Integrated Turbine-Boiler Control

Rankine Cycle Power Plant Control Room Operators, Supervisors, and Managers must be familiar with integrated turbine-boiler control. All Rankine Cycle plants must coordinate the control of the boiler and the turbine generator so that the load of the unit can be changed without creating plant upsets that could cause the unit to trip. Several modes of operation are commonly used for integrated turbine-boiler control, which includes, but are not limited to:

Boiler Following

• Rapid response that responds to changes in load demand.
• The operator responds to that increase in demand by increasing the turbine governor setpoint. This is done to open the turbine control valves further.  The steam flow through the turbine increases as soon as the valves open and, with the increase in steam flow, the turbine-generator load increases.
• At the same time that the steam flow increases, the boiler pressure falls.  The Firing Rate Master responds automatically by increasing the firing rate.  As the firing rate increases, the heat input to the boiler increases, thus restoring the boiler pressure.

Turbine Following

• More stable operation of the boiler.
• When the boiler pressure increases, the turbine control valves respond by opening to restore the pressure.  This results in an increase in turbine-generator load.  Similarly, when the boiler pressure falls, the turbine control valves respond by closing to restore boiler pressure, thus decreasing turbine steam flow and turbine-generator load.

Coordinated Control

• A compromise between the boiler following and turbine following modes of operation.
• This means that both the turbine and the boiler start responding to changes in demand at the same time.

Automatic Generation Control (AGC)

• The AGC allows the Electrical Power System Operator (PSO) to send demand signals from a central location to any (if not all) units in the Electrical Power System.
• For the older plants, the AGC demand signal is used to adjust the turbine governor setpoint.  For newer plants with a coordinated control system, the AGC demand signal is used to adjust the unit load setpoint in the coordinated control system

Constant Pressure Operation

• The load of the turbine-generator is determined by the position of the control valves.
• Boiler pressure remains constant.
• Lower efficiency at lower loads, but faster response to changes in load demand.

Sliding Pressure Operation

• The turbine control valves are held at Valves Wide Open (VWO) and the load is controlled by reducing changing boiler pressure with firing rate.
• Boiler pressure changes to, in turn, change turbine-generator load.
• Poor response to changes in load demand (slow).

Ensuring operators, supervisors, and management are well-trained on these various turbine-boiler control modes is critical toward maximizing plant efficiency, performance, and reliability. If there is a knowledge gap at your facility in these areas, FCS has extensive experience providing the training needed to get your people up to speed.

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Heat Rate Cost – Part 1

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