Steam Turbines:
A look into how a steam turbine functions

Stacy L. Rousseau EE '03

At a time when energy is such an issue in our country, it may be surprising to realize that the major technology involved in producing electrical energy today was developed over a century ago by Charles G. Curtis: the steam turbine. The aim of a steam turbine is to produce the maximum amount of electrical energy in the minimum amount of space. Today, steam turbines produce more electrical energy than any other system. They comprise approximately 78 percent of all generation capability in our country. Their largest manufacturer is General Electric. GE put its first turbine into operation in November 1901; it had 500 kW of capacity. Now, steam turbines encompass well over 500 million kW of capacity.


A General Electric D11 model of a steam turbine.

In a power plant, the steam turbine is attached to a generator to produce electrical power. The turbine acts as the more mechanical side of the system by providing the rotary motion for the generator, while the generator acts as the electrical side by employing the laws of electricity and magnetism to produce electrical power.

Terminology

To understand the functioning of a steam turbine, one must first be familiar with some of the terminology associated with it. The rotor is the spinning component that has wheels and blades attached to it. The blade is the component that extracts energy from the steam.

There are two basic types of turbine designs. One is called the "impulse" design in which the rotor turns due to the force of steam on the blades. The other is called a "reaction" design, and it works on the principle that the rotor derives its rotational force from the steam as it leaves the blades.

A single-flow turbine design has steam entering at one end. The steam then travels in one direction toward the other end of the section and exits the casing to be reheated, or passes on to the next section. A double-flow section, however, has steam entering in the middle and flowing in both directions toward the ends of the section.

Functioning

Steam is first heated in a boiler, where it reaches a temperature of approximately 1,000 F. It enters the turbine at a speed greater than 1,000 mph. The first valve that the steam encounters as it goes from the boiler to the turbine is the Main Stop Valve (MSV), which is either fully open or fully closed. The MSV does not control the steam flow other than to completely stop it. The terms "throttle pressure" and "throttle conditions" refer to steam as it is entering the MSV. The steam hits the first row of blades at elevated pressure. Its pressure is so high, in fact, that it can produce a torque with just a small surface area. The steam's impact causes the rotor to begin turning. As the turbine stages progress, however, the steam loses density, thus requiring increasingly large surface areas. For this reason, the size of the blades increases with each stage. When the steam leaves the turbine, it has dropped over 900 F and has lost almost all of its elevated pressure. Most of the pressure drop occurs across the diaphragm, which is a component between the outer wall and inner web. Its partitions direct the steam at the rotating blades.

The steam must strike the blades at a specific angle that will maximize the useful work of the steam's high pressure. This is where nozzles come into play. Stationary rings of nozzles are placed between blade wheels to "turn" the steam at the optimal angle for striking the blades. A thrust bearing is mounted at one end of the main shaft to maintain its axial position and keep the moving parts from colliding with stationary parts. The journal bearing supports the main shaft and restricts it from springing out of its casing at high speeds.

The exhaust hood guides steam from the last stage of the turbine, and it is designed to minimize pressure loss, which would decrease the thermal efficiency of the turbine. After the steam leaves the exhaust section of the turbine, it enters a condenser, where it is cooled to its liquid state. The process of condensing the steam creates a vacuum, which then brings in more steam from the turbine. The water is returned to the boiler, reheated, and used again.


Known as th "Grand Canyon," the factory floor of GE's steam turbine plant occupies the same area as 40 football fields.

The governor is a device that controls the speed of the turbine. Modern turbines have an electronic governor that uses a sensor to monitor the turbine speed by "looking" at the rotor teeth. The Ventilator Valve (VV) also aids in controlling the turbine's speed. The VV is normally closed, but in an overspeed situation, it drains steam. This steam comes from the reheat section, which is forcing steam back through the turbine, and is used to cool the high-pressure section.

Efficiency

The most basic steam turbine consists of a nozzle for steam to pass through and blades mounted on a rotor wheel rim. To make a more efficient turbine, however, a casing to confine the steam and valves to control the admission of steam to the nozzles are added. The thick-walled castings used for pressure-containing turbine sections are called shells, and are usually made from a chrome-molybdenum-vanadium alloy steel. Some designs employ both an inner and outer shell, which serve to balance the pressure drop and reduce the shell's thickness for thermal stress and starting and loading.

Multiple stage designs do not effect the overall operation of the turbine; they only serve to increase the efficiency. The type and number of stages that a turbine uses varies, as well as the shape and size of the blades. They depend on the steam pressure and temperature, the exhaust pressure, and the speed. How does one maximize the efficiency of a turbine? When the rotor wheel is held stationary, the steam flowing through the nozzle will hit the blades with full force, thus exerting the greatest amount of torque. However, since this occurs while the rotor is at a standstill, the work done is zero. On the other hand, if the speed of the rotor is equal to that of the steam, the steam will have no velocity component relative to the blades and the blades will not turn. Therefore, this case leads to zero torque and, once again, zero work. The maximum efficiency occurs between these two extremes.

Future

The future of steam turbine-generator technology lies in an increase in capacity, which will stem from larger and more efficient turbines. Nuclear fission will continue to compete with conventional fuel combustion in the production of steam for the turbine, but the heart of the technology will still lie in the technology of the steam turbine itself.






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