Protecting the world’s largest steam turbine

Our products are used to monitor and protect the largest machines and operations on the planet. That’s not hyperbole – that’s fact. In this series of articles, we examine three of the “world’s largest” and what we do to make them safe and reliable. We continue our series here with installment #2 and an examination of the world’s largest steam turbine: Arabelle.


The truth is that large machines aren’t necessarily monitored differently or with different equipment. They may have more bearings and more measurement points. They may have more complex voting logic. And they may qualify for the most extensive complement of protection and condition monitoring measurements because so much is on the line in terms of lost production when they fail. But it’s basically a matter of more – not different. The consequences of a missed alarm or false alarm is what sets these machines apart. Big means expensive and highly consequential. In this 3-part series we examine big machines and big plants. And not just big – the world’s biggest in terms of power output. We continue the series now with a focus on steam turbines – the workhorse of industry.

From Novelty to a Behemoth

The first steam turbine is often credited to Hero of Alexandria in the first century and was little more than a curiosity (Figure 1). But it was actually a Roman architect and engineer by the name of Vitruvius that should rightly take credit for its invention as he describes a steam turbine in his treatise, De architectura, published a century before Hero’s era. Hero’s contraption was little more than a spinning steam kettle that provided no useful work. Fast forward nineteen centuries to 1884 and James Parsons, the father of the modern steam turbine1.


Figure 1. Hero’s Aeolipile is generally credited as the first steam turbine, dating back to the first century.

Parson’s first turbine2 was unimpressive by today’s standards: it had an efficiency of only 1.6% and an output of 7500 Watts. But technology progressed rapidly and just 15 years later in 1899, Parsons had succeeded in building the first 1MW machine, delivered to a company in Germany for electrical power generation. Today, steam turbines approach power outputs nearly 2000 times larger.

Since that time, the largest steam turbines have always found their home in power generation applications, but none so big as nuclear power generation. Today’s champion in terms of raw power output is the Arabelle design from GE. It traces its roots to Alstom more than 20 years ago and is built specifically for nuclear applications ranging from 1.2 GW to 1.9 GW.

“Alstom (now GE) is no stranger to these big machines nor to holding the title of ‘world’s most powerful."

Why Nuclear?

The thermodynamics of nuclear reactors place constraints on the steam temperature. It’s cooler and wetter (saturated) than the superheated steam that is produced in conventional fossil-fueled plants, whether via a fired boiler in simple-cycle plants or via a heat recovery steam generator (HRSG) using the exhaust from gas turbines in combined-cycle plants. This means for a given mass of steam in a conventional plant, the steam contains more energy and a smaller turbine can be used to extract it. In contrast, the nuclear plant needs physically bigger machines with more turbine stages and larger wheel diameters to extract the same amount of power from a given mass of its lower-energy steam. Larger steam flows are required in nuclear plants and this translates to bigger machines and larger geometries. Economically, and until recently with the introduction of so-called SMR (Small Modular Reactor) technology, it simply did not pencil out to develop nuclear power plants with turbines smaller than about 1000 MW. In contrast, conventional steam turbine plants can be much smaller, ranging from sub-100 MW units up to about 1 GW; however, units in the range of 200-400 MW are the most typically encountered.

The Trophy Case

Alstom (now GE) is no stranger to these big machines nor holding the title of “world’s most powerful”. They are responsible for the world’s seven largest fossil-fired steam turbine units, arranged in cross-compound fashion and clocking in at an impressive 1.3 GW each. They are responsible for the first PWR-powered turbine to exceed 1 GW back in 19773. They are responsible for the first machine to break the 1.5 GW barrier in 19962. They are responsible for the first machine to exceed the 1.7GW barrier with the delivery of an Arabelle unit to France’s Flamanville 3 nuclear plant. And with a frame size able to deliver up to 1.9 GW, they were the first to build a pair of machines that are currently destined for a nuclear plant in the UK that will tip the scales at 1770 MWe each once commissioned4, becoming the world’s largest in operation.

Figure 2. Arabelle - the world’s largest steam turbine. Once commissioned, the two identical trains at Hinkley Point C will be protected and monitored by Meggitt’s vibro-meter sensors, VM600 protection and condition monitoring hardware, and VibroSight software.


If you speculated that Arabelle is perhaps a sentimental tribute to a significant female in some engineer’s life, you’d get credit for a very good guess – but you’d unfortunately be wrong. Instead, the charming name is a tribute3 to four important influences that made this amazing machine possible:

  • The company - "A" comes the Alstom energy business, which was acquired by GE in 2015.
  • The technology – “Ra” is a nod to Auguste Rateau, the French inventor who is responsible for inventing Rateau turbine stages.
  • The location – “Bel” stands for Belfort, France where GE’s mammoth factory is located that builds these mammoth machines.
  • The legacy – "Le" stands for Le Bourget, a flagship Alstom factory near Paris that was decommissioned in the 1990s.

The first Arabelle unit went into operation in 2000 at a nuclear plant in France and dozens have since been deployed around the world, monitored by our vibro-meter technology. But none will be bigger than those destined for Hinkley Point C, a PWR plant under construction near Somerset, U.K.

