Advancing steam turbine technology

Steam turbines are central to the future of thermal power plant

Credit: Siemens

David Appleyard looks at how steam turbine manufacturers are focusing on improving the design and performance of their machines.

Renewables may have struck a decisive lead in new worldwide power capacity investment, but King Coal is still to be dethroned. Indeed, according to GE, coal still represents nearly 30 per cent of global energy consumption – its highest share since 1970 – and provides 40 per cent of the world’s electricity. While this number is expected to fall, coal will remain the backbone of the power system in many countries.

Consequently, the global steam turbine market is projected by some analysts to grow from an estimated $12,872.5 million in 2015 to some $19,292 million by 2020.

Trevor Bailey, General Manager of Steam Power Systems at GE, comments: “Coal is still around and will be around for some time in certain parts of the world where it’s the most readily available fuel. We need to find responsible, environmentally friendly ways of using that fuel.”

With the drive to improve environmental performance across the power generation sector, manufacturers of key electromechanical equipment are under pressure to improve energy efficiency, but in a competitive market operations and maintenance considerations are always high on the agenda. In the case of coal-fired or other thermal plant, the steam turbine is one principal area of focus for the major OEMs.

Predicting operational integrity

Jiri Fiala, Director of R&D at Doosan Skoda based in the Czech Republic, emphasizes the role of detailed knowledge of a steam turbine in improving plant performance: “First, what is important for the operator? It is important to have a good picture of the turbine cycle. It means information and this is measurement connected with the control system. On the other hand, we can offer a remote monitoring system which means that we are always connected online and we are able also to advise the plant operator.”

Fiala explains how such a system can support plant operations: “We are asked by our clients that their turbines should be able to operate without a major overhaul for maybe ten years or eight years. Our answer is yes, but it is necessary also to take into account the correct operation of the turbine. For example, the purity of the steam – as mechanical impurities can damage some surfaces of th=e flow path – can damage some glands and seals. If the chemical purity of the steam is poor, there can also be wear on the rotor blades which can be, after some time, damaged by chemical effects on the material.

“We are able to extend the period between service intervals, but it is necessary for the operator to comply with some recommendations to keep the purity of the steam within some limits.”

This is a point picked up by Bailey: “There’s an element of erosion from the continuous flow of steam through the steam turbine that, over many years, does have some impact, particularly on the longer blades. The last stage or the low pressure section of the turbine, for example, can experience some heavy erosion.

“There’s also a mechanical integrity element to this, making sure the rotating equipment is operating safely and that it is capable of operating over extended periods of time, as many power plants are operating way beyond their original design life.

“Making life assessment studies and making sure the equipment is safe to operate is another part of the service portfolio.”

He continues: “Having that deep domain knowledge around how the machine is designed, how the materials are incorporated in the different parts of the rotating equipment, and being able to apply that to predict what remains of the in-service life of a piece of equipment is a core competency and obviously we use that extensively.

Detailed knowledge of a steam turbine can improve plant performance

Credit: Doosan Skoda Power

“We’re constantly, through our monitoring centres, gathering data and understanding trends on different fleets so we can start to predict failures on families of machines and therefore make that less traumatic for customers, because we can step in and advise them of the high probability of a failure in this operating mode.

“Predictive maintenance is a very important dimension, especially on equipment that’s running. It’s got a design life of 30+ years and could be running longer than that, and as we move more into the industrial digital environment we’re able to use digital applications to help customers operate their plant more efficiently, preventing forced outages by having more innovative, predictive maintenance and monitoring approaches.”

Dr Lutz Voelker, responsible for the Research and Development of Industrial Steam Turbines at Siemens, also emphasizes the importance of long-term predictability: “Material is one of the key elements in the design and it is important to know its behaviour after ten, 15, 25 years of operation.

“If you have a better understanding of the long-term behaviour of the material, you can apply this knowledge to the design philosophy. So if you have, in the past, used very heavy construction due to uncertainties in the long-term behaviour, now with new knowledge of the material’s behaviour you can optimize the design by wall thicknesses and main dimensions reduction, keeping same safety margins, as you know exactly what will happen after 20 or 25 years of operation.

