#1 What is decompression?
Explosive decompression occurs when the pressure in a system drops suddenly and the gas in an O-ring wants to escape faster than the gas permeability of the O-ring allows.
In principle, every O-ring is permeable to gases or liquids to a certain degree (technical term: permeability). How high the permeability is depends on the composition of the O-ring material.
Low gas permeability: NBR, FKM and FFKM
High gas permeability: SILICON
#2 How does decompression occur?
1. Initial situation: A compressed elastic O-ring in a system with a pressure of 50 bar and a gas as medium.
2 Due to the gas permeability, the O-ring contains compressed gas under a pressure of 50 bar.
3. The system pressure drops abruptly from 50 bar to ambient pressure (approx. 1 bar).
4. The gas is now still under 1 bar pressure and therefore wants to expand again.
5. The gas in the O-ring suddenly wants to escape because there is no longer enough space.
6. The gas permeability of the O-ring is too low, so that the gas makes its own way through the structure of the O-ring.
7. Cracks appear and bubbles form on the surface from which the gas escapes.
8. The O-ring can be damaged so severely that it loses its sealing effect.
#3 Rule of thumb for avoiding decompression
The higher the system pressure, the harder the O-ring must be. Ideally, the O-ring should have a Shore hardness of 90 Shore A.
If it is known that the system pressure fluctuates greatly and can change abruptly, then a material mixture developed for use with decompression should also be used. These materials all have 90 Shore A and the addition “Resistant to explosive decompression” or “Approval: Norsok M710”.