Once-Through Steam Generator Loop Seal Separator Thermal Fatigue
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Solution Overview
Problem
Once-through steam generators (OTSGs) face issues with thermal fatigue and reduced cycle life due to cold feedwater introduction during start-ups and potential water slugs during rapid load changes, leading to costly downtime and maintenance.
Innovation Solution
Incorporation of a loop seal separator and a start-up module within the OTSG design, allowing for wet and dry start-ups respectively, which enables water to remain in the tubes during shutdowns and controls temperature changes, minimizing thermal shock and stress on heat transfer tubes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If cold feedwater is introduced into hot heat transfer tubes during start-up, then steam generation can begin immediately, but large thermal fatigue stresses occur that reduce cycle life of the tubes
Solution Approach 1:
The loop seal separator is pre-filled with water before start-up. During start-up, this pre-existing water in the separator mixes with incoming cold feedwater, creating a thermal buffer that prevents direct contact between cold feedwater and hot tubes, thereby eliminating thermal shock while maintaining rapid steam generation capability
Solution Approach 2:
The loop seal separator acts as an intermediary component between the feedwater system and the heat transfer tubes. It contains a water seal that mediates the thermal interaction, allowing cold feedwater to be introduced without directly exposing the hot tubes to thermal shock, thus protecting tube integrity while enabling continuous operation
2Ease of manufacture
If water is drained from the tube bundle at every shutdown, then the system is prepared for next start-up, but costly boiler feedwater is lost and thermal fatigue occurs during subsequent start-up
Solution Approach 1:
The loop seal separator maintains a continuous water seal that persists through shutdowns. This allows water to remain in the tube bundle and separator without draining, enabling the system to maintain its thermal state and water inventory continuously, eliminating the need for draining and subsequent refilling operations
Solution Approach 2:
The loop seal separator is designed with sufficient water inventory beforehand to cushion against thermal shocks during any operational phase including shutdowns and start-ups. This pre-positioned water buffer protects the system from thermal fatigue without requiring drainage, preserving both water and tube integrity
3Reliability
If a steam drum is included in the HRSG design, then water circulation and steam separation are effective, but the drum walls are prone to fatigue failures from rapid temperature changes
Solution Approach 1:
The invention extracts the steam separation function from a large steam drum and relocates it to a compact loop seal separator. This removes the large thermal mass drum that is susceptible to fatigue, while preserving the essential steam-water separation capability in a smaller, more thermally resilient component
Solution Approach 2:
The steam generation system is segmented into distinct functional zones: the tube bundle for heat transfer, the loop seal separator for steam-water separation, and associated piping. This segmentation allows the separator to handle separation duties independently without requiring a large drum, reducing thermal fatigue risks while maintaining separation efficiency
4Ease of operation
If an OTSG design is used without a steam drum, then operation is simplified and start-up is faster, but water must be drained at every shutdown causing thermal fatigue during restart
Solution Approach 1:
The loop seal separator serves as an intermediary water reservoir that enables the OTSG to maintain water in the tube bundle during shutdowns. This simple addition allows the system to retain its operational simplicity while protecting against thermal fatigue by providing a water buffer that prevents direct thermal shock during restart
Solution Approach 2:
The invention changes the operational parameter of water inventory management by maintaining a permanent water seal in the loop seal separator. This parameter change allows water to remain in the system through shutdowns without causing harm, transforming the shutdown/start-up thermal profile from shock-prone to protected, thereby extending tube life while maintaining operational simplicity
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 extends the cycle life of heat transfer tubes by preventing thermal fatigue and reducing maintenance costs through controlled start-ups and operation during load changes, maintaining efficient steam generation without draining boiler feedwater at every shutdown.
Implementation Method 1
the loop seal separator is a water-filled device that utilizes a water seal to prevent gas flow between the evaporator section and the superheater section
Implementation Method 2
the loop seal separator separates the evaporator section from the superheater section, allowing for wet and dry start-ups
Implementation Method 3
the heat transfer tubes of the tube bundle are in a horizontal orientation, and the flue gas passes through the OTSG on an upward (vertical) path
Implementation Method 4
Feedwater is introduced into the tube bundle via feedwater delivery piping and then flows through the tube bundle in a direction opposite to that of the flue gas
Implementation Method 5
the evaporator section, where the water is converted into saturated steam
Implementation Method 6
the superheater section, where the saturated steam is converted to superheated steam that can be used to power a steam turbine generator
Data Source
AI summary
A once-through steam generator comprises a duct having an inlet end in communication with a source of a hot gas; and a tube bundle installed in the duct and comprising multiple heat transfer tubes. The tube bundle has an economizer section, an evaporator section, and a superheater section. A steam separating device may be positioned between the evaporator section and the superheater section, wherein, as part of a wet start-up, hot water collected by the steam separating device is delivered from the steam separating device to mix with cold feedwater before it is introduced into the economizer section. A start-up module may be positioned in the duct near the inlet end, wherein, as part of a dry start-up, cold feedwater is delivered into the start-up module to generate hot water that is then mixed into the feedwater stream before it is introduced into the economizer section.