoloration from last cargoes or due to tank cleaning.
v  NO free water or condensation in pump well/bottom, bulkheads, overhead and all associated line systems.
v  NO loose scale and blisters which could entrap previous cargo.
v  Heating coils clean and bright.
v  Cargo pump and piping clean and bright.
v  Hatch packings clean and free from odor.
v  Tank pump, lines, valves, and drain-cocks must all be fresh water rinsed to prevent damage to stainless steel components and prevent contamination of subsequent cargo.
v  Fixed Butterworth machine nozzles to be set in the point down position, and if drain hole fitted on bottom of machine, check that it is not blocked.
v  Cargo heat exchangers must be opened and visually inspected to confirm they are clean and free of previous product or water.

Thus in order to achieve this, what is important and required is that all cargo loading / discharge associated pipelines / valves / pumps are cleaned and all cleaning water to be removed from the system.


WALL WASH STANDARDS

This would include all the requirements of the White Wash standards, though to a higher degree of cleanliness, which is ascertained by certain tests, normally referred to as Wall Wash Test. These tests could be visual, physical & chemical checks. The following points are considered when deciding if a tank has reached a Wall Wash standard -

v  No or below required specifications of Chloride content.
v  No discoloration of the test sample
v  No presence of impurities, particles or residues of previous cargoes, visually or as determined by chemical tests.
v  No or below required specifications of Hydrocarbons in the test sample.

The wall wash test procedures normally subscribed to defer in various guides, company manuals (please refer to own company / charterer specific requirements), though generally and broadly the below is accepted procedures.



WALL WASH TEST KIT
For the purpose of Quality Control and to expedite cargo tank readiness, a Wall Wash Test Kit is aboard all chemical tankers.
Included within the kit are following:
  1. A List of Apparatus
  2. Instructions for performing the following tests:
              i)   Colour of clear liquids
              ii)  Detection of Hydrocarbons
              iii) Detection of chlorides
              iv) Permanganate Time Test
  1. MSDS of Nitric Acid and Hydrochloric Acid
  2. Instructions for cleaning the equipment.

Vessel will be advised when a particular test is to be undertaken and to which standard. It is therefore important that senior officers become familiar with all the apparatus and the safety procedures to be followed. However, very important to note that the chemicals supplied have a shelf life, usually, if procured through standard suppliers, is labeled on the bottles or containers. If not and if a description of expiry cannot be ascertained, it is advisable to procure new chemicals. Sometimes, onboard tests do not indicate presence of foreign materials being test, due to usage of expired chemicals.

Caution:  
NITRIC ACID AND HYDROCHLORIC ACID ARE CORROSIVE. THEY WILL BURN SKIN, EYES AND ARE IF SWALLOWED.

Great care should be exercised when using these substances particularly when "pipeting".
Chief Officers are responsible for the safe keeping of this kit and all its contents. To this end it is strongly recommended the kit is stored under lock and key. The contents of this kit are not to be used for other purposes.


WALL WASH TEST KIT EQUIPMENT

  1. funnel - flat side (1 piece)
  2. squeeze bottle, plastic (2 pieces)
  3. filter paper, i 1cm (1 piece)
  4. pH paper (o-14) (1 piece)
  5. plastic gloves (disposable latex) (1 box)
  6. stand for nessler tubes (1 piece)
  7. nessler tubes 100 ml with caps (( pieces)
  8. 100 ml graduated cylinder with glass stopper
  9. nessler tubes so ml with caps (4 pieces)
  10. nessler tubes so ml with stand and glass stopper (2 pieces)
  11. 1 ml pipet
  12. 10 ml pipet
  13. 500 ml glass flask
  14. potassium permanganate 10 x 0.1 gm
  15. 14a. potassium permanganate solution 500 ml (dark glass bottle)
  16. thermometer in Celsius
  17. pen light
  18. igloo bath
  19. silver nitrate 0.1 normal (200 ml) (in dark glass bottle)
  20. chloride standard solution 100 ppm (200 ml) (in dark glass  bottle)
  21.  2o. platinum cobalt colour standard (200 ml) (in dark glass bottle)
  22. 1 gallon of distilled water
  23. 1 gallon of reagent methanol
  24. 2.5 ltr. hydrochloric acid
  25. 2o % nitric acid (200 ml)

