Advances in steam turbines for modern power plants

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The major engineering solutions for improvement of commercial heat exchangers in thermal power plants steam turbine units must, first of all, be directed at enhancing the efficiency and reliability of their tube systems, since they are the most subject to wear during operation. The choice of approaches and modern engineering solutions for heat exchangers improvement must meet a number of specifications including the following [ 1 ]:. The improvement of heat exchangers is a rather complicated system problem. A large number of factors at different levels which influence the efficiency and reliability of a heat exchanger should be taken into account, along with its relationship to the engineering subsystems of steam turbine unit of a thermal power plant.

The choice of a modernization scheme involves solving a specific optimization problem where the target function or criterion determining the choice may be any of various characteristics, such as raising the thermal efficiency or the thermal capacity of heat exchanger, or increasing its operational reliability or repairability.

The approach determination for heat exchanger improvement can be divided structurally into three stages: a stage for analyzing the initial data, a stage of computations and accounting for a number of operational and design-engineering factors and a stage of technical and economic analysis and evaluation.

One of the most important stages in the analysis is the use of computational methods which take most complete account of the features and behavior of the processes occurring in steam turbine heat exchangers.

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These methods are used to calculate separately heat transfer coefficients for two heat transfer media, while individual factors affecting the heat transfer are taken into account by introducing corrections to heat transfer coefficients base values. When estimating a choice of improvement method, each specific heat exchanger is considered as a component of a particular engineering subsystem of steam turbine condensation unit, system for regenerative feed water heating, system of hot water heating, etc.

When improvements are made in heat exchangers at thermal power plants under operating conditions, it is necessary to analyze and take into account a large number of factors and performance indicators, including the specific operating conditions. Most often it is possible to optimize only the basic thermal and hydraulic operating characteristics of heat exchanger, leaving the others at levels corresponding to commercial apparatuses.

Over many years the authors have proposed, tested and brought to realization a series of new, by now commercial, technical solutions for modernizing a large number of different heat exchangers for steam turbine units; these have provided satisfactory solutions of the problems arising during heat exchanger operation, such as [ 1 — 3 ]:. Retrofitting of the oil supply systems of turbine units is intended not only to recover indices characterizing their efficiency and environmental security but also to carry out measures that will improve their characteristics without significant financial and material outlays.

These problems may be solved on the basis of technical considerations aimed primarily at oil coolers design. Our analysis of the efficiency and in-service reliability of over serially produced oil coolers, as well as the data of the Ural All-Russia Thermal Engineering Institute on the vulnerability to damage of oil coolers in service at steam-turbine units, has shown that the elements of these heat-exchangers most susceptible to damage are their tube bundles.

Advances in steam turbines for modern power plants

A summary of this data and also the results of tests on a number of apparatuses and experience with their operation under different conditions allowed us to formulate the main lines along which commercially produced oil coolers should be modernized. They are as follows:. When selecting the material for the structural elements of an oil cooler, several factors should be taken into account, such as the corrosiveness of the cooling water and the associated corrosion resistance of the heat-transfer tubes; the thermal and hydraulic characteristics of the tubes and their adhesive properties; the compatibility of different materials in one apparatus; the technological features of an assembly of apparatuses with the tubes made of the material selected; and also considerations of cost.

The casing of the oil cooler and its parts are usually made of carbon-steel sheets and the tube sheets — of carbon-steel plates or from different types of brass. At most thermal power stations, tubes of brass L68 are installed in the oil coolers; this does not comply with present-day requirements. If a feasibility study is conducted and tubes of corrosion-resistant steels are used in the oil coolers, water chambers and tube sheets made of steels 12Cr18Ni10Ti or Cr23Ni17Mo2Ti can be manufactured. Lately stainless steel tubes are being installed more and more frequently in oil coolers.

Considering the higher corrosiveness of the cooling water and the more stringent requirements for environmental safety of thermal power stations, in our opinion, the latter solution is more expedient. When using stainless-steel tubes in oil coolers, one should bear in mind that the heating capacity of the apparatuses has decreased, because the thermal conductance of the steel is 6—7 times lower than that of brass.

