Once-Through Steam Generator Enthalpy Control via Density Derivative
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Solution Overview
Problem
Existing predictive mass flow control methods for once-through steam generators, especially in waste heat boilers, face challenges in maintaining stable enthalpy during load changes due to inaccuracies in measuring feedwater mass flow and fluctuations in fluid density, leading to undesired temperature fluctuations and reduced service life.
Innovation Solution
Incorporating a correction value based on the time derivative of the density of the flow medium at the inlet and outlet of heating surfaces, using a derivative element to account for storage and withdrawal effects, and adjusting the feedwater mass flow to maintain stable enthalpy, particularly by considering the heat flow balance and enthalpy changes across the evaporator and superheater surfaces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If feedwater mass flow is controlled using predictive mass flow control based on heat flow balance, then the feedwater quantity can be pre-calculated to maintain stable enthalpy, but measurement inaccuracies of feedwater mass flow and fluctuations in fluid density lead to deviations from setpoint enthalpy
Solution Approach 1:
The patent implements predictive mass flow control by pre-calculating the required feedwater quantity based on heat flow balance before actual evaporation occurs. The control system uses the heat input into the evaporator heating surface and target enthalpy increase to determine the feedwater setpoint in advance, allowing the system to anticipate and prepare for enthalpy changes rather than reacting to them after measurement errors occur.
Solution Approach 2:
The patent incorporates feedback mechanisms by continuously monitoring actual enthalpy at the evaporator outlet and comparing it with the setpoint. When deviations are detected due to measurement inaccuracies or density fluctuations, the control system adjusts the feedwater mass flow to correct the enthalpy deviation, creating a closed-loop control system that compensates for measurement errors.
2Reliability
If feedwater mass flow is changed synchronously with heat input during load changes, then enthalpy deviation can be minimized, but rapid response requirements increase control system complexity
Solution Approach 1:
The control system performs preliminary calculation of feedwater requirements based on anticipated heat input changes. By using the heat flow currently being transferred from hot gas to the flow medium and the target enthalpy increase, the system determines the feedwater setpoint in advance, enabling synchronous adjustment without requiring overly complex real-time control mechanisms.
Solution Approach 2:
The patent adjusts the feedwater mass flow parameter in response to changes in heat input parameters. The control system monitors heat flow and enthalpy parameters, and when load changes are detected, it modifies the feedwater flow rate to maintain the desired enthalpy relationship, using parameter interdependence to simplify control logic.
3Measurement precision
If feedwater density at inlet of preheater is used for predictive control, then some inaccuracies are counteracted, but storage and withdrawal effects in heating surfaces are not fully compensated
Solution Approach 1:
The patent uses feedback from actual enthalpy measurements at the evaporator outlet to detect deviations caused by storage and withdrawal effects. When the actual enthalpy deviates from the setpoint, the control system adjusts the feedwater mass flow to compensate for these effects, ensuring enthalpy stability even during transient states when density-based predictions alone would be insufficient.
Solution Approach 2:
The control system dynamically adjusts the feedwater mass flow parameter based on actual enthalpy measurements and heat input conditions. By monitoring enthalpy deviations and modifying the feedwater flow in response, the system compensates for storage and withdrawal effects that occur during load changes, maintaining reliable enthalpy control beyond what density-based predictions alone can achieve.
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 the predictive mass flow control quality, ensuring a needs-based and stable feedwater mass flow, effectively compensating for storage and withdrawal effects, thereby maintaining consistent enthalpy at the evaporator outlet and reducing material stresses.
Implementation Method 1
the heating of a number of steam generator tubes, which together form an evaporator heating surface, leads to a complete evaporation of a flow medium
Implementation Method 2
complete evaporation of a flow medium in the steam generator tubes in one pass
Data Source
AI summary
The invention relates to a method for operating a continuous flow steam generator comprising an evaporator heating surface (4), whereby a target value (Ms) for a supply water mass flow (M) is fed to a device for adjusting the supply water mass flow (M). In order to improve the quality of a predictive supply water or mass flow control and to maintain the enthalpy of the flow medium at the evaporator outlet particularly stable especially when load changes occur, a correction factor (K) is taken into consideration during production of the target value (Ms) for the supply water mass flow (M), said correction factor being characteristic of the temporal derivative of enthalpy or the density of the flow medium at the input of one or more heating surfaces (2, 4).