Principle material used in ship building
Ship building materials:
In ship building there are a variety of materials used depending upon the location the material is being used and the amount of stress the material is expected to take at various stages. The following are commonly used amongst them,
Steel-There is different types of steel used in the ship building depending on the location it is to be used. For hull construction usually mild steel containing 0.15 to 0.23 % carbons and a high manganese content is used. Sulphur and phosphorus content in the mild steel are kept to a minimum, as low as 0.05 %.
High tensile steel have greater strength than that of the mild steel and are used in the regions where high stress concentrations are expected to occur. Using high tensile steel also allow the thickness of the plate to be reduced. During the design stage only it is recommended to consider the weld ability of the steel.
Stainless Steel-steel is mixed with alloy elements to form a corrosion resistant. This kind of steel is normally not used because of its high initial and fabrication costs. Only the ships which are designed to carry high corrosive cargoes are fabricated with this type of steel.
Steel castings are produced by open hearth, electric furnace or oxygen process is poured into a carefully constructed mould and then allowed to solidify to the shape required after removal this type of steel need to be heat treated by the following process annealing, or normalising and tempering to reduce the brittleness. Example of locations where this type of steel is used is rudder frames, stern frames, spectacle frames for bossing etc.
Steel which is being used for ship building is approved by the classification society after carrying out the entire required test.
The properties required for steel used in ship building are as follows:
Reasonable cost, easy to weld with simple equipment, easy to do repairs on, ductility and homogeneity, yield point to be a high proportion of ultimate tensile strength, while flame cutting it shouldn't harden, comparatively resistant to corrosion.
Properties of steel can be altered by the following processes.
Annealing:this is done by heating the steel at a slow rate to a temperature of 850 deg C and then cooled in a furnace at a slow rate. This is done to relieve an internal stress, to soften the steel or to bring to a suitable condition for subsequent heat treatment.
Normalising:this is done by heating the steel at a slow rate to a temperature same as that of annealing (850 deg C) then cooled down in air, this results in faster cooling and hence the steel becomes harder and stronger than the annealing process.
Quenching:this is done by a similar way like the above but cooled by water or oil. Hence cools faster than the above two process leading to a very hard structure with high tensile strength.
Tempering:in this process the steel is heated to a temperature of about 650 deg C and then cooled down very rapidly by quenching in oil or water. This relieves the internal stresses caused by the heat treatment for hardening. Makes the steel less brittle and retains the high tensile strength.
Aluminium:There are three main reasons for the increased use of aluminium over steel in a ship construction. They are
Aluminium is lighter than mild steel, up to 60 % lighter
High resistance to corrosion over steel
It is non magnetic
Care should be taken to insulate aluminium from steel to stop the galvanic action between them. A major disadvantage of aluminium over steel is that it is 8 to 10 % costlier than steel.
Aluminium is always used in the form of alloy due to its low tensile strength in the pure form. Aluminium alloy normally contains 3 to 5 % of magnesium. Aluminium is used in the building of the super structure and it is widely used in the passenger liner.
Glass reinforced plastic (GRP) -Glass reinforced steel is also known as fibreglass. It is widely used in the manufacture of lifeboats, binnacles, fixtures and fittings. It is mixture of high strength glass fibre bonded together by a resin of low strength. This material has both properties of resin and glass fibre. By changing the ratio of glass fibre and resin the property of the GRP can be changed as required by the particular application. It has excellent strength properties and is strong when compared, taking its weight and volume. It has good weather resistance and a high water resistance. It does not shatter. Most important property of being easily moulded to any required, complex shape makes it to be used largely. The biggest advantage is that GRP requires less maintenance than steel, wood etc. Protection of GRP against weathering is taken care by the gel coating which formed by the resin while manufacturing. This coating is then safeguarded by waxing at regularly as required. The eroding of gel is a very slow process but once it is eroded exposing the underlying glass fibres it becomes rough and pitted leading to a accelerated attack. The GRP can be polished by two types of polishes wax and silicone. Wax polish is thicker and easier to remove but requires a lot of buffing to making it gloss.
Timber/ wood:soft wood sheathing is normally used in supply boats, wooden platform used for the magnetic binnacle is mode of timber, Furniture, dunnage, doors bulkhead sheathing etc. The use of wood is slowing being eliminated due to the availability of plastic fixtures which are cheaper and easily maintainable.
