Material science
Piping system
Creep test:-
Where e, is the plastic strain which would be expected at the end of the first stage, this is important to the designer when considering tolerances, t is the time usually in hours.
Fretting can take place whenever low-amplitude vibratory sliding takes place between two surfaces. It is a common occurrence because most machinery is subject to vibration, both in transit and in operation. Examples of vulnerable components are shrink fits, bolted parts and rolling bearings.
Simple matellargy of cast iron:-
Creep, fretting and Brinelling
A. Explain Creep; Brinelling; Fretting; Fretting corrosion
1. Creep:-
Creep may be defined as the slow plastic deformation of a material under a constant stress. A material may fail under creep conditions at a much lower stress and elongation than would be ascertained in a straight tensile test. Hence tests have to be conducted to determine a limiting creep stress with small creep rate.
Creep test:-
The creep test consists of applying a fixed load to a test piece which is maintained at a uniform temperature. The test is a long term one and a number of specimens of the same material are subjected to this test simultaneously, all at different stresses but at the same temperature. In this way the creep rate and limiting stress can be determined, these values depend upon how the material is going to be employed.
Figure shows a typical curve for a metal, to obtain a minimum uniform creep rate V (i.e slope of line AB). It is necessary that the test be conducted long enough, in order to reach the second stage of creep. Hence, for a time t greater than that covered by the test, the total creep or plastic strain is given approximately by
$\displaystyle \mathrm{e=e_o+Vt}$ ,
Fine grained materials creep more readily than coarse grained because of their greater amorphous metal content, i.e. the structure-less metal between the grains.
2. Brinelling:-
Brinelling is the permanent indentation of a hard surface. It is named after the Brinell scale of hardness.
Brinell Test: This test consists of indenting the surface of a metal by means of a 10 mm diameter hardened steel ball under load.
The Brinell number is a function of the load applied and the area of indentation, thus:
Brinell number =Load in Newtons /Area of indentation in mm-sq.
Only the diameter of the indentation is required and this is determined by a low powered microscope with a sliding scale.
Tables have been compiled to avoid unnecessary calculations in ascertaining the hardness numeral. Loads normally employed are 30,000 N for steels, 10,000 N for copper and brasses and 5,000 N for aluminum.
Duration of application of the load is usually 15 seconds. (Industry is still using the old system of calculating Brinell numbers, i.e. load in kilograms/area of indentation in mm2. Hence, their Brinell numbers will be less by a factor of 10.)
3. Fretting:-
Fretting can combine many of the wear processes described earlier. The oscillatory motion causes fatigue wear, which can be enhanced by adhesion. The wear can also be combined with corrosion – principally oxidation – and the corrosion products can be abrasive. The fact that no macroscopic sliding takes place often means that wear debris cannot escape, but is trapped between the surfaces.
4. Fretting corrosion
Fretting is a phenomenon of wear which occurs between two mating surfaces subjected to cyclic relative motion of extremely small amplitude of vibrations. Fretting appears as pits or grooves surrounded by corrosion products. The deterioration of material by the conjoint action of fretting and corrosion is called 'Fretting Corrosion.' Fretting is usually accompanied by corrosion in a corrosive environment. It occurs in bolted parts, engine components and other machineries.
Factors affecting fretting
1 Contact load: - Wear is a linear function of load and fretting would, therefore, increase with increased load as long as the amplitude is not reduced.
2 Amplitude: - No measurable threshold amplitude exists below which fretting does not occur. An upper threshold limit, however, exists above which a rapid increase in the rate of wear exists. Amplitude oscillations as low as 3 or 4 nm are sufficient.
3 Number of cycles: - The degree of fretting increases with the number of cycles. The appearance of surface changes with the number of cycles. An incubation period is reported to exist during which the damage is negligible. This period is accompanied by a steady-state period, during which the fretting rate is generally constant. In the final stage, the rate of fretting wear is increased.
