Floating roof storage tanks seals are relied on to keep the gap between the roof and the tank shell properly contained. Without an effective seal, vapours can escape, product can be lost through evaporation, and tanks can fall short of safety and environmental standards.
However, not all seals do the same job. Most floating roof systems use two seals working together: a primary seal and a secondary seal. Understanding the difference between them, and why both matter, is key to keeping your tank compliant, efficient, and safe.
What Is a Primary Seal?
The primary seal is the first line of defence. It sits directly against the tank shell and the edge of the floating roof, forming the main barrier between the stored product and the atmosphere above it.
EWFM’s primary seal range includes options such as the Mechanical Shoe Seal, Tank Tube Seal, Pantograph Seal, Delta Seal, Mini Shoe Seal, and Graver Seal, each suited to different rim spaces and roof types. As the roof rises and falls with the liquid level, the primary seal flexes and moves with it, maintaining continuous contact with the tank shell throughout.
Because the primary seal handles the bulk of the sealing work, it takes the most wear over time. Even so, a primary seal alone rarely provides complete containment. Small gaps can still form, particularly around the rim, which is where the secondary seal comes in.
What Is a Secondary Seal?
The secondary seal sits above the primary seal, closing the gap between the primary system and the tank shell. Rather than replacing the primary seal, it backs it up, capturing any vapours or product that manage to bypass the first barrier.
EWFM offers several secondary seal types, including the Compression Plate Seal (CPI), Compression Plate Seal “U” (CPU), Compression Plate Seal “L” (CPL5), and Wiper Seal, along with a combined Double Compression Plate Seal (DL5) for tanks that need a higher level of containment. Since the secondary seal deals with a smaller volume of escaping vapour than the primary seal, it typically experiences less wear, but it still plays an essential role in overall tank performance.
Why You Need Both
Relying on a primary seal alone leaves a tank vulnerable to emissions and product loss that a secondary seal is specifically designed to catch. Together, the two systems provide layered protection: if the primary seal underperforms in one area, whether through wear, damage, or a manufacturing tolerance in the tank shell, the secondary seal is there to limit what escapes.
This layered approach also supports compliance. Many environmental and safety regulations for volatile liquid storage expect a secondary seal to be fitted alongside a primary seal, particularly for tanks storing higher-risk products. Beyond compliance, using both seals reduces product loss through evaporation, which has a direct impact on operating costs, and helps protect personnel working on or near the tank.
Choosing the Right Combination for Your Tank
The right pairing of primary and secondary seals depends on several factors, including the tank’s roof type (internal or external), the rim space available, and the product being stored. EWFM supplies a full range of primary and secondary seals for both internal and external floating roofs, and each system is tailored to match specific tank types and customer requirements. We also provide complete installation services for new tanks or replacement seals, delivering a fully integrated solution.
If you’re unsure which combination is right for your tank, or want to discuss a bespoke seal solution, get in touch with our team.
Meet Us at the Tank Storage Conference and Exhibition
We’ll be exhibiting at the Tank Storage Conference and Exhibition on Thursday 17th September at the Coventry Building Society Arena. If you’d like to discuss our range of Floating Roof Storage Tank Seals, or any other aspect of your storage tank equipment, come and find our team on the day at stand 62, we’d be glad to talk through your requirements face to face.
You have until tomorrow, Friday the 11th September 2026, to get your ticket to the most exclusive event in the tank storage industry. You can get yours by clicking the button below:
