Tuesday, December 28, 2010

EPOXY ZINC TANKS PRACTICAL ASPECTS ON CHEMICAL TANKERS -- CAPT AJIT VADAKAYIL

Chemical tanker officers working on coated ships must know the contents of this chapter. This is also important for new build superintendents and delivery masters.

It is a shame if a coated tanker , with millions of dollars in investment is spoilt by an ignorant officer in just one voyage.

Cargo tank coatings can be categorized into two maigroups:
a)  Inorganic coatings – zinc silicates and ethyl silicattypes
b)  Organic coatings- epoxy and modified epoxy systems

Inorganic coatings are one-layer coatings, comprising of inorganic silicates pigmented with high percentage of zinc powder. The paint filis porous so the cargo, after the discharge of the ship, can be completely removed from the coating by evaporation but it cannot be happened the same with high-density cargoes like lube oils. Generallthe life of those coatings is proportional tthe thickness of the coat.

Organic coatings consist of an organic resin system which when imixed with a hardener it forms a cross-linked array of chemical bonds between the resimolecules. Those types of coatings have the ability to resist imore strong acids or alkalies than inorganic coatings and they tend to absorb significant quantities of cargo and   contamination problems can still occur
Today’s state of coatings can be categorized as:
Pure epoxy
Epoxy phenolics
Epoxy isocyanates
Alkaline zinc silicate
Ethyl zinc silicate
Cyclosilicon epoxy  ( siloxirane )
One important factor, which determines the performance of coating, is the curing process and the adhesion of coating on the metal surface. Curing is done in ambientemperature or with high velocity hot air applied into the tanks. Very important factor regarding curing is the time and relative humidity.

If we consider a coating system as an epoxy/amine coating where the epoxy is the first component of the coating and the amine is the curing agent, then the epoxy and the amine will react together. As the curing continues the molecules will become greater and they will continue to grow until a “gel is formed. When the “gel is formed, the epoxy molecules cannot be more soluble to the solvent. This means that the molecules do not move free as previously.

When the greatest amount of reaction has taken place and the biggest amount of the solvent has escaped the systethen the system can be consider to be cured.

The coating system can be further be cured in order to achieve better properties of the coating by increasinthe “environment” temperature. In this way we obtain to assist further the molecule mobility and thus further reaction between epoxy and amine molecules. At the end most of the solvent has escaped the system. This heat treatment is known as the “post-curing” and the final film has better properties than the simple cured film.

A vessel with mild steel tanks costs almost the half that of Stainless Steel and further more applying the latest coating technology, the income could greater than the one with stainless steel cargo tanks.
Corrosion can be defined as “the destruction of a metal by an electrochemical reaction with its surroundings”. Experiments have shown that iron will not rust wheit is in dry air, nor in water which is free from dissolved oxygen so both oxygen and water  are  necessary  i the  corrosion  process.  The  process  of  corrosion  will  be accelerated with the presence of an electrolyte into the solution, especially when it is acid or base.
The chemistry of corrosion is described above.

In simple terms corrosion can be expressed by the chemical reaction

A+B=C+D
where A is the metal and B the non –metal reactant (reactants) and C, D are the products of the reaction. In other words it is an electrochemical reaction of metal with its environment
The iron reduces tiroions at anode, the oxygen is reduced by combining with water and electrons passed from the anode (by iron changing tions) to hydroxyl ions. The oxygen reacts with the Fe2+ to form ferrous oxides  (Fe2O3, FeO), which are a reddish brown loose deposit. In chemical tankers most of the cargoes are organic and inorganic substances.

So it is essential to coat them in order to prevent corrosion of tanks. Corrosion is accelerated more in pH ranges 0 to 3 and from 13 to 14 due to break down of coating system corroded areas and spots may be developed most of thtimes coating fail because of several reasons like bad application or weather conditions. The coating protection mechanisis composed bthree sub-mechanisms:

First mechanism:
By the barrier protection, the dried film blocks oxygen, moisture and any corrosive environment from the metal. However, all coatings cannot prevent by 100% their penetration by the corrosive elements and this property is called permeability. Typical barrier coatings are two-part epoxies (e.g. epoxy amines).
Second mechanism:
Coatings that protect by inhibition contain special pigments to inhibit the corrosion reactions on the steel surface.
Third mechanism:
Coatings with sacrificing action contain zinc ipowder form. Zinc is more active than steel. So if zinc is icontact with steel and this system is present into a corrosive environment, then the zinc will corrode to protect steelMost of the times zinc is contained in the primer.

Depending the kind of coating we are going to use, we definthe number of layers that steel is going to be coated. For example, zinc silicate coatings are applied as one layer while epoxies can be applied atwo or three.

So if we examine a three layer coating system, then each layer wilhave its purpose

The primer is the first coat applied to the surface. It is very important because it ensures the adhesion of coating on the surface. Surface preparation helpthe coating stick by removing contaminants that interfere with bonding and by creating a profile or roughened surface. Many primers for steel also contain anti-corrosive pigments that actively assist the control of corrosion.

The intermediate coat or undercoatis required in many coating systems and may provide one or all of the following functions: improve chemical resistance, serve as an adhesion coat between the primer and topcoat when the primer and topcoat are not compatible, and increase ththickness of the coating system.

The  finishing  coat  is  the  final  coat  applied.  Topcoats  are  formulated  to improve the chemical and weather resistance of the coating system, and provide characteristics such ascolour, gloss and wear resistance.

Generallthe coating is consisted by three components:

The pigment
The resin
The solvent

Each one haits purpose and their mixture providthe final coating properties.

The pigment is used to distinguish the types and number of coatings. Is a relativelinsoluble element of the coating system. It is well known for the colour characteristic, which gives to the coating. Additionally, it assists corrosion resistance, adhesion characteristics and decreases moisture permeability. They can be categorized as inorganic and organic. They are used because they enhance the anti-corrosive coating characteristics and for aesthetic purposes

The resin plays thmost important rolat the film formation. It holdthe pigment particles together and binds the coating to the metal surface. Also it has significant effect on the durability, strength and chemical resistance of the final film. Additionally forms the final membrane upon which depend many of its basic physical and chemical properties. Generally the coating systems are categorized according the
type of resin. 

