[PSUBS-MAILIST] K-sub hatch o-ring

[Sean T. Stevenson via Personal_Submersibles]

That particular drawing does not appear to call out the O-ring groove diameter by annotation. Is that on another drawing? I guess you could scale it off. It also appears to show a half-dovetail groove, with a 10° taper on the ID surface, which would negate the need for stretch to hold the O-ring in as long as the narrowest width of the gland is narrower than the O-ring. Even still, you probably want a bit of installation stretch for reasons given earlier.

What reference are you using for O-ring dimensions? Per ORD5700, 400 series are 0.275" width. A 0.275" ring in a 0.215" gland depth at zero extrusion gap is 28% squeeze, which is acceptable but at the upper limit of the acceptable range for 400 series face seals. A few percent of installation stretch would reduce the cross-sectional diameter just enough to make the resultant squeeze comfortably within the range.

Extrusion should not be a problem provided the groove is sufficiently wide that when the flanges come metal-to-metal, you don't end up with an excess amount of gland fill. I might shoot for 75% - 85% maximum. Particularly with a dovetail on the inside per that drawing, which will push the elastomer against the acute angle. Also, with 0% squeeze (where the extrusion gap still exists), you don't want a lot of area subject to the external pressure compared to the hatch area acting axially. As pressure increases, you want the squeeze to win over the tendency to extrude. 0% installation stretch on a 0.275" thick O-ring would start you at 0.060" gap. With 5% installation stretch you would start at 0.053" gap. Not significantly different.

Sean

-------- Original Message --------
On Jun. 9, 2021, 17:18, Jon Wallace via Personal_Submersibles wrote:

> Sean, thanks for that analysis. See attached image for reference to K350.
>
> I screwed up some of the K600 numbers based upon the plans which turn out not to be accurate. Specifically, the actual K600 hatch seal configuration I see does not match either the K600 plans nor the K350 plans, so somewhere along the way the details were changed without documentation (which Lloyds didn't catch) or I don't have the final set of K600 drawings.
>
> K600 groove - 23.5 ID, 24 OD, 23.75 mean diameter
>
> K350 groove - 23.25 ID, 23.75 OD, 23.5 mean diameter, according to the plans.
>
> BOTH have a groove width of 1/4 inch as shown in K350 image (including dovetail on ID side) and a depth of .215 inches.
>
> Cross section diameter of both 2-471 and 2-472 is .271 inches. On the K600 that's a 2.1% stretch which would be within the limits you wrote about, but now I'm wondering if Kittredge used the 2-471 on the K350 to reduce the cross-sectional diameter enough to counter the relatively hefty .271 o-ring inside a .25 x .215 channel. Too big of a diameter would cause extrusion at some point, wouldn't it?
>
> Jon
>
> On Wednesday, June 9, 2021, 05:25:03 PM EDT, Sean T. Stevenson via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>
> Per ORD5700, stretch should be limited generally to no more than 5%. A 2-471 O-ring (21.955" ID) on a 23.25" gland is 5.90% stretch. A 2-472 O-ring (22.940" ID) on that same 23.25" gland is only 1.35% stretch.
>
> Sometimes it is advantageous to specify an O-ring which must be stretched for installation in order to create friction against the gland in order to prevent the O-ring from slipping out of the groove if it isn't retained by other means (e.g. dovetail).
>
> At 5% stretch however, the cross-sectional diameter of the O-ring is reduced by about 2.5% (and at 5.9% squeeze, almost 4%) so the gland depth may need to be reduced accordingly in order to achieve the design squeeze on the O-ring. Per ORD5700, for face seal glands in sizes 425 through 475, that design squeeze is 21% to 29% with no extrusion gap.
>
> For face seals subject to external pressure, the gland is usually sized with its inside diameter equal to the mean diameter of the O-ring, with tolerance of +1% of that mean ID, but not more than 0.060". The reason for this is that by ensuring that the O-ring is situated on the correct side of the groove for the anticipated pressure, it won't experience premature failure as a result or being shifted across the groove on every pressurization cycle. For a 2-471, that mean diameter would be 22.230", and for a 2-472, 23.215", which would correspond to 1.25% and 1.20% installation stretch respectively, but these are recommended design minimums.
>
> My guess is that the 23.25" gland ID just provides enough stretch to keep the O-ring from slipping out, and performance in that embodiment was good enough. Provided the cross-sectional diameter reduction and associated gland depth changes are accommodated, the only downside to greater installation stretch is premature aging of the O-ring. (See ORD5700, 3.6).
>
> Sean
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.whoweb.com/pipermail/personal_submersibles/attachments/20210610/822b7caf/attachment-0001.html>