Steam Turbine

Steam turbines are one of the oldest prime mover technologies in general production. A steam turbine is a mechanically driven device that extracts thermal energy from a multistage expansion of steam and converts it into rotary motion. Because of the rotary motion the turbine generates, it is most commonly coupled with an electrical generator to create power. Steam turbines generate approximately 90% of the United States electrical power and have been used for over 100 years [114]. More efficient and less expensive, the steam turbine replaced its predecessor, the reciprocating steam engine.

Coppus Steam Turbine at MNSU
Coppus steam turbine at MNSU [129].


Basic Operation

The steam turbine operates by means of the thermodynamic Rankine cycle. The basis for this conventional power generation system is a heat source, most commonly a boiler or steam generator, which converts water into high pressure steam. The steam is run through the turbine, which converts its high thermal energy into kinetic energy that rotates the turbine blades. The pressurized steam expands through the turbine and is exhausted into a condenser. In some cases, an intermediate temperature steam distribution system exists, which delivers the remaining high temperature steam to the industrial or commercial utility. The condensate from the condenser or the steam systems returns to the feedwater pump and back to the boiler to continue the cycle.

How Stuff Works → The Turbine Blades

The steam turbine consists of both stationary and moving sets of blades called, respectively, nozzle and rotor blades. The two sets of blades function together to cause the steam to turn the shaft of the turbine connected to the mechanical load. The stationary blades accelerate the steam to high velocities, expanding it to lower pressures. The rotating blades, fitted into a wheel at an angle appropriate for the application, change the direction of the steam flow, which in turn pushes against the blades, creating torque and turning the shaft [127].

Turbine Blade Manufacturing

The steam turbine blades are manufactured from a large ingot or bar stock. Each blade is made individually and then attached to the turbine rotor. Steam turbine blades are manufactured in this manner because when the blades are inspected them during outages and a single blade has enough cracking or erosion pitting that it may fail, that individual blade can be replaced instead of the entire turbine.

The type of metal that is used to make the blades depends on the type of turbine. The metal used for the blade of a high pressure turbine (HPT) must be able to withstand the high temperatures and pressures that it is operated in due. On the other hand, low pressure turbine (LPT) must be constructed with materials that have an increased resistance to corrosion. This is because the lower pressures in the LPT allow for the steam to condense more. This causes the steam to have a higher moisture content in the form of small water droplets; at high velocities, the water droplets can damage the blades.

The piece of metal used to make the blades is prepared using a process in which heat treats the metal multiple times before it is subjected to chemical and alloying treatments in order to achieve the proper metallurgical properties for the blade. After the blade has been treated, it is placed on a computer-programed milling machine. The milling machine uses a variety of rotary tools to machine the turbine blade with precision.

Turbine Problems

Turbines are subject to pitting, corrosion fatigue, stress concentrations, deposit buildup, and erosion from water droplets and solid particles. Turbines can lose as much as 15% of their total generating capacity as a result of erosion and the buildup of deposits on the blades [115]. According to Jonas and Mancini’s article Steam Turbine Problems and Their Field Monitoring, it is estimated that 40% of

Faribault Energy Park steam turbine undergoing maintenance
Faribault Energy Park steam turbine undergoing maintenance [130].
utility turbines and 60% of industrial turbines in the United States operate with high concentrations of impurities in the steam [116]. Sodium and cation conductivity allow for the deposition of salts and hydroxides throughout the system. This can lead to corrosion, deposits on parts, and reduction in the amount of power generated. The deposits can build up within the nozzle inside the turbine which can alter the energy distribution and aerodynamic flow of the steam. These deposits can result from superheated steam, acid droplets, evaporation of moisture (liquid films) on surfaces, metal oxides formed in the steam cycle, and adsorption of gases and impurities in superheated steam. In many cases, magnetite, a ferrous mineral used to coat steam boilers, leaches from the superheater and steam pipes causing chemical interaction and erosion.

As the steam expands through the turbine, the solubility of contaminants in the steam decreases. The contaminants—including chlorides, sulfates, and sulfides—condense onto the blade surfaces where they concentrate at much higher levels than in the flowing steam. The concentrated contaminants exacerbate corrosion. Pitting, which can damage rotors and disks in particular, is localized corrosion caused by chloride deposits which create small holes in the metal. In order to address these potential problems, the content of the feedwater must be carefully monitored. When an appropriate chemical balance in the water is maintained, the steam turbine will run more efficiently and require less maintenance.