“We can, for example, improve the design philosophy, make the turbine lighter in total weight, and extend the application range and operational behaviour of the technology.”

Voelker adds: “To improve operational availability further, Siemens offers a remote monitoring system which allows tracking of some key values of the turbine. Based on knowledge over its lifetime, customers can get direct feedback as to whether the defined turbine overhauls are required or if the operation can be extended.”

Changing operational profiles

Noting the changing demand profile for thermal plant operation in many markets as a result of increasing volumes of variable output renewables, Voelker highlights another trend in steam turbine development: “Flexibility and customization. That means fast startup times or unlimited load changes while in operation to act on and support stronger green power generation. Steam units are not making base load as in the past. Further, steam turbines used in combination with green power generation such as solar plants must fulfil the special demands of this application. To be still successful with steam turbines, we have to follow these new market requirements for operation.”

Fiala also notes the changing marketplace: “Demands to improve partial load operation or fluctuations in demand have risen in recent years, for example in Europe where there are increasing volumes of variable-output renewable energy such as photovoltaics.

“Equipment connected to the inlet parts of the turbine will enable higher power output and also very high efficiency on lower power operation, very good dynamic efficiency.

“Increasing ramp-up rates and shortening startup procedures or startup times, for example increasing the number of starts, is a typical request from, for example, solar power plants which every day start and stop the turbines. This requires changes and modifications to the design, mainly on the rotor part to reduce concentration of the stresses on the rotor, to enable the rotor to start very rapidly if the number of starts and stops for the application is very, very high.

“It is possible to do this, but with some special design provisions. Measuring the temperature close to rotor or some stator parts, and using some evaluation in the control system, you can calculate the temperature on the surfaces of the turbine and inside the rotor. Based on this knowledge, you can change or evaluate the startup procedure to maintain limits within a ‘safe area’. Above this limit, definitely the lifetime of the material will expire very fast.”

He adds: “On the other hand, the control system can take all this information and evaluate, let’s say, a residual lifetime of the rotor.”

Voelker: “It is an improvement based on the latest developments in the flow path of the design.”

Sealing and steam path

In addition to improving predictability in various operational modes, improvements also cover the various sealing systems within the machine and steam path to boost thermal efficiency – for example, the introduction of abradable materials for the sealing concept or brush steel technologies to improve the efficiency and the internal performance.

As Ronald Schmidt, responsible for the industrial steam turbines business segment at Siemens, explains: “We had a very robust blading path design in the past. It was efficient but not as good compared to what we saw in the big steam units, but for different applications we have now introduced highly sophisticated blade path designs which are normally used for the big steam units, and in the 3DV blading which is a lean sweep blade path design with an improved sealing geometry that means more sealing strips per blade row to cover or reduce the leakages.”

Applying expertise derived from other areas of business is also a key consideration for GE, as Bailey observes: “GE acquired Alstom’s power generation and grid businesses late last year, and one area where we do have significant overlap is around steam turbine technology. That’s actually provided us with the opportunity to look at two ways of addressing how best to provide the most efficient steam turbines to make best use of all these fuels, and we are now actively looking at the technologies that are available to us.

The RDK8 single re-heat plant in Karlsruhe, Germany

Credit: EnBW AG

“GE has historically been a wheel-and-diaphragm-type architecture, more an impulse steam turbine, although in recent years with HEAT (High Efficiency Advanced Technology), primarily used in combined cycles, the firm has moved more into high reaction-type architectures. Alstom has a mix of technologies coming from both camps.”

And, as Bailey notes, one area of development is in the area of sealing within the steam turbine: “We have a mix of sealing technology that’s applied to different places on the steam turbine: brush seals, traditional labyrinth seals, abradable-type seals. With the combination of Alstom and GE technologies, we now have a broad range of sealing capability that can be used across all of these applications, and that can vary depending on the mission that the steam turbine has to operate within.

