Bubble caps rank among the oldest tower internals together with tray panels. They consist of a cap and a riser. The various degrees of freedom (Buble Cap diameter, height,...) mean that a large variety of different bubble caps exist. Only a few models can be marked as standards.
Risers and caps have to be fluidically and geometrically aligned with each other. Bubble Caps are normally used in towers with a low fluid loading and thus they are often used in combination with sealings.