Turbomachine Casing Fastener Bushing for Thermal Expansion Compensation
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Solution Overview
Problem
The dissimilar thermal expansion coefficients between the inner casing and the fasteners in turbomachines lead to a reduction in pretension, causing leakage, especially when modern steam turbines operate at higher temperatures with inner casings having lower thermal expansion coefficients than the bolts used to secure them.
Innovation Solution
A bushing with a higher thermal expansion coefficient than the fasteners is embedded in the casing to compensate for the difference in expansion, ensuring the fastener remains securely seated and maintaining pretension by expanding and extending further into the casing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If bolts with higher thermal expansion coefficient are used to secure inner casings, then the bolts can withstand higher steam temperatures, but the pretension in the bolts reduces due to greater thermal expansion than the inner casing
Solution Approach 1:
A bushing is introduced as an intermediary component between the bolt and the inner casing. The bushing has a thermal expansion coefficient higher than both the bolt and the inner casing, allowing it to expand more during heating and thereby maintain the pretension force by compensating for the differential expansion between the bolt and casing
Solution Approach 2:
The thermal expansion coefficient parameter is strategically selected for the bushing material to be higher than both the bolt and inner casing materials. This parameter change enables the bushing to undergo greater thermal expansion, which compensates for the pretension loss in the bolt and maintains the clamping force on the joint
2Temperature
If inner casings with lower thermal expansion coefficients are used to withstand higher temperatures, then the inner casing can operate at higher steam temperatures, but the clearance between inner and outer casings becomes insufficient
Solution Approach 1:
The thermal expansion coefficient of the inner casing material is reduced to allow operation at higher steam temperatures. The bushing's higher expansion coefficient compensates for this by expanding more during heating, effectively maintaining the required clearance and preventing interference with the outer casing
3Ease of manufacture
If conventional bolts are used to secure inner casings at high temperatures, then the joint can be simple and easy to manufacture, but leakage occurs due to reduction in bolt pretension from thermal expansion differences
Solution Approach 1:
The bushing serves as a mediator component that is relatively easy to manufacture and install. It maintains the joint sealing by compensating for thermal expansion differences, preventing leakage while keeping the overall joint structure simple and manufacturable
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 bushing effectively maintains bolt pretension across temperature changes, reducing fluid leakage and extending the lifespan of turbomachines by preventing the need for frequent replacements.
Implementation Method 1
The bushing, due to its thermal expansion coefficient, expands as the inner casing is heated in a manner that compensates for the difference in expansion of the fastener and the casing
Data Source
AI summary
A casing including: a first casing section and a second casing section configured to be joined to the first casing section to form the casing; a first hole extending through a portion of the first casing section; a second hole extending through a portion of the second casing section, wherein the first and second holes are configured to be in alignment while the first and second casing sections are joined; a bushing seated in the second hole; a fastener having a shaft configured to extend through the first hole, extend into the second hole, and seat in and engage the bushing, wherein a thermal expansion coefficient of the bushing is greater than a thermal expansion coefficient of the shaft of the fastener, and the thermal expansion coefficient of the fastener is greater than a thermal expansion coefficients for each of the first and second casing sections.