Ni-based Superalloy Valve Stem and Bush for Steam Turbines
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Solution Overview
Problem
In steam turbines, the sticking between the valve stem and valve bush due to oxide film deposition leads to increased steam leakage and reduced thermal efficiency, and existing solutions like building-up welding with Ni alloys are prone to corrosion from potential differences under high-temperature and vibration loads.
Innovation Solution
The valve stem and valve bush are made entirely of Ni-based superalloys, with the valve stem forged for mechanical strength and the valve bush cast for vibration absorption, ensuring uniform material properties and reducing corrosion risks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If building-up welding with Ni alloy is applied to prevent oxide film generation, then sticking is prevented, but corrosion due to potential difference occurs under high-temperature and vibration loads
Solution Approach 1:
The invention applies homogeneity by making both the valve stem and valve bush from the same Ni-based superalloy material, eliminating the potential difference between dissimilar metals. This prevents galvanic corrosion while maintaining the anti-sticking benefits, as the uniform material composition eliminates the electrochemical cell formation that occurs with building-up welding of different materials.
Solution Approach 2:
The invention extracts the Ni alloy coating from the solution by eliminating building-up welding entirely. Instead of applying Ni alloy only to necessary areas, the entire valve stem and valve bush are made from Ni-based superalloy, removing the interface between dissimilar materials that causes corrosion while preserving the material's inherent resistance to oxide film formation and sticking.
2Reliability
If clearance between valve stem and valve bush is increased to prevent sticking, then sticking is reduced, but steam leakage increases causing decreased thermal efficiency
Solution Approach 1:
The invention changes the material parameters of the valve stem and valve bush by using Ni-based superalloy with specific properties (linear expansion coefficient, hardness, surface finish). This material parameter change allows for reduced clearance while preventing sticking through the material's inherent resistance to oxide film formation, thereby eliminating steam leakage without compromising reliability.
3Adaptability or versatility
If different materials are used for valve stem and valve bush, then functional requirements are met, but corrosion occurs due to potential difference in high-temperature corrosive environment
Solution Approach 1:
The invention resolves the contradiction by using homogeneous material composition (Ni-based superalloy) for both valve stem and valve bush, eliminating galvanic corrosion while maintaining all necessary functional requirements through the superior properties of the Ni-based superalloy material itself.
Solution Approach 2:
The invention uses Ni-based superalloy, which is a composite material containing multiple elements (Ni, Cr, Mo, Nb, Ti, Al, Fe) that work together to provide simultaneous resistance to oxidation, corrosion, and sticking, while maintaining compatibility between contacting surfaces.
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 solution prevents sticking and corrosion, maintaining efficient steam flow and reducing maintenance needs by using Ni-based superalloys with optimized surface roughness and material properties.
Implementation Method 1
the main body of a valve stem and a bush be formed from materials having almost the same linear expansion coefficient, that a Ni alloy be building-up welded on the outer surface of a sliding part of the valve stem
Implementation Method 2
Under high temperature, the surface of a metal is in an activated state to react with the high-temperature steam present in the atmosphere to produce an oxide film
Data Source
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AI summary
Provided is a valve gear in which corrosion due to a potential difference does not occur and which can reduce the corrosion occurrence even when loads due to microvibrations are given. The valve gear according to the present invention comprises a valve stem 6 and a valve bush 7 that slidably supports the valve stem 6. The valve stem 6 is formed from a forged part made of a Ni-based superalloy and the valve bush 7 is formed from a cast part made of a Ni-based superalloy. The preferable surface roughness of a sliding surface of the valve stem 6 in contact with the valve bush 7 and the preferable surface roughness of a sliding surface of the valve bush 7 in contact with the valve stem 6 is 100 µm or less in units of Rz.