BRASS:This is non corroding, non magnetic and non sparking, these are used in fire hose nozzles, sounding caps, ports, brass tools are used in hazardous area.
Explain the causes of corrosion and material failure. What can be done to protect the steel?
Corrosion is a reaction of metal with the environment, the resultant product of this reaction is similar to the chemical composition of the original mineral of the metal.
There are various types of corrosion,
Atmospheric corrosion:the corrosion process is best prevented if it is not given a chance to start. The protection of steel against the atmospheric corrosion is important even during the construction of the ship when the steel is in the shop of in the building berth. Rusting may occur if the relative humidity is about 70 %. The rate of rusting is mostly determined by the pollution in the air namely smoke or sea salts.
Corrosion due to immersion:when a ship is in service the most part of the hull is immersed in the water in the boot top area is intermittently immersed in sea water. Under normal operating conditions these areas require a great amount of care to prevent them from corroding. These conditions favour the formation of electro chemical corrosive cells.
Electro chemical nature of corrosion:at normal temperatures in a solution the transformation of a metal atom into a mineral molecule occurs by the metal passing into solution. During this process the atom losses one or more electrons and becomes an ion. This reaction may only occur if an electron acceptor is present in the solution. Thus any corrosion reaction is always accompanied by a flow of electricity from one metallic area to another through a solution in which conduction of an electric current occurs by the passage of ions. Because of the high salt content in the sea water, it acts as a good electrolytic solution. In total immersion and atmospheric corrosion the moisture and oxygen content are the controlling factors. When the metal is exposed to the atmosphere there is enough oxygen and moisture becomes the controlling factor, in case of total immersion the moisture is present and oxygen becomes the controlling factor.
Bimetallic or Galvanic corrosion:this happens when two different metals form a corrosion cell. In ships the corrosion problems are generally associated with the coupling of different metals with different potentials which forms corrosion cells under certain operating conditions. The most common bimetallic corrosion formed in the ship is the mild steel hull with bronze or nickel alloy propeller and when aluminium superstructure is attached with the steel hull or the steel equipments being fitted on the aluminium super structure.
Stress corrosion:corrosion occurs in various areas due to the stress applied during the cargo operation or during the voyage. These areas can be judged by carefully investigating the general loading and discharge pattern being followed on board. The internal stresses caused by non uniform cold working are more dangerous than the applied stresses.
Corrosion erosion:erosion is a mechanical action but it is associated with electro chemical corrosion in producing two forms of metal deterioration. The air bubbles present in the flowing water hits the metals surface may damage the protective film locally, the eroded becomes anodic to the other surrounding surface and corrosion occurs. This is possible where ever there is a flow of water, will be more if there is a turbulence created because of the flow of water and greater is when warm water is discharged through over board discharges.
Failure of material:when a material is subject to action of repeated or fluctuating stresses it may fail at some stage. Materials can withstand more static stresses than dynamic stresses. Some steels can with stand unlimited number of stress cycles if the stress is reduced to a n amount the material can take, this amount of stress is called as the fatigue limit but some steels will fail after a number of stress cycles even if the amount of stress is reduced this is called as endurance limit.
The failure usually defined to happen in three stages,
Nucleation:a small defect in the material caused due to poor welding, corrosion, quench cracks in heat treated material, Faults in sharp bends etc.
Crack growth:when caused by fluctuating stresses the defective sides rub together and become polished.
Failure:ultimately when the material cannot take any more stress, fracture or break will occur and the fractured surface is crystalline in appearance. The crack develops slowly the fractures surfaces rub together due to the vibration and become polished. When the material cannot carry anymore stress, the final fracture occurs.
Few causes of stress changes are as follows;
a. Repeated bending or twisting in opposite directions
b. Alternate compression and tension
c. Repeated stress, minimum to maximum at short intervals.
Methods of Protecting steel:
There are basically three ways of controlling the corrosion. Each method applicable to different type of corrosion.
By separating the metal and the corrosive substance.
It is achieved by applying paint. Before applying paint the surface should be prepared carefully. Any rust, metal oxide, mill scale or contaminant in the surface should be removed before the coat of paint applied.
