Steam turbines: Big prospects for small units

Growing demand for distributed energy systems supplying heat and power is placing a premium on flexibility of design from manufacturers servicing an incredibly diverse market from mechanical drives, process heat and steam, and power generation, writes Gerd Seidel

Demand for industrial steam turbines supplying both heat and power is increasingly being driven by growth in decentralized energy applications.

Much of this growth may be attributed to the push for low-carbon energy, one of the biggest global drivers for this sector.

There are many other growth drivers though. For example, in Europe various environmental policies, such as the 2005 EU Directive prohibiting landfill of non-treated waste, have seen waste-to-energy activities rapidly intensify.

Steam turbines are one of the major technologies that can support this shift towards decarbonisation, a trend that is especially relevant for the power generation sector and across various industrial branches.

The steam turbine market for distributed heat and power is not only large and expanding, it is also extremely heterogeneous. Applications include the full range of power generation and mechanical drives, as well as the specific demands of various industrial processes.

An energy-from-waste plant in Sweden running on a steam turbine
An energy-from-waste plant in Sweden running on a steam turbine

For example, waste-to-energy (WtE), refineries, pulp and paper, the chemicals and fertilizer businesses and – increasingly relevant in the clean energy context – biomass, concentrated solar power (CSP) and waste heat recovery.

A natural consequence of the diversity found in the small and medium-sized steam turbine sector is seen in the range of supply requirements. While a typical biomass plant may be 5 to 15 or even 20 MW, a waste-to-energy plant may start in the 10-15 MW range and reach 80 or 100 MW.

Similarly, a pulp and paper plant may need a turbine of more than 100 MW, serving several hundred tonnes per hour of steam at, say, 30 bar, 12 bar and 5 bar for different processes such as drying and finishing wood products, while a brewery may have completely different requirements. Each of these sectors not only have different demands, but those demands may also vary between seasons and across the day.

Responding to the need for uniquely optimised steam turbines, major manufacturers have looked to support this diverse market with reliable, flexible high efficiency steam turbines that are based on a modular design philosophy.

The modular platform allows a wide range of bespoke designs to be developed suitable for application in a variety of industries – given different requirements for process steam, power and heat and requiring a variety of solutions regarding steam offtake, condenser and boiler arrangement.

An ideal steam turbine presents high-efficiency characteristics while supplying process steam and producing power across the load range for both outputs. Ultimately, the steam path – the number of stages, the angle of the blades, the positions of the various bleeds and so on – is key to good performance.

Within the small and medium-sized steam turbine sector, typically boiler steam conditions are subcritical, placing increased emphasis on efficiency. To achieve this, the engineering departments within MAN Diesel & Turbo share know-how between different disciplines and segments.

For instance, the company brings its experience in gas turbine technology to bear in its steam turbine portfolio: one example is seen in sealing technology. The multi-stage steam turbines also feature a blade family able to run with very high efficiency over a wide load range and specifically adapted to each application and its different load points.

Steam turbines offer distributed energy for numerous applications
Steam turbines offer distributed energy for numerous applications
Credit: MAN Diesel & Turbo

Highest efficiency

Similarly, a unique feature of the MAN steam turbine designs is the adjustable guide vane control for extractions, which facilitates highest efficiency especially in the part-load operational range. This variable guide vane geometry may add around 3 per cent to the total efficiency by delivering optimal steam flow through the machine.

Also this technology emerged from MAN experience with machines for other industrial segments, such as gas expanders. Meanwhile, metal or welding procedures may see a colleague from the segment of reciprocating engines engage or the compressor division may benefit from the experience within gas and steam turbine disciplines.

By adopting a modular platform, unique steam turbines precisely optimised with tailor-made extractions and bleeds can be designed and manufactured in order to reach shortest delivery times.

Characterized by a variety of design features for an optimized configuration to meet challenging process conditions and specific customer needs, the modular design concept keeps delivery time and cost at a minimum. All application characteristics, such as extractions and/or admissions can be covered, meeting relevant international design and manufacturing standards like API, ASME, VGB and others.

Typical performance data for power generation with the smaller steam turbine units under consideration here will feature maximum inlet steam conditions of, say, 100 bar (1450 psi) and 520à‚°C (968 à‚°F). For mechanical drive (Fixed frame sizes, API) of 1-16 MW, corresponding steam conditions are 40 bar (580 psi) and 400à‚°C (752à‚°F). Delivered as condensing or backpressure turbine, the modular design concept encompasses standardized inlets and exhaust casings, bearing housings, extraction/admission modules as well as standardized blades. Optional controlled extractions allow sophisticated steam supply at multiple pressures.

