Due to its excellent corrosion resistance, titanium is widely used in Marine environments and aerospace fields such as the chemical process industry.
Condenser and Heat Exchangers
Condenser and heat exchanger tubes currently used in power stations are either made of (a) copper based alloys like aluminium brass and cupro nickels 90:10 and 70:30 fortified with iron or (b) high chrome – molybdenum steels like A1-6X, MONIT and SEA CURE, or (c) C.P.
Titanium has the advantage of a low specific gravity, a low thermal expansion coefficient and superior corrosion resistance to both cupro nickels as well as stainless steels. Copper alloys show excellent thermal conductivity, while stainless steel has the advantage of high tensile strength, fatigue strength and Young’s modulus with the disadvantage of low thermal conductivity. Copper alloys suffer from pitting, stress corrosion cracking, impingement attack as well as cavitation. High Cr-Mo stainless steel is stiff against cavitation but occasional pitting and crevice corrosion have been reported.
Cost: On the basis of relative cost per unit length for 25.4 mm, OD condenser tubes, aluminium brass is the cheapest while cupro nickel and stainless steel tubes are almost the same in price. Ti tubes, however, becomes cheaper when the thickness is reduced to 0.5 and still lower at 0.3 mm
A similar comparison on the long term economic advantage of titanium shows that based on a service life of 40 years titanium tube (0.7 mm) is only one-fourth as expensive as cupro nickels (1.6 mm) or Al – brass (1.6 mm) under sea-water conditions. By using 0.3 or 0.5 mm thick titanium tubes, the cost advantage is enhanced since zero leakage condenser efficiency can be achieved with titanium tubes.
Tube thickness: In power and desalination plants, 0.5 mm and 0.6 mm thick tubes are specified whereas 0.7 mm tube was previously more common. In new plants which have freedom of design, 0.3 and 0.4 mm thick tubes are projected for desalination plants. Nuclear power plants, however, continue with the present stipulated thickness of 0.6 and 0.7 mm on safety grounds.
Titanium has an additional advantage in nuclear power plants. Due to higher capacity on the order of 1000 MW resulting from an absence of leakage, there is less shutdown and also an increase in the maintenance period by longer intervals.
Use of thin wall titanium tube: In Japan and France, the wall thickness of titanium tubes has been reduced in stages of 0.5 mm, 0.4 mm and 0.3 mm, thus improving the heat transfer efficiency as well as economy, in view of absolute freedom of titanium tubes to corrosion in seawater cooling system.
There was no deterioration of material even after 15 years of service including hydrogen embrittle ment at the edge protruding from the tube sheet. The reduction of heat transfer coefficient even after 15 years service was negligible. The suitability of 0.3 mm thin titanium tubing for sea water cooling was established by the trial. Fatigue testing ha s also concluded the reliability of 0.3 mm tube in service.
Biofouling in temperature and tropical waters: In coastal power stations situated at northern latitudes like Sweden, U.K., U.S.A. and Japan, where the intensity of fouling is low and only seasonal, biofouling in titanium tube was prevented by intermittently flown sponge balls or treatment with low dosage chlorine
Fabrication of titanium structures: Fabrication of titanium is comparable with that of stainless steel in method, degrees of difficulty and cost. Commercial grade titanium can be bent 105° without cracking and a radius of 2-2.5 times the sheet thickness. The bend radius of the alloys is as high as five times the sheet thickness. A loss of 15-25° in the included bend angle is normal due to springback action at room temperature. Heat is required to form most titanium metal parts.
In oil and gas refinery applications, titanium is suitable in environments of H2S. SO2, CO2, NH3, steam and cooling water. It is used in heat exchangers, condensers for fractional condensation of crude hydrocarbons, propane and desulphurisation products using sea-water or brackish water for cooling.
Sizes of titanium tubing: Seamless tubes varying from 6 to 60 mm in diameter are made in strengths of up to 860 MPa. Seamless extruded pipe is available up to 170 mm diameter In Ti-6A1-4V, with an annealed tensile strength over 900 MPa. This weldable grade is available up to 1000 mm dia, as fabricated pipework for diverse, seawater applications currently in use or likely to be used in the future.
Miscellaneous Application of Titanium and its Alloys in an Allied Environment
Due to continuous efforts made in the last four decades after titanium was first commercially produced, applications of titanium and its alloys have been developed in a large number of fields. Some of the applications involving sea-water and chloride environments are highlighted below:
Marine propeller shaft: In a certain class of high performance ships, when aluminium alloys are used as the hull material and copper alloys for the marine propeller T1-6A1-4V appears to be a promising material for the propeller shaft due to its high strength and good corrosion resistance to sea-water.
Tanker purger system: Fans in the tanks’ purger systems are preferred to be fabricated with titanium because low density has the added advantage in the prevailing marine atmosphere.
Deep sea submersibles:
Deep sea submersibles are required because they fill the following needs:
(a) Deep sea exploration of mineral resources
(b) Scientific research in the field of oceanography and geophysics
(c) Rescue of conventional and nuclear submarines in distress
Ti alloys are used in deep diving vessels for structures, pipework and the buoyancy sphere. The submersible is designed to be compact, lightweight and fabricated at a high strength to weight ratio with an exceptionally good resistance to sea-water corrosion. It is unrivalled in depths greater than 2000 meters as is evident from a comparative evaluation of Ti-6A1-4V-ELI with the previously used HY-100 steels (Table 18). Alvin, the first such vessel consructed with Hy-100 and Al alloy and launched in 1965 was recently renovated with titanium.
Naval ships and submarines: In view of its nonmagnetic properties, a variety of indispensable engineering components in fibre glass minesweepers, which are currently made of steel, can be replaced by titanium. Other uses include hydrofoil struts due to cavitation resistance, propeller shafts, sea-water trunking for fire fighting vessels as well as mast top radar components to reduce weight and stability with improved corrosion resistance. In the USSR, nuclear submarines have been fabricated with titanium in view of superior corrosion resistance and minimum hull weight for maximum buoyancy, thus increasing the underwater speed, maximum buckling strength to hydrostatic pressure with enhanced operational depth and non-magnetic property.
Unalloyed titanium has provided more than twenty years of outstanding corrosion resistant and reliable service systems in sea-water environments in (a) power Installations, (b) desalination plants and (c) oil refining and exploration installation. As a result of its immunity to ambient sea-water corrosion, titanium is considered to be technically the correct material for many critical applications including naval and offshore components.
In a sea-water environment, titanium has been found to be superior to the currently used corrosion resistant materials based on copper alloys, cupronickels as well as high Cr-Mo steels both In performance as well as cost.
Extensive use of titanium was forecast as early as 1972 in non-aviatlonal fields like ocean environments and chemical industries. However, in the absence of exhaustive corrosion data at that time, there was reluctance on the part of users to switch over to a new construction material. With the generation of a vast literature on the corrosion resistant properties of titanium in the last 2-3 decades, confidence to change over to the new metal has been created.
The present disadvantages of higher cost have been neutralised by longer life and greater service reliability of titanium structures In a sea-water environment. Development of a cheaper extraction process for commercial use will give a further boost to consumption of titanium in the coming decades.