12. Ducts or Flexible joints
For the protection of liquid such as solid matters, pulverulent bodies and gases the bellows are used that take into consideration the difference between inner and outer pressures.
Selection of appropriate materials is essential here and various materials, not necessarily metal, are used including rubbers of all types, plastics and fibrous materials. Metal frames placed inside and outside of the bellows are often used to cope with the difference between inner and outer pressures.