Lately, the problem of using titanium alloy tubes in the tube bundles of heat exchangers has been widely discussed.

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Note also that the cost of titanium is higher than that of other materials. The larger overall dimensions of oil coolers for high-power turbine installations required their developers and manufacturers to revise several basic concepts in the design of apparatuses associated, in particular, with the use of new heat exchange surfaces that augment the process of heat transfer, using variously profiled and finned tubes. We know oil coolers that have tubes with spiral rolled finning, longitudinal welded finning, spiral wire mesh finning, and other kinds of finning. The comparatively new MP and MP oil coolers for the K, K and K turbines manufactured by LMZ , with stainless steel tubes and cross fins manufactured by LMZ instead of the previously manufactured M and M oil coolers, are very efficient products.

However, these new designs have, in our opinion, one shortcoming — comparatively low oil velocity, which is due to the non-optimal tube bundle layout in the apparatus. Optimization of the tube bundle layout in heat exchangers, oil coolers included, is one of the most promising ways for improving them.

It should be carried out on the basis of a comprehensive calculation of the thermal, hydrodynamic, and reliability characteristics of every given apparatus. The technique for such an optimization calculation has been worked out for oil coolers [ 4 — 7 ] both with plain and with twisted profiled tubes TPT that are used in power engineering see Fig. It allows us to account for changes in the parameters of oil in different zones of the apparatus that have been selected downstream of the oil flow. One of the main factors determining the operating efficiency of oil coolers is the leakage of oil through the orifices in the intermediate partition plates and in gaps between the intermediate partition plates and the oil cooler shell.

This factor is accounted for in another way. The permeability of oil through an intermediate partition orifice in the oil cooler was studied at an experimental facility that simulated its geometrical, thermal, and physical performance parameters. The outside diameter of a working tube is 16 mm, the diameter of the orifice in an intermediate partition plate is The difference in the oil pressures on the partition plate ranged from to Pa. All the experiments were carried out with the tube centered relative to the orifice in the intermediate partition plate.

Note that the well-known physical laws of liquid flow through an annular slot were confirmed in this study, and the quantitative and qualitative results allowed us to find more exact operating characteristics of oil coolers and to account for them in their designs. The oil flow rate through the annular slot increases as the oil temperature t oil rises and as the difference in pressure across the slot increases.

The throughput capacity for the alternative with twisted profiled tubes TPT shown in Fig. In this connection, when designing and calculating oil coolers with TPT bundles and plain tubes, we should allow for a sufficiently high level of idle oil leakages through the annular gaps between the tubes and the orifices in the intermediate partition plates.

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In contrast to existing methods, the more exact technique developed by us for calculations of oil coolers employs the zone-by-zone approach and accounts for changes in the oil parameters along the oil flow through the tube bundle Fig. In this case, the oil space of the oil cooler is divided by the partition plates into a number of zones: the input zone I , the output zone N , and intermediate zones from I to N in between. The heights of the zones correspond to the distances between the adjacent partition plates.

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Then the weighted mean temperature of the oil in every flow is calculated. In the course of the calculation, at a certain viscosity, the average oil flow rate through the tube bundle became negative. This meant that the oil did not reach the extreme tube rows; that is, the apparatuses have zones of stagnation.

After making changes in the tube bundle layout, in particular, by decreasing the number of rows of tubes along the depth of the bundle, the thermal hydraulic calculation was repeated. Calculations executed in accordance with the above technique make it possible to optimize the layout of the tube bundles for the oil coolers and to determine such specifics as the number of rows of tubes that are removed from the center of the tube bundle; the distance between the tubes the back pitch ; the distance between the intermediate partition plates, that is, the number of passes for oil; and others.

It is known that one of the most important elements determining the operating reliability of an oil cooler is the joint attaching the tubes to the tube sheet. Operating experience with shell-and-tube apparatuses shows that mechanical flaring of tubes in the smooth orifices of the tube sheets the usual way of attaching the tubes does not ensure a reliable tightness of the joint.

