Control Variables

Speed control is the primary control loop for all steam turbines, since without it, the turbine cannot be started or operated safely. While one steam control device can control only one parameter, such as speed, application flexibility is increased when the controlled parameter's set point is influenced as a function of a secondary parameter, such as exhaust pressure. For example, exhaust pressure can be used to bias the speed set point in order to maintain a desired exhaust set point.

In electric generation applications, it is often desirable to control the turbine for load rather than speed. For condensing

Fig. 11-23 Multi-Valve, Controlled Extraction Steam Turbine Designed to Drive Electric Generator. Source: Tuthill Corp., Murray Turbomachinery Div.

turbines and admission turbines, load control is also usually the primary parameter. In the case of back-pressure and extraction turbines, load control may be secondary.

Typical driven equipment and/or process control parameters include pressure, flow, temperature, and speed. Often, it may be desirable to reduce turbine speed at low load to improve the efficiency of the driven equipment (a fan or centrifugal compressor, for example), even if the turbine itself would operate more efficiently at a higher speed. In such cases, the reduced power requirement of the driven equipment more than offsets the negative impact of reduced turbine efficiency.

In addition to speed and load, other important control parameters sometimes include the following:

• Inlet pressure

• Extraction pressure

• Induction pressure

• Exhaust pressure

Inlet pressure is controlled by manipulating the inlet governor valve(s). To control inlet pressure, a bias must be applied to the speed control loop that relates actual inlet pressure to set point values. That is, after comparing a pressure signal to a reference signal, the required speed reference is generated. The corrective signal repositions the actuator controlling the inlet steam valve. To promote stability and keep the turbine from reacting to minute changes, a dead band is established in the control system to provide a window within which no corrective action is taken.

Exhaust pressure control is used with back-pressure turbines when the exhaust steam is being used in a process that requires steam pressure control. Exhaust pressure is controlled by manipulating the inlet governor valve(s) in a manner analogous to that described above.

Extraction valves regulate flow to downstream stages

Fig. 11-24 Illustration of Governor Assembly and Overspeed Trip and Valve. Source: Elliott Company

of the turbine in order to maintain constant back-pressure at the exhaust. With extraction added to speed/load as a control parameter, the governor must control (ratio) the inlet valves and the extraction valves in such a manner that both speed/load control and extraction pressure are held at desired levels.

Induction (admission) control valves operate on a similar principle. Induction valves control flow into the downstream stages of the turbine, while maintaining constant pressure at the induction opening. As with extraction turbines, the control system must ratio the positioning of the inlet valve and the induction valve to maintain stable set point control.

For every application, a steam map, or envelope, must be established that provides a description of the operating range of the turbine and, therefore, establishes boundaries. Boundary lines include a maximum throttle (inlet) flow, maximum power, maximum exhaust flow, zero extraction flow, minimum power, and minimum exhaust. Within these boundaries are the combination of throttle and extraction flow that result in various power output levels with safe and stable operating conditions.

The minimum exhaust flow is usually of critical importance in order to avoid overheating the exhaust section. In addition, admission turbines must assure a minimum throttle flow so that enough steam flows through the inlet section to cool the forward section of the turbine rotor.

Figure 11-25 shows a schematic diagram of a digital control system for an automatic extraction steam turbine. The system features a 32-bit microprocessor-based digital control. Typical control functions include turbine speed control, load control/limiting, inlet and exhaust pressure control/limiting, import/export control/limiting, and isochronous load sharing. As shown, in this application, two parameters are controlled. In addition to extraction pressure, load, exhaust pressure, or inlet pressure may be controlled.

Instrumentation

There are many types of instrumentation that can be used depending on the type of turbine application. Typical instruments used include:

• Steam pressure gauges for supply exhaust and nozzle ring or first stage pressure

Fig. 11-25 Schematic Diagram of Digital Control System Configuration for Automatic Extraction Turbine. Source: Woodward Governor Co.

• Steam temperature gauges for turbines operating with superheated steam

• Oil pressure gauges for lube and control oil

• Oil cooler temperature gauges

• Vibration detectors

• Bearing temperature gauges

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