Once-through evaporator systems
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Solution Overview
Problem
Once-through evaporators in combined cycle systems face challenges in maintaining a constant steam temperature during start-up and low load operations due to non-linear relationships between valve positions and temperatures, leading to oscillatory issues and inefficient feedwater control.
Innovation Solution
A method is introduced to adjust the feedwater mass flow rate by determining changes in operational parameters, predicting steam temperature changes, and using a feedforward signal to dynamically offset these changes, ensuring a predetermined steam temperature is maintained through a controller and distribution valve system.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If conventional filling procedures are used to ensure adequate feedwater volume, then evaporator temperature control is improved, but water waste increases and start-up time is delayed
Solution Approach 1:
The system performs preliminary actions by pre-calculating the required feedwater volume based on predicted operating conditions and gas turbine parameters before start-up. This allows the evaporator to be filled with the exact amount of water needed, avoiding both overfilling (which causes water waste) and underfilling (which causes temperature control issues). The feedforward control system prepares the optimal water quantity in advance based on anticipated load changes.
Solution Approach 2:
The system uses feedback from level detectors and temperature sensors to continuously monitor evaporator conditions and adjust feedwater flow accordingly. The level detectors provide real-time information about water volume, allowing the control system to prevent both water waste from overfilling and temperature control problems from underfilling. This closed-loop feedback ensures optimal water usage while maintaining temperature control.
2Stability of the object's composition
If conventional closed loop control is used with steam temperature feedback, then temperature stability is improved, but control response is delayed during transient events
Solution Approach 1:
The system performs preliminary calculations to predict steam temperature changes based on anticipated feedwater flow changes and gas turbine operating parameters. By calculating the required feedforward compensation in advance, the system can respond to transient events before actual temperature deviations occur, eliminating the delay inherent in conventional feedback-only systems that must wait for temperature changes to manifest.
Solution Approach 2:
The system dynamically adjusts the feedforward control signal based on changing operating conditions, including load changes and gas turbine parameters. This dynamic adaptation allows the control system to maintain optimal performance across varying conditions, providing both rapid response during transients and stable temperature control during steady-state operation, unlike static feedback systems.
3Temperature
If distribution valves are used to control temperature in evaporator sections, then temperature distribution is improved, but non-linear valve-temperature relationships cause control oscillations
Solution Approach 1:
The system uses feedback from temperature sensors in each evaporator section to continuously monitor actual temperature distribution. This feedback information is combined with feedforward predictions to adjust distribution valve positions, compensating for the non-linear valve-temperature relationships. The feedback mechanism detects and corrects oscillations by comparing actual temperatures with target values, stabilizing control despite the inherent non-linearity.
Solution Approach 2:
The system changes control parameters by using multiple operational parameters (gas flow rate, gas temperature, load changes) in addition to simple valve position commands. This multi-parameter approach allows the feedforward control to predict temperature changes more accurately and adjust valve positions accordingly, linearizing the effective control relationship and reducing oscillations caused by non-linear valve characteristics.
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 stabilizes steam temperature control, reduces water consumption, and enhances the robustness of the control system during transient events and low load operations, improving the overall efficiency and stability of the once-through evaporator system.
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
the incoming flow of feedwater is completely evaporated before reaching the superheater
Implementation Method 4
a superheater to turn the flow of saturated steam into superheated steam
Data Source
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. 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 based on dynamically offsetting the combined predicted changes in steam temperature, and changing the mass flow rate of feedwater based on the feedforward signal.