Since almost all seals utilize the process liquid or gas to lubricate the seal faces, they are designed to leak. Process liquids and gases containing hazardous vapors, dangerous toxic chemicals or flammable petroleum must not be allowed to leak into the atmosphere or onto the ground. In these applications a second "containment" seal is placed after the primary seal along the pump shaft. The space in between these two seals is filled with a neutral or compatible liquid or gas [Generally nitrogen] called a "buffer [Unpressurized]" or "barrier [Pressurized]" fluid.
In a tandem seal [Face To Back], the seal will leak into the buffer fluid contained in the unpressurized cavity commonly known as thermosiphon pot. If the cavity registers a dramatic increase in pressure or fluid level, operator will know that the primary seal has failed this can be achieved by using pressure / level switches or transmitters. If the cavity is drained of liquid, then the secondary seal failed. In both instances, maintenance will need to be performed. This arrangement is commonly used when sealing fluids that would create a hazard or change state when contacting open air. These are detailed in API 682 [Currently 3rd Edition] Piping Plan 52
In a double seal [Generally Back to Back], the barrier liquid in the cavity between the two seals is pressurized. Thus if the primary seal fails, the neutral liquid will leak into the pump stream instead of the dangerous pumped fluid escaping into the atmosphere. This application is usually used in gas, unstable, highly toxic, abrasive, corrosive, and viscous fluids. These are detailed in API Piping Plan standards #53a, 53b, 53c; or 54. Plan 74 may also be considered a double seal piping plan, although it is used exclusively when describing a dry gas barrier seal support system. The barrier fluid used in a Plan 74 system is simply a gas, not a liquid. Typically, nitrogen is used as its inert nature makes it advantageous due to mixing with the process stream being sealed.
Tandem and double seal nomenclature historically characterized seals based on orientation, i.e., tandem seals mounted face to back, double seals mounted back to back or face to face. The distinction between pressurized and unpressurized support systems for tandem and double seals has lent itself to a more descriptive notation of dual pressurized and dual unpressurized mechanical seal. This distinction must be made as traditional 'tandem seals' can also utilize a pressurized barrier fluid.
