Rapid Gas Decompression
Because all elastomers are permeable, gases under pressure penetrate into the seal material. Naturally, the greater the pressure, the larger the quantity of gas forced into the rubber. When gas pressure around a seal is released after a soak period, gas trapped inside the seal expands and may escape harmlessly into the atmosphere, or it may form blisters on the surface. Some of these may rupture, leaving cracks or pits.
This phenomenon is called explosive decompression. The severity of the damage varies with pressure, the gas, the rubber compound, the size of the cross section, and other factors, such as pressure drop rate.
We rarely see problems when the pressure is below 27.6 Bar (400 psi), and generally carbon dioxide causes more swelling and damage than does nitrogen, as mentioned, although any pressurized gas may cause the condition. As mentioned, elevated temperature increases the damage, as does a rapid rate of pressure drop.
Where problems due to explosive decompression are anticipated, it may help to use a small cross section O-ring, as smaller cross sections are less subject to explosive decompression problems than are large ones.
Explosive decompression or gas expansion rupture is caused by high pressure gas trapped within the internal structure of the elastomeric seal element. Rapid decrease in system pressure causes the trapped gas to expand to match the external pressure and this expansion causes blisters and ruptures on the seal surface. If the volume of trapped gas is small, the blisters may recede as the pressure is equalized with little effect on seal integrity. Excessive trapped gas may cause total destruction of the seal. (Refer to Section III, O-Ring Applications, for more information on this problem.) Suggested solutions to explosive decompression are:
1. Increase decompression time to allow trapped gas to work out of seal material (pressure not to exceed 200 psi/min decrease is a good starting point).
2. Choose a seal material with good resistance to explosive decompression.
3. Minimize the time application is held at pressure.
4. Choose an O-ring cross section that is 0.210 in (5.33 mm)
5. Increase the amount of gland fill being careful to not exceed 90% maximum