4 Temperature:- The effect of temperature depends on the type of oxide that is produced. If a protective, adherent, compact oxide is formed which prevents the metal-to-metal contact, fretting wear is decreased. For example, a thick layer of oxide is formed at 650° C on titanium surface. The damage by fretting is, therefore, reduced at this temperature. The crucial factor is not the temperature by itself, but the effect of temperature on the formation of oxide on a metal surface. The nature and type of the oxide is the deciding factor.
5 Relative humidity:- The effect of humidity on fretting is opposite to the effect of general corrosion where an increase in humidity causes an increase in the rate of corrosion, and an increase in dryness causes a decrease in corrosion. Fretting corrosion is increased in dry air rather than decreased for metals which form rust in air. In case of fretting, in dry air, the debris which is formed as a consequence of wear on the metal surface is not removed from the surface and, therefore, prevents direct contact between two metallic surfaces. If the air is humid, debris becomes more mobile and it may escape from the metal surface, providing sites for metal-to-metal contact.
Fretting proceeds in three stages:
(a) The first stage is the metallic contact between two surfaces. The surfaces must be in close contact with each other. The contact occurs at few sites, called asperities (surface protrusions). Fretting can be produced by very small movements, as little as 10–8 cm.
(b) The second stage is oxidation and debris generation. There is a considerable disagreement between the workers on whether the metal is oxidized prior to its removal or after its removal. It is possible that both processes may occur, each process being controlled by conditions which lead to fretting. In either case, the debris is produced as a result of oxidation.
(c) Initiation of cracks at low stresses below the fatigue limit.
B. State, with reasons, where these may occur in ship propulsion System.
1.Common example of creep failure is the cracking of the expansion bellows after a long period of operation of the main engine. The failure occurs because of the alternate conditions of working state and idle state. During working condition the bellow is at about 3500 C and in idle state it is at about 350 C . This situation leads to creep failure.
2. Brinelling in the propulsion systems is the damage to the fuel pump, exhaust valve rollers and cam profiles.
3. fretting and fretting corrosion can be seen in the deformation or reduction in the shank diameter of the bottom end bolts of medium speed auxiliary engines. The wear in the shank diameter is caused by fretting action due to the variation in the stresses the bolt is subjected to and this wear is called fretting wear. Similarly the coupling bolts of the propeller shafting is subjected to fretting caused by the variation in transmission of torque due to heavy seas during bad weather.
Cast Iron
Cast iron is produced by remelting pig iron in a cupola (a small type of blast furnace) wherein the composition of the iron is suitably adjusted. The fluidity of this material, makes it suitable for casting; other properties include; machinability, wear resistant, high compressive strength.
Simple matellargy of cast iron:-
Carbon can exist in two states, crystalline and non-crystalline. In the former state, diamond and graphite, the latter is pure carbon.
Pure iron (ferrite) is soft and ductile with considerable strength, when carbon is added to the iron it combines with it to form a hard brittle compound. This compound of iron and carbon called iron carbide or cementite (Fe,C) lies side by side with ferrite in laminations to form a structure called pearlite, so called because of its mother of pearl appearance.
Pure iron (ferrite) is soft and ductile with considerable strength, when carbon is added to the iron it combines with it to form a hard brittle compound. This compound of iron and carbon called iron carbide or cementite (Fe,C) lies side by side with ferrite in laminations to form a structure called pearlite, so called because of its mother of pearl appearance.
As more carbon is added to the iron, more iron carbide and hence more pearlite is formed, with a reduction in the amount of free ferrite. When the carbon content is approximately 0.9% the free ferrite no longer exists and the whole structure is composed of pearlite alone. Further increases in carbon to the iron produces free iron carbide with pearlite reduction.
The steel range terminates at approximately 2% carbon content and the cast iron range commences. Carbon content for cast iron may vary from 2% to 4%. This carbon may be present in either the form of cementite or graphite (combined or free carbon) depending upon certain factors one of which is the cooling rate.
