As distinct from other modern turbines mentioned above, with their integrally shrouded LSBs, these turbines have free-standing LSBs without shroud or interblade ties of mm about 41 in in length. With a net efficiency of In , this level was exceeded by the Boxberg Q unit, with a net output of MW and steam conditions of According to heat rate field tests, its net efficiency was estimated to be As to the unit capacity of the Lippendorf turbines, this level was surpassed by another Siemens steam turbine, that of Niederaussem K Figure 3 , which entered commercial operation in autumn It has a gross output at the generator terminals of MW with a net capacity of MW.
The main difference is that the LSBs were increased in length, from mm A transition to titanium mm 56 in LSBs, with an annular exhaust area of This opens up the possibility in principle of increasing the output of a five-cylinder turbine to MW. In contrast with steam turbines for fossil fuelled power plants, all wet steam turbines for nuclear power plants, including those of the highest unit capacity, are of the single shaft, or tandem—compound, type. This is mainly explained by the fact that both the HP and LP sections of these turbines work with wet steam.
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The rotating blades of both sections are exposed to water drop erosion WDE and similar approaches to coping with the problem are required in each section, bearing in mind that the WDE rate is proportional to circumferential speed. If an extremely large exhaust area is needed to achieve the required steam flow, the turbine has to be designed to be half speed, that is, with a four pole generator. The main problem with such low speed machines is their manufacture, requiring heavier machining facilities, larger areas, more advanced technologies, and greater capital expenditures.
An advantage of high speed turbines is their higher internal efficiency in the HP stages, with longer vanes and buckets. The maximum output achievable for full speed high speed wet steam turbines without excessive exhaust losses on one hand or an absurd increase in the number of LP flows on the other, is, as with other turbine types, not fixed but depends on available LSB lengths, which tend to grow over time as the technology improves. For 50 Hz, the limit currently lies at a little over MW. For 60 Hz the level is about 1. Attempts so far to force the level up eg, by using a Baumann stage appear ineffective.
But the use of titanium LSBs with a length of mm 59 in would enable a full speed rpm wet steam turbine to achieve a unit capacity of about MW with two or three double flow cylinders, depending on the cooling water temperature. The largest high speed wet steam turbine to enter operation to date is rated at MW. This is the rpm Siemens turbine at the Trillo nuclear plant in Spain.
A larger high speed wet steam turbine, rated at MW, was proposed by BBC for the Graben nuclear plant in Switzerland, but this project never materialised.
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Transition to the low speed half speed concept, rpm in the case of 50 Hz applications, allows the turbine unit output to be increased by a factor of at least 1. Four such turbines have been in operation since the late s at the Chooz B and Civaux nuclear plants in France. The LSBs are mm 57 in long, giving a total annular exhaust area of The same area could now be achieved with four exhausts, using the mm 73 in LSBs developed by Alstom in more recent years. Such a turbine, with two LP cylinders, would be about 6 m shorter. On the other hand, a six exhaust turbine employing the mm LSBs would have a total annular exhaust area of about m2.
This would reduce the energy losses associated with the exit steam velocity to tiny values, despite an enormous steam flow rate. The Arabelle HP and IP sections are both single flow, which results in higher efficiency thanks to longer blades and reduced secondary energy losses. This is because of the greater rotation speed rpm and, as a result, the necessity to limit the HP stage dimensions, keeping in mind the WDE threat.
With a mm 47 in LSB, which provides a circumferential speed close to that of Arabelle, such a turbine has a total annular exhaust area of only For comparison, even to achieve a total annular area of as little as To provide a somewhat smaller unit capacity, around MW, with a speed of rpm, Alstom has developed four cylinder turbines, with one single flow HP cylinder and three double exhaust LP cylinders. Such machines have been supplied to South Korean nuclear plants.
The Olkiluoto turbine Figure 5 will comprise one double flow HP cylinder and three double flow LP cylinders with mm 66 in long titanium LSBs, providing an annular exhaust area of 25 m2 per flow m2 in total. The turbine is furnished with two vertical moisture separator reheaters instead of horizontal ones as situated on either side of the Arabelle units , which makes it more compact. Not to be outdone an advanced version of the Arabelle turbine Figure 6 , with a rating of MW gross, is to be supplied by Alstom to the Flamanville 3 EPR plant in France, due to start up in This will also use vertical MSRs.
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.
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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. 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. 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.
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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.
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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 You can change your ad preferences anytime. Steam turbine leyzerovich.
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