The primary role of solvent is for application. The solvent provides the coating tbtaken out of the can and be applied on the surface, dissolves the film-forminingredients and provides flow out of the coating once it is on the surface and contributes to the drying, adhesion, of the final film. Furthermore the solvent ithmai component which  help the  resi and  cure  agent  molecules  t react.

Additionallthe solvents evaporate, in a their greatest percentage, and they are not taken into account as parts of the dried film coating. Some times the coating system might contain more than one solvent each of which has a certain role like to dissolve the resin and control evaporation

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 witthe maximum zinc percentage.

Coatings, which are water based are the Alkaline Zinc Silicatesthey may be composed of water-dissolved sodium silicate, potassium silicate or lithium silicate. The curing of coating occurs by the reaction between the zinc powder (pigment) and the binder silica gel (binder). The binder is supposed to react also with the steel substrate, forming a chemical form that results in outstanding adhesion. This chemical bonding tsteel surfaces avoids undercutting of coating.

The curing mechanism and coatings formation is supposed to occur in three stages:
1.    Initial reaction involves concentration of the  components by water evaporation. This brings the zinc and silica into close contact, providing moist coating on the substrate. During this stage, wetting agentithe paint enhance contact othe coating film witthsteel surface.

2.   At the second stage insolubilisation of the coating film, caused by the reaction of zinc ions with the silicate, and formation of the initial zinc silicate will occur. After this reaction a solid coating on the metal surface has formed. Thmechanical and chemical properties are acceptable but the film has a porous structure.

3.   The third stage of reaction is composed by the action of carbonic acid formed by the carbon dioxide and moisture on the coating surface. The carbonic acid, when penetrates the coating film reacts with the free zinc particles completinthe formation of a dense zinc silicate matrix.

Curing or hardening of the coating takes place by hydrolysis of the soluble silicate followed by interaction witthe zinc to form an insoluble zinc/zinc silicate complex. For self -cure formulations only atmospherimoisture is needed to completthe chemical reaction since atmospheric CO2 creates carbonic acid witmoisture.

Ethyl Zinc Silicates are solvent borne coatings consisting of ethyl silicate and zinc powder. The curing procedure is similar tthe Alkaline Zinc Silicates but now instead of water, solvenis evaporated.

Alkaline Zinc Silicates, cannot be applied by airless spray equipment due to high content of metallic zinc powder, dehumidification and ventilation during the drying and curing stages 1&2 are critical, they are applied in a single coat with a range of 75~125 microns due to crack formation.

On the other hand, Ethyl Zinc Silicates can be applied by airless spray since organic zinc silicates have lower content of zinc powder, the drying conditions are less critical with relative humidity be greater than 60%. The main problem is the difficulty of respraying low dry film thickness areas since adhesion problemmay occur athe first coat.

Although the physical properties (i.e. hardness and abrasion resistance) vary according tthe type of silicate used, chemical resistance and cargo compatibility are very similar. These coatings are normally applied as a single coat of 75~125 microns to a blast clean metal surface. They are sensitive to quality of surface preparation and blast cleaninto a white metal finish is necessary

Generally, the above coatings have an extremely high resistance and tolerance to aromatic hydrocarbon solvent such as benzene and toluene, alcohols and ketones. They are not resistant to acids or alkalis, including sea water which has a slow deteriorating effect. Vegetable oils and animal fats are unsuitable but halogenated compounds are suitable provided that tank surfaces are free of moisture. Any moisture will react witthe cargo and release acids, which will damage the coating. Also the cargo should not contain any moisture for the same reason.

So it is important that both tanks and cargo will be free of moisture.

Epoxy coatings are generally suitable for the carriage of alkalis, glycols, seawater, animal fats and vegetable oils butthey have limited resistance to aromatics such as benzene and toluene, alcohols such as ethanol and methanol. In other words are blends of polymers of varying molecular weights. They contain curing agents in order to cure fast and they are 75~90 % solids by volume. They have very good chemical resistance and they applied as two or three layers.

These coatings have tendency to pick up slight traces of the product carried, especially those chemicals which have only a limited suitability. Alcohols, esters, ketones have a tendency to soften the coating and ithis condition the coating is more likely to absorb small amounts of cargo. A “fingernail test can be used to establisthe hardness of the coating. If the fingernail is able to penetrate the coating, it is still considered to be soft, in that case the tank is vented thoroughly before water washinis carried out.

Generally these coatings are suitable for the carriage of animal and vegetable oils provided the acivalue does not exceed 10 (i.e. free fatty acid content of 5%). However, oils or fats with acid value between 10 and 20 may be acceptable folimited time of carriage.

Example of Sigma paints:
The max acid value determined by ISO 660 ( 1996 ) are related to approx weight % of FFA as below—
Phenguard/ max acid value, no limit/  approx free fatty acid, 100%
Novaguard/ 100/ 30 to 50%
Guard EHB/ 20/ 6 to 10%
Silguard MC/ 5/ 1.5% to 2.5%
Molasses is acceptable in epoxy provided the PH is above 4, although dilute solutions may become acidic and attack the coating. Such situation is remedied by adding an alkali to keep PH in acceptable level.

Epoxy coating can be categorized according to the resin that they will be mixed as follows:

Pure  epoxy  coatings  are  based  on  bisphenol  and  epichlorhydrin  resins reacting, through their terminal epoxide groups, with hardeners having polyfunctional –NH2 groups which are called polyamines.
The properties of cured pure epoxy products depend on:

The type of epoxy

The type and quantity of hardener

The degree of cross-linking

The nature and quantity of additives

Chemical resistance and mechanical properties of epoxy coatings may vary. The factors which, influencing these properties are the molecular weighof resins, thtype of hardener (curing agent) and the pigmentation and solvent mixture.

Low molecular weight epoxy resins results in coating films with a higher densitof three-dimensional crosslinkings as well as a lower number of hydroxyl croups. Therefore, low molecular weight epoxy resins offer better chemical and water resistance than mediumolecular weight epoxy resins, which, on the other hand offer bettemechanical resistance and flexibility.