The steel range terminates at approximately 2% carbon content and the cast iron range commences. Carbon content for cast iron may vary from 2% to 4%. This carbon may be present in either the form of cementite or graphite (combined or free carbon) depending upon certain factors one of which is the cooling rate.
Grey or malleable cast iron is composed of pearlite and graphite and can be easily machined. Pearlite and cementite gives white cast iron which is brittle and difficult to machine and hence is not normally encountered in Marine work. The following diagram analyses the above in diagrammatic form.
PROPERTIES OF MATERIALS
The choice of a material for use as an engineering component depends upon the conditions under which it will be employed
The choice of a material for use as an engineering component depends upon the conditions under which it will be employed
Materials
Materials for cooling water systems.
The system should be initially filled with fresh water containing 5 ppm Ferrous sulphate solution, and this should be left for a period of one day. When the system is in use, Ferrous sulphate may be dosed for at least one hour every day, at a concentration of 5 ppm, in order to make up for any break in the original film. The criteria for judging whether the treatment is successful is — there should be a red-brown colour of the internal surfaces of the piping system, after treatment. Over-dosage causes no adverse effects, and the above treatment should also be carried out when any pipe lengths are renewed, or tubes renewed.
Valves
Valve materials must be sea water resistant and compatible with the materials used for piping. If not, there must be a protective coating, which will reduce the effects of corrosion. The coating must be thick enough to give satisfactory protection. This can only be done with simple shapes, such as Gate valves, which may be rubber-lined. More complex shapes, such as Globe valves, require the valve material to be corrosion resistant. The body of the Globe valve is preferably of Ni-Al-bronze, or leaded gunmetal. Coated cast iron or steel is not usually acceptable for sea water service. The seats and disc are of Monel metal or stainless steel. The stem could be of Monel alloy or Ni-Al-bronze. Gate valves are usually having the body of Ni-Al-bronze or leaded gunmetal. The seats and disc are of Monel alloy or stainless steel — copper base alloys are unacceptable for sea water service.
Butterfly valves may have the body of rubber-lined Nickel resistant cast iron or rubber-lined grey cast iron. The disc is usually of Ni-Al-bronze or stainless steel. The stem of Monel alloy.
Heat exchangers
Heat exchangers have tubes of 90 — 10 Cupro-nickel for general use, while condensers may use 70 — 30 Cupro-nickel. Tube plates are usually of 90-10 Cupro-nickel or Aluminium bronze or Naval brass. Water boxes, of shell and tube type heat exchangers, are of 90—10 Cupro-nickel or cast Ni—Al bronze.
Pumps
The casing is of Ni—Al bronze or gunmetal. The Impeller is usually of Monel alloy or stainless steel. The Shaft is made of Monel alloy or stainless steel. The wear rings are of Monel alloy 505 — for the casing wear rings and Monel alloy 400 for the impeller wear rings.
Consumables like 0-rings, joints and packings are made of a variety of natural rubber and synthetics called as elastomers. Vulcanised rubber is suitable for low temperature operation for most materials except when there is a contact with oil, in which case Neoprene is used. Nitrile rubber is used for o-rings which have to withstand higher temperatures, such as for sealing of cylinder liners.
There are a number of applications where plastics are being used, such as Perspex for level gauges or oil sight glasses. FIFE or Teflon is used for gland packings of pumps, since it has a low coefficient of resistance and is having a good chemical resistance. There is an increase in the applications of plastics in paints, thermal insulation, bonding cements and so on. Bearings have traditionally been made of white metal, which usually consists of Tin, Antimony and Copper. The use of tin makes it easy to absorb the small changes in alignment which occur between journal and bearing. This thus prevents cracking of the bearing surface. The presence of Antimony improves the resistance to wear, while Cadmium is used in traces to prevent fatigue by improving the toughness. Since components are subjected to varying stresses and conditions, great care must be taken in the selection of materials used for construction, to ensure that they have all the requisite properties, while keeping the costs reasonable.
