Primary Floating-Roof Seals — Selection and Lifecycle Performance
Primary seals are often compared by type name, but the project sources show that performance is created by geometry: where the seal is mounted, how long the shell-contact path is, how the vapour barrier accommodates movement,…
Matching sealing mechanism, rim-space range, materials and maintainability to the actual tank
Primary seals are often compared by type name, but the project sources show that performance is created by geometry: where the seal is mounted, how long the shell-contact path is, how the vapour barrier accommodates movement, and how the mechanism behaves as rim space changes. Mechanical shoe, compression-plate and spring/compression-shoe concepts each trade complexity, adaptability, material exposure and maintenance. Selection should therefore be based on the real tank geometry and service rather than a generic ranking.
Key points
- Liquid-mounted sealing generally reduces the vapour path but must remain immersed across the operating envelope.
- Long, stable shell contact and adequate membrane slack help maintain performance as the roof moves off centre.
- A simpler seal can be attractive to install but may have a narrower effective range.
- Material compatibility and flexural fatigue often determine lifecycle performance.
- Primary and secondary seals should be evaluated as a system, including fire-fighting and shunt interfaces where applicable.
Start with the mechanism
The seal comparison source separates mechanical shoe seals, double compression-plate concepts and simpler compression-shoe/spring arrangements. The useful comparison is not the product label but how each design maintains shell contact while the rim gap varies and the roof moves relative to the shell.
Liquid mounting and contact height
A liquid-mounted primary seal places the lower sealing element in the product and reduces the direct vapour path from the rim space. For this to remain effective, the shoe or skirt has to stay immersed at the minimum and maximum operating rim spaces. Contact height also matters because a taller, continuous contact surface is less sensitive to small local shell irregularities.
Membrane length and roof movement
The source comparison highlights the need for excess membrane length when the roof moves laterally. A membrane sized only for the nominal centred roof can become taut, lift from the shell or pull the shoe as rim space increases on one side. Mechanical linkages can reduce the amount of free membrane required, but the movement still has to be accommodated somewhere in the mechanism.
Complexity versus robustness
Mechanical shoe seals contain more moving parts and take longer to install, but can provide robust shell contact over a wide range when well designed. Simpler plate or spring concepts reduce installation effort and component count, yet may transfer more performance responsibility to the flexibility and durability of polymer components. The correct trade-off depends on rim-space range, product, inspection interval and maintenance philosophy.
Materials and service
Stainless spring elements, PTFE fabrics, NBR or other elastomers each have different roles and compatibility limits. The selection should distinguish liquid-phase exposure from vapour-phase exposure and consider aromatic content, temperature, UV where relevant, friction and long-term flexing. A material list that is chemically compatible but mechanically unsuited can still deliver poor seal life.
Inspection and performance interval
Seal inspection should measure gaps and also diagnose why they occur: shell geometry, loss of spring force, membrane ageing, local obstruction, roof eccentricity or damaged attachments. The useful inspection interval is therefore linked to the mechanisms that can degrade sealing performance, not simply to the structural inspection interval of the tank.
Conclusion
There is no universally best primary seal independent of the tank. A high-performing selection matches the sealing mechanism to the actual rim-space envelope, product chemistry and maintenance strategy. The objective is stable contact and vapour control over the full operating life, not only a good fit at installation.
Source basis and publication notes
This draft is a supplier-neutral synthesis of the project source material. Supplier-specific claims, branding and proprietary conclusions have been removed or reframed as general engineering considerations. No attempt has been made in this draft to update historical standards or regulatory references beyond what the source material itself states.
Principal source material
- Whitepapers.zip — seal comparison.docx
- Product Selection and Design Guide HMT.docx (seal design, rim-space and survey sections)
- wax scraper design.docx
- Evaluation Aluminator HFC.docx
Before publication: verify current editions and clause references for API, EEMUA, EN, NFPA, PGS and applicable local regulation; confirm any quantitative design limits against the project-specific code basis; and use project photographs only after branding/confidentiality review.