It depends on the following factors: the influence of heat cycles in different directions, the inherent and forced vibrations of the tubes in the bundle and in the apparatus as a whole, the corrosive effect of the cooling water, the natural aging of the material of tubes and tube sheets, etc. An improved tightness and reliability of the joint between the tubes and the tube sheets can be obtained by applying a new technology developed by the specialists of the St.

Petersburg State Nautical Technical University. It amounts to flaring the tubes using annular reliefs on the metal in the orifices of the tube sheet, which are formed with the aid of a special tool Fig. Method for attaching tubes to tube plates. To find the optimal dimensions, shape, position, and mechanical characteristics of the annular reliefs, computational studies and experiments were conducted that gave the following results:.

We checked the efficiency reliability of this technology of attachment on one-tube specimens, pilot modules, and a number of commercial apparatuses having tubes of different materials. On the basis of our investigations, we established that the given method provides a higher tightness than other previously known methods used in oil coolers do, and it is actually not inferior to the combined joint with flaring and welding. However, this may be justified. Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more. Start Free Trial No credit card required. View table of contents. Start reading. Book Description Advances in Steam Turbines for Modern Power Plants provides an authoritative review of steam turbine design optimization, analysis and measurement, the development of steam turbine blades, and other critical components, including turbine retrofitting and steam turbines for renewable power plants.

Presents an in-depth review on steam turbine design optimization, analysis, and measurement Written by a range of experts in the area Provides an overview of turbine retrofitting and advanced applications in power generation. Introduction to steam turbines for power plants Abstract 1. These will be the largest 60 Hz steam turbines in the world when they enter service in These turbines consist of one double flow HP cylinder and three double flow LP cylinders. The LSBs are mm 52 in long, providing an annular exhaust area of The increase in blade length inevitably reduces the aerodynamic qualities of the LSBs and makes their design more complicated because of the large length-to-mean-diameter ratio and the increased pitch of the meridional stage profile.

The maximum length-to-mean-diameter ratio for the longest full speed LSBs today reaches 0. The closer the LSB is to its maximum length, the smaller the gain in efficiency and the higher the costs. With an optimal circumferential-speed-to-steam-velocity ratio, the increased mean diameter means an increased enthalpy drop and, as a result, a greater difference in the specific steam volume values between the blade row entrance and exit. High, supersonic, steam velocities and their great variations lengthwise with row height hamper the attainment of optimal aerodynamic performance.

Of particular importance is that at low-flow operating conditions the last LP stages are especially prone to reverse vortex motion of the steam which can seriously threaten blade integrity. This applies not only to the LSBs but also to the second and even third LP stages counted from the exit, and the longer the LSBs and the greater the pitch of the meridional profile, the more probable is the appearance of this reverse vortex motion. It seems likely that just this phenomenon caused the failure of a rotating blade and high-cycle fatigue cracks at the root prongs of many other blades in the 12th 3rd from the exit LP stages of the Hamaoka 5 turbine in June see MPS January It is obvious that longer LSBs call for more attention to low-flow and high-backpressure operating conditions.

In addition, the erosion effects of wet steam become more pronounced the longer the LP stage blades and the greater their tip circumferential speed. But for low speed LSBs there is still sufficient margin in steel blades to increase their length further. The density of titanium alloys is about 1. Because of this, the length of titanium buckets can still be increased appreciably. On the other hand, titanium alloys are considerably more expensive than steel and are much harder to machine.

Nevertheless, even the strongest former opponents of titanium LSBs have now turned to developing and implementing them, and all the major turbine producers in the world employ or, at least, have at their disposal, titanium LSBs commercially available for their full speed turbines.

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  • For many years, the most widespread type of attachment bases for LSBs were prong-and-finger fork shaped roots with varying numbers of prongs. For example, Hitachi has made titanium LSBs with lengths of mm 40 in and mm 43 in with seven and nine prongs, respectively. However, most LSBs are now designed with curved-entry fir tree roots, and these are currently considered to be the best way of attaching the longest LSBs. The compactness of the dovetails allows a thinner wheel configuration, reducing the centrifugal stress in the rotor body.