a. Iron and steel pipes
b. Copper and brass pipes
c. Lead pipes
Sometimes piping is distinguished by the fluid carried
a. Refrigerant pipes
b. Hydraulic pipes
c. Pneumatic pipes
d. Steam pipes
e. Sea water pipes
f. Hot water pipes
Corrosion protection of sea water circulation systems
Corrosion; as well as marine growth, needs to be controlled, in order to provide sufficient- protection to the piping systems. A common method is the impressed current system, where direct current is applied to one or more copper anodes.
The copper ions are released into, the system at a controlled rate.
A second aluminium anode releases a precipitate of aluminium hydroxide, which collects these copper ions and distributes them over the entire system to be protected. The coating act as a current dispersing film, which provides suitable protection against electro-chemical corrosion.
The d. c. current is produced by the power unit, which converts the a. c. from the mains by means of a transformer and rectifiers. The protective anode is connected to the positive side of the circuit, while sea water system forms the negative side.
The main effects of corrosion can be seen in packets which do not get adequate coating. Thus good design will ensure that such locations are avoided.
All joints are to be sealed with a suitable filler. Epoxy coating / Paint should be having a certain minimum thickness, to ensure adequate protection. Smooth and rounded surfaces ensure less likelihood of any marine growth formation.
Steam pipelines
Efficient piping of steam is only possible if pipelines are having proper lagging. Besides the loss of heat from loss of lagging, there is also the danger of `water hammer', a condition when slugs of condensed water are hurled against bends in the pipelines, with a force sufficient to cause severe damage.
Water collects from the condensing in poorly lagged pipelines or from water carry-over while initially firing, or accumulated water from a line which has not been drained properly. If the steam lines are sagging, this leads to accumulated pockets of water. Steam blows out the accumulated water, which collects to form a slug of water, that travels along the pipeline at speeds in excess of 120 km/h. When there is a bend in the line, the in-compressible water slug hits the bend with a loud banging or hammering noise, hence the term water hammer. The gaskets nearest the water hammering are the most prone to blowing out, as are weak sections of the pipeline itself, due to corrosion.
Damaged piping is costly to repair, as well as troublesome, due to increased maintenance requirements and drop in steam supply pressure.
The system should be initially filled with fresh water containing 5 ppm Ferrous sulphate solution, and this should be left for a period of one day. When the system is in use, Ferrous sulphate may be dosed for at least one hour every day, at a concentration of 5 ppm, in order to make up for any break in the original film. The criteria for judging whether the treatment is successful is — there should be a red-brown colour of the internal surfaces of the piping system, after treatment. Over-dosage causes no adverse effects, and the above treatment should also be carried out when any pipe lengths are renewed, or tubes renewed.
Valves
Valve materials must be sea water resistant and compatible with the materials used for piping. If not, there must be a protective coating, which will reduce the effects of corrosion. The coating must be thick enough to give satisfactory protection. This can only be done with simple shapes, such as Gate valves, which may be rubber-lined. More complex shapes, such as Globe valves, require the valve material to be corrosion resistant. The body of the Globe valve is preferably of Ni-Al-bronze, or leaded gunmetal. Coated cast iron or steel is not usually acceptable for sea water service. The seats and disc are of Monel metal or stainless steel. The stem could be of Monel alloy or Ni-Al-bronze. Gate valves are usually having the body of Ni-Al-bronze or leaded gunmetal. The seats and disc are of Monel alloy or stainless steel — copper base alloys are unacceptable for sea water service.
Butterfly valves may have the body of rubber-lined Nickel resistant cast iron or rubber-lined grey cast iron. The disc is usually of Ni-Al-bronze or stainless steel. The stem of Monel alloy.