STANDARD METHOD OF TEST FOR DETECTING THE COLOUR
(Platinum - Cobalt Scale)  ASTM - D1209
This test method describes a procedure for the visual measurement of the colour of essentially light coloured liquids.
1.     From the stock solution prepare standards in accordance with Table I by diluting the required volumes to 100 ml with distilled water in the Nessler tube. Cap the tubes and seal the caps with shellac or a waterproof cement if long term storage is desired (Note: This standard would be stable for at least a year).
2.     Wall wash the tank using the appropriate solvent (e.g. Acetone or Methanol or Paraxylene etc. as applicable) and introduce 100 ml of the specimen into a Nessler tube passing the specimen through a filter if it has any visible turbidity.
3.     Report the standard number colour which most matches the specimen. In the event that the colour lies midway between two standards, report the darker of the two or otherwise report the range over which an apparent match is obtained.
TABLE I
Colour Standard Number
Stock Solution
ml
Colour Standard Number
Stock Solution
ml
1
0.20
25
5.00
2
0.40
30
6.00
3
0.60
35
7.00
4
0.80
40
8.00
5
1.00
50
10.00
6
1.20
60
12.00
7
1.40
70
14.00
8
1.60
100
20.00
9
1.80
150
30.00
10
2.00
200
40.00
11
2.20
250
50.00
12
2.40
300
60.00
13
2.60
350
70.00
14
2.80
400
80.00
15
3.00
450
90.00
20
4.00
500
100.00


STANDARD METHOD OF TEST FOR DETECTING HYDROCARBONS
This method of test describes a procedure for detecting the presence of hydrocarbons in methanol by observing any cloudiness or opalescence which occurs when the sample is diluted with distilled water.

1.     Wall wash the tank with methanol (about 1 m2) and dilute 15 ml of this sample with 45 ml of distilled water to a 100 ml colour comparison tube. Invert the tube several times to ensure complete mixing and allow it to stand for twenty minutes at room temperature.
2.     Fill up 100 ml blank tube with 60 ml of distilled water.
3.     Compare the sample to the distilled water blank by looking down through the tube against a black background to pass the test, no cloudiness or opalescence shall appear at any time during the test period.

STANDARD METHOD OF TEST FOR DETECTING CHLORIDES
This method of test describes a procedure for detecting chlorides ions.

  1. Measure 20 ml of distilled water into each of 3 Nessler tubes (50 ml tubes).
  2. Pipet 5 ml of the Methanol Wall Wash sample into one of the Nessler tubes (Tube A).
  3. Pipet 5 ml of the Methanol used in the Wall Wash into another Nessler tube (This will be the blank to check whether the Methanol used is contaminated). (Tube B).
  4. Pipet 0.25 ml of the Standard Chloride solution (100 ppm) into another Nessler tube (This will be 5 ppm Chlorides standard). (Tube C).
  5. Pipet 1 ml of 20 % Nitric Acid and 1 ml of the 0.1 Silver Nitrate Solution to all three Nessler tubes.
  6. Cap all tubes and mix well by inverting several times.
  7. Allow the contents to stand for 10 minutes in the dark before comparing the sample (the 1st tube) with the standard (the 3rd tube) by viewing the tubes against a black background.

If the sample cloudiness (Tube A) is less than that of the 5 ppm standard (Tube C) then report as less than 5 ppm chlorides.
Always check the blank (Tube B) to ensure that the Methanol used is not contaminated, it should not have any cloudiness and must be clear and transparent.
If higher values of chlorides are required then following volumes of the Standard Chloride solution (100 ppm) should be added in the Tube C (Step 4).

0.50 ml of Chloride Solution for 10 ppm
0.75 ml of Chloride Solution for 15 ppm
1.00 ml of Chloride Solution for 20 ppm
2.25 ml of Chloride Solution for 25 ppm
1.50 ml of Chloride Solution for 30 ppm
1.75 ml of Chloride Solution for 35 ppm
2.00 ml of Chloride Solution for 40 ppm
2.50 ml of Chloride Solution for 45 ppm
2.75 ml of Chloride Solution for 50 ppm


STANDARD METHOD FOR PERMANGANATE TIME TEST   
This method covers the detection in alcohol or ketones of the presence of impurities that reduce potassium permanganate.

Substances reacting with potassium permanganate in neutral solutions reduce it to manganese dioxide which colours the solution yellow. In the permanganate test the time required for the colour of the test solution to change to that of a standard solution is measured. The colour of the test solution changes from pink-orange to yellow-orange.

  1. Fill up a 50 ml glass stoppered Nessler tube beyond the mark with the sample under test and place it in a constant temperature bath (15o C for Methanol or 25o C for Acetone).  Maintain the water level in the bath approximately 25mm below the top of the tube.  When the sample has reached the bath temperature (about 5 min) bring the level to the 50 ml mark. Add with a pipet 2 ml of the potassium permanganate solution. Stopper the tube, invert once to mix the contents, return to the bath and note the time.