The most valuable property of epoxy resins is their ability to transform from the liquid state to tough, thermoset solids. The conversion is accomplished by the addition of a chemical compound, the curing agent. Depending the type, curation may occur at ambient temperature omay require post heating.

Amine  cure agents  provide  good  chemica resistance  to  epoxy  coatings, while polyamide-cured epoxies show more surface tolerance and better mechanical properties.  The  late are  more  preferabl because  they  offer  superior  solvent resistance.

Generally this type of coating is used because of its versatility, resistance range and application properties. A pure epoxy coating can be applied by airless spray at  medium  t high  dry  film  thickness  without  sagging cracking  or  pinholing. However, the maximum overcoating intervals are relatively short (three to five days),
requiring a tight application schedule

Epoxy Phenolics  are multifunctional epoxy resins made by the epoxidation of phenolics resins with Epichlorhydrin. This type of amine-cured resins result’s in polymers with very high crosslink density, offering outstanding chemical resistance. However, most epoxy phenolic coatings require heating to 50 ~70 C for four to five days to reactheir full resistance range.

Generally,  the  chemical  resistanc of  heat-cured  epoxy  phenolic against strong solvents and fatty acids is better than pure epoxies. From practical point of view, however, heat post-curing poses several problems. To keep the cargo tanks athe required temperature, they must be loaded with an inert cargo (i.e. lube oil) and heated with the heating coils. This procedure is usually insufficient to reach 50-70 in areas such as deck heads and bulkheads, requiring the use of auxiliary heaters in the double skin compartments as well as the construction and heating of provisional air casings (void spacemade of staging which trap into blown hot air) on the deck areas above tank ceilings. Today the heating can be easily achieved by blowing hot aiinto the tanks but it is quite difficult to ensure that all tank areas are kept constantly and uniformly at the required temperature for long periods.

It has been observed that without heat treatment, the chemical resistance of epoxy phenolics improves after a service tome of at least three months if only moderately aggressive cargoes are carried, but it does not acquire the full resistance range of heat –cured coatings.

Properly formulated epoxy phenolics coatings have application properties similar to pure epoxies but usualllonger overcoating times, making recoating less critical. On the other hand, they may create more overspray due to theistronger solvents, which are evaporated faster. A coating system with a dry film thickness of more than 700 to 800 microns, which may occur at critical areas such as angular welding seams on bottoms and ceilings may cause cracking through the whole coating film. Usually this phenomenon appears only after a salt-water test and cannot be detected during application

Epoxy Isocyanates:  Higher molecular weight epoxy resins can be crosslinked with polyisocyanate with polyisocyanate compounds. This reaction occurs at room temperature and thisocyanate reacts with the hydroxyl groups of epoxy resin. So a densely crosslinked structure with excellent chemical resistance is obtained.

Cured epoxy isocyanates offer a resistance range similar to heat-cured epoxy phenolics, the only exception ithat cannot carry alkaline cargoes with high concentrations.  Most  cargoes  can  be  carried  after   curing  of  ten  days.  Very aggressive cargoes such as methanol can be carried after a three-month service period.

It has been mentioned the cure occurs at ambient temperature however, epoxy isocyanates are more difficult to apply than pure epoxies or epoxy phenolics and they have more critical application properties. For example, because they need rapidly evaporated solvents, overspray may be a problem and they are sensitive to overthickness. So the dry filthickness of the whole system is smaland crack may occur at 150 microns. Most of the times the crack can be observed with naked eye after drying, however some times it is visible only witmagnifying lens because the crack does not split the whole coating film. Therefore, it will not result in rusty spots during thsalt-water test.

Areas usually affected by cracking are angular welding seems and corroded spots (pitting). Stripe coated areas, if overcoated before they are completely dry, can cause cracking or blistering. To eliminate this problem, each coat must be inspected for cracking and defected areas should be repaired. Paint defects such as sagging and orange peemust be also eliminated because they are associated witcracking.

These application problems as welas health problems are the main reason for reduced  usage  of  epoxy  isocyanates.  However,  i they  are  used  they  can  offer excellent resistance to aggressive cargoes, especially in the case of newbuildings. In the case of repair they may not be recommended because at heavily corroded steel it will be difficulto avoid overthickness on pitted areas.

Cyclosilicon EpoxiesThese coatings are based on a new resin, which is essentially a cyclic silicon structure with five epoxidised phenol groups, that are cured by means of catalyst to give a highly crosslinked polymer. In other words these coatings are a two-component paint based on Siloxirane, a patented polymer with an organic/inorganic matrix.

More precisely Siloxirane consists of SiO- rings as a backbone forming a homopolymerized thermoset (heat cured) coating resin with high chemical resistance and good mechanical properties.

The homopolymerized thermoset resin has an oxygen to carbon linkage with high dense and cross-linked molecular structure. Also the absence of –OH eliminates the failure of building other types of polymers.

Manufactures claim that cyclosilicon coatings can resist up to 98% of the seatrade cargoes, including cargoes which are unsuitable for stainless steel. Additionallthese coatings have very low absorption characteristics. As a resulthey can offer significant advantages over conventional coatings regarding the cargo range, cargo handling antank cleaning.

The coating is applied as a two-component paint. It can be applied like a conventional organic coating with partial curing taking place at room temperature, then the curing time will range from four to five days. However, for the full chemical resistance range, heat curing at 80 C for at least eight hours with hot air is necessary. Moreover, the coating system is sensitive to overthickness. Maximum dry filthickness should not exceed 500 microns because of the risk of solvent entrapment or cracking. Also overthickness could be a problem when recoating older tankers with corroded structures sincthe barrier of 500 microns could be exceeded the coating should have a degree of elasticity to remain on the steel surface without any crack to occur.

Coatings are said to have good slip when they have a low coefficient of friction and poor slip when they have a high coefficient of friction Slip is an important characteristic of coated tanks for it ithe propertthat allows easy removal of cargo during tank cleaning.