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    In addition, the fir-tree roots are free from potential stress concentrators such as sharp edges or pin holes. This is especially important for blades made of titanium alloys, which are relatively brittle and sensitive to notches. A notable feature of the curved-entry fir-tree dovetails is the uniform distribution of load over all the blade root hooks. Insufficient rigidity of the prong-and-finger attachment base, unavoidable presence of stress concentrators, and uneven stress distribution in the root may have contributed to the failure and cracking of the 12th stage rotating blades at Hamaoka 5.

    Modern rotating blades, including LP ones, are usually integrally shrouded, that is, made with shrouding elements milled together with the bucket airfoil profiled body. The shrouding elements of individual buckets are connected together by means of special outside inserts and wedge-shaped grooves in the shrouds like a dovetail joint, or the shroud pieces are designed with special wedge-shaped edges that mesh with the blades when subjected to centrifugal forces. Their edges also engage under the action of centrifugal forces.

    As a result, when the turbine rotates, all the stage blades are tied together, forming a continuous ring of blades. One of the major advantages of such an annular blade structure, compared with blade groups several units of several blades each connected with wire ties , is that it has fewer resonance points during rotation. The resulting blade structure with two contact supports tie-bosses at the blade mid-height and integral shroud at the blade tip provides well defined and easily controlled vibration modes and significantly reduces the buffeting stresses arising when the LSBs are subjected to low-steam-flow and high-back-pressure conditions.

    Free-standing LSBs, not connected by shrouds, mid-span damping wire ties, or tie-bosses have also been successfully used eg by Siemens and ABB. Modern CFD methods combined with extensive model trials, together with precise manufacturing techniques, make it possible to completely eliminate the need for any vibration damping elements, including shrouds. Even though shrouding the blades typically reduces tip leakage losses, this is completely compensated for by the more effective peripheral water separation of unshrouded blades.

    In turn, mid-span damping devices increase the airfoil thickness in their neighbourhood, considerably increasing the profile losses. In addition, all the obstacles in the interblade channels like tie bosses or wire ties disrupt the steam flow and lead to additional energy losses. Of importance also is that any local wetness concentration in the stage channels greatly exacerbates blade erosion. This is a particular issue for wire ties and tie bosses between the blades and provides another reason for using free-standing LSBs, as well as shrouded blades without any additional ties in the preceding stages.

    But there is a length limitation for free-standing LSBs. So the latest titanium LSBs from Siemens, providing annular exit areas of The low pressure rotors of large modern steam turbines with their long LSBs and large root diameter experience large centrifugal forces. To withstand them, the LP rotors are solid forged without a central bore or welded. Nevertheless, rotor strength can be among the factors limiting turbine output increase. At turbine start-ups, the steam admission sections of LP rotors can encounter large tensile stresses due to the superposition of centrifugal and unsteady thermal stresses.

    To protect LP rotors from brittle fracture, their thermal stress state must be monitored during start-up, with the aid of mathematical modelling, as is done for high temperature HP and IP rotors. This is especially important for the steam turbines of fossil fired plants with elevated reheat steam temperatures. As turbine capacity increases, so do the diameters of rotor journal necks and therefore journal bearings too, with a consequent increase in the loads on them. This was declared to be the largest diameter journal bearing ever employed in rpm turbines.

    But this was not the case as LMZ had already deployed journal bearings of mm and mm in it MW supercritical-pressure machine and in MW wet steam turbines for nuclear power plants.

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    To provide vibrational reliability and decrease bearing friction losses, large steam turbines are often equipped with segmented, or multi-wedge, bearings. In this design, the journal neck interacts not with an entire bush of the bearing but with a few self-adjusted segments, each of which can turn independently. The lubricant is introduced to each segment, forming separate oil-covered wedges, which hold the journal neck.

    This noticeably increases rotational stability.