This is also achieved by electroplating, by dipping the metal to be protected in molten tin or zinc, this coating protected the covered metal and in many cases it is anodic to the metal so that in the event of a break in the surface the covering acts in a protection for a short time.
By changing the environment surrounding the metal.
This is done by eliminating one of the corrosion controlling factors either moisture or oxygen. In the factories storage area if the air conditioning system is supplying warm and dry air it may reduce the chance of corrosion. In the cargo hole if an inert atmosphere is maintained then the oxygen content is kept to a minimum thus minimising the rate of rusting. The only way to protect the outer part of the hull and accommodation is applying paint.
Interfering with the galvanic action:
Whenever there is a galvanic cell formation, the area with more negative potential i.e. an anodic area which corrodes and the area with more positive potential i.e. cathodic area which doesn't. Hence if the whole area is made into a cathode then there will be no corrosion. Two ways of achieving it is by
The sacrificial anode system:
This is done by placing anodes the appropriate amount of anodes in the required place.
Zinc, aluminium and magnesium alloys are the anodes which are usually used. Zinc and aluminium are used in to protect the hull and magnesium is used in the fresh water tanks and for specialised purposes. Magnesium is however not used in tanks which are subject to carry oil because they are liable to create spark if they drop off.
A galvanic cell is formed between the steel hull and the bronze propeller. Bronze being the cathodic cell and the steel hull being anodic, the steel tends to corrode. Since zinc is further apart from bronze in the galvanic table it forms a better galvanic cell with bronze than the steel hull, thus protecting the hull. The disadvantage is that the anode wastes away and they only protect the area around their anchoring points and the anode needs to be replaced during the dry dock or by underwater welding.
The impressed current system:
This works in the same basic principle, but the current is not from the natural potential difference between the metals, it is obtained from the DC supply to the hull which makes it a cathode. The impressed current is to be controlled carefully just to overcome the corrosion current formed by the hull and propeller. As long as the current is supplied to the plate the hull is protected.
Describe a typical planned maintenance system. Discuss reasons why it is required?
Planned Maintenance System:
In the process of designing a planned maintenance system the following should be taken into account,
* The plan should be adaptable for various weather conditions.
* It should be flexible so that it is not affect greatly by the changes of orders or cargo.
* The trading pattern of the vessel.
* It works in up keeping the standards laid down by the classification society or any statutory.
* It should integrate dry docking and the jobs to be carried out in a dry dock.
* Manufactures instruction for maintenance should be incorporated.
* There should be provision to keep track of the spares being used, and a method of ordering. So that the required spares are always maintained onboard all the time.
* It should be made in time phases to suit a particular vessel, could be short term, long term or maintenance due to operational requirements.
* By all means try and eliminate the need for break down maintenance by planned and regular maintenance.
Most important feature of a planned maintenance must be preventive maintenance, which is achieved by continuous assessment and action. In a commercial point of view, due to the lack of repair and spare facilities while at sea, preventive measure should assess, then repair or replace the required part which will prevent the system from a total breakdown which may bring along the cost of emergency spares, services and delays. Planned maintenance should ensure that the equipment is ready and reliable at times. It also should ensure that the resources are used to full advantage, resource like man power, spares etc. And no area in the vessel is overlooked or neglected. A plan is made to continuously assess the efficiency of equipment and its maintenance aspects; this will result in fewer hazards and more efficient operation of the equipment.
This will also ensure that new personal are aware of the maintenance schedule to be carried out and the history of the equipment. The system should ensure that spares are also available for carrying out the regular maintenance as per plan.
Referring to the scenario, describe the processes involved for making the repairs and any subsequent painting. Include in your assignment health and safety aspects to be considered.
Process involved in repair:
The following factors should be evaluated before the process for repair is commenced.
The degree of damage, the cause for the damage (it could have been due to progressive corrosion or impact while operating etc) whether size of renewal or fabrication that section is required, the condition of the surrounding metal plates (can be assessed by thickness gauging), which will be supporting the new plate which is to be inserted. After it is ascertained if renewal of plate is required the thickness to be used is decided taking into account the thickness of the plates in that area. If this not considered and a plate of the design thickness is inserted where the thickness of the surrounding plates have reduced due to corrosion. The stress distribution between the plates will vary which will result in early failure.
Various methods of cutting:there are various methods to cut the steel plate, suitable method is determined by the location and resources available in hand.