Turbine Startup

Due to the high pressure levels that a steam turbine is typically operated under, the turbine must be built with thick casings to withstand the load. The thick, heavy duty construction of the turbines equates to a high thermal inertia. Thermal inertia is a measure of the materials resistance to change in temperature. Because of this, the startup of a steam turbine must be done properly in order to avoid excessive fatigue. The steam must have the proper superheat before being routed through the turbine or else it will condense and damage the blades, but too much superheat and there will be a large temperature difference between the steam and the turbine, which can damage the steel.

Steam Turbine Blades at FEP
Steam turbine blades at Faribault Energy Park [130].
Bypass valves are used in this case in order to divert the steam around the turbine to heat it more or to slowly meter in steam. During startup, the decision is sometimes made to start the turbine faster when the demand is high and power is needed faster, though this causes more fatigue on the turbine. Depending on the initial conditions of the turbine and boiler as well as the size of turbine and boiler, startup time can range from minutes to upwards of ten hours. Typically, the slower the steam turbine is started up, the longer it will last.

Cogeneration System Performance

Physical sites that use a combined heat and power system:
Minnesota State University, Mankato
Faribault Energy Park

Power is produced at a cogeneration plant in the same way that it is at any other steam power plant. However, the overall efficiency will increase. The steam exiting the turbine is still at high temperature, in normal steam power plants, this energy will be released to the surrounding as waste heat. Yet, at a cogeneration plant, the steam will be distributed to nearby commercial or residential areas for heating or other industrial purposes. By using the energy that would otherwise be wasted, overall thermal efficiency will be a lot higher for the cogeneration plant than a stand-alone steam turbine power plant. For example, the average fossil fuel steam power plants have an efficiency of 33%. In comparison, the efficiency for cogeneration plants using steam turbine can reach up to 80%. The better efficiency will translate to lower greenhouse gas emission, lower operating cost and higher profit [117].

Minnesota State University, Mankato
 
In 1995 Minnesota State University decided to look into the idea of a cogeneration cycle for campus.

Coppus steam turbine at MNSU
Coppus steam turbine at MNSU [129].
The university accepted a bid that included a Coppus model RLHA 24 single stage turbine and a Reliance Frame E5010S electrical generator with an estimated total cost of $453,000 (in 1995). This system was designed to provide an additional 434 kW of power to campus using excess heat from the boilers. Energy produced by the excess heat was extracted from steam fed into the Coppus steam turbine at 150 psig and 366°F and converted to mechanical power by a rotating shaft.

The Coppus Model RLHA 24 steam turbine is a horizontal/axial split single stage turbine. The axial split construction of the RLHA steam turbine enables operators to remove the upper half of the turbine casing to inspect and maintain the internal components. The RLHA model is designed for applications of up to 2500 hp (1865 kW). The RLHA 24 is designed to rotate at a speed of up to 6300 rpm using a single, two-row Curtis impulse-type. The Coppus steam turbine, with a 6-inch diameter inlet and 10 in diameter outlet, has a maximum inlet pressure and temperature of 900 psig and 950°F. The compact design—55 inches long and 44 inches wide—of the steam turbine is ideal for driving lube oil pumps, feed water pumps, process pumps, fans, compressors, and generators.

Faribault Energy Park
 

GE A10 steam turbine at Faribault Energy Park [129].
Faribault Energy Park generates electricity and steam in a different way than is done at Minnesota State University Mankato’s site. The main source of power comes from a GE 7FA gas turbine which directly drives a generator. When more power is needed, the hot exhaust gasses from the gas turbine are sent to the CMI Heat Recovery Steam Generator (HRSG) which turns water into steam by using the hot exhaust gas. The steam is then directed into the GE A10 steam turbine which then drives a second generator. When the steam turbine is used, a maximum of 252 MW is produced compared to 143 MW when only the gas turbine is used for electricity production [118].

The steam from the HRSG that drives the steam turbine is at a temperature of 1050°F. The GE A10 is a 12-stage turbine with a high pressure of about 1400psi, intermediate pressure of about 300 psi, and a low pressure of about 60 psi. The maximum inlet pressure of the steam turbine is rated at 1900 psi, but the steam pressure in the plant can vary from between 1150 and 1800 psi depending on the electricity demand on a given day. Between the first and second stages of the steam turbine, the steam is sent through a reheater in order to raise the temperature of the steam to generate more power and raise the efficiency level of the plant. Approximately 2.4% of the total power generated by the plant is used to operate the facility.

Here is a video from the Basin Electric Power corporation as the Laramie River Station receives a steam turbine upgrade.
Want to learn about the Rankine Cycle? Read this Description.
Video showing the machining process of a steam turbine blade.
Want to know about corrosion and deposit problems and solutions on steam turbines? Read this research paper.
This video explains the concept of thermal inertia.
What are the design options and benefits of a HRSG? Click here.