Plastic second stage failures

[cerich]

No debate. The real issue is a housing failure can make the second stage inoperable but shouldn't freeflow. I've had all metal US Divers second stage housings fail and diaphragms split on Poseidons. Dumping gas at a high rate is an unacceptable failure mode.

agree

 

[ams511]

The yellow 2nd stage pictured is a US Divers octo, I have one. I think any of the USD plastic 2nd's like the SE or SEA series are exactly the same construction. I do inspect them but have not had a failure.

My understanding is that older US Divers second stages were prone to cracking at the hose inlet. This is a known issue and the second stages need to be inspected in that area. I am not sure if the cracking is due to a manufacture's defect, overtightening of the hose, or just plain old age.

 

[ams511]

Dumping gas at a high rate is an unacceptable failure mode.

Looking at the schematic, a hairline crack in inlet should not cause a free-flow. This must have been a pretty good crack, that allowed the orifice to move. I am thinking that the hose pulled the on the orifice pulling it away from the seat. This would cause the regulator to free flow like a continuous purge.

I agree this is not a acceptable failure mode. However, I think this is more of a problem with a down-stream valve than a barrel poppet design. I am no sure of other barrel poppet designs would have the same problem.

 

[tech3324]

From the incident report it sounds like the LP hose with inlet attached pulled out of the plastic regulator body:

"On close inspection the diver discovered that the plastic octopus regulator had completely broken away from the hose. The regulator second stage fell to the bottom, and the hose was left in a free-flowing state."

 

[Akimbo]

Looking at the schematic, a hairline crack...

The article makes it sound like the hose broke loose from the second stage housing and the hose was unrestricted. Can you make sense of the article?

 

[KeithG]

I claim the design of the housing depected in the DAN article is suspect. It features a metal hose mount that is threaded into the plastic.

This is a failure waiting to happen. Anytime the hose mount is tightened it will stress the plastic and eventually lead to the cracking described. The Metal on plastic thread design presents an opportunity to create a failure.

My sherwood plastic seconds have a metal to metal thread design. There is a smooth through wall hole in the housing with the internal metal lever support going through the hole and threading into the orifice housing (located outside the second stage housing) which the hose then threads to. The threads are all metal to metal with the plastic housing clamped in between.

 

[KeithG]

The article makes it sound like the hose broke loose from the second stage housing and the hose was unrestricted. Can you make sense of the article?

yes - the hose mount broke lose from the second stage (likely due to plastic thread failure ). There was no hose failure. The metal hose mount (still attached to the hose) was torn away from the plastic threads in the housing leaving the end of the hose open to ambient water.

 

[ams511]

The article makes it sound like the hose broke loose from the second stage housing and the hose was unrestricted. Can you make sense of the article?

Yes, I re-read the article more carefully this time. Looking at the schematic, the hose is attached to the orifice (part 36). The orifice is held in place by the plastic of the case, if the plastic breaks then the orifice will be pulled out of the regulator with the hose. The broken case would still have the diaphragm, poppet, spring, and lever left inside.

The hose that was whipping around would still have the orifice attached to it as there is no way it would unscrew. However, the hose would not need to come lose to have a free flow. All that is needed is for the inlet to crack enough so the orifice moves away from the seat. I have seen these crack before, but I never seen or heard of one totally break.

 

[tech3324]

Anytime the hose mount is tightened it will stress the plastic and eventually lead to the cracking described.

That depends. If a backup wrench is used on the Inlet Fitting flats then no force is applied to the plastic when tightening the hose. I make the LP hose only finger-tight at the 2nd stage which may have prevented cracks on this 20+ year old plastic reg.

 

[KeithG]

That depends. If a backup wrench is used on the Inlet Fitting flats then no force is applied to the plastic when tightening the hose. I make the LP hose only finger-tight at the 2nd stage which may have prevented cracks on this 20+ year old plastic reg.

I was unclear - i was referring to tighthening the host mount onto the housing. Since it is a sealing surface and has an o ring, removal and replacement of the metal hose mount will be part of normal service.