  1. Dissolve 0.1 g of KMn04 in 500 ml of distilled water, from this standard solution take 50 ml and put in a 50 ml stoppered Nessler tube and keep in the same bath as the sample.

  1. At the end of the minimum time specified for the material being tested remove the tubes and compare to the colour of the standard by viewing downward through the tube against a white background.

  1. If the residual pink colour of the sample is greater than the standard, report the permanganate time as "greater than X minutes". If the residual pink colour of the sample is equal to that of the standard report the permanganate note time as "X minutes". If the residual pink colour of the sample is less than the standard report as "less than X minutes" where "X minutes" is the minimum time specified for the material being tested.

CLEANING OF WALL WASH TEST KIT EQUIPMENT
The following notes are intended as guidelines in the care and cleaning of the equipment. Cleanliness of equipment is paramount to ensure test results are accurate.

  1. On completion of each test all equipment should be drained, rinsed with D.I. water and allowed to dry.
  2. If equipment is not going to be used for sometime then all apparatus should be carefully stowed in Igloo Box provided.
  3. Immediately prior to use the equipment to be used must be thoroughly cleaned by :
              i) Rinse with small quantity of Hydrochloric Acid and drain
              ii) Rinse with D.I. water, drain and allow to dry

PERSONAL SAFETY PRECAUTIONS ARE ALWAYS TO BE OBSERVED WHEN HANDLING COMPONENTS OF THE WALL WASH TEST KIT


While the idea of writing this blog is not to get into the procedure of Tank Cleaning as generally that is understood to all as a process of following steps – applied in various details basis last and next cargoes –

1.     Pre-cleaning preparations, including collecting tank cleaning data, safety precautions and preparation of tank cleaning equipment.
2.     Subjects touched upon by ISGOTT or Chemical Tanker Safety Guides are to be ensured with respect to washing in inerted / lean / too-rich atmosphere and other safety precautions.
3.     Pre washing
4.     Cleaning
5.     Rinsing
6.     Flushing
7.     Steaming
8.     Draining
9.     Drying
10.  Wall-wash testing and final preparations

It is very imperative to note here is the record keeping, which should be adequately done. Most of the times, the person in-charge, which is mostly the Chief Officer is so busy, that he puts this to the last job, of filling papers, mostly by memory. It is better to depute another officer to note down the details, as the job progresses. Putting it broadly, as a Chief Officer and later as a Master, I’d ensured this is done by the Bridge duty officer, keeping in touch on the Walkie Talkie. A run-down with him prior to commencement of cleaning would ensure his awareness and with least efforts an intricate record could be maintained.

Why is record-keeping important? Ask yourself, how often you’ve followed exact procedures advised by the charterers or vague procedures for that matter…and that has lead to tank’s being unaccepted. You instantaneously know that your hard work has gone for a toss. But can you prove that there was hard work involved? Can you prove that you really did the needful?


Now let’s touch on the subject of tank coatings… believe that was the original objective. It is always easy to deviate from the topic, which somehow, my friends complain I’m capable of, especially when I’m sharing experience. After all the aforesaid by itself is a big topic, and I do not feel like leaving it half shared. Anyhow, back to the subject.


Cargo tank coatings and tank cleaning is complex and dependent to each other.
While a lot has to be understood in terms of basic tank cleaning approach, one often neglected feature is the failure to understand the impact of either that can lead to prolonged and inefficient tank cleaning or subsequent cargo contamination claims.

The primary role of the coating is to prevent corrosion of the steel structure of the vessels. Various paint manufacturers strive to produce coatings with the best all-round corrosion protection and chemical resistance that are effective for the trading patterns in which the vessels are trading in.

Cargo tank coatings are mostly a compromise. There is nothing called a  the 'perfect' coating. Increasing the pigment volume concentration (PVC) in organic coatings can improve the apparent chemical resistance, but not for every cargo.

Unfortunately, not much is said about the paints in company manuals. Surf the net, information is freely available.