The compressed air used for abrasive blasting should be oil free, be cooled after compression and must not have higher temperature and humidity than the air fed int the  tanks  by  the  dehumidifiers.  By  balancing  th ventilation  of  the  tanks, oxidation of the blasted surface ieffectively prevented.
The abrasive should to be used should be dry, sharp, of good quality with a content osolublsalts which should not exceed a specific limit.

An easy test for detecting oil/grease on a surface is the "water break method", where a drop of water is added on to the prepared surface. The drop will spread out rapidly on the surface if no oil/grease is present, but will remain on the surface in a drop-shaped form in the presence of oil/grease.

After the finish of blasting we use suitablindustrial-type of vacuum cleaners in order to remove residual grit and dust from surfaces

 Sa 1: Light blascleaning

When the surface is examined using the naked eye, it has to seen to be free of traces of  oil grease,  dirt  and  lightl attached  mill  scale,  rust ol layers  of  protective coatings and other bodies.

Sa 2: Thorough blast cleaning

When the surface is examined using the naked eye, it has to be seen to be free of traces of oil, grease and dirt and of the greater part of attached mill scale, rust, ollayers of protective coatings and other bodies. Any remaining dirt has to be well attached to the surface.

Sa 2.5: Very thorough blast-cleaning

When the surface is examined using the naked eye, it has to be seen to be free of traces of oil, grease and dirt and of the greater part of attached mill scale, rust, ollayers of protective coatings and other bodies. Any attached dirt has to have the form of light stainithe form of narrow strips or points.

Sa 3: Blast-cleaning to visually clean steel

When the surface is examined using the naked eye, it has to be seen to be free of traces of oil, grease and dirt and of the greater part of attached mill scale, rust, ollayers of protective coatings and other bodies. The surface should have a metallic shine.

In the shipbuilding industry, we often encounter an initial condition A (good condition)  or   (rust  condition),  whereas  the  required  preparatio grades  are, according to regulations Sa 2, Sa 2.5 and Sa 3. Preparation grade Sa 3 is desirable but requires expensive cleaning installations whereas at the same time it produces an increase in surface roughness beyond the desired level. Thus, in superstructures a preparation grade of Sa is often sufficient whereas for underwater regions of the hull and the inner structure of compartments a preparation tgrade Sa 2.5 is required.

Steel temperature, can be measured by a contact thermometer The steetemperature should always be 3 C above the dew point before painting.

The dew point is the highest temperature at which moisture will condense from  the  atmosphere.  The  dew  point  i essentia t determine  if  the  climatic conditions are acceptable for pain-work. The calculation, can be done from tables or by a so-called dew point calculator When you have measured the dry and wet bulb temperature, the dew point and relative humidity can be read from the dew point calculator.

The lining material, which is stored under controlled temperature is mixed and thinned ithe correct proportion before use, and after mixing must be used within the specified “pot life” set by the manufacturers. To avoid errors in mixing ratios the components are supplied appropriately sized containers.

The application of coating starts from the bottom of the tank tthe ceiling, because during application the evaporated solvents go to the bottom of the tank. So the air ithe tank is both renewed and dehumidified to keep clean atmosphere and steady temperature and humidity conditions.

The advantages of air-less spray include the provision of a smooth paint film with less change of air entrapmentgreater versatility for the operator, less turbulencithe spray pattern and greatly reduced risk of contamination witmoisture and oils from improperly cleaned compression equipment.

After the first coat is sufficiently dry the tank should be inspected and any uncoated spots shoulbe coated.

However, there are some critical areas where due to their structure it iimpossible to achieve the appropriate dry film thickness. For that reason stripe coats are applied with roller or brush. Typical stripe coat areas are:

As the stripe coatings have been applied and inspected we apply the second coating, which has different color frothe firstusing the air-less equipment.

When the coating system has adequately dried then dry film thickness measurements are done At the same time hardness of the coating are taken using a hardness-pen and inspection for any uncoated areas is done for the wholtank.

The tanks may be charged with sea -water and then discharged in order any uncoated areas to be discovered. These areas will be corroded by the sea -water.

At the last stage all heating coils and pumps are fitted on their initial places. Hot air is applied into the tank continuously to enhance curing of coating. This procedure is called post-curing and it can be done even while the ship is in service with these tanks empty.

Temperature determines the maximum amount of moisture which air can hold. When warm air is trapped ithe tank, it is cooled by contact witthe cold structure of the vessel so the relative humidity level rises until the dew point is reached and water droplets begito form on the cold surfaces. So in order to avoithe above mentioned phenomenon we try to keep steel temperature few degrees of C above the aitank temperature.

Ventilation is required during each stage of the process: blast cleaning, application of coating, and curing of coating. It can be described in terms of airflow and the exchange of clean incoming air and dirty outgoing air. The balancing of incoming and outgoing air is an important feature of a ventilation system. If a high volume of clean air is blown into the tank while a lower volume of dirty air will be extracted then aiturbulence will be created.

Spaces of 60 m3 and less shalhave an air change every minute.
Spaces from 60 m3 to 850 m3 shall have an air change every three minutes.
Spaces from 850 m3 to 2,800 m3 shall have an air change every five minutes.
Spaces over 2,800 m3 shall have an air change every ten minutes.

The ventilation rate should sufficient to dilute solvent vapour to 10% or less of the lower explosive limit (LEL) of the specific solvents being sprayed. LEL is the lower limit of flammability or explosiveness of a gas or vapour at ordinary ambient temperature. It is expressed in percent of the vapoiair by volume.

Additionally we force the solvents to leave the coating system so that molecules can move easily and further hardening reaction take place.

Proper ventilation is obtained witequipment fomoving air, directing the air, anthe efficient set-up of the equipment. The major air movement components of a ventilation system are fans, ducting, and system layout
The reason of setting the suction pipes near the bottom of the tank ibecause dust particles and vapour solvents are heavier than air they concentrate at the lower tank height. Additionally as both blasting and painting are applied from the tank bottom tthe top, we keep the atmosphere for workers clean.

