Evolutionary Development

Upgrading the performance of gas turbines has been common since the first gas turbines were introduced more than a hundred years ago. Often turbines were introduced with relatively conservative outputs and efficiencies followed by uprates, after that operational experience had been gathered. Many performance improvements could then follow the first.

Siemens recently introduced a new uprate of its SGT-800 industrial gas turbine. The turbine has been on the market for 20 years, during which time Siemens said numerous steps have been taken to improve performance while still maintaining the unit’s high operational reliability. The recent upgrading is the seventh since the launch in 1997, giving the unit an output of 57MW and a simple cycle efficiency of more than 40%. Over the more than seven million accumulated operating hours, a reliability of around 99.5% has been achieved for the fleet.

“Product development in small steps, a progressive evolution, gives the advantage of fine-tuning the design based on available operating experience,” said Mats Blomstedt, technical manager, Product Positioning, Siemens Industrial Turbomachinery, Sweden.

The first SGT-800s was rated 43 MW and throughout the years the power rating has increased by some 35%. The original SGT-800 gas turbines and the first two upgrades are called the A-platform by Siemens; the last five upgrades make up the B-platform. Both output and efficiency have been increased at each performance step.

In 2011, with the B-platform, the next compressor generation was released with increased air flow capacity and increased efficiency, but with the air flow path remaining unchanged.

“The uprate from the A- to the B-platform included a modification of the first five stages of the compressor as to blade profiling, but with the basic design of the blades, like lengths and roots, unchanged,” said Blomstedt. “The first and third blades of the turbine were also modified related to the increased performance.”

When an updated design or a service schedule is released by Siemens to the market, older versions of the SGT-800 are always considered. This is made by launching service products, where the latest design of components may be implemented on the existing fleet as well. This gives existing customers the opportunity to implement upgrades, when a planned inspection is performed. The latest such performance upgrade released by Siemens for the A-platform offers an 11% output increase.

The development of the initial version of the SGT-800 was started in 1995 and introduced to the market in 1997. It is a single shaft gas turbine for power and heat generation, where the performance/efficiency has been optimized for combined cycle applications. Over the 20 years that the unit has been on the market, more than 350 units have been sold to all continents.

“The evolution is always based on the same basic design principles: same rotor speed, same amount of stages in the compressor and turbine section and the same type and number of burners,” said Blomstedt. “By keeping these basic design features intact, it makes it possible to utilize the evolutionary developed components on existing (older) units as well, and to be installed as upgrades during planned inspections.”

The evolution of the SGT-800 is based on four cornerstones, each of which contributes to the enhancement of the design. The first mentioned by the company is the prototype testing. Here, a number of tests are performed to full load and speed in the test stand providing clear views of the design boundary conditions.

Secondly the operating experience gained from long operation at sites has been essential. Over the years since the introduction, more than 200 hot section and compressor inspections and overhauls have been performed. More than 300 borescope inspections of the SGT-800 fleet have also been carried out.

The third point is that over the 20 years since the introduction of the gas turbine, the software technology & computational power and model updates and calibration have developed together with the calculation capacity of the computers. This gives more accurate results of the calculations. To make well-founded conclusions, it is also necessary to have the right input data and boundary conditions. Here the operating experience and actual feedback makes it possible to calibrate the models.

And lastly, the manufacturing procedures are developing as well as components and materials and their properties give further opportunities to optimize the design of the components. Examples are 3D-printed components, and new alloys with better resistances to oxidation, creep, corrosion and low cycle fatigue.

“The enhancement of the design is done by utilizing the experience from operation over time, and by calibration of calculation and design models. As a result a fine-tuned design with just further small step improvements a well proven and reliable SGT-800 gas turbine is achieved,” said Carina Elneros, portfolio manager SGT-800, Siemens Industrial Turbomachinery, Sweden. “And still the latest technologies are included.”   

The combustion system in the SGT-800 is referred to as the 3^rd generation dry low emission (DLE) system, which is used also in the Siemens SGT-600 and SGT-700 gas turbines, providing significant fleet experience gained from various operation regimes. Both the experience gained at different sites and engine validation load testing runs are fed back into the design. One result of this is expanding the approved fuel specification as well as further reduction of emission levels. When first introduced to the market in 1997, 25 ppm NO_x emissions for gas operation was offered. Today, related to tuning of both the hardware and the software, a level of 15 ppm NO_x can be guaranteed. In certain cases even 9 ppm NO_x can be offered including the latest knowledge and development efforts. On liquid fuel operation 42 ppm NO_x emissions is most commonly offered, but 25 ppm NO_x can also be guaranteed. Also, the CO emissions of 5 ppm is low and competitive, said the company.

Throughout the years the SGT-800 has used the same type of DLE-burner. Emission levels have normally been manually tuned at commissioning with a certain margin to combustion dynamics. In 2017 an automatic control was introduced, where feedback signals from dynamic pressure measurement are utilized for the burner control software to minimize the emissions.

The world today has a strong focus on the environment, and there is clearly an interest in how various fuels are used for production of electricity, and for transport of goods and people. Like many other manufacturers Siemens is putting a great deal of effort in making smarter use of fuels to make operation as clean as possible. The fuel specification for the SGT-800 standard design incorporates heavier hydrocarbons and inert gases; examples are methane, ethane and propane. By adopting the latest additive manufacturing methods like 3D-printing, the burners can be optimized for high reactive fuels.

One of the key requirements of customers and operators of gas turbines is high availability and reliability of the equipment. In 2010 the standard maintenance periodicity for B-inspections (or hot gas path inspections) for the SGT-800 was extended from 20 000 Equivalent Operating Hour (EOH) to 30 000 EOH. This has since then been the basis for maintenance planning.

In 2017 a “Tailored Maintenance Program” was released. Algorithms have been developed for different SGT-800 configurations of components, where consumed and remaining life of each component can be established based on ambient conditions, load and other operating factors.

“This gives the opportunity to make a tailored maintenance program for the SGT-800, where the previous rigid EOH-based program can be replaced with a more flexible approach and an optimal use of components,” said Blomstedt. “A number of digital offerings are available, where collected data from the customer is used for analyzing the status of the unit and giving advice about planned service scope, remote inspection and operational support in general.”

We asked Siemens how they view the emerging 3^rd party services on gas turbines, like currently on the SGT-800.

“Competition drives ingenuity and as long as all respect the proprietary copyrights of others, it’s good,” said Christian Svedung, director, Product Positioning, Siemens Industrial Turbomachinery, Sweden. “Our SGT-800 customers tell us they appreciate the unit’s high reliability and predictable maintenance costs. We can provide them with not only an excellent product but also a combination of service excellence and domain knowledge. That, combined with the experience we can derive from fleet data coming from all types of operating conditions, gives us a distinct advantage.

“So far 3^rd party service suppliers for the SGT-800 have not performed any major inspections so they don’t have the experience that comes from validated services,” said Svedung. “We should also point out, that we take safety very seriously and perform risk analysis on a regular basis for each generation of the design.”

“The evolutionary development is always based on previous experience,” said Elneros. “The fleet operating statistics show, that the reliability of the SGT-800 has been maintained at a very high level. The average reliability value over the last nine years is about 99.5%.”

–Bo Svensson