Once-Through Evaporator Feedwater Control for Stable Start-Up
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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 valve relationships and inadequate feedback, leading to delayed steam production and reduced efficiency.
Innovation Solution
A once-through evaporator system with distribution valves and level sensors, controlled by a controller that adjusts valve positions using dynamic feedforward signals based on fill levels and temperature feedback, ensuring optimal feedwater flow and temperature control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the evaporator is filled completely prior to start-up to ensure adequate feedwater volume, then the evaporator temperature control is improved, but a large amount of feedwater is wasted and steam production is delayed
Solution Approach 1:
The system performs preliminary feeding of feedwater to evaporator sections before full start-up, but only to the extent needed to prevent overheating, rather than complete filling. This preliminary action ensures temperature control is maintained while avoiding the waste associated with traditional complete filling methods.
Solution Approach 2:
The control system applies different feedwater feeding strategies to different evaporator sections based on their specific conditions. The level sensor in each section enables localized control, feeding water only where and when needed, rather than uniformly filling all sections.
2Temperature
If conventional closed loop control is used with main feedwater control valve and distribution valves, then temperature control is attempted, but oscillatory issues occur due to non-linear valve relationships and insufficient feedback
Solution Approach 1:
The system employs level sensors in each evaporator section that provide direct feedback on feedwater levels. This feedback is used by the controller to adjust distribution valve positions, creating a reliable closed-loop control system that prevents oscillations by responding to actual level conditions rather than relying on non-linear valve position relationships.
Solution Approach 2:
The control system dynamically adjusts distribution valve positions based on real-time level sensor feedback and controller calculations. This dynamic adjustment capability allows the system to adapt to changing conditions and maintain stability, overcoming the limitations of static or non-linear valve relationships.
3Device complexity
If steam temperature alone is used as the control parameter, then simple control is achieved, but it is not a reliable or timely indicator of required feedwater mass flow during transient events
Solution Approach 1:
The system introduces level sensors as intermediary measurement devices that directly measure feedwater levels in each evaporator section. These level measurements serve as more reliable indicators of feedwater mass flow requirements than steam temperature alone, especially during transient events, while still maintaining a manageable control system through the use of a controller that processes these level signals.
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 enables efficient start-up by minimizing water waste, stabilizing steam production, reducing oscillations, and improving control stability, leading to quicker steam temperature stabilization and reduced start-up time.
Implementation Method 1
a level sensor in each of the once-through evaporator sections
Implementation Method 2
The controller provides the distribution valve with a position set point and biases the position set point via a feedforward signal based on a fill level
Implementation Method 3
an evaporator to turn the flow of feedwater into saturated steam
Implementation Method 4
The heat recovery steam generator may extract heat from the hot combustion gases from the gas turbine to produce steam
Implementation Method 5
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 6
controlling the feedwater flow rate via the dynamic feedforward signal and temperature feedback control based on once-through evaporator section exit temperatures
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 with a distribution valve and a level sensor and a controller in communication with each distribution valve. The controller provides the distribution valve with a position set point and biases the position set point via a feedforward signal based on a fill level as determined by the level sensor in each of the once-through evaporator sections.