O-ring permeability in vacuum 1

[Gerry02]

I am trying to calculate what the effect on vacuum would be from an o-ring sealing water - vacuum for different o-ring materials looking specifically at permeation.

I found the following formula for calculating o-ring permeability:

Q = K A (P1 - P2) / D

Q is the permeation rate (cm3/sec)
K is the permeation coefficient (cm3 cm/sec cm2 atm)
A is the area (cm2)
P1 - P2 is the pressure gradient (atm)
D is the thickness (cm)

https://www.marcorubber.com/o-ring-permeation.htm

I thought i could use the formula for gas load (Q = P.S)

My issue is the units for Q do not match. Q for the permeability equation is volumetric flow rate where as the Q for the gas load equation is pressure - volumtric flow rate.

Its my first time working with vacuum so still getting up to speed with the technology.

Thanks

 

[Prometheus21]

Hi,

It is a nice formula, but what are you trying to accomplish? Sealing against vacuum is tricky depending on the sealing configuration.

Static, dynamic, face seal? Dovetail configuration? Half-dovetail? If it is hot water then an EPDM material should be considered as a material.

Further I would consider the squeeze rate and lubrication carefully. Increasing the squeeze does reduce the leakage rate significantly, as well as lubricating the O-ring with high vacuum grease. Now if you have higher squeeze percentages (30-50%) the effects of the grease will be lower than if you have lets say 15 % squeeze.

Not to mention the gland configuration where an increased squeeze reduces the flow rate by increasing the length of the physical path the gas has to travel (width of the O-ring) and decreasing the area available to the entry of the gas (gland depth). A softer O-ring might also work better if you have imperfections in the gland. and to end it all you might even have to consider back-up rings to avoid extrusion.

I would recommend you reach out to an O-ring supplier like Marcorubber or Parker and ask their technical department for advise directly. They are usually quite helpful (in my experience).

 

[Gerry02]

Hi,

Thanks for your response

It is a static face seal. Sealing from water (cold) to vacuum is not something we usually do and when you compare the Permeation Coefficients of water to other gases it is much higher and therefore something I would like to evaluate first. I will do as you say and reach out to Parker.