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Threads cracked under pressure?

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Threads cracked under pressure?

Unread postAuthor: daberno123 » Sat Jan 12, 2008 5:36 pm

Alright so i just finished my piston valve and i screwed it onto my cannon and began to pressurize it. I left the room and came back when it was at 30psi. I notice a small leak occurring at the threads. I try to screw it on tighter when BAM! the the whole valve pops off and hits me in the face. Did i over tighten it? the female threaded part was cracked down all the way and into the fittting below it. was it because the pvc had lost its properties because of how cold my basement is? Both threaded parts are rated NSF-pw. I don't understand why this happened.
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Unread postAuthor: Hubb » Sat Jan 12, 2008 5:40 pm

It seems that the fitting may have already had the crack before you pressurized it. When you went to tighten it, it agitated the crack and, well it had 30psi in it.

Good thing you didn't get hurt.
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Unread postAuthor: daberno123 » Sat Jan 12, 2008 6:00 pm

Yeah luckily I was holding onto it pretty tightly. Didn't really expect it. Luckily it didn't even leave a bruise. Just cut my lip where it hit my braces brushteeth

I have trouble believing the piece already had a crack in it because it was ordered from mcmaster and generally everything they sell is pretty high quality
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Unread postAuthor: bigbob12345 » Sat Jan 12, 2008 6:05 pm

That is strange the only possible explination would be that it was faulty or there was some type of manufacturing defect.
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Unread postAuthor: daberno123 » Sat Jan 12, 2008 6:13 pm

Oh well. I guess i'll just have to rebuild the parts that cracked.

Luckily i left enough pipe in between the fittings so i can do this.

Also going to rebuild the piston housing to make it so it doesn't screw on, just to be safe.

Man and i thought it was finished. I'll post it once it is
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Unread postAuthor: clide » Sat Jan 12, 2008 8:04 pm

How tight did you screw it on? I believe you are only supposed to go one or two turns past hand tight. And hand tight would be just turning by the female fitting not including extra leverage from having a tee or something attached.
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Unread postAuthor: SpudBlaster15 » Sat Jan 12, 2008 11:10 pm

As clide said, the failure was likely cause by an overtightened threaded connection.

At 30psi, assuming you used sufficient amounts of thread sealant, threaded connections should seal when threaded on 2 - 3 turns by hand. The fact that your connection developed a leak at such low pressures suggests that the threads were insufficiently sealed, and your attempt to create a seal led to overtightening of the fittings, which initiated the crack.
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Unread postAuthor: MrCrowley » Sat Jan 12, 2008 11:15 pm

<A HREF="http://www.spudfiles.com/forums/the-true-power-of-a-piston-valve-t11276.html">Rings a few bells</a> :D

Different cause, similar outcome :roll:
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Unread postAuthor: frankrede » Sat Jan 12, 2008 11:56 pm

SpudBlaster15 wrote:As clide said, the failure was likely cause by an overtightened threaded connection.

At 30psi, assuming you used sufficient amounts of thread sealant, threaded connections should seal when threaded on 2 - 3 turns by hand. The fact that your connection developed a leak at such low pressures suggests that the threads were insufficiently sealed, and your attempt to create a seal led to overtightening of the fittings, which initiated the crack.
and if you throw in the brittleness due to the cold.
It seems highly likely
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Unread postAuthor: daberno123 » Sun Jan 13, 2008 1:32 pm

Yeah clide i probably overtightened it since i had the tee on top and was gripping both ends to screw it on i had extra leverage on it. Not sure why it leaking at such low of a pressure, since i had gobs of thread tape on it. All this plus its a little colder in my basement probably caused the crack.

Well, out to the hardware store to buy parts to fix it
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