Bypass Controller for Combined Cycle Turbine Start-Up
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Solution Overview
Problem
Combined cycle power plants face challenges in reducing start-up time due to thermal stress and thermal expansion differences between high-pressure and intermediate-pressure steam turbines, limiting their operational efficiency and service life.
Innovation Solution
The implementation of a combined cycle power plant configuration that includes a gas turbine, high-pressure and intermediate-pressure steam turbines, an exhaust heat recovery boiler with bypass paths and regulators, and a bypass controller to manage thermal effect margins, allowing both turbines to operate near their thermal effect limits, thereby reducing start-up time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If steam temperature and flow rate are rapidly increased during start-up to reduce start-up time, then productivity is improved, but thermal stress on the turbine rotor increases and service life is reduced
Solution Approach 1:
The system performs preliminary calculations of thermal stress and thermal expansion differences before actual start-up operations. The control device computes predicted values of thermal stress and thermal expansion differences based on current plant state quantities, and determines the steam temperature increase rate in advance to ensure these predicted values remain within allowable limits during rapid start-up.
Solution Approach 2:
The control device continuously monitors actual thermal stress and thermal expansion difference measurements during start-up, compares them with predicted values, and adjusts the steam temperature increase rate dynamically. This feedback mechanism ensures that thermal stress and thermal expansion differences are kept within safe limits while enabling rapid start-up.
2Productivity
If steam temperature is significantly increased during start-up to reduce start-up time, then productivity is improved, but difference in thermal expansion between turbine rotor and casing increases causing potential contact and damage
Solution Approach 1:
The system calculates the predicted thermal expansion difference between the turbine rotor and casing before start-up operations begin. Based on this preliminary calculation, the control device determines an appropriate steam temperature increase rate that will keep the thermal expansion difference within safe limits, preventing rotor-casing contact during rapid start-up.
Solution Approach 2:
The control device continuously monitors the actual thermal expansion difference during start-up and compares it with predicted values. Based on this feedback, the steam temperature increase rate is dynamically adjusted to ensure the thermal expansion difference remains within allowable limits, thereby preventing rotor-casing contact while enabling rapid start-up.
3Reliability
If the amount for plant operation is determined based on the smaller thermal effect-amount margin to ensure safety, then reliability is improved, but start-up time increases due to conservative operation of one turbine
Solution Approach 1:
The control device calculates thermal effect-amount margins separately for each steam turbine (high-pressure and intermediate-pressure) based on their respective thermal stress and thermal expansion characteristics. Each turbine operates with its own optimized steam temperature increase rate determined by its specific thermal effect margin, allowing different turbines to have different operational characteristics rather than being constrained by the minimum margin of all turbines.
Solution Approach 2:
The system dynamically adjusts the steam temperature increase rate for each turbine based on real-time thermal effect measurements and predictions. The control device continuously updates the operational parameters for each turbine according to its current thermal state, enabling both turbines to operate near their respective thermal effect limits during start-up, thereby reducing overall start-up time while maintaining safety.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration enables both high-pressure and intermediate-pressure steam turbines to operate near their thermal effect limits, reducing thermal stress and expansion differences, thus shortening start-up time and extending turbine service life.
Implementation Method 1
a high-pressure superheater which superheats high-pressure steam and supplies the superheated steam to the high-pressure steam turbine
Implementation Method 2
a reheater which reheats exhaust steam from the high-pressure steam turbine and supplies the reheated steam to the intermediate-pressure steam turbine
Implementation Method 3
an exhaust heat recovery boiler that uses exhaust gas from the gas turbine as a heat source
Data Source
AI summary
There is provided a combined cycle power plant in which a high-pressure steam turbine and an intermediate-pressure steam turbine can operate in a state where amounts of thermal effect thereof are close to a limit value, and capable of reducing start-up time. A combined cycle power plant includes: an exhaust heat recovery boiler that includes a high-pressure superheater which superheats steam for a high-pressure steam turbine, and a reheater which reheats steam for an intermediate-pressure steam turbine; bypass pipes through which steam bypasses the high-pressure superheater and the reheater; bypass valves that regulate flow rates of steam which flows through the bypass pipes; and a bypass controller that controls the bypass valves such that a difference between thermal effect-amount margins of the turbines is decreased.