 Dehumidification may be required or desired for three reasons. One reason is that a coating specification may require a maximum relative humidity (RH) that is belothe ambient RH of the tank For most shipyards in the USA, Europe, and Asia, the ambient RH will normally be higher than 50%, so dehumidification is needed tmeet the specification.

A second reason for requiring dehumidification is to prevent condensation on a steel substrate. Condensation will occur when the dew point is at or abovthe surface temperature. Most coating specifications require the surface temperature to be at least 3 degrees C above the dew point temperature. If these conditions are not met, dehumidification can be used to lower the dew point. These conditions will then allow blasting and painting to proceed. Some shipyards prefer a spread of 6 or 8 degrees C, especially for tanks. In many locations, the surface temperature is less than 3 degrees C above the dew point temperature, so without dehumidification, there would be a substantial risk of condensation.

A third reason for requiring dehumidification is that it can create working conditions that can improve productivity. Dehumidification can raise or lower the ambient temperature while reducing the RH in a tank. Coating work on tanks under dehumidification can continue despite cooler ambient temperatures and high RH. Dehumidification can alslower the ambient air temperature, so it can reduce hot and humid conditions inside tank that make the workers tired.

Generally the coatings are porous, this means that cargo can physically penetratthe film and be captured into it. The sequent of this behaviouis the possible reaction between the previous and the following cargo, which might lead to cargo contamination. Regarding inorganic coatings (i.e. zinc silicatcoatings), very volatile cargoes can be easily removed using evaporation-ventilation techniques from the coating because this type of coating does not absorb large quantities of cargo. However, “heavier” cargoes like lube oil cannot be easily removed from the film. That might cause contamination of the next cargo, especially when the next cargo is a “good” solvent The  organic  tank  coatings,  despite  they  are  more  resistant  t corrosive environments they tend to absorb greater quantities of cargo than zinc silicate.

The main factors influencing absorption-desorption characteristics are:

Coating thickness
Temperature
Tank cleaning

Also  som coating  absorption/desorption  characteristics  are  influence by  water. Some coatings have considerably lower rate of absorption when they are saturated with waterThe coating breakdown has a form of blistering, which increases the tendency of the coating tabsorb cargo.

It has been observed that the absorption rate of a substance into a coating film is rapid and increases in a linear way and then falls to zero when the film becomes saturated. In other words no more cargo is absorbed by the coating. On the other hand the desorption ratis rapid too, at the first stages, and at the end it falls to a steady value. That means than the absorbed substance has not fully escape the film absorbed cargo quantity becomes maximum during the first three days and until the 13th day the absorption rate relatively does not change. By the 13th day desorption occurs, and lasts four days. As we can see the desorption rate does not change during the last two days (17-19), which means that an amount of absorbed substance will retaiinto the coating.

        Cargoes having small molecules are able to penetrate organic coatings easiethan those cargoes with greater molecules. For this reason methanois one of the most aggressive cargoes.

High cross-linked molecule structure of coating the percentage of adsorbed cargo is reduced• The absorption/desorption characteristics of the paint systems differ significantly. Some paints absorb less amount of cargo than others and desorb the cargo more efficiently. The selection of such coating system reduces the risk of contamination.

Allow coatings to desorb as long as possible. The rate of desorption increases as thtank temperature increases. An important point is that continuous ventilation of tanis not as effective as the increased air temperature ithe tank. Avoid the stowage of “sensitive” cargoes such as ethanol, methanol, isopropanol etc., in tanks where incompatiblcargoes have been previously stowed.

It is often necessary to clean or ventilate cargo tanks when changing cargoes in order tprevent undesired interactions between cargo residues and the next cargo. Such interactions can form substances that may attack the coating system, enhance danger of steel corrosion and contaminate or discolour the next cargo.

For example, when residues of a cargo, which contains ester groups iits chemical composition, may create acetic acid by hydrolysis. Hydrolysis will take place as soon as the residues of a cargo will comin touch with water molecules. This reaction will cause corrosion and may attack the coating. The same will happen when, cargo residues, contain chlorinated hydrocarbons. They can form hydrochloric acid upon contact with water or water containing cargo.

To avoid such interactions, all esters and chlorinated hydrocarbons must be transported in dry cargo tanks. Methanol cargoes can be especially problematic. Besides having a softening effect on organic coatings, methanol residues in a coating can cause water vapour permeability, causing osmosis between coating and steel substrate. In addition, methanol can extract residual solvent and low molecular weighmaterials from the coating. This induces stresses ithe coating that can lead to cracking.

Only highly crosslinked coatings are resistant tmethanol. Most coatings suppliers do not allow transportation of water-containing cargoes after transportation of methanol. Additionally cargo-compatibilittables result of collaboration of chemical companies and organizations are available to ensure the cargo purity and corrosion steel prevention . Guidelines for tank cleaning procedures when changing cargo should be followed carefullto ensure that cargo residues are sufficiently removed before loading a new cargo.  Organi tank  lining  systems  can  absorb  materials  from  cargoes,  and  the amounts after different time periods are not well defined.

Variable and unpredictable absorption/desorption characteristics are found not only among different coating types but also within the same generic type of coating from different manufacturers. In addition, different rates of absorption/desorption are found among different cargoes. This can make idifficult to selecthe correct cargo tank coating system.

According to the coating manufacture specifications, the coating should have a specified d.f.t. of 300 microns and the minimum d.f.t. should not be below the 29microns. In other words, the final d.f.t. should not have a diminution more than 10% of the specified.

Temperature and humidity sensors can be attached on the tank plates using magnets in order tmonitor aitemperature and humidity. These sensors can be removed after the completion of curing.


CAPT AJIT VADAKAYIL ( 28 YEARS IN COMMAND )

4 comments:

  1. Capt. Ajit Vadakayil
    April 20, 2015 at 10:05 AM
    hi s,

    time to have a OBJECTIVE and DISPASSIONATE look at osho.

    this man was indeed intelligent. he does NOT need valium / nitrous oxide which instead of helping will degrade his consciousness.

    cannabis can increase your consciousness.

    PEA BRAINED HILLY BILLY YANK AND JOHN BULL CANNOT GUAGE OSHO.