Pigment Volume Concentration (PVC) 
When pigments are added to a particular coating that are added based on the PVC. What is PVC you say?  It is the pigment volume concentration. The pigment volume concentration is essential in determining the amount of a particular pigment that can be added to the polymer of the coating. The pigment has to have sufficient "wetting" by the polymer to create a protective coating. By wetting I mean, that there must be sufficient polymer, or binder, to completely wet or surround all the pigment particles. There must be enough polymer to completely fill the voids between the pigment particles.  How do you calculate the PVC?  The following equation is used: 
% PVC = 100 * Vpigment / (Vpigment + Vnon-volatile binder)
Vpigment                   = pigment volume 
Vnon-volatile binder = non-volatile binder volume

 The point at which there is just sufficient polymer to wet the pigment particles is known as the critical pigment volume concentration (CPVC).  Below the CPVC there is sufficient polymer for pigment wetting and above the CPVC there is not.  There are abrupt changes in the coatings properties at the CPVC.  Below is a depiction of PVC and property changes that occur at the CPVC. 


As you can see by the picture above that both gloss and blistering properties decrease as one reaches the CPVC, while permeability increase above the CPVC.  Permeability properties increase because above the CPVC there are voids in the coating filled by air and the coating becomes discontinuous.  Some of the properties that can be evaluated above and below the CPVC are blistering, gloss, rusting, permeability, enamel hold out, scrub resistance, tensile strength, and contrast ratio.
The filling of pigments in a coating is similar to fillers in composites.  In a composite the polymer matrix must be in intimate contact with the fiber reinforcing material.  If there is not intimate contact then the fiber acts as a defect.  The same theory applies here.  If there is not enough polymer to wet the pigment then the pigment becomes the defect and the properties of the coating decrease. 


CALCULATIONS ON PAINT FORMULAS
1.      Density
Density paint is:
Total sum by weight of each ingredient in paint
Total sum by volume of each ingredient in paint 

2.      Solid content
Solid content by weight of paint is:
Total sum by weight of each solid ingredient in paint  x 100%
Total sum by weight of each ingredient in paint
Solid content by volume of paint is:
Total sum by volume of each solid ingredient in paint  x 100%
Total sum by volume of each ingredient in paint 

3.      Pigment Volume Concentration
PVC paint is:
Total sum by volume of all pigments + extenders in paint  x 100%
Total sum by volume of each solid ingredient in paint

4.      Critical Pigment Volume Concentration
The best way to determine the critical PVC of a paint is to measure the volume of the dry pigment/extender mix of a paint in a centrifuge and to calculate the CPVC as follows:
CPVC = (calculated volume pigment/extender mix / volume after centrifuge) x 100
or
CPVC = ( f x G x 100) / (dPi x Va)
f           = ratio weight pigments and total weight solid content paint
G         = weight of dry film
dPi      = density of pigment/extender mix
Va       = geometric volume of paintfilm (e.g. 250 micron wet filmthicknes x length x width) 
A theoretical calculation which estimates the approximate CPVC is:
CPVC = (volume pigment+extender x 100) / (volume pigment+extender) + ((average o.a. pigment+extender) / 0,93)

5.      Extender replacement
To calculate an extender replacement by weight by another extender and maintaining PVC: 
E = A1 x D2
      A2 x D1  
E = factor to multiply with original extender weight to calculate weight substituting extender
A1 = OA of original extender
A2 = OA of substituting extender
D1 = density of original extender
D2 = density of substituting extender 

6.      Crosslinking percentage of two component products Epoxies

Equivalent weight epoxy component base paint is:
Weight base paint                                                   
Weight epoxy resin (1) 
  +   Weight epoxy resin (2)  +  
Eq.w.epoxy resin (1)             Eq.w. epoxy resin(2) 

Equivalent weight amine component harder is: 
Weight harder                                                   
 Weight amine (1)          +     Weight amine (2)   +
 Eq.w. amine (1)                    Eq.w.amine (2)

Crosslinking percentage Weight harder x Eq.w. base component   x 100%
                                               Weight base x Eq.w. harder component 
Polyurethanes
Equivalent weight polyol component base paint is: 
Weight base paint                                                   
 Weight polyol resin (1)   +   Weight polyol resin (2)  +  
 Eq.w.polyol resin (1)             Eq.w. polyol resin(2) 
Equivalent weight isocyanate component harder is: 
Weight harder                                                                    
 Weight isocyanate (1)             +     Weight isocyanate (2)   +
 Eq.w. isocyanate (1)                        Eq.w.isocyanate (2) 

Crosslinking percentage =
Weight harder x Eq.w. base component   x 100%
Weight base x Eq.w. harder component
 7.    Calculation the amount of amine to neutralize water-reducible resins 
Equation:  A = R (AN) E
                       56.100
Where:
A = weight of amine
R = weight of resin, non volatile
AN = acid number resin, non volatile
E = equivalent weight of amine
 