There are clearly a lot of base possibilities for such a diverse steam turbine portfolio and a basic design can be developed by a bespoke MAN Diesel & Turbo tool. Once the base design has been established it is then finalised and uniquely engineered within defined standards.

A small steam turbine providing heat and power for a German utility
A small steam turbine providing heat and power for a German utility
Credit: MAN Diesel & Turbo

MAN Diesel & Turbo furnishes this as complete packages for a frictionless installation on site, reducing manpower, time and cost. Focused on a ‘plug and play’ design, shipped turbo generator units include six core modules – the steam turbine, gearbox and generator, together with the lubrication oil module, control oil module and the control cabinet. With systems like oil lubrication integrated into the base frame, the design also represents a compact, skid-mounted solution.

Beside all auxiliaries, the OEM also provides comprehensive service packages that go far beyond shear spare part supply, e.g. operator training modules or technical consulting. This service, delivered under the MAN PrimeServ brand, goes up to long-term service agreements (LTSA) that allow to define customized support even for decades.

Increasingly, designs are now including more sophisticated instrumentation and control systems, as well as additional options for remote monitoring. According to the requirements of the numerous customer branches, remote monitoring and diagnosis can further reduce downtimes, also bringing the advantages of predictive maintenance and predictive control.

This is especially true where the OEM is also serving as a service provider: In-service development allows that knowledge gained from research and development or executed maintenance jobs is integrated into existing systems at other plants. With an additional 24 hour hotline, a remote operator can connect to the site to check system data at short notice, even without the need for constant data flow.

Alongside efficiency, for industrial steam turbines flexibility, reliability and safety are also central considerations. The ability to precisely respond to the demands of the application represents a significant economic benefit.

An industrial steam turbine must not only be able to adapt to variable heat demand, but also take advantage of this variation to maximise revenue potential. For instance, as process steam demand falls, condensing machines with controlled extraction points can utilise the additional steam availability to increase power production.

Should an industrial process trip for any reason, perhaps several hundred tonnes per hour of steam production may need to be diverted within about 1 second. A double extraction condensing machine is able to handle these process requirements by converting this amount of steam into additional electrical power, representing a silver lining in an otherwise economically and technically challenging situation.

Indeed, one trend seeing growing interest is additional power production capability. Whilst typically combined heat and power machines are sized for the appropriate heat demand, as steam turbines have become more efficient the commercial benefits available from supplying grid power are becoming increasingly attractive. In some applications more than half of the revenue can come from power supply.

The very high reliability and safety of steam turbines can thus pay real dividends. For example, a paper mill is usually centred on a large biomass-fuelled plant running on waste products from the process, while the whole plant may represent an investment in the billion dollar/euro range. At its heart is the steam turbine. Flexibility and reliability is key in this kind of process application, which may run for more than 8400 hours per year and where a single day’s loss of production may mean some €100.000 in lost revenue alone.

Thermal fatigue is another example of a potential reliability issue for steam turbine designers tasked with delivering high numbers of start-stop cycles. Even a medium-sized steam turbine may need hours to warm up to operating temperature.

This effect is particularly acute in concentrated solar power (CSP) plants without heat storage capability. The stop-start and partial load cycle of CSP plants also demands a rapid ramp and grid synchronisation when the solar resource is available.

Such a demanding cycle requires specialised designs which feature thinner-walled components among other novel features. MAN Diesel & Turbo has a specific design adapted for this load cycle with a design life of 20,000 starts and has delivered more than 10 steam turbines for solar power generation to date, with capacities spanning 4 MW to 125 MW.

Generally speaking, this reflects the manufacturer’s capability of delivering steam turbine systems for various demanding applications. Be it power generation from waste in Germany or from biomass in Southern Europe, be it a CSP station in Thailand or a US refinery using process steam for mechanical drive, steam turbines are at the heart of all of these processes.

Looking forward, there is a clear and growing demand for more energy, coming together with the mega trends of decarbonisation and digitalization.

Finally, more energy needs to be produced more efficiently, and from renewables such as biomass and solar. Also, maximising the use of potential heat energy for power generation or mechanical drive further illustrate this drive towards a cleaner world. Small and medium-sized steam turbines suitable for a wide range of diverse applications in a distributed set-up represent an efficient, reliable, safe and flexible solution to this.

Gerd Seidel heads the sales and contracts department for small steam turbines that are manufactured by MAN Diesel and Turbo at its Hamburg plant in Germany.

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