NGT - Tandem Dry Gas Seal without intermediate Labyrinth
The tandem seals are used where a little process gas leakage to the atmosphere side are acceptable. Tandem seals could be like two or more single seals connected by end to end. It is similar to single seal that the buffer gas is just process gas.
Normally the primary seal handles the full product pressure and the secondary seal is a back-up seal that normally operates at low pressure but can handle the full pressure in the event of a primary seal failure