Aero and marine engine turbine components
Seawater gas turbine engines
A gas turbine engine blade is a specific component that develops the turbine section of a gas turbine engine. The blades receive energy from the elevated temperatures; a combustor releases gas at high temperature. The blades are usually a limited component of gas turbines. To operate in this severe condition, turbine blades usually use exotic materials such as super alloys and several various methods of cooling like internal air channels, boundary layer cooling and thermal barrier coatings. The blades fatigue failure is a major source of outages in the steam turbines and gas turbines that is because of dynamic stresses caused by blade vibration and resonance within the service range of machinery. To secure the blades from these extremely dynamic stresses, friction dampers are utilized. Wind turbine blades and water turbines are made to work in the various conditions that usually include smaller rotational speeds and temperatures.
In the gas turbine engines, each turbine compartment is constructed of a disk or hub that keeps several turbine blades. A turbine section is linked to a compressor through shaft and the compressor can be axial or centrifgual. Compressed air, increased pressure and temperature are sent through various compressor stages in the engine. The temperature is widely increased by fuel combustion in combustor that lies among compressor and turbine stages. The elevated temperature and high pressure exhaust gases then travel through the turbine stages. The turbine stages take out energy from this flow, decreasing the pressure and temperature of air and send the kinetic energy to the compressor stages through the spool. This process is comparable to the service of an axial compressor in reverse manner.
The count of turbine stages changes in various kinds of engines with large bypass ratio engines tending to keep the several turbine stages. The count of turbine stages can leave a significant effect on the design of the turbine blades for each level. Various gas turbine engines are twin spool designs, states that a high and low pressure spool lie. Other gas turbines utilize three spools, including a moderate pressure spool among them. The large pressure turbine is subjected to the warmest, largest pressure air and small pressure is exposed to cool and low pressure air. The variation in conditions directs the structure of turbine blades working in these compartments that are considerably different in materials and quenching options even though the aerodynamic and thermodynamic principles are same. In these intense service conditions, in the gas and steam turbines, the blades operate at elevated temperature, large stresses and very high vibrations the steam turbine blades are crucial parts in the power production units that transform the linear motion of the elevated temperature and large pressure steam traveling down a pressure gradient in a rotary motion of the turbine shaft.
The drawback in the traditional jet engines was the service of the construction materials for the warm parts. The demand for suitable materials forced to conduct more studies on the development of alloys and production methods and the research offered a latest list of advanced materials and methods that create the modern gas turbines. The traditionally used alloy is Nimonic.
Production of superalloys through vacuum induction melting has significantly improved their temperature capability of turbine blades. Moreover processing methods such as hot isostatic pressing enhanced the alloys utilized for turbine blades and improved service of turbine blades. The turbines blades usually use nickel based superalloys including chromium, cobalt and rhenium.
Beside from alloy enhancements, a chief infiltrate was the invention of directional solidification and single crystal production methods which help significantly in increasing fatigue strength and creeping by aligning grain boundaries in single direction or by preventing grain boundaries jointly.
The major enhancement to turbine blade material technology was the development of thermal barrier coatings. Where directional solidified and single crystal developments enhanced creep and fatigue resistance. Thermal barrier coatings enhanced corrosion and oxidation resistance that become the greater concerns with increase in temperature. The coatings increased temperature of turbine blades by 200oF or 90oC. They also enhance blade life and hence doubling the life of turbine blades in some conditions.
Many blades are produced by investment casting that includes producing a precise negative die of the blade shape that is filled with wax to produce the blade shape. If the blade is hollow, a ceramic core in the shape of the flow is inserted into the middle. The wax blade is coated with heat resistant material to create a shell which is filled with blade material. This stage can become more complex for directionally solidified and single crystal materials however the process is similar.
Heanjia has been forging aircraft engine and turbine alloys for above 30 years. With our expertise, knowledge in producing the critical components and handling the super stainless steel, duplex steel, nickel alloys, high temperature and cobalt alloys. We are ISO 9001 certified, so our customers feel confident for receiving the best quality products.
Engine and turbine forged components made from the superalloys are:
Rolled rings, rotor shafts, drive shaft, impellers, turbine wheel discs, bearing blocks, blade and vanes and valve stems.
Role of superalloys in turbine engine propeller shaft in aerospace and marine industry
The super alloys are commonly employed for the combustor and turbine parts in the engines. It is because the service conditions, specifically the temperature and attacking media are extensive at these locations.
Standard components include turbine blading, nozzle guide vanes, turbine discs, combustor cans and support casings. The nozzle guide vanes send the warm gases on the turbine blades that are connected to the turbine disc. The performance of blade or disc assemblies is to drive a shaft that is used to produce electricity (for example in land based turbine), fan or compressor (in aircraft engine) or propeller (in ship). The combustor chamber is also made from them in which the compressed air is combined with fuel and combination is burned prior supplied to the turbine. The ceramic coatings limit the temperatures subjected to super alloys, specifically in combustor and turbine blades.
