Once-through evaporator feedwater control
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Solution Overview
Problem
Once-through evaporators in combined cycle systems face challenges during start-up, including inefficient water management, temperature control issues, and oscillatory problems due to non-linear relationships between valve positions and temperatures, especially at low loads, which affect steam production and overall system stability.
Innovation Solution
A once-through evaporator system with upstream distribution valves and downstream temperature sensors, utilizing position controllers, temperature controllers, and a deadband flow control system to manage feedwater flow and temperature, ensuring efficient steam production and stability by predicting static head and pressure drops, and using dynamic feedforward signals to adjust feedwater flow based on heat balance considerations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the evaporator is completely filled prior to start-up to ensure adequate feedwater volume, then the evaporator temperature control is improved, but water is wasted and steam production is delayed
Solution Approach 1:
The system performs preliminary heating of the evaporator using combustion gases before introducing feedwater. This pre-heating action ensures the evaporator is ready for immediate steam production upon start-up, eliminating the need to fill it completely with cold water beforehand, thus avoiding water waste while maintaining temperature control
Solution Approach 2:
The invention implements dynamic control of feedwater flow rate based on real-time evaporator temperature and heat input conditions. Instead of static filling, the system continuously adjusts feedwater flow to match thermal demands, enabling adequate temperature control without excessive water filling that would be wasted during start-up
2Ease of operation
If conventional closed loop control is used with main feedwater control valve, then the system is simple to operate, but oscillatory issues occur due to non-linear relationship between valve position and temperature
Solution Approach 1:
The system employs multiple temperature sensors positioned at different locations within the evaporator to provide comprehensive feedback about thermal conditions. This multi-point feedback enables the control system to detect and correct temperature variations and oscillations more effectively than conventional single-point feedback, maintaining stability while preserving operational simplicity
Solution Approach 2:
The invention changes the control parameter from valve position to feedwater flow rate, and from single temperature measurement to multiple temperature measurements. This parameter transformation linearizes the control relationship and provides more accurate temperature stability control while keeping the system easy to operate through automated control
3Device complexity
If steam temperature alone is used as control indicator, then the control system is simple, but reliable and timely control of feedwater mass flow is not achieved during transient events
Solution Approach 1:
The system uses multiple temperature sensors to provide advance indication of thermal conditions before they manifest as steam temperature changes. This preliminary thermal feedback allows the control system to anticipate and prepare for transient events, adjusting feedwater flow proactively rather than reactively, thereby improving reliability without significantly increasing complexity
Solution Approach 2:
The invention adds spatial dimension to temperature measurement by placing sensors at multiple locations within the evaporator. This multi-dimensional temperature monitoring provides more comprehensive information about thermal states and transient developments, enabling more reliable control decisions while maintaining relatively simple control logic
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 solution reduces water consumption, enhances control stability, minimizes oscillations, and improves steam temperature consistency, leading to faster start-up times and increased component lifetime by optimizing feedwater distribution and temperature control.
Implementation Method 1
The heat recovery steam generator may extract heat from the hot combustion gases from the gas turbine to produce steam to drive the steam turbine
Implementation Method 2
an evaporator to turn the flow of feedwater into saturated steam
Implementation Method 3
a superheater to turn the flow of saturated steam into superheated steam
Data Source
AI summary
The present application provides a once-through evaporator system. The once-through evaporator system may include a number of once-through evaporator sections having an upstream distribution valve and a downstream temperature sensor and a position controller in communication with each distribution valve.