Gas cutting:oxygen and acetylene are combined to produce the flame for cutting. It is basically a chemical/thermal reaction with iron and iron alloys. A small area of the metal to be cut is preheated by the flame, when the required temperature is achieved a confined stream of oxygen is then blown through the heated iron. The iron is oxidised in this area and the molten oxide and metal is removed by the kinetic energy of the oxygen stream. Oxygen and acetylene onboard are stored in gas cylinders which are regulated to the hand held torch where the required mixture of oxygen and acetylene are then adjusted as required by the dials in the hand held torch.
Plasma-Arc cutting:this employs a high velocity of jet of high temperature gas to melt and displace material in its path. Plasma is a mass of ionised gas which conducts electricity. An electrode is connected to the negative terminal of the DC supply and a gas shield is supplied to the arc. The arc produced between the electrode and the material creates a high temperature that will cut the steel. The current used varies from 20 to 1000 amps and it can be used to cut plates of thickness 0.6 mm to 150 mm. The gas used varies; some of the examples are nitrogen or argon and hydrogen mixture. It will be advisable to do this cutting in a water table as it will absorb the dust particles and will reduce the noise and ultra violet radiation.
Water jet cutting:this type of method is used when thermal cutting is not possible. The cutting tool concentrates a high pressure jet of water, approximately 1000 bars. The jet may contain abrasives and is directed onto the work piece via a nozzle, where the water is travelling at 2.5 times the speed of sound. Water cutting can be used on a range of materials timber, plastic, rubber, steel, aluminium alloys, etc. Being a cold cutting process the heat affected zone, mechanical stresses and distortion are left at the surface. It is slower than most of the thermal cutting process and is not a portable machine tool.
After the cutting is made, in order it is important to prepare the surfaces for welding,
The edge of the plate must be prepared so that the surface to be welded is clean, dry and free of contaminants. When plates of different thickness are to be welded the thicker plate is chamfered. Excessive gaps between the plates must be corrected. A welding sequence must be pre planned to prevent locked in stresses.
Various types of welding:
Electric arc welding:this is the most common method widely used in the shipyards. The basic principle of electric arc welding is that a wire or electrode is connected to a source of electrical supply with a return lead the plates to be welded. If the electrode is brought into contact with the plates an electric current flows in the circuit. By removing the electrode by a short distance about 3 to 5 mm from the plate so that it is possible for the current to jump the jump the gap, an electric arc of high temperature is created which will melt the plate edges and the electrode to form a molten state of metal between the two plates once this cools down the molten metal solidifies and forms a solid joint. The slag which forms on top is then chipped away. The electrode consists of a wire core, coated in a material called flux. As a general rule the metal in the core of the electrode should be similar to the metal being welded and can be of 1.5 to 9 mm in thickness. Flux is used for various reasons; it provides a easy strike and a stable arc and also provides a gas shield around the weld pool. If oxygen is allowed to combine with the hot metal it will form oxides which would weaken the joint. Oxygen could also combine with the carbon in the steel and form carbon monoxide. This results in the solidified joint containing blow holes. Nitrogen will react with the hot metal to produce hard and brittle steel. There are various types of electric arc welding they are slag shielded process, manual welding electrodes, automatic welding with cored wires, submerged arc welding, stud welding, gas shielded arc welding.
Tungsten inert gas weldingalso known as TIG welding: in this type of welding process the arc is drawn between the water cooled non consumable tungsten electrode and the plate. An inert gas shield is provided to protect the weld metal from the atmosphere and the filler metal may be added to the weld pool as required. Ignition of the arc is obtained by means of high frequency discharge across the gap since it is not advisable to strike an arc with the tungsten electrode; the restriction is only a plate of less than 6 mm thickness can be welded by this process. It is mostly used for welding on aluminium.
Metal inert gas welding or MIG welding:it is similar to a TIG welding but for the electrode being a consumable metal wire. A wire feed motor supplying wire via guide rollers through a contact tube in the torch to the arc. An inert gas is supplied to the torch to shield the arc, and electrical connections are made to the contact tube and work piece. This is done by establishing a the arc, then the wire is fed into the arc until it makes contact with the plate, resistance heating of wire in contact with plate, pinch effect, detaching heated portion of wire as droplet of molten metal. And finally re-establish the arc for further welding.
