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

Wed Jun 9 19:18:08 EDT 2021

 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

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