Chapter 1
Introduction to Steam Turbines

1.1 Why Do We Use Steam Turbines?

Steam turbine drivers are prime movers that convert the thermal energy present in steam into mechanical energy through the rotation of a shaft. Industrial steam turbines fit into one of two general categories: generator drives and mechanical drives. Generator drives include all turbines driving either synchronous or induction generators for power generation. In this book, we will cover primarily steam turbines used in the petrochemical industry as mechanical drives for centrifugal pumps and centrifugal compressors. In mechanical drives, the rotational energy is transmitted to a process machine that in turn converts it into fluid energy required to provide flow for a given process.

Heat energy Steam energy Rotational energy Fluid energy

Figure 1.1 General purpose steam turbine. (Courtesy of Elliott Group)

1.2 How Steam Turbines Work

Steam turbines are relatively simple machines that use high-velocity steam jets to drive a bladed wheel that is attached to a rotating shaft. Figure 1.2 depicts an impulse-type steam turbine in its most basic form: A steam nozzle and a bucketed, rotating wheel.

Figure 1.2 Basic impulse steam turbine.

In this design, high-pressure steam is accelerated to a high velocity in the stationary nozzle and then directed onto a set of blades or buckets attached to a wheel. As the steam jet impacts the buckets, it is deflected and then leaves the scene. The change in momentum involved in the steam's deflection generates a force that turns the wheel in the direction opposite of the incoming steam jet. If the wheel is affixed to a shaft and supported by a set of bearings, rotational power can be transmitted via the output shaft.

To produce useful work in a safe and reliable manner, an impulse-type steam turbine, at a minimum, must contain:

1. A bladed wheel that is attached to a shaft.
2. A set of stationary steam nozzles capable of accelerating high-pressure steam to create high velocity jets. (See the steam nozzle in Figure 1.3.)
3. A pressure-containing casing.
4. Seals that can control steam leakage from traveling down the shaft. (See carbon packing end seals in Figure 1.3.)
5. A governor system capable of controlling rotating speed within design specifications. (Speed governor in Figure 1.3.) Governor systems fall into two main categories: hydraulic and electronic.
6. A coupling that can transmit power from the steam turbine to an adjacent centrifugal machine.

Figure 1.3 Cross section of an impulse steam turbine.

Steam turbines can be rated anywhere from a few horsepower to around a million horsepower. They can be configured to drive generators to produce electricity, or mechanical machines such as fans, compressors, and pumps. Steam turbines can be designed to operate with a vertical or horizontal rotor, but are most often applied with horizontal rotors.

1.2.1 Steam Generation

Steam is either generated in a boiler or in a heat recovery steam generator by transferring the heat from combustion gases into water. When water absorbs enough heat, it changes phase from liquid to steam. In some boilers, a super-heater further increases the energy content of the steam. Under pressure, the steam then flows from the boiler or steam generator and into the distribution system.

1.2.2 Waste Heat Utilization

Waste heat conversion is the process of capturing heat discarded by an existing industrial process and using that heat to generate low-pressure steam. Energy-intensive industrial processes-such as those occurring at refineries, steel mills, glass furnaces, and cement kilns-all release hot exhaust thermal energy in the form of hot liquid streams that can be captured using waste heat boilers (see Figure 1.4).

Figure 1.4 Waste heat boiler.

The steam from waste heat boilers can be utilized for heating purposes or to power steam turbines.

Steam systems all tend to have the following elements:

• Boiler-A process subsystem that uses a fired fuel or waste heat to turn condensate into high-pressure steam. Steam is typically collected in a steam drum (see Figure 1.5)
• Steam Turbine-A rotating machine that converts high-pressure steam energy into shaft power
• Process Waste Heat Recovery or Condenser-A part of the process that recovers sufficient lower pressure steam heat to condense all the steam back to condensate
• Boiler Feedwater Pump-A liquid pump that raises condensate pressure back to boil pressure so that it can be returned to the steam boiler

Figure 1.5 Steam drum.

1.2.3 The Rankine Cycle

The Rankine cycle is the thermodynamic basis for most industrial steam turbine systems. It consists of a heat source (boiler) that converts water to high-pressure steam. In the steam cycle, water is first pumped up to elevated pressure and sent to a boiler. Once in the boiler, liquid water is then heated to the boiling temperature corresponding to the system pressure until it boils, i.e., transforms from a liquid into water vapor. In most cases, the steam is superheated, meaning it is heated to a temperature above that required for boiling. The pressurized steam is: (a) transmitted via piping to a multistage turbine, where it is (b) expanded to lower pressure and then (c) exhausted either to a condenser at vacuum conditions or into an intermediate temperature steam distribution system. Intermediate pressure steam is often used for other process applications at a nearby site. The condensate from the condenser or from the industrial steam utilization system is returned to the feedwater pump for continuation of the cycle.

Primary components of a boiler/steam turbine system are shown in Figure 1.6.

Figure 1.6 Components of a boiler/steam turbine system.

1.3 Properties of Steam

Water can exist in three forms, ice, liquid and gas. If heat energy is added to water, its temperature will rise until it reaches the point where it can no longer exist as a liquid. We call this temperature the "saturation" point, where with any further addition of heat energy, some of the water will boil off as gaseous water, called steam. This evaporation effect requires relatively large amounts of energy per pound of water to convert the state of water into its gaseous state. As heat continues to be added to saturated water, the water and the steam remain at the same temperature, as long as liquid water is present in the boiler.

The temperature at which water boils, also called boiling point or saturation temperature, increases as the pressure in the vapor space above the water increases. As the water vapor pressure increases above the atmospheric pressure, its saturation temperature rises above 212 °F. The table below titled, "Properties of Saturated Steam" illustrates how the saturated steam temperature increases with increasing steam pressure.

Figure 1.7 Tea kettle producing steam.

If heat is added after the steam has left the boiler, without an increase in steam pressure, superheated steam is produced. The temperature of superheated steam, expressed as degrees above saturation corresponding to the pressure, is referred to as the degrees of superheat. Adding superheat to steam is a good way to prevent steam from condensing as it makes its way from a boiler to a steam turbine.

In general, we can say that the higher the steam pressure and its corresponding temperature the more energy it contains to perform useful work. In order to get a feel for typical saturated steam pressure and temperature, we will provide a few realistic examples. Refer to the "Properties of Saturated Steam" (Table 1.1) as you consider the following examples:

Table 1.1 Properties of saturated steam.