Once-through evaporator systems
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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 position and temperature, especially at low loads, which affect steam production and overall system stability.
Innovation Solution
The implementation of a combined closed loop and open loop control system that includes dynamic feedforward signals, PID controllers, and a start-up filling system with level indicators to optimize feedwater distribution, temperature control, and steam generation, using sensors and processors to predict feedwater flow demands and adjust valve positions dynamically.
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 about half of the initial fill is ejected to waste and the procedure cools down the heat recovery steam generator and 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 when sufficient feedwater has been added, preventing both under-filling and over-filling waste. This preliminary action ensures the evaporator is properly prepared for operation without requiring complete filling that would later be wasted.
Solution Approach 2:
Level indicators provide real-time feedback on the feedwater level in the evaporator during start-up. This feedback mechanism allows the control system to stop filling when the appropriate level is reached, preventing the waste of feedwater that would occur with conventional complete filling methods. The feedback ensures reliable operation while eliminating unnecessary water loss.
2Temperature
If conventional closed loop control is used, then temperature control is achieved, but the non-linear relationship between valve position and temperature at low loads causes oscillatory issues
Solution Approach 1:
The control system dynamically adjusts the feedwater flow control strategy based on operating conditions. At low loads, where non-linear relationships cause oscillations, the system modifies control parameters and uses feedforward signals to anticipate temperature changes. This dynamic adaptation maintains stable temperature control across the full operating range, eliminating the oscillatory issues that plague conventional fixed-parameter control systems.
Solution Approach 2:
Feedforward signals act as an intermediary mechanism that anticipates temperature changes before they occur. By calculating required feedwater flow adjustments based on predicted heat input changes, the system smooths out the non-linear response at low loads and prevents oscillations. This intermediary control layer works in conjunction with closed-loop feedback to achieve stable temperature control.
3Device complexity
If steam temperature alone is used as feedback, then temperature control is simplified, but it is not a reliable or timely indicator of required feedwater mass flow during transient events
Solution Approach 1:
The system introduces feedforward signals as an intermediary that complements steam temperature feedback. These signals calculate the anticipated feedwater flow requirements based on heat input changes and system dynamics, providing timely guidance during transient events before temperature changes fully manifest. This dual approach maintains control simplicity while significantly improving the accuracy of feedwater flow indication during transients.
Solution Approach 2:
The control system performs preliminary calculations of required feedwater flow based on predicted heat input changes and system state. This preliminary action provides advance guidance on feedwater flow requirements during transient events, allowing the system to respond more accurately and timely than relying solely on steam temperature feedback, which lags during transients.
4Temperature
If multiple distribution valves are used to control temperature in each evaporator section, then temperature distribution is improved, but the non-linear relationship between valve position and temperature creates control challenges
Solution Approach 1:
The control system dynamically adjusts distribution valve positions based on real-time temperature measurements and feedforward signals. By continuously adapting valve positions rather than relying on fixed linear relationships, the system maintains optimal temperature distribution across all evaporator sections despite the inherent non-linearity of valve-temperature relationships. This dynamic control approach simplifies operation while achieving precise temperature management.
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 approach reduces water consumption, enhances control stability, minimizes oscillations, and improves steam temperature consistency, leading to quicker start-up times and increased component lifetime by providing precise feedwater management and temperature regulation.
Implementation Method 1
Each once-through evaporator section may have a distribution valve upstream thereof. The system may also include a number of level indicators in communication with each once-through evaporator section.
Implementation Method 2
A processor may calculate a dynamic feedforward signal based on a change in a process parameter... The system may include a heat recovery steam generator that may include an economizer to heat a flow of feedwater, an evaporator to turn the flow of feedwater into saturated steam
Implementation Method 3
an evaporator to turn the flow of feedwater into saturated steam
Implementation Method 4
an evaporator to turn the flow of feedwater into saturated steam
Data Source
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AI summary
The present application provides a method of adjusting a feedwater mass flow rate to maintain a constant steam temperature in an evaporator section (130). The method may include the steps of determining a change in a number of operational parameters, predicting a change in steam temperature based on the number of operational parameters, combining the predicted changes in steam temperature, determining a feedforward signal (320) based on dynamically offsetting the combined predicted changes in steam temperature, and changing the mass flow rate of feedwater based on the feedforward signal.