8. Starting point amounts of hydroxy / carboxy / amide containing resins in melamine / urea resins combinations in oven cured systems 
Let:    
x = hydroxyl number (mg of KOH to neutralize the organic acid required to   esterify the hydroxyl groups present)
y = acid number (mg of KOH required to neutralize one gram of resin)
z = amide number (mg of KOH equivalent to reactivity of the amide groups present-by calculation)
Then:  56 grams (mol.wt. KOH)  = gram mol. of resin to be crosslinked
            x + y + z (mg) 
Example: the equivalent weight of a melamine resin is 130 
If a polyester resin has a hydroxy number of 60, an acid number of 5 and an amide number of 0, the gram-mol weight of the polyester is:
56 grams       = 861,5 grams
60+5+0 mg 
Or the starting levels for the binder may be polyester / melamine resin = 861,5 / 130
Or polyester / amino = 87 /13 by weight on solids                                 
9.Volatile Organic Compounds (VOC’s with vapour pressure > 0,01 kPa at 20 C)
 VOC = weight volatile organic material   (g/l)
            Volume paint (without water 
VOC = 100 – NV% - water%   x 1000 (g/l)
             (100/density) – water% 
For solvent based coatings the formula simplifies to: 
VOC = (100 – NV%) x 10 x density (g/l)
 
10.Theoretical spreading rate paint at given filmthickness: 
Volume solids                            = liters/m2
Filmthickness in microns : 10 
Volume solids                                              = kg/m2
(Filmthickness : 10) x density solids paint 
§  Price per m2 at given filmthickness:
Price per liter                               
Theoretical spreading rate in liters 
Relation thickness steel construction and surface: 

Thicknes s of steel in mm
1
2
3
4
5
6
7
8
9
10
15
25
50
Surface in m / tonne steel
254
127
85
63
51
42
36
32
28
25
17
10,1
5,1


11. Required air quantities to stay below MAC levels:
∑ ( TLV per ingredient x weight %)
 
12.Required air quantities to stay below lowest explosion limits:
∑ ( LEL per ingredient x weight %)

13.General formulae for mixing liquids 
A = C – B 
B = C (a – c)
        a – b 
C = B (a – b)
        a – c 
Where:
A = weight of original liquid            a = its content in % by weight
B – weight of diluent                      b = its content in % by weight
C = weight of prepared mixture       c = its content in % by weight
For water as diluent, b = 0
 
14. HLB (hydrophylic – lipophylic – balance) of surfactants and their activity features 
0 – 4               = defoaming
3 – 7               = emulsifying
7 – 15             = penetrating
7 – 20             = emulsifying
12 – 20           = detergency
15 – 20           = solubilising              

15.Mixing rules by using mixture-cross
How much of each solution has to be mixed to get a 62% solid solution of a 54% solids solution with a 92% solids solution? 
30 parts by weight of 54% solid solution must be mixed with 8 parts by weight of 92% solid solution to yield a 62% solid solution.
92 - 62 = 30 pbw 54% solution
62 - 54 = 8 pbw 92% solution

Increasing PVC will also generally increase the permeability of the coating, which allows for a freer movement of cargoes 'in and out' of the coating. This reduction in residency time tends to reduce the contamination potential for volatile cargoes. However, the increase in permeability may also 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 the chemical resistance and the flexibility of organic coatings. In general, the higher the cross link density, the better the chemical resistance, but the lower the flexibility. 


ORGANIC COATINGS
There are three types of organic coating in common use today,
1.     phenolic epoxy and
2.     straight (amine cured) epoxy
3.     MarineLine
First two are primarily different in their chemical resistance with Phenolic epoxy providing a much higher level of chemical resistance compared to straight epoxies. MarineLine is extra-ordinary coating because while it is considered as an organic coating, this is not strictly correct, because of its make-up which provides for many inorganic coating features.

Phenolic epoxies are more expensive than straight epoxies because of greater chemical resistance. As such, phenolic epoxies tend to be utilised far more widely in the chemical trade as opposed to straight epoxies which tend to be utilised in the vegetable oil, easy chemicals and CPP (Annex I) trades. They absorb cargoes to high levels (depending on cargo), and have a relatively slow cargo absorbtion rate.
This may appear to be misleading statement to all. Slow aborbtion rate and absorbs cargoes to high levels. The slow absorbtion rate would mean slow release of absorbed cargo.
Epoxy coatings release absorbed cargo very slowly. Furthermore, small traces may be retained by the epoxy coating, contaminating the next cargo.
Epoxies are a three dimensional “cross linked array” of chemical bonds between resin molecules. Amine cured epoxy is used on chemical tankers. Polyamine cured epoxy having no resistance to Aromatics is used on oil tankers usually.