    Osho revelled in making a CHOOT of of his white skinned devotees.

    he would tell them black is white in the morning and in the evening he would tell them white is black.

    the MANDH BUDDHI white skinned man is at the lowest gross plane of wisdom.

    the men of wisdom knows what must EVAPORATE must CONDENSE .

    this is the dance of shiva/ vishnu/ brahma

    http://ajitvadakayil.blogspot.in/2013/05/dvaita-vedanta-of-sanatana-dharma-or.html

    the white man followed osho because he has RAW animal magnetism.

    i give leadership lesson to my chief offiicers preparing them for ship's command.

    when any officer leaves my ship for marriage i also tell him HOW TO FUCK HIS WIFE - so that she does NOT become a chutney mary -- making her pickle john husband make bed tea for her and wash her bloodied undies.

    HOW TO FUCK A WOMAN IS BELOW-

    http://ajitvadakayil.blogspot.in/2011/09/muringakai-quantum-medicine-and.html

    when a woman is fucked well she does NOT care to write about her PHOREN jaunts in her blogs to impress her BLOG DOSTS .

    HEY BLOG DOSTS - I HAVE BEEN TRAVELLING TO ZURICH, PARIS - NOW I AM BACK TO COMPLETE MY PORN BOOK STARTING WITH S !

    getting back-

    one chief officers disagreed with my ANIMAL MAGNETISM leader virtue thing.

    so i told him to tell me one DRY BORING topic over which i would speak to my full officers/ crew next day for 4 hours continuously--while STANDING ON FEET erect -in a line .

    if one crew member even moved - i would give him an agreed amount of beer/cigarettes .

    he chose RISK ASSESSMENT which is more boring than boring-totally technical.

    what i spoke is now a blog post ( condensed form ).

    first the speaker has to exude magnetism ( like OSHO ) -secondly if he can , he must hold the interest.

    speakers like K Krishnamurty could NOT hold his audience's interest for more than 5 minutes -- as HILLY BILLY YANKS were NOT perceptive enough to understand the brillaince of his words. but OSHO spoke in small sentences like automatic rifle bursts.

    THIS PLANET KNEW ONLY "STATIC" RISK ASSESSMENT .

    IT WAS A MAN NAMED VADAKAYIL WHO INTRODUCED "DYNAMIC " RISK ASSESSMENT.

    http://ajitvadakayil.blogspot.in/2010/04/dynamic-risk-assessment-on-chemical.html

    I STRETCHED THE POST ABOVE IN A LECTURE OF FOUR HOURS - NOT ONE SAILOR MOVED OR LEANED BACK ON THE WALL 12 INCHES BEHIND HIM.

    I HAVE DONE THIS FOUR HOURS SPEECH THING AT LEAST 8 TIMES - TO PROVE TO MY CHIEF OFFICER -- ON DIFFERENT SHIPS- WITH DIFFERENT NATIONALITIES.

    DYNAMIC risk assessment was always done in my mann mandir - before ant critical task. People would ask me why I am doing something totally opposite . If he could understand the flammability composition diagram I MAY tell him--otherwise it was NOT worth it.

    http://ajitvadakayil.blogspot.in/2011/07/flammability-composition-diagram.html

    above few sentences ARE THE MOST VALUABLE AMONG MY 51587 COMMENTS TILL TODAY-- so understand this -nobody on this planet can explain better ( this is NOT a boast so read the line again ) .

    ug krishnamurthy ( a wannabe J ) was an ordinary man. J krishna murthy was on a higher plane.

    Jiddu left a legacy, but nothing of this sort for UG, who lacked the self conviction and self confidence.
    .
    UG was volatile and that too on a shaky foundation, and hyper-fragile Parveen Babi who accepted his a her guru, was catalyzed to self destruction. UG died bitter and alone- he could NOT rein in his inner devils.

    capt ajit vadakayil
    ..

    ReplyDelete
  2. Shruti Khajuria
    October 7, 2015 at 4:53 PM
    Hello captain.
    Today i asked my teacher about the role/power of president to avoid parliament sessions being washed out- like this monsoon session.

    He said that president cant do anything n it is in the hands of speaker(inshort pm) who can do something.

    Could u plz she'd some light on it.

    ReplyDelete
    Replies
    ###########################
    Capt. Ajit Vadakayil
    October 7, 2015 at 7:55 PM
    hi sk,

    WHY WAS CAPT AJIT VADAKAYIL THE MOST POWERFUL SHIP CAPTAIN AT SEA?

    WHY WERE HIS BOSSES MORTALLY SCARED OF HIM?

    WHY WAS THIS SHIP CAPTAIN LIKE THE "SLAVE MORE POWERFUL THAN CAESAR "?

    OK-- THE EASY WAY OUT IS TO SAY THAT THIS CAPTAIN KNEW MORE THAN OTHERS .

    SORRY-- THE COOKIE DOES NOT CRUMBLE THAT WAY.

    I KNEW MY POWERS VESTED IN ME BY THE PRESIDENT OF INDIA BY A DOCUMENT--THE MERCHANT SHIPPING ACT.

    http://vikaspedia.in/social-welfare/unorganised-sector-1/the-merchant-shipping-act1958

    I ONCE HAD A BOOS WHO SAILED ON MY SHIP FOR 2 DAYS .

    FROM HOUR ONE HE WAS AFTER MY ASS -TRYING TO TEAR IT APART.

    I ASKED MY PURSER TO SIGN HIM ON ON THE SHIPS" ARTICLES OF AGREEMENT"

    LEGALLY AT SEA THERE CAN BE NO PASSENGERS ON A MERCHANT SHIP WITHOUT PROPER DOCUMENTS.

    HE REFUSED TO SIGN IT- SAYING THAT HE HAD NEVER DONE IT BEFORE.

    IMMEDIATELY I TOLD THE PURSER AND CHIEF OFFICER -- YOU ARE WITNESS , CARRY ON.

    THIS MANGY BOSS WENT RUNNING AFTER THE PURSER AND SIGNED IT-- AND NOW HE WAS PART OF MY CREW.