The engine components are produced from cast or wrought alloys however processes including machining and welding are often employed for finishing purposes. The production of super alloys for such applications has significantly contributed to the economy of fuel and thrust/weight ratio of these engines. It can be judged by considering the turbine entry temperature (TET), stated as the temperature of the hot gases supplied to the turbines- the service of engine is significantly enhanced if the TET can be increased. The TET of an advanced aeroengine is above 1500oC- exceeding the melting point of various super alloy grades and is about 700oC more than the traditional gas turbine engines. It can be seen that super alloys have significantly contributed in the success of current jet engine sector.
Main functions of compressor are:
- To improve the pressure of air supplied to the combustor and deliver it an ideal count with suitable radial flow characteristics.
- To provide bled air for engine sealing, anti-icing and aircraft environmental controls
- To offer for power off-take needs
Nickel based superalloys are used in the various jet engine parts. Engine’s weight can be controlled to decrease the fuel cost such as for transatlantic flights. The temperatures in fan and compressor parts are low about 600oC, therefore titanium is recommended in account of their density that is about half typical super alloy.
For big civil engines that employ two core modules, a triple shaft compressor layout is recommended and offers a flexible and robust system permitting every module to operate at its adequate speed. It also has the benefit of decreasing the count of variable vane stages and therefore engine complexity. Small civil engines and combat engines possess a single core and dual shaft axial layout. The centrifugal compressors have extensive applications in helicopter and cruise missile engines where size is an essential factor. In the whole layouts, the material selection and challenges are same.
The current material selection for compressor rotor, blade and impellors is traditional nickel based alloys. Nickel based superalloys are widely used in compressors. Steels as stators where titanium based alloys are incapable to meet the erosion and fire resistance needs and nickel based superalloys such as Inconel 718, as blades and discs where titanium alloys are unable to meet the temperature needs. In various conditions, these alloys are approaching the limits of their service and surface engineering techniques are applied to the standard materials to secure them from the conditions or to improve their service performance. The treatments include shot and laser shock peeing, erosion resistant coating and corrosion, oxidation and sulfidation security apparatus.
Rotor and stators, compressors also need an abradable coating offers gas seal around the rotor blade tips. They are often thermally sprayed coatings that comprise of a matrix and a dislocator phase. The compressor blade tips cut a path by an abradable on the initial turning the engine hence developing a labyrinth seal over the blade tip. There are several abradable options that are common in the industry and are often propriety materials prepared and delivered by the supply chain.
In the main civil engine air may release the compressor at about 150 m/s. It is hence very high to permit stable combustion to occur hence the air travels by a series of diffusers and baffles and enters an area of small velocity recirculation downstream the fuel spray nozzles where stable combustion can be maintained. It is called as basic region.
Basically, the temperature of gases emitted by combustion process is about 2100oC that is very hot for the downstream parts to withstand. Hence to decrease gas temperature cooling air is introduced in the secondary zone downstream of the main zone. The air also has a significant role in limiting the emissions. Eventually in the dilution region at the end of the combustor exit. It is important that combustion is complete before the dilution air enters else the combustion flame will continue downstream in the turbine causes thermal distress and overheating.
Core turbine systems
The count of turbine stages is based on the engine design. High compression ratio engines often keep minimum two shafts with dual turbines driving large and small pressure compressors. The benefit of this design is that it permits the whole compressor and turbine stages to serve at their adequate speed hence increasing the service.
The traditional turbine system is an assembly of alternate static vanes and rotating disc mounted blades joined to shafts. The blades and vanes are present in a divergent casing. The turbine offers rotational power output with the shaft. It often drives to fan, compressor and engine accessories such as in aero gas turbine however in energy and marine based applications, it generates shaft power for a propeller, rotor, pump and compressor.
The turbine blades and their supporting discs have been needed to serve at increasing temperatures such as in the range of 1500oC. The creeping resistance in blades and fatigue strength in discs has been considered as service limiting. Therefore production and development of alloys has been followed to two directions:
- High creep resistance nickel based superalloys for blades
- Optimized superalloys made for good fatigue resistance in discs and supporting structure.
Creep strength at the elevated temperature has prolong been known to be widely affected by grain boundary deformation procedures and through inherent thermal stability of the strengthening phases. Earlier decades have observed the development of blades with directionally solidified aligned columnar grain structures and single crystal obtained by advances in casting technique.
Superalloy composition has developed over the same time period with early alloys fit for single crystal blades comprising of chromium, cobalt, molybdenum, tungsten and aluminum, titanium, tantalum, niobium or vanadium to produce solid solution strengthening and big volume fractions of stabilized ordered intermetallic particles. Chromium and aluminum inclusions have the further advantage of controlling the chemistry of the surface oxides critical for oxidation resistance. An alloy composition in specifically revisiting the benefits of rhenium and ruthenium inclusions at the cost of reductions in refractory metal inclusions of chromium, cobalt, molybdenum and tungsten, specifically decrease the precipitate coarsening rates and increase the reinforcing effects of like phase.
