The temperature variations encountered during filter drying operations will result in condensation formed between the flexible bellows and the agitator shaft. With the PSL agitator design, the bellows is attached via an RJT coupling and sealed using PTFE encapsulated o-rings. Although this offers a seal in normal operation, in the event of a bellows rupture, this condensation would contaminate the product.

This contamination risk is avoided by purging this void with nitrogen gas. As the agitator is lowered, the bellows will expand and the pressurised nitrogen will fill the available space. As the agitator is raised the nitrogen pressure within the purge system will increase, as it cannot pass back through the mechanical seal inlet, due to a non-return valve. When the pressure has increased sufficiently to overcome the relief valve, nitrogen will pass through the mechanical seal outlet.

When this pumping action has occurred a number of times the oxygen content within the system will be less than 5%, replaced by an inert nitrogen atmosphere