As said earlier MarineLine is extra-ordinary coating because while it is considered as an organic coating, this is not strictly correct, because of its make-up which provides for many inorganic coating features. It is neither a phenolic nor a straight epoxy and its chemical resistance is said to be derived from a very high cross link density, balanced with unique characteristics, which allow flexing of the product after it has been applied and post cured. MarineLine provides more chemical resistance than other coatings, including Phenolic Epoxies, zinc and stainless steel. It is resistant to most of the frequently carried cargoes.
Furthermore, MarineLine is resistant to all IMO cargoes including the following: methanol; EDC; acrylonitrile; fatty acids; super phospohoric; caustics; MTBE; MEA; methylene chloride and sodium hydroxide. MarineLine's virtual non-absorbency (absorption only takes place after 45 days immersion) results in decreased downtime with faster tank cleaning. The MarineLine coating provides a faster drying time, and does not normally require the use of any cleaning chemicals. With less slops, it also ensures less environmental problems.
MarineLine coating is actually based on silicon chemistry, which arguably puts it into the inorganic coatings category. However, the claim for high resistance to various cargoes is based not only on the high level of cross linking, but also on the fact that the coating and the cargoes being carried are chemically opposite.
MarineLine provides superior resistance to the following: acids, alkalies and solvents; thermal shock (-40°C to +200°C); flex stressing; wear and abrasion; product absorbtion; impact and undercreep corrosion.

Benefits of MarineLine coatings and linings

  • Superior bond strength and adhesion
  • Maximum cargo flexibility and product cycling
  • Repairable
  • Can be applied to pitted corroded steel
  • Can carry food grade cargoes
  • Maintains product purity
  • Virtually non-absorbant

MarineLine is 2½ times smoother than phenolic epoxy, five times smoother and 50 times less slippery than stainless steel:
  • MarineLine - surface roughness = 0.7Ra - 09.Ra; slipperiness = 34.0dynes/cm
  • Phenolic Epoxy - surface roughness = 1.8Ra - 2.1Ra; slipperiness = 80dynes/cm
  • Stainless steel - surface roughness = 3.2Ra-4.5Ra; slipperiness = 1,800dynes/cm

MarineLine offers:
  • Easy, safer and faster cleaning and drying
  • No contamination from cargo to cargo
  
Cleaning Cargo Tanks with Epoxy Coatings
As discussed the vast majority of bulk liquid cargoes are organic in composition and as such it is fair to assume that there is a natural affinity between these cargoes and the organic coatings. If one then looks at the permeability of organic coatings it can be quickly understood that aggressive and penetrating organic solvents are not ideally suited to organic coatings.
Many phenolic epoxies can actually be used to carry such cargoes, but there are restrictions, particularly after the cargoes are discharged and some of such organic solvents are prohibited to carry by the paint manufacturer.
Straight epoxies can be almost immediately destroyed in such solvents and are thus considered unsuitable.
But apart from the carriage of aggressive organic solvents, most epoxy coatings are quite versatile and suitable for the carriage of a wide range of cargoes; non aggressive organic solvents and derivatives, clean and dirty petroleum products, acidic and alkaline based products, vegetable oils, waxes.
In terms of tank cleaning, most epoxy coatings are very smooth, which generally restricts the amount of clingage of previous cargoes and as such surface tank cleaning materials tend to very effective at removing
previous cargo residues. It is also found that epoxy coatings are quite resistant to extremes of pH, so there are less risks using alkaline or acid based tank cleaning materials certainly compared with zinc silicate, which is also important to consider.
The absorption of certain previous cargo residues into organic coatings is of most concern when formulating tank cleaning plans, because removing these residues is not easy. Unsaturated and aromatic based cargoes
are particularly challenging because once they have been absorbed into the coating, these residues can stay there for a long time, if the coating is not exposed to conditions that actively desorb these residues.

It is a misconception that if an organic coated vessel is loaded with intermediate or buffer cargoes after the carriage of an absorbing cargo, the residues of the absorbing cargo will be removed and will no longer pose a threat to a cargo that is particularly sensitive to these residues. This is not strictly correct, as it depends completely upon the chemical nature of the intermediate cargo and has often resulted in cargo claims.
There are really only two ways of removing residues absorbed into an organic coating.