Plasma welding:in this type of welding the arc is formed between a pointed tungsten electrode and the plate. But, with the tungsten electrode positioned within the body of the torch, plasma arc is separated from the shielding gas envelope. Plasma is forced through a fine bore copper nozzle which constricts the arc. By varying the bore diameter and plasma gas flow rate three different operating modes can be achieved. Namely micro plasma, medium current and keyhole plasma.
Safety precautions to be taken while carrying out hot work:
Welding or flame work which is carried out anywhere other than in a designated area, should be done only after obtaining a permit to work.
Person carrying out the operation should be fully aware of the equipment and the special precautions that should be taken.
Sufficient illumination and access should be arranged before the work is even commenced.
Provision for ventilating the harmful gases which are likely to be released during welding and cutting should be arranged.
If this process is being carried out in a confined space, breathing apparatus should be used.
Personal protective equipment to be used as mentioned in the code of safe working practises for seaman.
The protective gear one should normally wear while doing hot work is welding shields, leather gauntlets, leather apron, long sleeved boiler suit.
Clothing should be free from oil, grease or any other flammable substance.
Welding equipments should be inspected for any defects.
Lines should be checked for any blockage.
Adjacent area should be checked for any flammable materials or electric cables passing through.
Portable fire extinguisher of an appropriate type should be kept in readiness.
Suitable protection like screens should be in place to avoid the sparks falling in the hold.
The progress of the work and equipments should be rechecked at regular intervals.
The welding equipment should be firmly secured to the work piece.
Contact of the earthing for the work piece should be made sufficiently.
The wire cables being used should be adequately insulated and of appropriate length.
Means of emergency stop should be provided at the location where welding is being carried out.
An assistant should be provided for continuously attending the operator and he shall also be adequately protected with personal protective gears.
The operating area should be maintained as dry as possible, since body sweat, hot or humid condition will reduce body resistance.
While doing gas welding the pressure of oxygen is always kept high to prevent the flow of acetylene into the oxygen cylinder.
Non return valves should be fitted adjacent to the torch in the oxygen and acetylene supply lines.
Flame arrestors should be provided in the oxygen and acetylene lines.
Harmful radiations of the electrical welding should be taken into account.
The welder should never stand on water and carry out welding.
Electrode should not be left in the holder when the work is not in progress. (Even during breaks)
Electrodes should be stored dry in a container until it is required.
Oxygen should never be used to ventilate, cool or blow dust off.
After the work is completed the hoses containing gases should be purged and all other equipments properly secured.
Tests for welding:After the welding is carried out it should be inspected for any defect in the welding: there are two kinds of tests which can be carried out to test the quality of the welding. A. Destructive test where the sample welded piece is tested till failure occurs, B. Non destructive testing, here the sample will be subject to no damage.
A destructive testis carried out to check the amount of stress the weld will be able to take. Various forms of destructive tests are as follows:
1. A tensile test- samples tensile strength should be sufficient
2. A bent test- the sample is bent to right angles. There must be no cracks at the edge of weld.
3. An impact test- sample is subject to impact and it should with stand it.
4. Electrode test- the sample is cut through and the outline of the weld is etched with hydrochloric acid.
The non destructive testis not always possible to be carried out. In order to find the defect in a weld it should be subject to non destructive testing as follows,
Visual inspection- during this inspection the dimensions of the weld, the penetration in joints welded from one side and surface defects are checked.
Dye-penetrant test-a coloured liquid dye is poured on to the weld. If there is any crack in the weld the dye seeps into it, then the surface is wiped off the dye and chalk powder dusted on the weld, this will absorb any dye which is present in the cracks thus indicating the presence of the crack.
Magnetic particle test:in this test a magnetic field is passed along the weld, if there is a crack in the weld a north and south pole will form causing the magnetic force to leak to the surface. If a detecting fluid consisting magnetic particles coated in fluorescent dye is sprayed on to the weld they will concentrate in the area of the crack and can be observed under an ultra- violet light.
Radiography-the above three non destructive test will help us in checking the surface of the weld, in order to detect the defects inside a weld radiography is used. An x-ray beam is directed at the weld with a photographic plate on the opposite side. If the weld is homogeneous then equal amounts of radiation will be absorbed by the weld metal and the photograph will appear the same shade throughout. If there are holes or pours present in the weld this will be evident in the negative film, will shows up as dark spots of the same shape as the pore or hole.
