Steam Generator Control Circuit for Rapid Load Changes
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Solution Overview
Problem
Once-through steam generators in combined cycle power plants face challenges in maintaining optimal steam temperature and subcooling during varying operating states, particularly during rapid load changes, which can lead to unstable flow and risk of material damage.
Innovation Solution
A control method and circuit that adjust the valve opening for the feedwater supply line based on temperature set-points and measured values, incorporating PID control steps to manage steam superheating, subcooling, and feedwater flow, including a bypass line for subcooling control during rapid unloading, ensuring stable operation and preventing overheating.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a once-through steam generator is used to rapidly respond to load changes, then productivity and adaptability improve, but steam temperature stability and subcooling control deteriorate
Solution Approach 1:
The control system is segmented into multiple independent PID controllers (first PID controller for superheating, second PID controller for subcooling, third PID controller for feedwater flow) that operate in parallel to manage different aspects of the steam generation process. This segmentation allows each controller to independently optimize its specific function without interfering with others, enabling rapid load response while maintaining stability.
Solution Approach 2:
The system dynamically changes control parameters by using different setpoint values for different operating conditions. The first PID controller adjusts the degree of superheating setpoint, the second PID controller adjusts the degree of subcooling setpoint, and the third PID controller adjusts the feedwater flow rate setpoint based on actual measurements, allowing the system to adapt to varying load conditions while maintaining optimal performance.
2Reliability
If feedwater flow is increased to prevent overheating, then reliability improves, but steam temperature control precision deteriorates
Solution Approach 1:
The system implements feedback control through three PID controllers that continuously monitor actual conditions and adjust control actions accordingly. The first PID controller uses feedback from actual steam temperature and degree of superheating to adjust the superheating setpoint. The second PID controller uses feedback from actual feedwater temperature and degree of subcooling to adjust the subcooling setpoint. The third PID controller uses feedback from actual feedwater flow rate to adjust the flow rate setpoint, ensuring both reliability and precision.
Solution Approach 2:
The control system performs preliminary actions by proactively adjusting feedwater flow and heat input before overheating occurs. The second PID controller anticipates potential overheating by adjusting the degree of subcooling setpoint based on feedwater temperature trends, and the third PID controller pre-adjusts feedwater flow rate to prevent temperature excursions, thereby maintaining reliability without compromising precision.
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
The solution ensures optimal steam superheating and subcooling across all operating states, preventing evaporation and overheating, thereby ensuring stable and reliable operation of the high-pressure evaporator with reduced risk of material damage.
Implementation Method 1
feedwater is evaporated and superheated by means of heat exchange with hot exhaust gas from the gas turbine
Implementation Method 2
heat in each tube row, over which the exhaust gas flows, is proportional to the temperature difference between the exhaust gas and the fluid in the heat exchanger tubes
Implementation Method 3
heat exchange with hot exhaust gas from the gas turbine and in counterflow to the hot exhaust gas
Implementation Method 4
opening of a valve (12) for regulating the flow volume in a feedwater supply line (14) is controlled in dependence upon a temperature set-point value for the steam for the steam turbine (DT) and also upon a measured steam temperature at the outlet of the at least one superheater (3) and by means of a plurality of control steps (PID1-4) in series
Data Source
AI summary
In a method for controlling a waste heat recovery steam generator of the once-through steam generator type in a combined cycle power plant, the flow volume of the feedwater into the steam generator is controlled based on a measured steam temperature at the outlet of a superheater and on a set-point value for the steam temperature for a steam turbine. A degree of superheating at the outlet of a high-pressure evaporator, a degree of subcooling at the inlet into the high-pressure evaporator, and the measured current flow volume of the feedwater are integrated in the control system in a plurality of control steps. For an optimum operation during rapid load changes, the method especially comprises additional controlling of the degree of subcooling of the flow medium at the inlet into the high-pressure evaporator.