1.     Raise the temperature of the steel inside the cargo tank to a level where the residues are evaporated from the coating. This is feasible for low boiling residues, but not practical for residues with a boiling point in excess of around +75 deg C.
2.     Load the cargo tank with a cargo, usually solvent based, that will extract the residues without itself becoming contaminated with the residues.

The tendency is to over-clean organic coatings because of their noted resistance to most cleaning materials and this is another area where problems can occur, particularly if the vessel is cleaning to a wall wash standard.

As the washing temperature increases, the coatings will start to open and absorbed residues will be liberated, however it has to be considered that the residues may have been accumulating for many voyages and while the most volatile residues will be liberated first, the heavier residues, azeotropes and/or reaction products of the previous cargoes, may stay behind in the coating, even after prolonged hot washing.

Solvents may also be used to clean the coating, because they can actively penetrate inside the coating and remove the most stubborn residues that remain after cleaning with water/detergents. It should always be
considered that not all of the residues will be removed in one go and wall wash samples may fail, even after cleaning with a solvent.

What should also be noted after cleaning organic coatings at high temperature or with a solvent is that the coating will be softened for a period of time after the cleaning has been completed. During this time the coating will cool down and harden and it is very common to see wall wash results improve over a period
of time, without any additional cleaning.

The most important consideration when cleaning an organic coating is the quality of the next nominated cargo and the understanding of the nature of the residues that may be absorbed in the coating from previous voyages. If the next nominated cargo is particularly sensitive to aromatic residues and the previous cargo was a medium boiling point aromatic solvent, it is unlikely that any amount of tank cleaning will remove sufficient of the previous cargo to prevent contamination of the next cargo.

Due to stringent cleaning standards demanded, one of the most successful tank cleaning operations today prior to the carriage of the most sensitive cargoes, does not involve any tank cleaning at all, it is more preventative management and involves loading intermediate lower grade cargoes that remove residues known to be a threat to the higher grade cargoes.

Cargoes tend to be “absorbed” like a sponge rather than “adsorbed”.  It reacts with water/ solvent mixtures which penetrate coating. So if methanol or ethanol is discharged the coating must not be allowed to get wet prior to venting. The problem of absorption can be understood by imagining 1% absorbed previous cargo could cause a concentration of 10 ppm in the next cargo if completely dissolved.

The best way to restore epoxy coating is  with an air temperature of  about 30C-- dry air with a relative humidity below 45%. At 15 C it takes more than 3 times the time. High temperature or heated air is not recommended as will leave the coating soft, and may cause severe blistering of paint on tank cleaning water jet. Ventilate with dehumidified air for fast results. This is the reason why epoxy coated chemical tankers must have dehumidifier and heater inherent with the fixed blower.

The following cargoes generally swell epoxy and MUST be ventilated prior washing with jets -
  • Acetate
  • Acetone
  • Acrylonitrile
  • Adiponitrile
  • Butyl acetate
  • Cyclohexane
  • EDC
  • Ekta solve DP
  • Ekta solve EB
  • Ekta solve EE
  • Ektapro EEP
  • Ethyl acetate
  • IPA
  • Isopropyl acetate
  • MEK
  • Methanol
  • Methyl acetate
  • Styrene monomer
  • Vinyl acetate monomer

IT IS THE MASTERS DUTY TO ADVISE WHOM SO EVER IT MAY CONCERN ABOUT THE DANGERS OF DESORBED CARGO (FROM SPONGY EPOXY CONTAMINATING THE WHOLE SHIPMENT). WHEN THINGS GO WRONG ALL AND SUNDRY (WHO WERE AVERSE TO TIME LOSS DUE TO VENTILATION) NOW WILL WASH OFF THEIR HANDS LEAVING THE SHIPOWNER ALL ALONE WITH A HUGE FINANCIAL BURDEN AND LOSS OF IMAGE.



Inorganic coatings
Zinc silicate. This coating type is quite different from organic coatings because the chemical resistance comes from the fact that the fully cured coating is inorganic and the vast majority of liquid cargoes shipped are organic. The coating and the cargo being carried are chemically opposite. A zinc silicate coating absorbs cargo quickly. It retains oil-like cargoes, endangering a subsequent cargo contamination. Zinc silicate coatings limit back hauling capability, and are not resistant to acids, caustics, acid-containing oils or urea.
Again one should also consider the permeability of the coating, because this property does change during the life of zinc silicate coatings. New zinc silicate is extremely porous, and there are some who say it would be a better coating if it stayed like this, based on the free movement of cargoes in and out of the coating and little or no retention of those cargo residues. However on the contrary, Zinc silicate is quite reactive, which also restricts the type of products that can be carried and upon exposure to water, cargoes and the atmosphere, there is a steady build up of zinc salts, which reduces the permeability of the coating, at the same time increasing the resistance to organic cargoes.
Zinc salts can cause other problems and there is tendency to try and remove them by scrubbing or using cleaning materials that actually dissolve the salts, but as long as there is sufficient 'free' zinc available in the coating, the salts will return and it is perhaps better to deal with the coating including the salts than to try and change the characteristics of the coating.

