The basic layout of the sealing system is the same for all Flygt products: an outer seal, a seal housing with buffer fluid, and an inner seal. The two main components in the sealing system are the outer and inner mechanical face seals. These should not be seen as a primary and a secondary seal, but rather two independent seals with slightly different functions. The outer seal is exposed to a harsh environment and must be able to cope with fibrous clogging matter, hard abrasive particles, chemically aggressive media, high pressure, impacts, etc. Therefore outer seals have in general a sturdy design that can withstand a lot of abuse. Hard seal faces are the only option for the outer seal.
The inner seal operates in a controlled environment that is less taxing. Since there are no abrasive particles in the buffer fluid, carbon faces can be used as a cheaper option instead of hard faces. Carbon has the benefit of excellent sliding properties, but is not as durable as other seal face materials.