Heat exchangers
Heat exchangers have tubes of 90 — 10 Cupro-nickel for general use, while condensers may use 70 — 30 Cupro-nickel. Tube plates are usually of 90-10 Cupro-nickel or Aluminium bronze or Naval brass. Water boxes, of shell and tube type heat exchangers, are of 90—10 Cupro-nickel or cast Ni—Al bronze.
Pumps
The casing is of Ni—Al bronze or gunmetal. The Impeller is usually of Monel alloy or stainless steel. The Shaft is made of Monel alloy or stainless steel. The wear rings are of Monel alloy 505 — for the casing wear rings and Monel alloy 400 for the impeller wear rings.
Consumables like 0-rings, joints and packings are made of a variety of natural rubber and synthetics called as elastomers. Vulcanised rubber is suitable for low temperature operation for most materials except when there is a contact with oil, in which case Neoprene is used. Nitrile rubber is used for o-rings which have to withstand higher temperatures, such as for sealing of cylinder liners.
There are a number of applications where plastics are being used, such as Perspex for level gauges or oil sight glasses. FIFE or Teflon is used for gland packings of pumps, since it has a low coefficient of resistance and is having a good chemical resistance. There is an increase in the applications of plastics in paints, thermal insulation, bonding cements and so on. Bearings have traditionally been made of white metal, which usually consists of Tin, Antimony and Copper. The use of tin makes it easy to absorb the small changes in alignment which occur between journal and bearing. This thus prevents cracking of the bearing surface. The presence of Antimony improves the resistance to wear, while Cadmium is used in traces to prevent fatigue by improving the toughness. Since components are subjected to varying stresses and conditions, great care must be taken in the selection of materials used for construction, to ensure that they have all the requisite properties, while keeping the costs reasonable.
Piping system
Piping on board ship is distinguished by the material used :a. Iron and steel pipes
b. Copper and brass pipes
c. Lead pipes
Sometimes piping is distinguished by the fluid carried
a. Refrigerant pipes
b. Hydraulic pipes
c. Pneumatic pipes
d. Steam pipes
e. Sea water pipes
f. Hot water pipes
Corrosion protection of sea water circulation systems
Corrosion; as well as marine growth, needs to be controlled, in order to provide sufficient- protection to the piping systems. A common method is the impressed current system, where direct current is applied to one or more copper anodes.
The copper ions are released into, the system at a controlled rate.
A second aluminium anode releases a precipitate of aluminium hydroxide, which collects these copper ions and distributes them over the entire system to be protected. The coating act as a current dispersing film, which provides suitable protection against electro-chemical corrosion.
The d. c. current is produced by the power unit, which converts the a. c. from the mains by means of a transformer and rectifiers. The protective anode is connected to the positive side of the circuit, while sea water system forms the negative side.
The main effects of corrosion can be seen in packets which do not get adequate coating. Thus good design will ensure that such locations are avoided.
All joints are to be sealed with a suitable filler. Epoxy coating / Paint should be having a certain minimum thickness, to ensure adequate protection. Smooth and rounded surfaces ensure less likelihood of any marine growth formation.
Steam pipelines
Efficient piping of steam is only possible if pipelines are having proper lagging. Besides the loss of heat from loss of lagging, there is also the danger of `water hammer', a condition when slugs of condensed water are hurled against bends in the pipelines, with a force sufficient to cause severe damage.
Water collects from the condensing in poorly lagged pipelines or from water carry-over while initially firing, or accumulated water from a line which has not been drained properly. If the steam lines are sagging, this leads to accumulated pockets of water. Steam blows out the accumulated water, which collects to form a slug of water, that travels along the pipeline at speeds in excess of 120 km/h. When there is a bend in the line, the in-compressible water slug hits the bend with a loud banging or hammering noise, hence the term water hammer. The gaskets nearest the water hammering are the most prone to blowing out, as are weak sections of the pipeline itself, due to corrosion.
Damaged piping is costly to repair, as well as troublesome, due to increased maintenance requirements and drop in steam supply pressure.
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