Mechanical Seal Face Stability via Two-Piece Shell
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Solution Overview
Problem
Mechanical end face seal assemblies face challenges in high-temperature, corrosive environments due to material compatibility issues and structural integrity problems, leading to leakage and reduced seal life, especially when handling unstable and toxic liquids.
Innovation Solution
A two-piece primary ring shell design made from Alloy 718, with optimized geometrical shapes and interference fits, providing enhanced face stability and structural integrity, and capable of handling high temperatures and pressures with reduced leakage and increased seal life.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a single-piece primary ring shell is used with interference fit, then manufacturing is simplified, but bending stresses concentrate at the hinge area causing structural integrity problems
Solution Approach 1:
The primary ring shell is divided into two separate pieces: a front-piece shell and a back-piece shell. This segmentation eliminates the hinge area that concentrates bending stresses in single-piece designs, while still allowing for simplified manufacturing of each individual piece. The two pieces work together to provide both ease of manufacture and structural integrity.
2Stability of the object's composition
If high interference fit is applied between shell and primary ring, then face stability is improved, but thermal expansion differences cause stress concentration and potential failure
Solution Approach 1:
The front-piece shell is designed with a specific geometric configuration where the back surface of the engaging foot portion is substantially planar and extends axially to form an interference-fit engagement surface. This local geometric optimization distributes the interference fit stresses more evenly across the engagement surface, reducing stress concentration while maintaining face stability. The planar engagement surface ensures uniform contact between the shell and primary ring.
3Stability of the object's composition
If Alloy 42 is used for the shell, then thermal expansion matches the primary ring materials, but the shell becomes susceptible to corrosive attack from high temperature organic acids and sulfur compounds
Solution Approach 1:
The shell is constructed from Alloy 718, which provides both the required mechanical strength and corrosion resistance for high-temperature corrosive environments. While Alloy 718 has different thermal expansion characteristics than Alloy 42, the optimized geometric design of the engaging foot portion compensates for thermal expansion differences by distributing stresses evenly, maintaining face stability across temperature ranges.
4Object-affected harmful factors
If chrome plating is applied to Alloy 42 shell, then corrosion protection is extended, but the plating is not effective against high temperature corrosive compounds and only prolongs inevitable corrosion
Solution Approach 1:
The shell material is changed from Alloy 42 to Alloy 718, fundamentally altering the material parameters to achieve both high-temperature strength and superior corrosion resistance. Alloy 718 contains higher amounts of nickel, chromium, and molybdenum, providing inherent resistance to corrosive attack from organic acids and sulfur compounds without requiring protective plating.
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 design achieves thermally insensitive face stability, reduced leakage, and longer seal life by distributing contact pressure evenly, minimizing bending stresses, and accommodating different thermal expansion coefficients, making it suitable for high-temperature corrosive applications.
Implementation Method 1
The amount of interference for a given seal size depends on the nominal interference diameter, the differential thermal expansion coefficients of the shell and primary ring materials of construction, and the maximum operating temperature.
Implementation Method 2
primary ring 514 is radially and axially fixed to primary ring shell 512. In press fitting, mating parts, on which the outer dimension of the interior member is the same as or slightly greater than the interior dimension of the exterior member, are forced together.
Implementation Method 3
The seal assembly can include either a single seal or a double seal where a buffer fluid pressure is supplied at a pressure higher than the process fluid to be sealed
Data Source
Figure 1~2
Figure 3~4
Figure 5~5A
AI summary
A seal assembly for use with a rotating shaft. The seal assembly comprises a seal ring, a seal ring shell and bellows. The seal ring defines an axially extending annular surface and a radially extending seal face. The seal ring shell has a front piece and a back piece secured together. The front piece includes a foot portion defining an axially extending engagement surface for interference-fit engagement with the annular surface of the seal ring. The front piece further includes a radially extending shin portion connected to the foot portion and located radially outward of the foot portion. The foot portion includes an inner foot portion extending axially from connection of the foot portion with the shin portion. The foot portion at its engagement surface has an axial length. The inner foot portion at its engagement surface has an axial length. The shin portion has an axial length. The bellows defines an effective diameter at zero pressure applied on the seal ring. The interface of the foot portion with the seal ring defines an interference diameter. It is preferable that the axial length of the foot portion is greater than the axial length of the shin portion. It is also preferable that the ratio of the axial length of the inner foot portion at its engagement surface to the axial length of the foot portion at its engagement surface is greater than 0.5. It is further preferable that the interference diameter is within +10 % and -10 % of the effective diameter of the bellows at zero differential pressure.