Turbines: Types and Operation

Michael
Nasser

Turbines are machines for producing continuous power in which a wheel or rotor, typically fitted with vanes, is made to revolve by a fast-moving flow of water, steam, gas, air, or other fluid. In this article, you will learn about the general operating properties, the different types, applications, and maintenance and repair considerations of turbines.

Turbine Operating Principle

Courtesy Energy Education

Basic functionality is achieved by initiating movement of the inlet rotor assembly through external means. Once the rotor assembly begins moving, the blades compress the inlet media. The compressed media is then directed through consisting of shaft or drum with blades on it. The stream of fluid acts on the blades and they start rotating and transfer the rotational energy to the rotor.

Types of Power Turbines

There are several types of industrial turbines. Common turbine groups are gas, hydro, wind, and steam.

The working principle in each type of turbine is to convert energy in a fluid stream to useful work. The operating principle for each type varies significantly and will be covered in more detail.

Gas Turbines

A gas turbine intakes air, compresses it, and then ignites the air-gas mixture. The turbine may be used to turn a shaft to do mechanical work, but most commonly powers a generator, producing an electric current. Rotor assembly is initiated through a startup mechanism, which is typically an electric motor. Once the rotor assembly begins moving, the blades compress the inlet air. As the air molecules become tightly packed they become hotter and pressure becomes high. That air is directed via a specially designed nozzle into a combustion chamber where fuel is ignited.

Courtesy GE Power

The ignited fuel is directed onto blades in the combustion chamber (also known as buckets). These combustion blades turn, allowing power to be generated. Industrial gas turbines can broadly classified into two types: mainframe and aero-derivative.

Mainframe Turbines

These types of turbines are physically large in size and are characterized by lower pressures ratio (ratio between discharge pressure and inlet air). Their compression ratio is typically below 20. They have very high power outputs, typically in the range of around 100 megawatts to 600 megawatts.

Aero-derivative Turbines

These turbines are comparatively compact in size and are used where smaller power outputs are required. As the name implies, these turbines are spinoffs of engines used in jets. These turbines can operate up to a compression ratio of 30 to 1. They can typically offer a power solution in the range from 20 megawatts to 65 megawatts.

Steam Turbines

Steam turbines convert thermal energy in steam to useful work or mechanical energy. Such turbines have air-foiled shape blades which create pressure difference whenever high energy fluid passes over it. This pressure becomes the cause of lift force. This lift force rotates the turbine and so the energy in the fluid has been transformed into mechanical energy.

Steam power is flexible as it can be configurated in a variety of different ways, depending on how steam is provided and the purpose of the outlet stream. These turbines operate from a power range of around 100 kW to over 250 MW. Steam may be delivered at a variety of pressures and temperatures. As pressure and temperature increase, the efficiency of the steam turbine increases.

At times, steam turbines are combined with gas turbines, creating a combined cycle set-up. The definition of a combined cycle is an assembly of engines that work of the same source of heat. A well designed combined cycle can deliver efficiencies up to 65% versus approximately 35%-40% for single-cycle operations.

Hydro Turbines

Hydro turbines utilize falling water energy to produce rotatory motion in turbines. This procedure converts kinetic energy into mechanical energy. Hydro turbines use water falling from a height as in dams and pass it down to pipes called penstocks.

Text Box: Courtesy 5 TranspowerNZ

A scroll case, present at the end of these penstocks, curls around the turbine. It is designed so that the pressure is consistent from all sides when the water enters the turbine. Wicket gates are openings that control the water flow into the turbine. The opening and closing of these wicket gates is controlled by an outside governor system.

Courtesy 6 TranspowerNZ

Beyond the wicket gates, blades of turbine are present. These blades extract the spin from the moving water in the most efficient manner. These blades connect to the generator via a shaft called the “runner”. Converted energy is transferred to the generator, producing electricity.

Courtesy 7 TranspowerNZ

Reaction Turbines

Reaction turbines utilize the combined effect of pressure and velocity of flowing water to create the rotatory motion. In this type, the runner (shaft) is placed directly into the water stream flowing over the blades. The rotor of a reaction turbine is placed inside a pressure casing.

Impulse Turbines

These turbines are solely based on the velocity of the flowing water to rotate the turbine. The turbine operates at atmospheric pressure, however, there are pressure changes at the nozzles that direct the water into the turbine.

Wind Turbines

Wind turbines convert wind power into a useful form, most typically electricity. When the wind causes the wind turbine blades to turn, an attached generator produces electricity using this kinetic energy. The blades turn by creating a lift force whenever the wind moves over an airfoil. An important phenomenon of these moving blades on relative wind velocity.

                                    Vrelative = Vwind – Vblade

As blades start moving at a higher velocity, the relative wind velocity is greater towards the tip of the blade. This inclination results in a continuous twisting effect from the root and tip of the blade. These rotations are very low in terms of RPMs, necessitating an increase in speed to generate electricity. A gearbox coupled to the blade assembly further increases the speed.

Text Box: Courtesy 8 EngineerLive.com
Courtesy Engineer Live

Most turbines begin to produce electricity at a wind speed from six to nine miles per hour. This is the “cut-in” speed. Turbines automatically shut down when wind speeds are too high to prevent damage to the turbine.

Maintenance and Repair Considerations

Each turbine manufacturer will have specific turbine maintenance requirements. While predictive and proactive maintenance schedule is generally considered best practice, preventive and breakdown maintenance schedules are still frequently deployed. The type of preventive maintenance schedule depends on the age of the equipment and the dollar value of the equipment. For instance, small turbines that are considered “disposable” may be on a breakdown maintenance schedule.

Courtesy Theseus

There are also specific maintenance considerations based on the type of turbine.

Gas Turbines

  • Air purity is crucial to the proper operation of gas turbines. As such, routine replacement of inlet air filters is highly crucial.
  • If the turbine has been shut off, a purge is done. The starter motor is turned on and rotated at a low RPM to remove in potentially explosive vapors in the turbine.
  • During routine inspections, key operating variables such as vibration,
  • Heat treatment is important for the parts operating at higher temperatures to rejuvenate the material’s grain structure and reduce the possibility of fatigue cracks.

Steam Turbines

  • High-pressure steam turbines can experience problems with clearances becoming too loose, affecting efficiencies.
  • Low-pressure blade erosion is a frequent concern, leading to inefficient operation and even debris downstream.
  • Uncontrolled rotations of the turbine rotor must be prevented to ensure the outer governor remains in proper working condition.

Hydro Turbines

  • Pipelines must be debris-free and in good operating condition.
  • Visual inspection for silt deposition, erosion of the wetted parts, and “water hammer” is critical.

Wind Turbines

  • It is recommended to tighten the bolts and nuts that hold all the blades twice a year.
  • The gearbox is known to be an area of potentially poor reliability. Proper and routine lubrication of the gearbox will ensure a smooth operation.