Once-Through Evaporator Start-Up Control for Stable Steam Temperature
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Solution Overview
Problem
Once-through evaporators in combined cycle systems face challenges during start-up due to non-linear relationships between valve position and temperature, leading to oscillatory issues and unreliable steam temperature feedback, particularly at low loads, resulting in inefficient steam production and increased water consumption.
Innovation Solution
The implementation of a once-through evaporator system with a start-up filling system using level indicators and dynamic feedforward signals, along with PID controllers and a controlled integration sequence, to regulate feedwater flow and temperature, ensuring stable steam production without initial filling and reducing water waste.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional filling procedures are used to ensure adequate feedwater volume, then evaporator temperature limits are maintained, but water is wasted and start-up time is delayed
Solution Approach 1:
The system performs preliminary actions by pre-calculating the required feedwater volume based on evaporator geometry and operational parameters before start-up. Level indicators are positioned to detect when adequate water volume is present, eliminating the need for excessive preliminary filling. This allows the system to initiate operation with precisely the required amount of water rather than overfilling conservatively.
Solution Approach 2:
The invention implements feedback through level indicators that continuously monitor feedwater volume in the evaporator. This feedback mechanism allows the control system to adjust feedwater supply in real-time, maintaining adequate water levels without requiring excessive initial filling. The feedback loop prevents both water waste and temperature control failures by dynamically matching water supply to actual evaporator needs.
2Stability of the object's composition
If complete evaporator filling is performed before start-up, then smooth transition to steam production is ensured, but start-up time and pressure buildup are delayed
Solution Approach 1:
The system transitions from static pre-filling to dynamic water level management. During start-up, the feedwater pump operates dynamically to maintain water levels based on real-time evaporator conditions rather than relying on static pre-filling. This dynamic approach allows the evaporator to reach operational conditions faster while maintaining stability, as water is supplied at the precise rate needed rather than being pre-loaded in excess.
Solution Approach 2:
The invention replaces the mechanical pre-filling procedure with an automated control system using level indicators and feedback control. Instead of mechanically filling the evaporator to a fixed level before start-up, the system uses sensors and control logic to manage water levels dynamically. This substitution enables faster start-up while maintaining the stability benefits of adequate water volume.
3Temperature
If conventional closed loop control is used, then temperature regulation is attempted, but oscillatory issues occur due to insufficient feedback and non-linear valve-temperature relationships
Solution Approach 1:
The invention introduces feedwater flow rate as an intermediary control variable between the control system and steam temperature. Rather than directly controlling temperature through non-linear valve positions, the system controls the linear feedwater flow rate, which then indirectly controls temperature. This intermediary approach linearizes the control relationship and eliminates oscillations by providing a more direct and predictable control path.
Solution Approach 2:
The system changes the controlled parameter from direct temperature control to feedwater flow rate control. By controlling the flow rate parameter instead of temperature directly, the system achieves more stable and linear control characteristics. The feedwater flow rate serves as a more reliable control variable that has a predictable, linear relationship with valve position, eliminating the non-linear oscillatory behavior observed in conventional temperature-based control.
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 enhances start-up consistency, reduces cycling of dry-wet conditions, stabilizes steam temperatures, and decreases overall start-up time, while minimizing water consumption and maintaining component longevity.
Implementation Method 1
the first and second level indicators determine that each of the evaporator sections has a sufficient amount of water
Implementation Method 2
an evaporator to turn the flow of feedwater into saturated steam
Implementation Method 3
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
Data Source
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AI summary
The present application provides a once-through evaporator system (100). The once-through evaporator system (100) may include a number of once-through evaporator sections (130) with a distribution valve (170) and a level sensor (340) and a controller (290) in communication with each distribution valve (170). The controller (290) provides the distribution valve (170) 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 (340) in each of the plurality of once-through evaporator sections (130).