Although due to large cost and significance of providing of these crucial elements alternative alloy development paths are now being discovered. The aggression of the turbine condition has crucially needed security of parts from the thermal effects and air thermal barrier coating systems with electron beam plasma vapor accumulated zirconia are common in both military and civil applications.
It is not simply the accepted effects of fatigue and creep that are limiting turbine parts. Elevated temperature crack growth, sulfidation and oxidation attack are becoming widespread as turbine temperatures increase whilst in marine and power production applications they are complicated by the contamination from the condition and low grade fuels.
Land based turbines
Superalloys are broadly significant for land based industrial gas turbines for the production of electricity. The combustion turbines are categorized into aeroderivatives and heavy duty kind. Aeroderivatives are basically aircraft engines serving as gas generators that then work as power turbine. Both types are significant. The aeroderivatives are lighter for a specific power output as a result the initializing period is smaller, the engines are also often more easily maintained. They are in 10 -40 MW and are hence used for back up purposes. The frame engines are of big size, latest engines are rated at 280 MW and bigger ones are constructed for base load systems.
Nickel based super alloys are commonly used for components in combustor and turbine parts. It is a significant incentive to produce the rotor inlet temperature as much as feasible as it increases the efficiency with which electricity can be produced. This condition has offered the technological advantage for the discovery of new kinds of super alloy for the parts of industrial gas turbines.
Considering the electricity production units, the lifetime is an essential factor. The duration between planned outages has standard been up to 3 years however in some cases it has been increased to 5 years. The needed plant service as complete is generally 30 years or 250,000 hours, however life extension programs can increase this out further. The conditions put significant emphasis on ensuring the integrity of super alloy parts that are employed for these applications.
The onboard steam condensers, oil coolers and other heat exchangers serve in marine water. A standard heat exchanger is a nest of tubes initializing and ceasing at a tube plate covered by a cylindrical shell. A cooling fluid of condensable steam travels inside the tubes and cross- travelling of quenching sea water lead by baffles passes over the outside. The equipment is kept in a suitable secured cast iron or steel shell.
The materials for heat exchangers have emitted from practical experience over the long duration. The tubes should prevent attack not simply to keep their reliability for design life even also to stay free from the surface attack that interrupts with heat transfer. Alloys offering suitable service are cupronickel 70/30, Brass and cupronickel 90/10. Titanium tubes are used in the aggressive environments or where need prolong design life to return on investment.
The tubeplates should be strong to support the tube bundle at the service temperatures and galvanically compatible with the tube material. The proven material for tubeplates is rolled brass that is attuned with copper-nickel alloy 90/10 however not with 70/30 cupronickel that causes brass’s dezinfication.
Super alloys are commonly employed for applications other than aircraft gas turbines and electricity production unit. These include – exhaust valve, warm plugs and valve inserts in reciprocating engines, hot working tools and dies for metal treatment, trays, fixtures, fans and furnace mufflers for heat processing systems, reaction vessel pipes and pumps in the chemical and petrochemical plants, scrubbers, flue gas desulphurization apparatus in the pollution control system, heat exchangers, reheaters and pipes in coal gasification and liquefaction units, tubing, bleaching circuit equipment in pulp and paper mills, specific automotive parts like turbochargers and exhaust valves, medical parts such as dentistry and prostheses and rocket engine such as space shuttle engine.
There is also a demand for modern super alloys for use in super-critical steam producing electricity unit as service temperatures are surpassing those at which ferritic steels can perform. The super alloys for other applications such as valves in automotive engines and joints for high temperature fuel cells.
Gas turbine engines based originated from aerospace engines power, variety of the warships such as frigates and demolishers. Factors of corrosion and its prevention that implemented to aircraft engines in enhanced form to engines in the marine service. The choice of alloy for turbine blades is considered for extensive vulnerability to warm corrosion and sulfidation caused by entrance of salt – laden air and of the smaller temperatures at which seawater gas turbine engines work.
On the other hand, type – 1 hot attack is a major factor for high temperatures in aircraft engines, the smaller temperatures in marine engines enhance type – 2 hot attack in which the sodium sulfate dew has a basic role. Although, the extremely high temperature strength and consistency of alumina developing super alloys for aircraft engine turbine blades is not needed at the smaller temperatures. Hence, the chromia producing super alloys can be utilized to take advantage of their significant resistance to basic fluxing.
Nickel based alloys are used for external cast underwater components including propeller shaft brackets and rudder fittings. The biocidal effect of copper ions release these alloys from the organisms fouling when subjected to the marine water. The copper-nickel alloy 90/10 is employed for marine pipes in ships.
Board oil tanks consist of fans to sustain an inert gas enclosure over the oil cargo to prevent burning and explosion risks through exhaust gas from engines scrubbed with marine water. This condition is variable and hence intense that fans constructed from titanium offer the adequate performance.
Heanjia Super-Metals is producing super alloys for meeting the demands of turbine engine construction industry. Contact us to place your order today.