Modern Design & Latest Achievements on Steam Turbines

Steam Turbine Capabilities in Modern Days

A steam turbine is a turbo-machine for generating mechanical power from the energy of steam at high temperature and pressure; in other words, it converts the thermal energy of steam into useful mechanical work. Steam turbines can deliver constant or variable speed and are capable of close speed control; drive applications include pumps, compressors, electric generators and many more. Steam turbines have always been important parts of industrial plants.

Steam turbines are classified by mechanical arrangement, as single-casing, cross-compound (more than one shaft side by side), or multi-casing (tandem-compound) which is two or more casings in a single train. Steam turbines are also identified by steam flow direction “axial” for most, but “radial” for few. Steam turbines can be categorized by steam cycle, whether condensing, non-condensing, automatic extraction, or reheat type.

In a steam turbine, the steam flow proceeds through directing devices and impinges on curved blades mounted along the periphery of the rotor; by exerting a force on the blades, the steam flow causes the steam turbine rotor to rotate. Unlike a reciprocating steam engine, a steam turbine makes use of the kinetic rather than the potential energy of steam.

The steam turbines proved to be very convenient drivers. They can be designed to operable at the speeds of driven equipment. They are more compact, lighter, better balanced, and more economical than the other drivers such as reciprocating engines, gas turbines, electric motor, etc. Steam turbines has been substantially developed because of extensive use in last 90 years, the efficiency has been significantly improved, output capacity has also been increased, and specialized steam turbines were designed for various applications.

Steam turbines have developed in the direction of multistage axial designs, in which the expansion of steam was performed in a row of sequentially arranged stages. Such staging permitted a considerable increase in the power output of steam turbines, while preserving suitable speed required for the direct coupling of the driven equipment.

Impulse versus Reaction

In an impulse steam turbine, the steam jets are directed at the turbine's bucket shaped rotor blades where the pressure exerted by the jets causes the rotor to rotate and the velocity of the steam to reduce as it imparts its kinetic energy to the blades. The blades in turn change the direction of steam flow, however, its pressure remains constant as it passes through the rotor blades since the cross section of the chamber between the blades is constant. The next series of fixed blades reverses the direction of the steam before it passes to the second row of moving blades.

Ideal reaction stages would consist of rotating nozzles with stationary blades (buckets) to redirect the steam flow for the next set of rotating nozzles. The expansion in the rotating blades causes a pressure force (reaction) on them that drives them. However, it is impractical to admit steam to rotating nozzles. The expansion of steam in the stationary nozzles of a practical reaction steam turbine is an impulse action. Therefore, the reaction stage in actual turbine actions is a combination if impulse and reaction principles. The reality is modern steam turbines use a combination if impulse and reaction concepts.

A reaction stage is a row of nozzles followed by a row of moving nozzles. Multiple reaction stages divide the pressure drop between the steam inlet and exhaust into numerous small drops, resulting in a pressure-compounded turbine. Impulse stages may be either pressure-compounded, velocity-compounded, or pressure-velocity compounded. A pressure-compounded impulse stage is a row of fixed nozzles followed by a row of moving blades, with multiple stages for compounding. A velocity-compounded impulse stage is a row of fixed nozzles followed by two or more rows of moving blades alternating with rows of fixed blades. This divides the velocity drop across the stage into several smaller drops. A series of velocity-compounded impulse stages is called a pressure-velocity compounded turbine.

Steam Turbine Market

Steam turbine manufacturers (the number of manufacturers) in the world has been declining over last 40 years. The logical ‘few per nation’ rule which was popular 3 or 4 decades ago has given place to the more modern ‘few per continent’ rule. Different manufacturers merged and technical co-operation have amalgamated technological elements of very different philosophies within a single turbine manufacturer.

Competing concepts and designs can now be compared and critically evaluated before being offered to the market. These changes lead to a sharper selection of development projects, but also to more flexible answers to market needs. Steam turbine engineering and manufacturing have embedded in a changing world.

Advances made in materials science, mechanical or aerodynamic analysis methods may strongly affect steam turbine technology. The same holds true for controls, data processing, manufacturing technologies, production and operating procedures and many other domains.

