Once-Through Evaporator Control for Steam Temperature Stability
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Solution Overview
Problem
Once-through evaporators in combined cycle systems face challenges during start-up, including inefficient water management, cooling of the heat recovery steam generator, reduced pressure, and delayed steam production, due to conventional filling procedures and non-linear valve temperature relationships, which lead to oscillatory issues and unreliable steam temperature feedback.
Innovation Solution
A once-through evaporator system with integrated sensors, processors, and controllers using open loop feedforward and dynamic feedforward signals to control feedwater flow and temperature, along with a start-up filling system that adjusts water distribution based on level indicators to prevent overheating and ensure stable steam production, and a controlled integration sequence that predicts feedwater demands and stabilizes steam temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the evaporator is filled completely prior to start-up, then adequate feedwater volume is ensured for smooth transition, but water is ejected to waste and evaporator cooling delays steam production
Solution Approach 1:
The system performs preliminary filling of the evaporator with feedwater before start-up to ensure adequate volume for smooth transition. The level indicators detect the filling status and trigger control actions to maintain proper water levels during operation, preventing both waste from overfilling and deficiency from underfilling.
Solution Approach 2:
Level indicators provide continuous feedback on feedwater levels in the evaporator sections. This feedback enables the control system to adjust feedwater flow rates dynamically, ensuring adequate volume for temperature control while preventing excessive filling that would lead to water ejection and waste.
2Measurement precision
If conventional closed loop controls are used, then steam temperature is monitored, but non-linear valve temperature relationship causes oscillatory issues and delayed response
Solution Approach 1:
The control system uses feedforward signals based on predicted feedwater demands to adjust valve positions in advance, rather than relying solely on reactive closed-loop control. This preliminary action compensates for the non-linear relationship between valve position and temperature, preventing oscillations before they occur.
Solution Approach 2:
The system dynamically adjusts control parameters based on operating conditions. The controller modifies feedwater flow rates and valve positions in real-time based on predicted demands and actual temperature measurements, adapting to the non-linear characteristics of the evaporator system across different load conditions.
3Measurement precision
If steam temperature alone is used as feedback, then temperature control is attempted, but it is not a reliable or timely indicator during transient events
Solution Approach 1:
The system uses feedforward control based on predicted feedwater demands to anticipate temperature changes before they occur. By calculating required feedwater flow rates in advance based on heat input and operational parameters, the system responds proactively to transient events rather than reacting after temperature deviations occur.
Solution Approach 2:
The control system introduces intermediate parameters such as predicted feedwater demand and heat input rates as mediators between the steam temperature measurement and the control action. These intermediate variables provide earlier warning of temperature trends and enable more timely adjustments to maintain temperature stability during transients.
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 quicker start-up times and reduced dry-wet cycling, while maintaining component longevity and efficient steam generation.
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
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AI summary
The present application provides a once-through evaporator system (800). The once-through evaporator system (800) may include a number of enlarged once-through evaporator sections (810), a first superheater (870) positioned immediately downstream of the enlarged once-through evaporator sections (810), a second superheater (880) positioned downstream of the first superheater (870), and an attemperator (890) positioned between the first superheater (870) and the second superheater (880).