Zinc silicates are generally a two system formulations, consisting of zinc powder which has particles size of 5-9 microns and inorganic or organic binder. The zinc powder may be blended with lead and iron oxides to provide improved spray application properties. The silicate binder may be water based with potassium (inorganic) silicate blend or alcoholic (organic solvent) solution, in order curing take place.
Post-cured silicates normally have an aqueous base and require application of a chemical curing solution to harden properly. Self-cured silicates may be aqueous or solvent-based and do not require application of curing solutions Most of the times are applied as one coat, which acts as a barrier between steel and corrosives. However, they are not resistant to strong acids and bases. This means that in practice these coatings are suitable only for cargoes, which have pH range of 6 to 9.

Zinc silicates are unusual coatings, are one of the few coatings which are designed so that all of the solid pigment particles are not coated with polymer and all of the gaps between particles are not filled with polymer, i.e. they are designed to be porous films.

It is obvious that the best performance in chemical resistance will be achieved with the maximum zinc percentage.

Cleaning Cargo Tanks with Zinc Silicate Coatings
First, the number of cargoes that are acceptable for loading in zinc silicate is considerably smaller, primarily because of the nature of the zinc, which reacts with products that have anything but an almost neutral pH.
A pH range of 5 - 9 is normal for most zinc silicates so this immediately rules out all alkaline and acid based products and also vegetable oils that have a significant fatty  acid content.
It is found that most fuels are neutral pH, so clean and dirty petroleum products do not pose any problems, but where zinc silicate really comes into its own is in the carriage of aggressive organic solvents, because the zinc silicate is chemically opposite and thus completely unreactive to any neutral organic product, solvent or otherwise. Tank cleaning zinc silicate is also completely different to cleaning organic coatings because while zinc silicate absorbs organic solvents, it does not retain them.
Perhaps more significantly though, unlike the surface of organic coatings the surface of zinc silicate is far from smooth; in fact in many cases it is extremely rough to the touch.
The latter point creates very challenging tank cleaning issues because it is found that non volatile cargo residues are readily adsorbed on to the surface of the coating and also absorbed into the matrix of the coating.
So when cleaning from oils it is potentially very difficult to clean.

This problem is made more challenging for two reasons:
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 metasilicate, which have a pH in the region of 12 or 13 and are thus prohibited for use on zinc silicate.

One other point to note at this time is the effect of fuel generated inert gas in zinc silicate coated cargo tanks, prior to loading and during the discharge of flammable clean petroleum products. In short, inert gas is acidic and with prolonged contact to the coating it is quite common to observe yellow/brown dust on the surface of the cargo tanks, which is most likely a reaction product of the zinc silicate and the acidic inert gas. 
This powder massively increases the surface area of the coating and significantly increases the potential of previous cargoes becoming adsorbed to the surface and trapped just underneath the surface layer. The only way to remove this problem is manual scrubbing, which is time consuming and limited in its effectiveness.


CONCLUDING THIS BLOG:
How the previous cargo residues are expected to be presented  - absorbed, adsorbed, retained in a surface profile – is critical to the correct choice of tank cleaning chemical and the duration of each cleaning cycle.

And of course the resistance of the cargo tank coating to the cleaning chemical is also extremely important, not only to effectively remove the previous cargo residues, but also to prevent short and long term damage to the coated surface.

Finally the quality of the next loaded cargo should never be overlooked, particularly if it is known that a coated surface is contaminated with a previous cargo residue. The objective of any tank cleaning procedure is to clean to a condition where the next cargo can be loaded without risk of contamination. By careful understanding and appropriate monitoring of each tank cleaning step, this objective is readily attainable.

Further comments and sharing of experience is welcome on the subject. This topic will always remain incompletely addressed at any forum since, as said earlier…a perfect coating does not exist.


Capt. Rajesh Baran

Most of the information in this blog is freely available on the net through various sources. As I heard in a movie, knowledge is floating every where, absorb it from anywhere, you get it.

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