Full Steam Ahead

The transition to more gas-fired generation and the need to preserve much of America's coal-fired power means there will be a lasting demand for the rehabilitation of steam turbines in the U.S.

Since the late 19th century, steam turbines have been used for the generation of electricity. A prototype steam turbine, designed by Charles Parsons in 1890, had 15 expansion stages and was one of the first steam turbines to be connected to an electric generator for the generation of electricity. At the turn of the 20th century, steam turbines were designed for an inlet pressure of about 200 psi and a temperature of 500 F. Today, steam turbines are available with steam conditions of 4,500 psi, inlet temperatures of 1,200 F and reheat temperatures of 1,250 F.

Most of the electricity in the U.S. is produced by steam turbines. What's more, most of the nation's new capacity will be generated by combined cycle power plants, which feature gas and steam turbines. The trend in power plant design is the combined cycle, which incorporates a steam turbine in a bottoming cycle with a gas turbine. Steam generated in the heat recovery steam generator (HRSG) of the gas turbine is used to drive a steam turbine to yield additional electricity and improve cycle efficiency.

Most, if not all, of the nation's new gas-fired capacity will be met by combined cycle power plants. Combined cycle plants are expected to account for 50.5 percent of U.S. power production by 2038, up from 25 percent this year, according to a report released earlier this year by Black & Veatch.

Although a large chunk of U.S. coal-fired generation will be retired amid stricter emission standards for air and water, the majority of America's coal-fired generation will survive as power producers spend billions to bring these aging coal-fired units into compliance with new and pending emission limits on a wide range of pollutants.

Coal will remain the dominant source of power generation in the U.S. through 2035, according to the Department of Energy. To remain online for another 20 years, many of these coal-fired plants will require a steam turbine rehabilitation. Worn and tattered after decades of operation, many of the rotating components in the steam turbine must be replaced to extend the life of these coal-fired units.

"With the modern design tools, manufacturing practices and sealing practices, you can redesign the steam path and optimize the HP or IP steam path to gain an additional 3 to 5 percent in design efficiency," said William Ray, vice president of Sales Service and Marketing for Mitsubishi Hitachi Power Systems Americas. "If you have additional available steam flow, you can add that on top of the efficiency gain."

The failure of components in high pressure (HP) turbines is responsible for the greatest loss of capacity and de-rating of the units. On the other hand, low pressure turbines (LP) contribute to more forced outages.

The incentives for upgrading most steam turbines are longer life, increased output, greater efficiency, and improved reliability, including improved solid-particle erosion capability."

"When you do these steam path retrofits, you're really making things new again," Ray said.

To justify the expense, an improvement in performance must be realized. Any upgrade should focus on optimizing the entire design system, including balance of plant.

"Most of the efficiency gains are derived from managing the flow in the turbine," Ray said. "The larger the unit, the greater those advantages are. A 900-MW unit will see a greater efficiency change than a 250-MW unit."

The most common problem with aging coal-fired units involves erosion of the steam turbine's blade path. More efficient blading will increase the steam output and improve the unit's overall heat rate.

"The heat rate trails the output," Ray said. "When you make a more efficient HP section, that steam has a lower energy content. It goes back to the reheater and is going to draw more heat to get it back up to design reheat temperature. That extra energy absorbed in the reheat section makes the heat rate go up."

Although the market for new steam turbines in conventional coal-fired generation is suppressed, the need for new steam turbines in combined cycle configuration is increasing.

Installation of integrally shrouded IP blades at Mitsubishi Hitachi's Savannah Machinery Works. Photo courtesy of Mitsubishi Hitachi Power Systems Americas.

When properly operated and maintained (including proper control of boiler water chemistry), steam turbines are extremely reliable.

Because of the high pressures used in steam turbines, the casing is thick. Consequently, steam turbines exhibit large thermal inertia. Steam turbines must be warmed up and cooled down slowly to minimize the differential expansion between the rotating blades and the stationary parts. Large steam turbines can take over ten hours to warm up.

One maintenance issue with steam turbines is borne from solids carried over from the boiler that deposit on turbine nozzles and other internal parts. This degrades turbine efficiency and power output. Some of these solids are water soluble but others are not. Three methods are employed to remove such deposits:

• Manual removal
• Cracking off deposits by shutting the turbine off and allowing it to cool
• For water soluble deposits, water washing while the turbine is running

Steam turbines have many stages and components. Degradation of each stage or component affects subsequent stages and other components. Several mechanisms cause the degradation of rotating machines. Fouling is caused by the adherence of particles to flow path surfaces. Corrosion is the loss or deterioration of machine component material exposed to fluids. Erosion is the abrasive removal of material from the flow path by hard or incompressible particles impinging on flow surfaces. Abrasion is caused when a rotating surface rubs on a stationary surface.

"One of the things in designing the new steam paths, especially for an impulse machine, is that you're able to manage the solid particle erosion much better," Ray said. "There are some inherent design changes that go into it that make it more tolerant to erosion and therefore makes the steam path sustain its efficiency over a greater period of time."

Three major effects determine the performance deterioration of rotating machines:

• Increased clearances,
• Changes in rotating part geometry
• Changes in flow path surface quality

The first two effects typically lead to non-recoverable degradation. The latter effect can be partially reversed by some corrective processes such as online washing. Degradation will force all stages to work at off-optimum efficiencies. Degradation also limits the operating range.

Corrosion and erosion tends to alter the flow path of steam turbines in two ways. It increases the surface roughness, but it may also remove material at leading edges, trailing edge, and nozzles. Increased surface roughness can cause thicker boundary layers on moving components such as blades, impellers and stationary components, and thus reduce the flow capacity. Changes in the flow capacity of the rotating machine will subsequently alter the operating point. Corrosion and erosion will affect total performance of the steam turbine.

Stress corrosion cracking (SCC) occurs in areas subjected to high stress and when corrosive ions are present. Blade root fastenings, where corrosive compounds can accumulate between the blade root and the wheel, are very susceptible to SCC. As the fleet of U.S. steam turbines gets older, plant operators are discovering SCC in many of their LP turbine rotors. Solutions include:

• Removal of the first stage blading
• Weld repair
• Installation of new rotors
• Complete upgrade of the steam path
• Longshanking

The aerodynamic performance of the steam turbine blade is crucial to achieving acceptable performance. Turbine manufacturers now provide bowed blade designs in the name of greater efficiency and reduced erosion, Ray said.

The most common problem with aging coal-fired units involves erosion of the steam turbine's blade path. More efficient blading will increase the steam output and improve the unit's overall heat rate.

These designs "manage the steam flow to minimize sidewall losses," he said. "You're directing more steam flow into the body of the blade, which minimizes those losses."

Power producers are installing a range of pollution control equipment to comply with stricter emission limits and preserve their coal-fired assets. Adding emission control technology to a coal-fired power plant can cause a meaningful reduction in output, as much as 20 percent in some cases. Much or all of that lost output can be recovered through efficiencies achieved with a steam turbine rehab.