Steam Cycle and Steam Turbine

The thermodynamic cycle for the steam turbine is the “Rankine” cycle. The cycle consists of a heat source (boiler, heat recovery unit, etc) that converts water to high pressure steam. In a steam cycle, water is first pumped to elevated pressure using boiler-feed water pumps (BFW pumps), which is medium to high pressure depending on the size of the unit and the temperature to which the steam is eventually heated.

The steam is then heated to the boiling temperature corresponding to the pressure, boiled (heated from liquid to vapor), and then most frequently superheated (heated to a temperature above that of boiling). The pressurized steam is expanded to lower pressure in a multistage steam turbine, then exhausted either to a condenser at vacuum conditions (condensing) or into an intermediate temperature steam distribution system (non-condensing) that delivers the steam to other applications. The condensate from the condenser or from the industrial steam utilization system is returned to the boiler-feed water pumps (BFW pumps) for continuation of the cycle.

The steam turbine itself usually consists of a stationary set of blades (called nozzles) and a moving set of adjacent blades (called buckets or rotor blades) installed within a casing. The two sets of blades work together such that the steam turns the shaft of the turbine and the connected load.

The stationary nozzles accelerate the steam to high velocity by expanding it to lower pressure. A rotating bladed disc changes the direction of the steam flow, thereby creating a force on the blades that, because of the wheeled geometry, manifests itself as torque on the shaft on which the bladed wheel is mounted. The combination of torque and speed is the output power of the steam turbine.

Material Selection and Design

The evolution of the classical steam units has always been coupled to advances made in high-strength steel alloys. Steam admission temperatures have continuously been improved battling severe restrictions in boiler and turbine operating flexibility. Old fashion alloy steels were used up to 550°C. For 580°C and beyond, modern alloy steels should be used. The creep has been an important consideration for high-temperature steam turbine applications and proper super alloys are needed to keep creep deformation within acceptable limits.

In addition of temperatures, centrifugal forces put high stresses on rotor and blade materials. A high yield strength combined with good fracture toughness is an important requirement. However, these are metallurgical contradicting properties, making it difficult to increase them simultaneously. An important challenge becoming evident during the last three decades was the occurrence of some stress corrosion cracks (SCC) in highly stressed discs exposed to wet steam.

Many steam turbine rotor blades have shrouding at the top, which interlocks with that of adjacent blades, to increase damping and thereby reduce blade flutter. In large steam turbines, the shrouding is often complemented, especially in the long blades of a low-pressure turbine, with lacing wires. These wires pass through holes drilled in the blades at suitable distances from the blade root and are usually brazed to the blades at the point where they pass through. Lacing wires reduce blade flutter in the central part of the blades. The introduction of lacing wires substantially reduces the instances of blade failure in large or low-pressure turbines.

Part-load or off-design conditions occur on many occasions in steam turbines, for instance, at start-up, shutdown, and at part-load operation, operation below the rated speed and rated load. Particular attention is required for part-load operation of steam turbines with regulated steam extraction system which could supply defined flow of LP (low-pressure) steam in addition of part-load requirement of the machinery train.

Steam Turbine Lubrication

Steam turbine oils are subjected to a wide range of conditions such as extreme heat, entrained air, moisture, contamination by dirt and debris, inadvertent mixing with different oil, and others; all these degrade the integrity of the hydrocarbon base stock and deplete the additive chemistries, causing irreversible molecular changes. There are two primary degradation mechanisms in steam turbine applications: “oxidation” and “thermal degradation”.

The oxidation is a chemical process where the oxygen reacts with the oil molecules to form a number of different chemical products, such as carboxylic acids. The rate at which this occurs depends on a number of factors. The temperature is perhaps the most critical factor, since the rate of oxidation doubles for every rise of 10°C. The temperature above which this occurs is influenced by the oxidation stability of the oil and the presence of catalysts and pro-oxidant conditions such as water, air, certain metals, fluid agitation and pressure.

Thermal degradation is the breakdown of the oil molecules by heat (high temperature), forming insoluble compounds that frequently are referred to as soft contaminants. Over time, it has become clear that the oxidation performances of the different base stock classes are quite different.