    SO I TOLD HIM--

    AT NEXT PORT I WILL SIGN OFF MY CHIEF ENGINEER AND I WILL ASK THAT YOU BE MADE MY CHIEF ENGINEER TILL I GET A SUITABLE REPLACEMENT.

    ( THOSE DAYS I WAS THE BLUE EYED BOY )

    YOU SHOULD HAVE SEEN HIS FACE --HE LITERALLY SHAT AS WE COULD SMELL SHIT.

    ALWAYS KNOW YOUR POWERS.

    THE INDIAN PRESIDENT MUST KNOW HIS POWERS.

    DID NOT TN SESHAN ( ELECTION COMMISSIONER ) POONGI BHAJAO ALL THE HIGH AND MIGHTY POLITICIANS ? SAB KA GAAAN EKKATTA PHAAD DIYA . .

    THEY WERE ALL CRYING MUMMMMYYYYY !

    HE KNEW HIS POWERS.

    IF TN SESHAN WAS PRESIDENT -- THE FUTURE PRESIDENTS WOULD HAVE BEEN ON A BETTER WICKET.

    when i wanted to interpret the merchant shipping act--i would call my wife and ask her to think like a landlubber ( like a lawyer in a court of law ) .

    see, i would be tied down by PRE-CONCEIVED NOTIONS.

    at sea i tool officers and crew who had NIL chemical experience and I moulded them like RAW CLAY.

    dog's tail cant be straightened !

    capt ajit vadakayil
    ..

    ReplyDelete
  3. HERE IS AN ADVISE TO MODI -

    A LEADER MUST KNOW HOW TO CONTROL FAKE NEGATIVE PROPAGANDA.

    TAKE THIS FROM A LEADER OF MEN , 30 YEARS AS A SHIP CAPTAIN . .

    MY FIRST 9 YEARS OF COMMAND WAS IN INDIAN SHIPPING COMPANY SCI . . .

    I GREW UP IN THE COMPANY AND I KNEW HOW THE CREW / OFFICERS PUT THE CAPTAIN ON THE BACKFOOT -- BY INSINUATING THAT HE MADE MONEY ON FOOD . . .

    TO BE FRANK, MOST OF THESE ALLEGATIONS WERE TRUE.

    COMPANY GAVE OUT A CIRCULAR TO FORM MESS COMMITTEES ON ALL SHIPS , WITH A MEETING EVERY MONTH AND A REGISTER FOR COMPLAINTS.

    IN EVERY MESS COMMITTEE MEETING CREW/ OFFICERS WOULD THROW SHIT ON THE CAPTAINs FACE -- THE COMPLAINT REGISTER WOULD BE FULL OF FOUL ABUSE , BAD LANGUAGE AND SLIMY INNUENDO.

    TO BE FRANK, WHEN YOU HAVE A CREW OF 60 IT IS DIFFICULT TO KEEP ALL HAPPY. PUNJABIS DO NOT WANT SAMBHAR, TAMILAINS DO NOT WANT PARATHA AND SO ON.

    ONE FINE DAY CAPT AJIT VADAKAYIL TOOK OVER COMMAND OF A SHIP , WHERE HE WAS SECOND IN COMMAND AFTER A MONTH OF LEAVE.

    I KNEW THE TROUBLE MAKERS-- THE STAGNANT CATEGORY LIKE ELECTRICAL OFFICER/ PURSER/ CHIEF ENGINEER WHO HAD NO MORE PROMOTIONS.

    I CALLED FOR A MESS COMMITTEE MEETING IN ADVANCE ON MY TIME TABLE.

    AS USUAL THE RABBLE ROUSERS STARTED OFF --WE WANT THIS , WE WANT THAT-- WE THINK SO AND SO.

    I GOT UP FROM MY CHAIR AND TOLD LOUD AND CLEAR-

    I WANT TO HEAR I NOT WE

    I WANT TO HEAR I WANT THIS, I WANT THAT, I THING SO AND SO.

    AFTER I SIT DOWN ON THIS CHAIR AGAIN, IF I HEAR THE WORLD WE AGAIN , IT WILL GO AGAINST HIM. THERE IS NO SAFETY IN NUMBERS-- ALL ARE ON THEIR OWN STARTING FROM NOW-

    AND I BANGED MY PALM ON THE TABLE-- WITH A EAR SPLITTING THUDDDDDDDD.

    YOU SHOULD HAVE SEEN EVERYBODYs FACES.

    NOBODY DARED TO UTTER A WORD .

    THEY ALL KNEW I WAS THE COMPANYs BLUE EYED BOY ...

    THEN I TOOK THE OLD COMPLAINT REGISTER -- TORE IT UP AND SUBSTITUTED THE COMPLAINT RESISTER WITH A SUGGESTION REGISTER., ADDRESSED TO CAPTAIN AJIT VADAKAYIL.

    I SAID--

    WHATEVER YOU WRITE HERE , WILL BE ADDRESSED TO ME, THE CAPTAIN --NOT THIN FUCKIN AIR.

    YOU CAN WRITE WHAT YOU WANT-- I EXPECT YOU TO LOOK AHEAD , NOT BEHIND .

    YOU HAVE TO SIGN EVERY ENTRY .

    NO ANONYMOUS ENTRY WILL BE ACCEPTED. EVERY SENTENCE WILL BE ADDRESSED TO ME ALONE.

    AND THEN I LOOKED AT THE CHIEF STEWARD AND CHIEF COOK -- I WANT GOOD FOOD. GET ORAL POSITIVE FEEDBACK DAILY FROM THE OFFICERS AND CREW. AND I WANT TO HEAR THAT DAILY . . LOOK IN THE REGISTER DAILY. I WILL SEE THE REGISTER ONLY ONCE A MONTH.

    MY SHIP NEVER HAD A PROBLEM.

    AFTER 3 MONTHS THE CHIEF STEWARD ASKED ME-- DO YOU HYPNOTISE OR DO SOME MAGIC ?

    YES-- LEADERS MUST KNOW HOW TO LEAD.