Ultrasonic's-it is an alternative method to inspect the quality inside the weld like the radiography. An ultrasonic transmission is made on one side of the plate across the weld and a sensitive receiver is placed on the opposite side. Small defects in the path of the sound wave reflect some of the signal and hence reducing the received signal; if the defect is large there will be a complete loss of signal. Reflection technique can also be used in a way similar to the echo sounder the echoes will be reflected by the defects in the weld and results can be made to display on the cathode ray tube.
Defect of weldingare identified as,
Undercut-in a process of electric arc welding if the current set is too high than required then excess metal is melted than it is required to join the two plates, this will result in a groove formation in line of the weld, reducing the thickness of the plate at the weld.
Cracks-this may appear after the cool down period of the welding. Can be identified by dye penetrant technique or sometimes visually.
Porosity-bad technique can lead the formation of metal oxides or air bubbles which might be trapped in the welded part in great quantities. This will reduce the strength of the weld.
Slag inclusion-if the slag is not chipped away after each layer of weld, there will be unsatisfactory connection between the welded layers.
Fusion faults-this will happen if the current set is too low, the current may be enough to melt the electrode but not to melt the original plates, this will result in insufficient melting and fusion between the weld and the original plate.
Root fault-this happens in case of insufficient or irregular penetration when laying the first layer along the root of a butt joint.
The cases are well illustrated in the diagrams attached.
Once it is made sure that there are no defects in the welding the welded joint should be protected against corrosion, this is done by painting, and it is very important that the welded surface is prepared for painting.
There are various defects that can be present in the surface which will reduce the efficiency of the paint coating; hence these should be eliminated before the paint is applied.
Some of the defects of welding which may affect the effectiveness of painting are as follows:
Spatter:if this is not removed before the coat is applied. A contour of spatter will produce both too low dry film thickness and a shading effect upon spray painting.
Slag-is formed due to high temperature during the weld and certain mechanical means of cleaning doesn't remove the slag. Care should be taken to remove these.
Smoke-from alkaline electrodes may deposit an alkaline water soluble substance that can cause osmosis.
Sharp edges, dents and burrs:applying the paint coat on top of any of the above will result in too low paint film thickness and thus cause premature damage to the coat and thus earlier corrosion.
Once all these are removed the steel may still contain other contaminants like salts, pitting or anti spatter agents,
Saltsare not removed by mechanical methods. It will cause osmotic blistering of the coating, reduced adhesion between paint and steel resulting in under rusting. Salt is removed by high pressure fresh water hosing, it might be needed to brush them while hosing. Pits invariably contain salts hence pitting should be cleaned off the salt and epoxy fillers used to fill then before coating. Some antis patter agents may be incompatible with the coat being applied these may be removed by water if they are water soluble. Other types may require solvents for cleaning.
Mill scale:if mill scale is not removed before the coating is applied it will result in galvanic corrosion between the mill scale and the steel since mill scale is more noble than the steel. Mill scale will peel off along with any coating on top of it.
Dust-will make the paint less adhesive with the steel hence will result in peeling of the coating. After the fresh water hosing down is carried out, the paint should be applied as soon as the water dries up.
Water jetting-if the water jet used is insufficient in capacity or condition it will result in either delay in the operation or insufficient preparation. Some of the reasons for this could be leakage in the lines, the pressure may be insufficient, wrong technique may be used which will result in insufficient removal of contaminants. To avoid this, leakages in the lines should be rectified and equipments too small for keeping the required should be replaced and proper techniques practised.
Air temperature while painting:too high air temperature may lead to dry paint being sprayed to the plate which will result in poor formation of coating, if the temperature is too low it will lead to slow drying risk of solvent retention, sagging and for two components paints insufficient cure and risk of side reactions. This may result in poor corrosion and chemical resistance. Poor adhesive of subsequent coats. The only remedial action for areas with dry paint due to high temperature must be scraped or sanded to remove dust spray and applied an extra coat. It is important that the extra coat applied forms a uniform paint film free of porosities. In severe case the damaged coating should be removed by blasting. If the temperature is too low then more drying time should be allowed for before over coating. Before chemically curing paints the temperature must be arranged to be increased to an acceptable range. Condensation and rain should be considered for and protection against them arranged for. Before over coating the surface should be checked for sweating.
