HRSG Attemperation Control with Feedforward and Feedback
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Solution Overview
Problem
Conventional temperature control systems for steam in heat recovery steam generation (HRSG) systems face challenges in preventing temperature overshoots and ensuring stable operation, leading to potential damage and reduced lifespan of equipment due to inadequate control during transient periods and thermal lag.
Innovation Solution
A control system incorporating a feedforward controller and a trimming feedback controller to determine the desired flow of attemperation fluid, compensating for inaccuracies, and manipulating a control valve to perform attemperation upstream of the superheater, thereby stabilizing outlet steam temperatures and preventing subsaturated fluid flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional non-model-based control techniques are used with outer and inner loop PID controllers, then the control structure can establish set point temperatures, but temperature overshoots occur during transient changes in gas turbine output
Solution Approach 1:
The control system performs preliminary action by predicting the future steam temperature based on current process conditions and thermal lag characteristics. The predictor calculates what the temperature will be after the thermal lag period, allowing the controller to take preemptive control actions before the overshoot actually occurs, rather than reacting after the temperature has already exceeded the setpoint.
Solution Approach 2:
The system uses feedback by continuously monitoring actual steam temperature and comparing it with the predicted temperature. The error signal between predicted and actual temperature, along with the setpoint temperature, feeds back to the controller to adjust the attemperator valve position, enabling dynamic correction of temperature deviations and prevention of overshoots.
2Quantity of substance
If the mass of metal in the finishing high-pressure superheater is increased, then the thermal energy storage capacity increases, but thermal lag increases causing temperature overshoots
Solution Approach 1:
The predictor component performs preliminary calculation of the thermal lag effect by analyzing the relationship between superheater metal temperature and steam temperature. It computes the time delay and magnitude of temperature response, allowing the control system to anticipate and compensate for the thermal lag caused by the large metal mass, thereby preventing overshoots while maintaining the beneficial thermal energy storage.
3Temperature
If attemperation control is applied to reduce high outlet steam temperatures, then temperature control is achieved, but the temperature may go too low causing subsaturated attempertor fluid flow
Solution Approach 1:
The control system uses feedback by monitoring the actual steam temperature and comparing it with both the setpoint and predicted temperature. The controller adjusts the attemperator valve to maintain temperature within the safe operating range, preventing both overshoots above setpoint and excessive cooling that would cause subsaturated flow conditions. The feedback loop continuously corrects deviations while considering the thermal lag dynamics.
Solution Approach 2:
The system dynamically changes the attemperator valve opening parameter based on real-time temperature conditions and predicted thermal lag effects. By adjusting this key parameter, the controller modulates the amount of attemperation fluid injected, thereby controlling the steam temperature to remain within the optimal range and avoid subsaturated flow conditions that would compromise system reliability.
4Ease of operation
If conventional control systems are used, then basic temperature monitoring is provided, but a great deal of tuning is required to verify satisfactory operation during all potential transients
Solution Approach 1:
The predictor-based control system changes its operational parameters dynamically based on real-time process conditions. The predictor adapts to different transient scenarios by calculating the appropriate thermal lag compensation for each situation, eliminating the need for extensive manual tuning. The controller automatically adjusts its control actions according to the predicted temperature response, making the system easy to operate across all potential transients without requiring lengthy verification cycles.
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
The solution effectively prevents temperature overshoots and ensures stable operation by dynamically adjusting attemperation fluid flow, reducing thermal lag and simplifying tuning processes, thus enhancing the longevity and efficiency of HRSG systems.
Implementation Method 1
an inter-stage attemperator for injecting an attemperation fluid into the steam path
Implementation Method 2
a control valve coupled to the inter-stage attemperator. The control valve is configured to control flow of attemperation fluid
Implementation Method 3
HRSG systems may produce steam with very high outlet temperatures
Data Source
AI summary
A heat recovery steam generation system is provided. The heat recovery steam generation system includes at least one superheater in a steam path for receiving a steam flow and configured to produce a superheated steam flow. The system also includes an inter-stage attemperator for injecting an attemperation fluid into the steam path. The system further includes a control valve coupled to the inter-stage attemperator. The control valve is configured to control flow of attemperation fluid to the inter stage attemperator. The system also includes a controller coupled to the control valve and the inter-stage attemperator. The controller further includes a feedforward controller and a trimming feedback controller. The feedforward controller is configured to determine a desired amount of flow of the attemperation fluid and the trimming feedback controller is configured to compensate for inaccuracies in the determined amount of flow of the attemperation fluid to determine a net desired amount of flow of attemperation fluid through the control valve into an inlet of the inter-stage attemperator based upon an outlet temperature of steam from the superheater. The controller also determines a control valve demand based upon the flow to valve characteristics. The controller further manipulates the control valve of the inter-stage attemperator, and injects the desired amount of attemperation flow via the inter-stage attemperator to perform attemperation upstream of an inlet into the superheater.