Novel strategies boost turbine efficiency

Paul Boughton
With power demand growing and capital in short supply, plant managers are turning to new strategies in an effort to extend asset plant and optimise plant performance.

One example of this is the project by NVision's contract service division to reverse engineer the complete core of a steam turbine for a major original equipment manufacturer (OEM) in only six weeks compared to the six months that the OEM had budgeted for the project using less sophisticated measurement methods.

"Measuring the critical blade geometry to high levels of accuracy made it possible for the turbine manufacturer to perform simulations that helped to redesign the blades and diaphragms to substantially improve the energy efficiency of the hundreds of existing turbines," said Steve Kersen, NVision president.

Computational fluid dynamics (CFD) technology gives engineers the opportunity to understand how flow affects the performance of turbine blades and quickly and inexpensively evaluate alternative geometries by determining their impact on energy efficiency. In order to run CFD simulation it's essential to have a CAD model that accurately depicts the as-built turbine geometry Nearly all of the turbines that are prime candidates for design upgrades were designed without a CAD model so reverse engineering is an essential first step to improving the turbine blade design.

The turbine rotor in this application measures 11 feet in length and 6 feet in diameter and was not available as a CAD model. NVision contract service division technicians scanned all of the turbine components in only three weeks using the NVision handheld non-contact scanner and touch probe at the OEM's site and the MAXOS scanner in NVision's Wixom, Michigan facility.

The NVision MAXOS scanner can measure complex geometry even if it has a shiny surface without the need for spraying and it is unaffected by the limitations of ball radius compensation from which traditional coordinate measuring machines suffer. According to the company, MAXOS is the most widely used optical blade measurement system used by OEMs in-house, including Toshiba GE, Alstom, and Siemens.

Next NVision technicians used company software to convert the point clouds to STL file format. The STL model was converted to a fully parametric CAD model which took another three weeks. NVision engineers edited the resulting CAD models by hand to correct machining inaccuracies in the as-built parts. The CAD models provided by NVision were used by the turbine manufacturer as the basis for CFD simulations that were used to design new blades and diaphragms that saved considerable amounts of energy by improving the efficiency of hundreds of existing turbines.

"Quickly obtaining a model of the original turbine geometry gives us a distinct advantage over our competitors," said Joe Hackett, metrology manager for Power Systems Manufacturing (PSM), part of the Alstom Group.

As well as working with NVision, PSM has also opened its new 76,000 ft2 gas turbine component repair and reconditioning facility at its HQ in Jupiter, Florida. According to the company, the new state-of-the-art shop was designed to incorporate green technologies and principles and is the first Leadership in Energy and Environmental Design (LEED) registered gas turbine reconditioning facility in the world. The workshop also includes advanced F class process technologies such as laser welding, high temperature brazing and heat treat, CNC (Computer Numerical Controlled) machining, HVOF (High Velocity Oxygen Fuel) and plasma coating, digital x-ray, and a complete metallurgical laboratory.

PSM's product line includes stationary and rotating airfoil parts, low-emission combustion systems, and advanced components for Frames 6B, 7E/EA, 7FA, and 9E, as well as the GT6-5000F and 501F machines.

Meanwhile, Siemens technologies took first place in two categories of the first Innovation Prize for Climate and Environment awarded by Germany's Federal Environment Ministry and the Federation of German Industries (BDI). German Environment Minister Norbert Röttgen and BDI director general Werner Schnappauf presented the awards - totalling EUR125,000 - to winners in five categories in Berlin in March.

For the world's most efficient gas turbine, Siemens Energy placed first in the 'Green products and services' category.

The SGT5-8000H gas turbine developed by Siemens for the power plant in Irsching was completed in Berlin in 2007. With an output of 375 megawatts (MW), it is the largest and most powerful gas turbine in the world.

"The award proves that innovations for climate protection and sustainability are the right approach," said Barbara Kux, member of Siemens' managing board and the company's chief sustainability officer. "Technological leadership is helping us continue our success in these difficult economic times. Our environmental portfolio grew 11 per cent in 2009 and is, thus, a key stabilising factor in our business. At some EUR23 billion, the portfolio accounts for nearly a third of our total revenue, making Siemens the world's largest supplier of green products and solutions."

The gas turbine is the centerpiece of a combined-cycle power plant (CCCP) operated by EON in Irsching, near Ingolstadt, Germany. The turbine is more than 13 metres long, five metres high and, at 444 tonnes, weighs more than the world's largest passenger airplane (Fig. 1). On its own, the gas turbine generates 375MW of electricity. Once it is connected with a steam turbine, output will rise to roughly 570MW, an amount sufficient to meet the electricity needs of some 3.4 million people - roughly the entire population of Berlin.

Siemens says the plant will achieve a world record efficiency level of more than 60 per cent, which will benefit both the environment and the climate: annual carbon dioxide emissions from each new plant of this type will be about 700,000 tonnes below the average emission level for power generation worldwide - a reduction equal to the total emissions of 350,000 cars driven 15,000 km/y.

Typical turbines contain about 3000 blades and Siemens is also working on technology to make these more efficient (Fig. 2). After a special heat treatment, a laser drills cooling holes into the turbine blades.

During operation, large volumes of air are pumped from inside through these openings, generating the life-prolonging cooling which flows evenly over the blades as a film of air.

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