Steam Turbines

A turbine is a device that converts chemical energy into mechanical energy, specifically when a rotor of multiple blades or vanes is driven by the movement of a fluid or gas. In the case of a steam turbine, the pressure and flow of newly condensed steam rapidly turns the rotor. This movement is possible because the water to steam conversion results in a rapidly expanding gas. As the turbine’s rotor turns, the rotating shaft can work to accomplish numerous applications, often electricity generation. In fact, it is estimated that 88 percent of electricity generation in the United States uses steam turbines.

Types of Steam Turbines

There are complicated methods to properly harness steam power that give rise to the two primary turbine designs: impulse and reaction turbines. These different designs engage the steam in a different method so as to turn the rotor. As water converts into steam, the molecules grow further apart. While steam can exert pressure, it cannot exert the correct pressure needed to spin the rotor quickly enough to generate electricity. Thus, a special design of rotor is required to properly harness the steam and spin.

In an impulse turbine, nozzles direct the steam towards the rotors, which are equipped with concave panels called buckets. The nozzles are able to project a jet of steam that spins the rotor at a loss of roughly 10 percent energy. As the jets change their position, they can increase or decrease the rate of rotor spin.

A reaction turbine works opposite the impulse turbine. The steam nozzles are attached to the rotor blades on opposite sides. The nozzles are so positioned that when they release jets of stream, they propel the rotor in a spinning motion that keeps it rotating as long as steam is being expelled. It can reach high speeds because the nozzle designs focus the steam into a thin stream, although the initial warm up period may take several moments.

Turbine Function

There are different methods for producing steam to propel a steam turbine. Condensing steam turbines typically employ low-pressure steam that is not fully condensed—it is usually approximately 90 percent steam. When steam has lower pressure than the atmospheric pressure surrounding it, it can be expanded to a greater degree for turning standard piston engines. Non-condensing steam turbines also work with low pressure steam, usually at refineries or pulping plants, where low pressure steam is typically available. These turbines take advantage of exhaust steam, a product of other applications.

Turbines also require a governor, or speed limiter, which controls the speed of the rotor rotation. Turbines require a slow warm up period to prevent accidents or damage. The governor can control the pressure and amount of steam emitted so as to properly monitor and control the speed of the spinning rotors. This is necessary in applications like electrical generation. The electrical grid in the United States and in other countries utilizes droop speed control. When a plant is functioning in a full-load output capacity, it runs at 100 percent speed, while it runs at 105 percent speed when at no-load. The speed variance is required because of the myriad power plants operating simultaneously, which need to provide dependable frequency despite constant changes, drop offs and capabilities of power.

Nuclear Power Generation

One common application of steam engines is in nuclear power plants. As stated, when water is condensed into steam, its molecules grow farther apart. In a nuclear power plant, pressure is applied to the gaseous steam to drive these molecules closer together, resulting in what is called a “supercritical fluid.” This is a gas that has the properties of a liquid. Nuclear plants create supercritical steam, which has higher temperature efficiencies than more typical types of steam. More recent innovations have employed flash steam technology to heat water so quickly that it doesn’t even boil, but rather becomes steam immediately. These turbines are also capable of higher heat and output capabilities than other turbine types.