text: In modern expansion joints, the metal bellows are often produced with a so-called multi-ply design. To increase both flexibility and stability, multiple thin plies of metal are layered to form the bellows walls. There are two basic design types: The multi-ply and the multi-walled bellows structure. The multi-ply structure consists of a pressure-tight, longitudinally welded outer and inner cylinder of stainless steel. In between these cylinders is an open, spiral cylinder which forms multiple plies depending on the design. The multi-walled structure consists of several concentric, longitudinally welded cylinders. Each cylinder forms a pressure-tight and closed "wall". 
  The main advantages of multi-walled bellows:
  * Resistance to high and very high pressures
  * Large movement absorption
  * Small dimensions
  * Small adjusting force rates
  * Optimal compensation in a very small space
  * Early leak indication (in case of damage) via standard check hole
  * Complete burst resistance
  *Possibility of permanent leak monitoring in critical media
  * Economical use of high-quality, corrosion-resistant materials such as Inconel, Incoloy, Hastelloy, titanium and Tantal
  * Insulation against structure-borne noise up to 20 dB
  This design has both technical and economic advantages. For example, the bellows can be constructed of different materials, such as high-alloy stainless steels for the pipes in contact with the medium (inside and/or outside), and low-alloy stainless steels for the intermediate plies.