Electric Power Generation and Distribution

The Role of Couplings in Power Plants

19 May 2017

Power Plant at NightPower Plant at NightHow many times a day do you flip on the switch or push a button and the lights come on or the dishwasher starts? You surely know that the electricity that makes that possible comes from a power plant, and you may know the process the power plant uses to create that electricity. But you’ve probably never thought about the components that are required to keep that equipment running. One such component is a highly engineered mechanical power transmission component called a coupling.

Power plants (or power stations) convert coal, oil, natural gas, biomass, hydropower, nuclear power, wind, wave or solar energy into electricity using electrical power generators. In addition to generators, power plants employ many different types of rotating equipment, including gas turbines, steam turbines, gearboxes, motors, pumps, fans, diesel engines and reciprocating engines. Whenever a connection between two pieces of rotating equipment is made, such as a generator and a turbine, a coupling is used.

Couplings are power transmission components used to connect two shafts together. Main coupling types include rigid, flexible and fluid. Flexible couplings and fluid couplings are the types of couplings most commonly found in power plants.

Flexible couplings are designed to handle small shaft misalignments and are available in many designs for a host of different applications. Flexible coupling styles include beam, bellows, chain, diaphragm, disc, gear, grid, jaw, Oldham and Schmidt. The right type depends on the application and operating conditions.

Fluid couplings, also referred to as hydraulic couplings, are torque-transmitting couplings that use hydraulic oil or water to transmit power. A fluid coupling is made up of three main parts:

  1. Pump, or driving impeller, mounted to the input shaft, typically an electric motor
  2. Turbine, or driven impeller, mounted to the output shaft
  3. Housing with a fluid tight seal that contains the fluid used to transmit the torque.

Couplings for critical applications such as those found in power plants are specially engineered to ensure maximum life and equipment uptime in harsh and demanding operating environments. Power plant equipment runs at high speeds and torques, high temperatures and in possibly corrosive environments. These couplings must conform to industry standards such as:

  • 810-2015 - IEEE Standard for Hydraulic Turbine and Generator Shaft Couplings and Shaft Runout Tolerances
  • ISO 10441:2007 - Petroleum, petrochemical and natural gas industries – Flexible couplings for mechanical power transmission – Special-purpose applications
  • API 671 - Special Purpose Couplings for Petroleum, Chemical and Gas Industry Services

Bibby Turboflex Gas Turbine Coupling Developed for GeneratorsBibby Turboflex Gas Turbine Coupling Developed for Generators

Steam-turbine power plants generate approximately 90% of the electricity in the United States. This includes plants that use coal, nuclear, natural gas, solar, and biomass as the fuel. One thing all of these plants have in common is that they all require many couplings to connect their rotating equipment. Coal is moved to the plant on bulk handling conveyors, requiring couplings between the motor, gearbox and conveyor drum pulley shafts. The coal is then pulverized, using additional couplings for the pulverizer drive. Water is pumped to a cooling tower and boiler, and couplings are employed between the motor and the pump. The most critical couplings are between the steam turbine and the generator shafts. Steam turbines drive generators rotating at constant synchronous speeds, most commonly 3,000 revolutions per minute for 50 hertz systems and 3,600 revolutions per minute for 60 hertz systems. In addition to these high speeds, these couplings operate under great loads. Turbine and generator couplings require specially engineered solutions to endure these conditions while ensuring maximum uptime. Unexpected downtime can take hours or days to resolve and are extremely costly. Fluid couplings are commonly used because there is no mechanical connection between the input and output shaft, and therefore no wear. They also provide smooth and controlled acceleration as well as effective damping due to shock or load variations.

Hydroelectric power is created by rotating turbines in fast moving water that are connected to electrical power generators. As is the case with steam-turbine power plants, the turbine and generator shafts are connected to each other using specially designed couplings.

Sometimes, the application requires a custom design in order to meet the unique demands. For example, Ameridrives, a major coupling manufacturer with a history of successful power plant implementations, was tasked with designing a custom coupling between a boiler’s water feed pump and steam turbine. The plant the coupling was designed for is the first coal-fired power plant to use ultrasupercritical boiler technology. For this application, the water changes directly from a liquid to a superheated steam, reducing the plant’s carbon dioxide emissions. The challenge was that the coupling needed to be disconnected periodically for preventive maintenance in order to stop the water flow, allowing the turbine to continue to rotate on the turning gear. The company designed a technically advanced diaphragm disconnect coupling that is now part of their standard offering for power plants.

Uptime is a critical operating parameter for electrical power plants, and fault-free operation of couplings is a requirement. Downtime at a natural gas power plant can cost as much as $11,000 an hour, or $264,000 a day and a recent economic study estimates that power outages cost the US economy $150 billion annually. Even the material used in couplings for power plants is engineered for maximum uptime and reliability. For example, Bibby Turboflex manufactures every coupling bolt out of aviation grade S5.99 stainless steel. Each bolt is laser etched with cast numbers and serial numbers for traceability of the entire manufacturing process.

The next time you turn on your lights, realize that it is only possible due to the implementation of highly engineered couplings.

To contact the author of this article, email ken.thayer@ieeeglobalspec.com

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Discussion – 1 comment

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Re: The Role of Couplings in Power Plants
2017-May-27 1:57 AM

I am really surprised to see that couplings mentioned are of olden age. Now there are Magnetic Non-Contactable couplings and are preferred by most of the Power, Mining, Cement and Steel Plants.

Magnetic Couplings offers many benefits which are not there with rigid, flexible and fluid Coupling. Would request please update your knowledge before such articles are posted.

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