Secondary Seals

Understanding of the loss mechanisms which apply to even well designed, installed and maintained primary seals led to the development of secondary seals.

The main objective of secondary seals was to reduce the loss of stored product from the primary seal area and two main types emerged:-

i) Those attached to the primary seal.
ii) Those entirely separate from the primary seal.

There is no doubt that the second category offers the better protection and reference to the sketches noted will illustrate the difference between the two categories. Fig. 4 shows a small wiper seal mounted on top of the shoe plate of an MN1 (mechanical primary seal). This is a category i) seal and, while it prevents wind-scouring vapour losses from the primary seal, it gives no protection if the MN1 continuous seal material should become damaged. Fig.5 is also a category i) seal but its secondary continuous seal material connects the wiper seal to the floating roof pontoon and does give some protection against a perforated primary continuous seal.

Both seals in Figs. 4 and 5 would become ineffective if there was a gross failure of the metal shoe plates in the primary seal. If say a pantagraph or bazooka failure occurs then the shoe plate can move away from the tank shell, taking the secondary seal wiper with it and permitting vapour leakage from the resulting gap.

Fig. 4

Fig. 5

Fig. 6 shows a category ii) seal where the secondary seal is completely independent of the primary seal and continues to function even where gross failure of the primary seal has occurred. The example shown has a metal compression plate (similar to early windshields but with each leaf joined and sealed to its neighbours). This gives additional protection to the primary continuous seal from falling debris etc. and therefore is clearly preferable to category i) seals.

Variations of the category ii) seals are:-

Fig. 7 similar to Fig. 6 but with a suspended vapour barrier of polyurethane connecting the wiper seal to the floating roof instead of using sealed joints in the metal compression plates. This is likely to be more expensive than the sealed plate type and the polyurethane membrane may be adversely affected by water vapour.

It should be noted that fitting a secondary seal to a tank could involve slight loss of useable liquid space since it projects above the primary seal. This is only true of course if the tank does not have a windskirt at the top of the shell and if the secondary seal must be kept below the highest level of the tank shell. The Motherwell Nayler Petroseals secondary seal (Fig. 8), can be taken beyond the top of the tank shell without damage to the seal system, provided prior notice is given. Of course, the full benefits of the secondary seal will not be available at those times when the seal is above the shell top.