Preventive measures-if the temperature is too high the job can be carried out in shelter or can be painted during the night time or the paint can be tinned down slightly than it is specified in the specification. Thinner used should be of recommended one which is compatible with the paint. For low temperatures provisions should be made to increase the temperature, for confined spaces heaters should be installed and insulation provided.
If the surface temperature of the steel is too high it will lead in quick drying, resulting in poor film formation with poor adhesion. If too low it may cause condensation on the substance preventing adhesion of thee paint which will result in peeling off. Same will be faced if the temperature of the paint is high or low. And ventilation is important after the paint is applied for the solvent to evaporate from the paint at a normal rate. Care should be taken when painting in confines spaces.
Paint preparation:each kind of paint has its own characteristics, product data sheet should be consulted before it is being handled. Make sure the paint is being stored in a suitable location, the quantity of paint available for the proposed job, correct mixing ratio is used and is being mixed properly by stirring to obtain a uniform colour throughout without any lumps or sediments. Some paints like the zinc primers have heavy pigments and so the paint should be continuously stirred while painting. Thinners should not be used unless it is necessary to obtain a good flow through the nozzle and film formation. Two component paints should be mixed before thinning takes place and should be painted within a certain period of time after mixing the two components. Pot life of the paint should be checked. If the pot life has exceeded then the paint should not be used.
Brush:application by brush is slow but very good application tool, in areas where it is difficult to spray paint. Good quality brush should be used for stripe coating the welded seams. Brush application will penetrate through rough surfaces and helps in achieving better coating in critical areas like sharp edges.
Rollers:these are only suitable for applying final coats on a already well protected surface. If rollers are used for applying primers then it should be applied evenly without stretching the paint. 2-4 coats may be needed to substitute coat applied by spray.
Air spray:provides a lower production because of the spray dust, only low thickness is achieved even with high build paints. Hence this should be used only for touch up or repairing a small area.
Airless spray:this is the fasted and most efficient way of coating. The following should be checked to ensure its efficiency, the spray pumps volume, output pressure and capacity. The nozzle size and fan width are correct for the paint type. Correct specifications can be obtained from the product data sheet.
While painting the following are few of the critical areas where additional attention is required
All sharp edges, welded seams, corrugated plates, light holes, underside of coamings, etc, Should be applied with stripe coats by brush before spray paint. Intervals between the coats should be as per the manufactures instructions and allowances made for any present condition which may affect them.
It is always important to have a final check on the dry film thickness (DFT) to make sure the required thickness is achieved if not additional coats may be required to meet the target. The area should be marked or fenced till the paint is completely dried.
Safety while painting can be divided into different sections,
Material safety:the paint should be stored in neat and tidy manner. All the paints and solvents should be covered and the store should be well ventilated. All the material data sheets of the paints should be available.
Equipment safety:airless spray should be used with caution as it can develop static electricity it can cause explosion, hence it should be earthed. All high pressure hoses should be properly connected and damaged hoses should be replaced. Since the airless spray is always under an hydraulic pressure any leak any leak will cause the paint stream to penetrate the human skin, hence medical aid should be immediately available.
Personal safety:as far as possible the contact of paint with skin should be avoided. Cover as much as possible with sensible working clothes. Gloves and eye protection should be worn at all times. When working in confines spaces air fed mask should be in use. Safe access and lighting should be present at all times while the work is in progress. Keeping the work area clean will enhance the safety to a great extent. All safety regulations should be complied with.
Job site safety: safe access should be marked as far as possible; all dangers should be marked or highlighted where ever possible. Any spillages should be removed immediately.
Fire hazard:naked flames and smoking should never be allowed in the paint store or in the place where spray painting is in progress. Ventilation should be carried out in confined spaces to remove the paint fumes. Emergency procedures should be established, and as far as possible powder or carbon di oxide type of extinguishers should be used. Avoid using water.
code of safe working practise. imo.
college, s. t. class notes.
Eyres, D. (2007). Ship Construction, 6th edition. Butterworth-Heinemann.
protection, h. p. (2004). hempel paints.