“If you’re in combined cycle where the machine may be operating for prolonged periods at part load or could be stopping and starting every day, the sealing technologies can be different to a machine like a nuclear application where it’s more a baseload-type application.”

Bailey concludes: “From a GE point of view, a lot of sealing technology for rotating equipment has flowed down from our aviation and gas turbine capabilities and is now being applied in a steam environment. Alstom obviously comes at that from a different direction, looking at it from a steam turbine point of view from the outset. We’re going through that process of learning from each other and being able to draw upon the best technologies and looking at the operating experiences from both companies. I think there are going to be some exciting developments as our engineers really get to grips with what we now have available to us.”

Improving steam condition

One well-known route to improving the thermal efficiency of power plant systems is to boost the temperature and pressure characteristics of the steam.

Bailey says: “Operating steam temperatures we continue to push. We’re operating at 600oC live steam inlet conditions, pushing re-heat temperatures further, and that means materials development continues, obviously pushing 650oC inlet temperature and longer-term to 700oC, potentially beyond that even.

“A number of programmes are in place to drive materials development, which GE is actively engaged in, along with the production techniques required.

“It’s not something that will change dramatically, it’s a more evolutionary process. From a steam turbine point of view it’s not such a big challenge; other components such as boilers and some of the interconnecting pipe work actually are somewhat more challenging from a materials point of view. We could move to higher temperatures now with a steam turbine, but you’ve got to bring the rest of the power plant with it.”

Nonetheless, GE has revealed a breakthrough in re-heat technology.

“An area that is of interest to us is around double re-heat technology. In the past, large coal power generation has been based on a single re-heat Rankine cycle. We’re looking at technologies now where we can take two re-heat cycles back to the boiler and pass it through a second intermediate turbine, which gives us a significant efficiency boost,” says Bailey.

“We’re pushing the materials capability that we have available to us too. We’re using proven technology with a slightly different steam cycle. It delivers a significant boost in cycle efficiency in excess of 1 per cent in overall net power plant efficiency. We’re calling it LE2 – Leading Efficiency, Lower Emissions – as clean-as-it-can-be power generation using solid fossil fuels.”

The double re-heat cycle follows on from the operating experience of the world’s most efficient single re-heat power plant at the RDK8 installation in Germany. GE says the plant at the Rheinhafen-Dampfkraftwerk facility in Karlsruhe has achieved 47.5 per cent net thermal efficiency while producing 912 MWe.

“If you looked at a double re-heat in the same location as RDK8, it would be in the range of 49.5 per cent plus net power plant efficiency,” says Bailey.

Doosan’s Fiala also picks up on the push towards 700oC steam. “We have in operation ultra-supercritical turbines which operate at about 600oC. High steam parameters mean high efficiency and 600oC is fairly typical. On the other hand, many companies, including us, are thinking about 700oC. For 700oC conditions we have prepared some materials for the rotor, components welded from several parts. We have done some tests on this welding process and are developing some components for this 700oC plant, which we suppose is the future. Of course, that will depend on the cost or the price of the electricity because everything should be calculated economically, and higher parameters also mean higher cost or higher investment, so everything should have sufficient payback.”

There are supply chain considerations too, as Fiala says: “600oC conditions are common in the market so that is why, for example, P92 material for the casting or hot casting or main pipelines is available. On the other hand, when we speak about 700oC then it is necessary to speak about alternatives such as nymonic alloy or chromium alloy combination of the rotor. This is not an easy task and that is why, over the last two, two and a half years we have developed such a welding procedure. It does also depend on the supplier because we can develop some welding procedures or heat treatment procedures but we need to buy good quality forgings.”

Siemens is also extending turbine steam conditions for industrial steam turbines: “With the enhanced platform, we have increased the lasting capability for our building block system, which now goes up to a maximum of 565oC, 180 bar pressure level for continuous operation. Compared to the former design, that is an improvement of 25o Kelvin higher temperature and roughly 40 bar higher pressure level,” says Voelker.