As can be seen at the left of Figure 2, Arabelle employes a unique design feature whereby the HP and IP stages are integrated on the same rotor and share the same case. Most nuclear turbines do not have an IP turbine and instead consist of only an HP section and anywhere from two to four LP cases. The LP cases are very important and account for up to 50% of the total output on many machines. By employing an HP section, more power can be extracted by Arabelle than prior designs consisting only of HP and LP cases. The HP/IP rotor for an Arabelle unit destined for the Akkuyu Nuclear Power Plant in Turkey is shown in Figure 3. There are nine HP stages and four IP stages.

Figure 3. HP/IP rotor arriving at Turkey’s Akkuyu Nuclear Power Plant. All four Arabelle machines at Akkuyu are designed to produce 1.1 GWe.

Last Stage Blades (LSBs)

LSBs receive a lot of attention on turbines for nuclear plants. As explained earlier, the steam in nuclear plants contains less energy per unit mass than in a conventional fossil plant. In order to extract all of the energy entrained in the steam, large mass flows and very large low-pressure turbine stages are required. As the steam expands through the turbine, the wheel diameters become increasingly larger as the steam passes through each stage, losing velocity and energy. LSBs thus have the largest diameter. At Hinkley Point C (HPC), not only will the power output be the largest ever, but the wheel containing the LSBs will also be the largest ever made as each blade is 1.9 m (75 inches)5 in length (Figure 4). A challenge with blades this long, and a fundamental aspect of the physics involved, is to limit the tip speeds the blades will be subjected to6.

This tip speed turns out to be a problem for almost all turbines in nuclear power generation service if they were to rotate at line frequency (3000 rpm or 3600 rpm) versus half of line frequency. Consequently, almost all machines in nuclear service are designed to run at half of line frequency. For 50 Hz machines, such as those at HPC, this means the rotor turns at 1500 rpm – not 3000 rpm. To then produce 50 Hz power, the generator must incorporate a 4-pole design instead of the usual two-pole design, allowing it to produce two cycles of electricity for each shaft rotation.

Figure 4. Arabelle’s LP rotor for Hinkley Point C at GE’s Belfort factory. This rotor is designated LP75 indicating the last stage blades are 75 inches long – the longest ever produced. Photo courtesy of GE.

What We Monitor

As with all other large steam turbines, every Arabelle unit in service – including those at HPC – receives a full complement of Turbine Supervisory Instrumentation (TSI). The HPC machines have 10 bearings and the usual arrangement of X-Y proximity probes.

Each machine we will be monitoring at HPC consists of 45 measurement points. We will also be monitoring valve condition for the Main Stop Valves, Main Control (Governor) Valves, and the re-admission valves prior to the LP stages after the steam exits the IP stages and is routed through the Moisture Separator Reheater (MSR). You can read more about our valve condition monitoring solution in a series of articles that will run in late 2023.

Our VibroSight software will also be supplied, resulting in a system that not only protects the machines, but provides comprehensive condition monitoring capabilities.


Execution of a project this large, and particularly for the nuclear industry, entails substantial amounts of documentation. The specification alone to describe the monitoring system in sufficient detail was 150 pages in length. More than 64 documents, reports, and process descriptions were entailed in ensuring that we execute to the customer’s expectations.

Numerous services are being delivered in conjunction with the project including the configuration of special voting logic to conform with the plant’s alarming philosophy whereby setpoint violations from multiple bearings are required for shutdown. The shutdown loops must meet SIL2 and will thus rely on the SIL-certified versions of our VM600’s MPC4Mk2 modules. We will also be conducting Factory Acceptance Testing, integration services, and site commissioning services.


After 70 years, we have learned the many nuances of protecting machinery and monitoring its condition. And although we monitor thousands of machines around the world, collectively encompassing TW of power, there is still special significance in being entrusted with the world’s largest, most powerful steam turbine.

In our next installment of this 3-part series, we’ll conclude with an examination of China’s Three Gorges hydroelectric power plant – not only the largest hydropower plant in the world, but the largest power plant on earth with an output of 22.5 GW or approximately 12 Hinkley-sized Arabelles.

In the meantime, we invite you to learn more about our offerings for nuclear power plants and steam turbines on our informative application pages and by contacting your nearest vibro-meter sales professional.


1 Although de Laval introduced a steam turbine design a few years earlier, as an impulse turbine it suffered from many practical problems compared to Parson’s compound design relying on reaction turbine principles.

2 Scaife, W. Garrett. “The Parsons Steam Turbine.” Scientific American, vol. 252, no. 4, 1985, pp. 132–39. JSTOR, Accessed 14 Feb 2023.

3 Lamarque, F., Deloroix, V. “Arabelle: The most powerful steam turbine in the world.” United States: PennWell Conferences and Exhibitions (PowerGen Europe ‘98). Accessed 14 Feb 2023.

4 “Two Largest Steam Turbines Ever Made are Heading for the English Countryside. Here's Why.” GE Reports, Sep 16, 2016. Accessed 14 Feb 2023.

5 “GE Steam Power designs and manufactures the largest-ever last-stage blade for Hinkley Point C’s Arabelle steam turbine” GE Press Release. March 15, 2021. Accessed 14 Feb 2023.

6 Zachary, J., Koza, D. “The Long and Short of Last Stage Blades” Power Magazine. pp. 40-52. Nov 2006. Accessed 14 Feb 2023.

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