The leg seal (Non-Vapor Cap) is used on open-top storage tanks, often in situations where mixers are used. It reduces emissions from roof legs considerably, nowadays developing into a standard for tanks storing volatile hydrocarbons.

 It also ensures that the support legs remain operational by preventing rainwater ingress. The locking pin is completely surrounded by the leg seal, preventing the support legs from dropping down as a result of pin displacement caused by mixer vibrations.