Boiler Header Thermal Stress Reduction During Shutdown
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Solution Overview
Problem
During boiler shutdown, the rapid cooling of tubed heat exchanging surfaces in superheaters and reheaters leads to thermal stress when the boiler is restarted, due to the temperature difference between the steam and the headers, potentially reducing the lifespan of these components.
Innovation Solution
A method to regulate the temperature of the headers by maintaining a controlled steam flow and pressure within the boiler during shutdown, using the boiler's pressure storage capacity and heat content, to align the headers' temperature with the expected steam temperature at startup, thereby minimizing thermal stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If the boiler is shut down and the tubed heat exchanging surfaces cool rapidly, then the shutdown process is completed quickly, but thermal stress increases on the headers when restarted
Solution Approach 1:
The patent applies preliminary action by maintaining steam flow through the headers during shutdown to pre-cool them gradually. This preliminary cooling action ensures that when the boiler restarts, the temperature difference between the headers and incoming steam is minimized, preventing thermal stress while still allowing relatively quick shutdown completion
Solution Approach 2:
The patent changes the temperature parameter of the headers during shutdown by controlling steam flow through them. By adjusting the steam flow rate and maintaining pressure, the headers are cooled from their high operating temperature to a lower temperature that matches the incoming steam temperature at restart, thereby eliminating thermal stress conditions
2Stress or pressure
If steam flow is maintained through headers during shutdown to control temperature, then thermal stress is reduced, but energy consumption increases
Solution Approach 1:
The system uses its own steam, which would otherwise be wasted or vented during shutdown, to cool the headers. The steam that is already present in the boiler and steam lines is redirected through the headers during shutdown, allowing the system to self-regulate the header temperature without requiring external energy input or additional cooling systems
Solution Approach 2:
The patent converts the potentially harmful effect of steam being vented or wasted during shutdown into a beneficial cooling action for the headers. By redirecting this steam flow through the headers, the energy that would have been lost is instead used to prevent thermal stress, turning a waste stream into a useful resource
3Speed
If the headers are cooled rapidly during shutdown, then the shutdown process is faster, but the temperature difference between headers and steam at startup increases, causing thermal stress
Solution Approach 1:
The patent implements feedback control by monitoring the temperature of the headers and the steam, and adjusting the steam flow rate through the headers accordingly. This closed-loop control ensures that the headers are cooled at an optimal rate that balances shutdown speed with preventing excessive temperature differences at restart, thereby avoiding thermal stress while maintaining efficiency
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 controlled cooling method reduces thermal stress on the headers, extending their lifespan by maintaining a consistent temperature gradient during shutdown and startup, without the need for additional fuel consumption.
Implementation Method 1
the steam contained within the tubed heat exchanging surfaces 16 of the superheater 13 and reheater 14 sensibly drops
Implementation Method 2
the headers 17, 18 have a large wall thickness and therefore they also have a large thermal storage capacity
Implementation Method 3
the temperature difference between the steam and the headers, potentially reducing the lifespan of these components
Data Source
Figure 1
AI summary
The method for managing a shut down of a boiler (1) having a duct (12), heat exchanging components, the heat exchanging component having tubed heat exchanging surfaces (16) within the duct (12) and headers (17, 18) outside the duct (12). The method comprises regulating the temperature of the headers (17, 18) during shut down to a temperature close to the one expected for the steam moving from the tubed heat exchanging surfaces (16) into the headers (17, 18) at a starting up following the shut down.