# How many O-rings would a mechanical engineer use here?

#### Roy S Ramirez

Hello everybody!

Happy 2019! I'm trying to design a mechanical face seal for the part shown below, and I'm struggling with the number of o-rings I'm supposed to use. There are two cavities (rings) for two different fluids. The liquid dynamic pressure for both the internal and external fluids is different but somewhere between 165 and 200 psi. I know I need at least two o-rings between them (as shown in the picture), but for a moment I thought maybe using three o-rings, with two of them located between both cavities would work better :/

How many o-rings would a mechanical engineer use in this case?

Best,

Roy S. Ramirez

School of Aeronautics and Astronautics
Purdue University - Class of 2021
Management and French Minor

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#### trurle

Hello everybody!

Happy 2019! I'm trying to design a mechanical face seal for the part shown below, and I'm struggling with the number of o-rings I'm supposed to use. There are two cavities (rings) for two different fluids. The liquid dynamic pressure for both the internal and external fluids is different but somewhere between 165 and 200 psi. I know I need at least two o-rings between them (as shown in the picture), but for a moment I thought maybe using three o-rings, with two of them located between both cavities would work better :/

How many o-rings would a mechanical engineer use in this case?
If you have seal ring material perfectly compatible with both fluids, use 2 seal rings. If you have a doubts about chemical compatibility, use 3.

#### sandy stone

If you have special concerns about cross-contamination between the two fluids, maybe use 3 o-rings, and vent the space between the 2 that are separating the fluids.

#### Baluncore

The grooves to take the seals appear to be the same depths, so the seals will all be the same thickness. The outer groove is about half the width of the inner groove, so there is room for two inner ring seals between the fluids.

O-rings seal by being pushed into a hard corner by differential pressure. For that reason, venting the space between two o-rings cannot be effective, unless they are seated in quite separate grooves.

#### sandy stone

Yes, of course, I should have been more clear in my previous post.

#### Roy S Ramirez

Thank you very much to all of you! The o-rings are most likely going to be Viton which is compatible with both fluids therefore, I guess two grooves would work just fine. My main concern was not some low level of cross-contamination itself, but sealing failure due to both fluids squeezing the o-ring towards the groove's center.

#### trurle

Thank you very much to all of you! The o-rings are most likely going to be Viton which is compatible with both fluids therefore, I guess two grooves would work just fine. My main concern was not some low level of cross-contamination itself, but sealing failure due to both fluids squeezing the o-ring towards the groove's center.
From mechanical perspective, double seal rings are advantageous to single rings if you have a solids (i.e. dirt) contaminating your fluid (likely not your case?). Dirt particles in multiple-ring system are less likely to result in leakage, because force exerted by particle lodged between flange and one ring is less effectively transferred to other ring, preserving other ring contact with flange and maintaining the seal.
Concerns about mechanical failure of (less rigid and strong) multiple-ring system similar to one you voiced above are typically not valid, unless you have very demanding application approaching the mechanical limits if the seal.

#### Roy S Ramirez

From mechanical perspective, double seal rings are advantageous to single rings if you have a solids (i.e. dirt) contaminating your fluid (likely not your case?). Dirt particles in multiple-ring system are less likely to result in leakage, because force exerted by particle lodged between flange and one ring is less effectively transferred to other ring, preserving other ring contact with flange and maintaining the seal.
Concerns about mechanical failure of (less rigid and strong) multiple-ring system similar to one you voiced above are typically not valid, unless you have very demanding application approaching the mechanical limits if the seal.
Thank you very much @trurle

"How many O-rings would a mechanical engineer use here?"

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