Decompression

#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”.

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