Question about burst disks on doubles manifold

[elgoog]

Hi all -

I had an odd issue with my doubles last week and wanted to hear your thoughts on it.

I ended a dive with ~400psi and then went home and cleaned out and liberally purged my 2nd stages using these tanks - I had at least a couple hundred psi in them after all this. When I took them to my LDS to get filled, I noticed that the tanks were completely empty. At the time, I suspected that one of the valve knobs may have gotten slightly bumped at some point and the gas just leaked out. There was no audible indication of any gas loss at home or when I was loading/unloading the tanks from my truck. I carry them around on my backplate so I'm not really grabbing valves. I had a fresh visual done, including a check of the valves and the manifold (not because of this incident alone - I was going to get one anyway as I had suspicions of a wet fill a few weeks ago). Turns out, the burst disks hadn't been installed properly (or got loose) and were the result of the leak. Included in the visual, all the o-rings and burst disks were replaced and my set should be "fixed" now.

Here's some questions I had based on the above.
1. Would going down to 200psi or so leave tanks susceptible to leaks? I thought any partial pressure would prevent this. I realize that SPGs are not accurate below 500psi but I definitely had gas in there when I was done rinsing my rig.
2. Do burst disks usually require regular inspection or tightening? What could have caused them to come loose? I bought the dual outlet manifold brand new from DGX in January and have done about 20 dives on my doubles since with no indications of any leaks.

Thanks in advance,
elgoog

 

[guruboy]

1) It might have been a slow leak. Were the tanks full when you started the dive?

2) Not if you torque them properly at install.

 

[boulderjohn]

I once had a set of doubles filled in anticipation of a dive that didn't happen. A few weeks later I was ready to use them, but they were only half full. Putting them in a tub of water revealed the very tiny leak through the burst disk. It had probably been there for quite some time, but it had never been noticed because I dived soon enough after filling that the gas loss was not noticeable.

 

[boulderjohn]

This is a totally different story, but you might find it amusing.

I left my doubles off at a dive operation for filling, and the guy who did it told me he noticed a leak in my burst disk. Rather than simply tighten it, he had replaced it. He was proud of the good, free service he had provided. I didn't give it a lot of thought. This was before an OW dive, and I was getting them filled at roughly their standard fill pressure, which for LP steel tanks is 2640 PSI. I used them for a few weeks and then took them cave diving, and the shop there gave me a standard cave fill, which is about 3800 PSI. It was not until I came back to pick up my tanks that it occurred to me that the shop employee had replaced my high pressure burst disk with a standard low pressure disk. I'm glad I wasn't there when everyone else found out.

 

[decompression]

Could be a small leak almost anywhere, next dive do a very thorough bubble check.

 

[elgoog]

1) It might have been a slow leak. Were the tanks full when you started the dive?

They were full at the start and the SPG reading was practically the same as what I had recorded at the time of the fill. The pressure drop during the dive is not what is surprising to me. My SCR on this dive was in my normal range. Having said that, I'm not precluding a slow leak that may have always been there.

I once had a set of doubles filled in anticipation of a dive that didn't happen. A few weeks later I was ready to use them, but they were only half full. Putting them in a tub of water revealed the very tiny leak through the burst disk. It had probably been there for quite some time, but it had never been noticed because I dived soon enough after filling that the gas loss was not noticeable.

I went about a month once between the fill and dive and didn't notice any drop in pressure on the dive day as compared to what I had written down when I analyzed gas at the time of the fill. Usually, it's about 2 weeks tops.

Could be a small leak almost anywhere, next dive do a very thorough bubble check.

Yeah, for sure. We always do one but it's usually a quick check for obvious leaks. Next time I'll have my buddy look a lot more carefully.

 

[Caveeagle]

I was helping tear down a set of doubles (not mine) last weekend and found the burst disks on both sides were barely more then finger tight! And these tanks were pressurized when dropped off, so it's safe to assume they were not pre loosened.

And fwiw, I had a witness to this. Just the weight of a craftsman 3/8" drive wrachet with a light touch broke them loose.

Oh hell no! I'm now checking all my valves that I did not personnel rebuild.

 

[Peter69_56]

Bursting disks can be the pits. Its one of those things I check after servicing a valve as they can have a tiny leak which is enough to dump a full cylinder over time.

Also I make sure I seat the valve seat by turning it on off a few times. Got caught once after servicing a valve and lost most of the gas, but a few turns on and off and the seat was fine, it just hadn't seated properly.

Easiest way to find leaks is to mix up a small jug of soapy water and pour over the valve assembly, any leaks will pour out bubbles. Also make a nice big bubble on the outlet face of the valve (like kids do with bubble makers) in case the seat leaks. Then I tip the cylinder so the bursting disc circle is facing up (if it has a protective ring) and filling it with clear water, looking for tiny bubbles, often a nip up with a socket stops the leak, however if it doesn't stop, then it will mean dump the air and replace the disc.

 

[Jack Hammer]

I've personally seen O-rings that seal and hold when the pressure is high and leak when the pressure is lower. +1 for the soapy bubble check.

 

[DEdiveguy1]

Torque specifications for most burst discs is pretty low, and I've seen many instances when they leak after installation. Good service shops will always leak test valves after any disassembly/reassembly to insure there are no bubbles.