    MR NARENDRA MODI -- LEARN HOW TO BE A LEADER .


    http://ajitvadakayil.blogspot.in/2015/06/ethos-logos-pathos-capt-ajit-vadakayil.html

    http://ajitvadakayil.blogspot.in/2010/11/blue-ocean-leadership-capt-ajit.html

    PSSST-

    MY LEADERSHIP WAS ON A DIFFERENT TRACK - MY CREW LOVED ME WHEN I GOT ANGRY.

    I COULD MAKE PEOPLE STRIP , IF I WANTED -- THIS IS MAGIC.

    http://ajitvadakayil.blogspot.in/2010/11/humor-management-capt-ajit-vadakayil.html

    IN FOREIGN COMPANIES CAPTAIN COULD SACK ANYBODY WHOM HE DID NOT LIKE.

    ONLY IN INDIAN COMPANIES IT WAS FREE FOR ALL.

    capt ajit vadakayil
    ..

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  4. STOP PRESS:

    SOMETIMES MY READERS WONDER HOW I GET SO MUCH OF UNKNOWN INFORMATION..

    A CHUNK OF IT CAME BY "SHAKING DOWN " PEOPLE.

    MAKE SOMEONE FEEL THAT IF HE DOES NOT CONFESS- HE WILL GO TO JAIL-- AND I HAVE THE POWER TO DO IT.

    THIS IS WHO I CAME TO KNOW SECRETS OF "INDIAN SHIP MANAGEMENT COMPANIES "

    EXAMPLE?

    ONCE I WAS PUT IN A HOTEL ROOM WITH SAME SEROUS DEFICIENCIES --SAY THE ROOM DID NOT HAVE A WINDOW

    I WOULD CALL FOR THE TOP MANAGER AND SHAKE HIM DOWN.

    BUT HEY--

    BUT I MAY RE-CONSIDER , IF HE TELLS ME HOW MUCH KICK BACK PERCENTAGE HE GIVES TO THE MANNING OFFICE WHO PUTS SHIP OFFICERS IN HIS HOTEL.

    PAT- THE ANSWER WILL COME-- 40 %

    THIS MEANS IN A HOTEL ROOM COSTING 5000 BUCKS A DAY 2000 BUCKS GOES TO THE POCKET OF OWNER/ MANAGER OF THE MANNING OFFICE.

    I HAVE SHAKEN DOWN SHIPPING ACCOUNTANTS IN FOREIGN NATIONS . NOBODY LIKES JAIL

    I HAVE COME TO KNOW OF SLIMY SHIP MAMANGEMENT INDIAN COMPANIES WHO PAY AN OFFICER 6000 DOLLARS A MONTH-- BUT THEY FORGE THE WHOLE SIGNED SHEET AGAIN ( ENTIRE CREW ) IN THE OFFICE , AND CHARGE THE JAPANESE SHIP OWNER 30% EXTRA.

    THAT MEANS IF THE ACTUAL WAGES GIVEN ARE 2 LAKHS DOLLARS PER MONTH 60,000 DOLLARS ARE SKIMMED OFF BY THE INDIAN SHIPMANAGEMENT COMPANY--WITHOUT THE CREW OR THE JAP SHIPOWNER KNOWING.

    OFFICERS AND CREW ARE MADE TO PAY FOR THE "FLAG DOCUMENTATION" SAY--PANAMA/ MONROVIA ETC SAY 800 DOLLARS-- BUT THEY COLLECT THIS MONEY FROM THE JAP OWNERS TOO--

    FLAG DOCUMENTATION-- CERTIFICATE OF COMPETENCY, CDC, SPECIAL VERTIFICATES LIKE OIL / CHEMICAL/ GAS ADVANCED COURSEs ETC

    I HAVE THE POWER TO JAIL SEVERAL HIGH FLYING MAHA MERUS WITH PROOF. I HAVE REVEALED JUST 2%.

    SOME OF THEM ARE IMMORAL SONS OF BITCHES-- THEY WILL SELL THEIR OWN MOTHERS TO WHORE HOUSES.

    WELL THEY WILL READ THIS TODAY--AND ALL WILL SHIT IN THEIR LANGOTS AND SPEND SEVERAL SLEEPLESS NIGHTS

    capt ajit vadakayil
    ..


    Capt. Ajit VadakayilFebruary 26, 2016 at 2:31 PM
    SOMEBODY CALLED –

    ASKED ME CAN YOU REVEAL SOME MORE ABOUT THESE BASTARDS RUNNING INDIAN SHIP MANAGEMENT COMPANIES—

    OK— TWO MORE

    AAA- MONTHLY ALLOTMENTS TO HOME BANK REACHING LATE.
    ##################################

    LET US SAY THAT I SENT 15000 USD AS MONTHLY ALLOTMENT TO MY WIFE

    IT WILL REACH MY HOME ONLY AFTER 20 DAYS

    THE FIRST 20 DAYS , THEY MILK OFF THE INTEREST ( HUGE AMOUNT FOR THE WHOLE CREW ) IN SOME BANK.

    AS A BONUS TO THESE BASTARDS I WILL NOT REVEAL THEIR FCNR / NRE MONEY CHANNELING VIA THE MAURITIUS ROUTE OR WHATEVER -- FERA - AS OF NOW


    BBB- A LOT OF COMPANIES DEMAND THE FIRST MONTHs SALARY AS BRIBE.
    ######################################

    I STOPPED THIS WHEN I CAME TO KNOW ABOUT THIS.

    EXAMPLE: I HAD A BANGLADESI OFFICER WHO HAD TO PAY HIS RECRUITING MANNING AGENT ( A SHORE CAPTAIN ) AT DHAKA , HIS FIRST MONTHs SALARY- BEFORE BOARDING THE SHIP.

    THIS MEANS YOU WORK THE FIRST MONTH WITHOUT SALARY.

    I CALLED UP THE TOP HONCHO IN DENMARK ( MY DIEHARD FAN ) AND STOPPED THIS NONSENSE , THAT VERY MINUTE.

    Capt ajit vadakayil
    ..

    ReplyDelete