He adds: “For the area of industrial units we are not considering actual 700oC steam conditions. The industry is currently focussed in the area of 560oC-580oC as we see no benefit in improving or increasing live steam conditions on industrial units. If you are talking about higher live steam conditions, then it goes more in the direction of the big steam and CCPP units. Big steam is at the level of 600oC, 280 bar and CCPP at the level of 600oC, 177 bar that we already have in the field. Especially for CCPP, Siemens is working to increase the live steam conditions further.”

New manufacturing techniques

One further trend concerns the use of novel manufacturing technologies to reduce lead time and costs.

Schmidt says: “I believe what is next on the operations side of manufacturing is the full digitalization chain, which we are working on. Obviously R&D gives us a very flexible product with lots of alterations possible for the specific customer order. Unfortunately, from a manufacturing perspective, the one-off which this creates in terms of manufacturing is the challenge we have to manage.

The RDK8 steam turbine

Credit: EnBW AG

“We believe the digitization initiative we are running there will give us a little bit of an edge as well. The concept really is that when R&D designs, let’s take a casing as an example, they provide their particular design; this is altered to the customer-specific order in our customer order engineering team, and then this creates a 3D model, which is then submitted to the supplier which casts the part.

“The difficult portion is really that we need to know the ‘as-is’ dimensions for the next manufacturing step. The vendor, in this case the casting house, provides the ‘as-is’ model and this is then submitted to the manufacturing team in Goerlitz, and while the transportation is being done from China or wherever the vendor is located, we can prepare manufacturing of the part already with the ‘as-is’ dimensions of the scanned part we received from the supply base. Then we can optimize manufacturing technology with the blades. In the end we simulate the manufacturing of the part, so when the actual part arrives in the plant we can immediately start machining without spending a lot of time on setup.

“We will have an optimized programming of the machine, and the results are documented electronically as well. So if we go to a unit that is running in the field, we know immediately what the dimensions are because we have the electronic models, and then we can service the unit much better going forward.

“It’s the 3D model of the components, the simulation of the process and the machine tool itself, these three things need to be brought together, really, to do this in a concurrent way rather than sequential.”

Schmidt also notes the use of laser sintering in the manufacture of components from the gas turbine arena: “We do use 3D printing, but for various gas turbine components. For steam turbines we utilize this technology rather in the area of machining fixtures. We need very variable parts so the fixtures need to be variable as well, and if you do that in metal all the time it is very costly and takes quite a long time. Now you do the stimulation of the part, then you print the insert for the machining fixture. This is a specific example where 3D printing helps to save costs and gain time.”

He concludes, though: “In the steam turbine unit, laser sintering doesn’t seem to be that attractive at the present moment, first and foremost because of cost.”

The cost issue is also noted by Fiala: “Of course there could be a use of some different up-to-date technology for some components, such as laser sintering, but definitely I think, for example, gas turbines which use higher temperatures are a different story. Steam turbines are more or less conservative: definitely we use some special materials, but typically smaller rather than big components – I mean, for example, some sealing parts – because new techniques or technologies usually are also more costly, so that is why we need to evaluate the cost of the material and manufacturing.

“In the future, for example, there will be some 3-D printing mainly for small components, but maybe not always for small components.”

He adds: “There are also some standard procedures with some scope for improvement of the surface, for example hardening of the inlet stages of the last or last-but-one stages, which operate in wet steam to improve erosion protection.”

Future turbine developments

Looking ahead, Voelker suggests further improvements in performance are anticipated: “What we are working on currently is, of course, some further improvements in efficiency, and strong interest and activity in the field of further development in the last stage blading area. That is one key component in the overall steam unit, to reach higher performance levels.”

In mid-2015 Siemens also delivered a steam turbine that operates almost entirely without lubricants, with the bearing systems consisting of air-cooled, active electromagnetic bearings. The first 10 MW turbine equipped with magnetic bearings was installed at Vattenfall’s lignite-fired Jàƒ¤nschwalde steam power plant in the German state of Brandenburg.

Voelker concludes: “We believe that flexibility, going forward, is key in both dimensions: cost and efficiency. That needs to be supported as well by a flexible manufacturing system.”

David Appleyard is a freelance journalist

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