Crossbar and isolator valve: do they help?

[leadduck]

I actually had an interesting one last year. On the surface, thankfully. We were setting up a downstream Emerald dive. My doubles were assembled and checked, valves open, and upright in my truck bed. We're going over the details in the parking lot and I hear a "POP-WHOOSH" as gas starts dumping out of my doubles. I tried to shut down the right post but couldn't do it. So we isolated and tried to figure out what it was, as gas was coming out of the handwheel. Ended up being an extruded bonnet nut o ring. That was the root cause, at least. For whatever reason, the valve stem got jammed up when that happened and we had to basically let that side drain and break down the valve to close the HP seat with a flathead. Would've been bad news underwater on a straight bar manifold.

If you're concerned about this kind of valve failure underwater, consider that the isolation valve is a single point of failure for the issue you had. If the same defect your right post valve had occurs at the isolation valve then you lose all backgas quickly but can't do anything about it.

Usually this point about the isolation valve being a single point of failure is made by sidemount divers or those with independent doubles, but dismissed by the DIR divers because they think it's very unlikely to happen and so you shouldn't be concerned about it but rather prefer to have the option to save more backgas in case of a regulator failure. But you had it happen, and you are concerned about it, so just saying that the isolation valve did not eliminate your single point of failure and you rather gave a reason for independent tanks.

 

[helodriver87]

Could it happen? Sure. Anything can fail in principle. My manifold might explode next time. Reality is, I probably won't ever see that failure again, as it was technically a compound failure with the valve stem jamming up in addition to the extruded O ring. I guarantee I'll see another reg failure though. And when that happens, I'd rather not be on independent tanks.

 

[markmud]

But you had it happen, and you are concerned about it, so just saying that the isolation valve did not eliminate your single point of failure and you rather gave a reason for independent tanks.

Hi leadduck,

Your points are well taken. As a former professional mariner, l learned early that EVERYTHING on the ocean is a compromise. Manifolding redundant systems creates more problems. On one vessel, an owner wanted the ability to run both generators on the same bus which would require phase matching. That never got out of the starting blocks even though I hired an electrical engineer. Too complicated, to many failure points, and human nature could have caused both systems to be ruined. We split the bus instead.

Simplex Sigillum Veri: As a faux tech diver (Tech 40) I understand the need for a segment of the tech diving community to use a manifold with an isolator valve. In project management, we are taught to tailor our project planning for the job. We used the process groups that were needed, and those that were only needed.

Training and maintenance need to be stepped-up because of additional failure points and the possibility for human error.
scuba diving manifold with isolator valve diagram - Bing images
I see three additional mechanical failure points in the picture above. Not to mention human failure points.

I guarantee I'll see another reg failure though. And when that happens, I'd rather not be on independent tanks.

Everything on the ocean is a compromise.

cheers,
m²V2

 

[rjack321]

Training and maintenance need to be stepped-up because of additional failure points and the possibility for human error.
scuba diving manifold with isolator valve diagram - Bing images
I see three additional mechanical failure points in the picture above. Not to mention human failure points.

The double orings on the cross bar really never fail. Doubles can fall out of pickup trucks and the manifold bend all screwy so its ruined and they still don't leak. Those 2 joints are pretty bulletproof.

Being 1) closed during fills or during the dive or 2) leaking via the valve stem (there's only a single tiny stem oring and its constantly under pressure if the center valve is open 24/7 like it should be)
are the vast overwhelming majority of the manifold failures.

 

[leadduck]

This has been discussed a few times, e.g. Manifold failure - what should you do?
that had examples of tank o-ring failures, or a guy who damaged his manifold while scootering in a cave and barely made it out. But these cases are very rare and there can still be a fallback; the buddy's long hose or a stage cylinder. So I didn't want to argue against the manifold, just said that @helodriver87 's risk of a fatal valve failure is not eliminated by adding only one isolation valve, but a burst disk failure or tank o-ring failure would've been.

Actually there are manifolds with two isolation valves:
Manifold – Nurkopedia (scroll down), where no valve is a single point of failure. Only one of my buddies has it. Not nearly as popular as the single isolation valve.

I think there's little value in arguing about the very small probabilities of post valve failure vs manifold valve failure, because looking at actual accidents, both seem to be very small, and the risk of the manifold or isolation valve failing is much smaller than the risk that the diver accidentally fills or dives with the isolation valve closed. Hence my reasoning against the isolation valve is only about its (considerable) hazard of human error, not about its (very low) risk of mechanical failure.

 

[helodriver87]

I actually use dual isolators on my rebreather with rack mounted dil/BO. You get double the chances of ending up with a human error problem. Which is still effectively zero if you make it a point to check them on setup, pre-dive on the surface, and anytime they leave your control, to include analyzing both tanks if there's any doubt whatsoever in the fill process. I don't disagree that the risk exists, but it can be completely eliminated with proper procedure. You can't say the same for the remote potential of a valve failure. I think the added risk of an isolation manifold lies entirely in substandard training and complacency. But it gives you a tool to deal with more or less any failure that could arise, assuming proper gas planning and training. As was mentioned earlier in this thread, it should be a thinking man's game.

 

[stuartv]

Here's an anecdote for you. I picked up my doubles from being filled on Friday. They had Nitrox in them. I requested them to just be topped up with air (in prep for very shallow lake diving this past weekend).

Because of this thread, I analyzed both posts using my Divesoft Trimix analyzer (with Pro Flow Limiter). They came out one full percentage point different. 22.7 on one post and 23.7 on the other.

I think I will always analyze both posts now. Even if I fill them myself. And especially if I got or put Helium in.

 

[cerich]

I actually use dual isolators on my rebreather with rack mounted dil/BO. You get double the chances of ending up with a human error problem. Which is still effectively zero if you make it a point to check them on setup, pre-dive on the surface, and anytime they leave your control, to include analyzing both tanks if there's any doubt whatsoever in the fill process. I don't disagree that the risk exists, but it can be completely eliminated with proper procedure. You can't say the same for the remote potential of a valve failure. I think the added risk of an isolation manifold lies entirely in substandard training and complacency. But it gives you a tool to deal with more or less any failure that could arise, assuming proper gas planning and training. As was mentioned earlier in this thread, it should be a thinking man's game.

I think you may be putting too much faith in your training and not enough into human factors to throw a wrench. YMMV

 

[helodriver87]

I think you may be putting too much faith in your training and not enough into human factors to throw a wrench. YMMV

Possibly. But I like knowing that I have tools available to deal with any situation that might come up. It's a good reminder not to be complacent with those tools.

 

[leadduck]

Here's an anecdote for you. I picked up my doubles from being filled on Friday. They had Nitrox in them. I requested them to just be topped up with air (in prep for very shallow lake diving this past weekend).

Because of this thread, I analyzed both posts using my Divesoft Trimix analyzer (with Pro Flow Limiter). They came out one full percentage point different. 22.7 on one post and 23.7 on the other.

I think I will always analyze both posts now. Even if I fill them myself. And especially if I got or put Helium in.

So the isolation valve was open while filling, but still got 1% difference. Did you open your isolation valve fully or just a little? Did you fill quickly from a storage tank or slowly from a compressor?
If the valve is just a quarter turn open, it will equalize during the dive and allow for a quick v-drill, but maybe it'll cause a pressure differential between the tanks during a fast fill?
Out of air with half-full tanks - Cave Diver Harry