The high natural oxidative resistance of some superior turbine oils combined with specific anti-oxidants employed (usually based in phenol and amine compounds) provide a nonlinear behavior in terms of their molecular degradation over time.

As a result, the majority of standard oil analysis tests offer little to no warning as the lubricant starts to degrade and generate system deposits. Instead of degradation occurring in a linear and predictable fashion, many of the modern turbine oils fail rapidly.

Changes in the oil’s molecular structure due to additive depletion and the development of insoluble particulates are among the first oil degradation conditions that affect steam turbine performance. The sequential process will be the formation of sludge and varnish, which are common occurrences in steam turbines. Besides these oxidation and thermal degradation byproducts being the main contributors for the development of varnish and deposit problems in steam turbines, they interfere with other important properties in steam turbine lubrication oils, such as demulsibility. Therefore, it is vital that appropriate diagnostic analysis be performed to detect these conditions.

Corrosion and Erosion

Corrosion is the most common damage mechanism resulting from deposits in the steam turbines. Increased surface roughness acts to increase deposition. The “corrosion fatigue” (CF) and “stress corrosion cracking” (SCC) of steam turbine components have been consistently identified among the main causes of steam turbine unavailability.

Both phenomena are characterized by two stages: initiation and propagation. In steam turbines, initiation most frequently occurs at micro-cracks that emanate from pits that form when deposits become corrosive during unprotected shutdowns. Cracks can, however, also initiate on locations of fretting, manufacturing defects, inclusions, microscopic imperfections, and at areas where specific absorption of species has locally reduced surface energy.

These locations are where deposition will be preferential. Propagation of CF and SCC is driven by cyclic or steady stress situations.

Pitting and localized corrosion are important precursors to more extensive damage from stress corrosion cracks (SCC) and corrosion fatigue (CF), although extensive pitting of blades can cause significant loss of stage efficiency or, in extreme cases, weaken component integrity to the point of failure.

Pitting and localized corrosion are unlikely to originate during steam turbine operation due to the absence of oxygen in the liquid films on the steam turbine surfaces during operation. Rather, pitting results from corrosive deposits absorbing moist air during steam turbine shutdown.

During non-protected shutdowns where the blade and disk surfaces are open to the atmosphere, any deposits, particularly chloride or sulfate, which have formed on steam-path surfaces during operation can become moist and lead to local, conductive, aqueous environments that contain ppm levels of oxygen. These local environments initially lead to breakdown of the blade metal passivity, then to metastable pit formation, and finally to stable pits after repeated shutdown cycles.

Each shutdown period is followed by operation where the dynamic situation of droplet formation, liquid films and deposition occur. Once a steam turbine has resumed operation, liquid films can re-passivate areas where passivity was lost during shutdown and some pits had formed. However, deposition continues to occur during operation, and deposits associated with a loss of passivity that caused a metastable pit during one unprotected shutdown will lead to further growth of that pit during the next extended unprotected shutdown.

Repetition of this process will eventually lead to a stable pit. Too often, these pits are not visible, but because they have resulted from an active corrosion mechanism during shutdown the internal surfaces will be rather irregular. Therefore the different environments which exist during the repetitive operation and shutdown periods eventually lead to the initiation and growth of a number of pits on the surface.

Steam turbine components may also be attacked by flow-accelerated corrosion (FAC) when liquid films form on steam turbine components in the presence of two-phase wet steam. Poor steam purity can cause low “pH” in such films, and thus trigger or enhance FAC. The use of suitable alloys (such as Cr-alloyed steels) can mitigate and even prevent FAC.

Particle erosion is another significant problem in steam turbines. The liquid erosion is commonly reported, particularly for saturated steam and condensing steam turbines. Solid particle erosion is commonly caused by iron oxide particles that scour the surface of blades, mainly in the initial stages of each steam turbine casing. The source of such particles is oxide on super-heater and re-heater tubes and piping that exfoliates during transient operation such as startup and shutdown. The growth and exfoliation of these oxides is not often related to steam chemistry.