O-ring size for 10 year old Guppy tank

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MosesM

Member
Apr 21, 2015
63
7
PA, United States
My friend's Guppy is about 10 years old. After being just fine for about 9 1/2 years, it blew out the o-ring between the tank and the valve assembly at 4500 psi, a couple hours after I had refilled it. At the time it was sitting untouched on the floor, hadn't been moved in over an hour. I got a couple o-rings from Joe B's place, they both blew in the mid-3000's. I got an assortment of o-rings I thought might work, had a 14mm x 2.5mm BUNA 70 last for about a day, then it blew at 4200. Got some information that tanks from that time period had a washer that was slightly thinner and larger than the o-ring between the tank and the valve assembly (though there hadn't been a washer originally on this tank), so I tried that, it started leaking almost immediately. Does anybody have any idea what size o-ring I should be using? Any other ideas?

Thanks

Moses in PA
 
Someone may come along that knows the answer from intimate familiarity but a picture of the O-ring and its groove would go a long way for those of us who are familiar with the nature of O-ring glands and seals more generally. Your description sounds like a typical extrusion failure from insufficient support of the O-ring or maybe just needs to be a higher durometer. With typical part tolerances and >3000psi, you’ll want 90 durometer.
 
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Valve assembly with 14mm x 2.5mm 90 Durometer o-ring still on, you can see where it extruded 
valve2.1644003818.jpg

 
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Okay so the O-ring presses against a chamfer rather than a traditional squared-off groove. I think you’re on the right track about a smaller cross-section being necessary. You want the valve to be able to thread in completely until its shoulder bottoms out, while simultaneously getting just a little squeeze on the O-ring. If the O-ring is so fat the parts can’t fully mate, it will squirt out into the gap when the pressure acts on it for a while.
 
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