OP
1/2 inch valve
- Thread starter dead dog
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Luis,
I just googled and found this: "Threaded pipes such as NPT seal tightly and are used to move liquids and gases. NPSM is not typically used to transport water or gases without an O-ring. Instead, they are used as structural pipes unless a sealer like an O-ring or tape is applied."
Any idea why the scuba industry moved away from what seems to me to be a simpler design (no O-ring, and therefore would seem to require less service)? Cutting NPT threads is more labor-intensive? Sealing an NPT scuba connection requires a chain vise? Something else?
Thanks in advance.
Safe Diving,
rx7diver
I can't tell you exactly why the industry went away from the tapered threads, but I can give many reasons why I dislike them (for this application) and why I would avoid ever using any tapered tanks if I can help it.
I worked for dive shops from the early 70's and a bit into the 80's. Many cylinders were the 1/2" NPT type. They are a pain.
The neck on the 1/2" is small and a hassle to deal with during inspection and tumbling. Large industrial cylinders do use the larger tapered threads, but there are lot of differences in use. Industrial cylinders rarely have the internal corrosion issues that scuba tanks experience and therefore do not need yearly visual inspection.
An O-ring seal is a extremely reliable seal (when installed correctly) and does not require high torque values to seal. All dive shops used to have large chain vise to clamp tanks. Many do not bother anymore. I still have a large chain vise (at home), but rarely use it.
I can honestly say that I have never had a tank leak from the neck (tapered or O-ring) that I have put together, but I have seen many, normally in rental fleets (AL 80s).
Tapered threads do wear from the constant removal and re-installation. Tapered pipe threads are fine for piping and pluming that is installed and rarely ever taken apart, but it doesn't make any sense for a cylinder that required regular inspection and occasional cleaning.
You can see very old tank valve tapered thread combinations that the valve threads are barely showing anymore because after so much use the valve thread area is slightly deformed (compressed?).
Tapered threads also produce very high hoop (circumferential) stresses on the female threads. They are not allowed in a lot of application (shipboard, Navy, etc.) due to this stresses. They should also not be used with aluminum cylinders for the same reason. I am fairly certain that they are not allowed in any DOT cylinders, but I have read that in Europe there were a few cylinders designed wit tapered threads. I don't know if they are in service anymore.
---------- Post added July 20th, 2014 at 12:10 PM ----------
Machining and re-taping a DOT pressure vessel is forbidden by the CFR. Any such modification will condemned a DOT cylinder. When I have some time, I try to look for the CFR paragraph that described this violation, but I clearly remember reading it.
If you have your own compressor and don't care about the DOT codes, IMO, you are probably still taking some risk by machining and re-taping a pressure vessel.
Machining a valve to fit the tank is not an issue, but machining the tank is.
I HATE tapered threads in any plumbing application with temperatures low enough for an O-ring. They work fine in specialized applications like drill pipe (oil drilling) and some artillery, but are terrible in most cases.
There is no reliable way to know how tight they need to be. As a result, they are a can of worms in compact tubing assemblies because their engagement distance varies. It can be a royal pain to tighten leaking pipe threads in long or complex assemblies.
Tapered threads gall badly in stainless, Aluminum, and Titanium. Gas-tight engagement often causes tiny metal particles to come loose in the gas stream, which is a common cause of Oxygen fires in high pressure systems. Tapered threads require a lot of torque to seal where a straight thread O-ring sealed fittings go in hand-tight and just get snugged up with a wrench.
Dont even ask me about compression fittings on tubing.
OP
dead dog
Contributor
" Machining a valve to fit the tank is not an issue, but machining the tank is."
How so ?
How so ?
An old friend (a dive store owner and my tech instructor, and also a former master auto mechanic) had one of his Sherwood Y-valves machined down and re-threaded to fit one of his PST HP (3,500 psig) 80's. The final result was quite a neat little package!
Of course, this type of procedure did NOT involve cutting away part of the thickness of the neck of a pressure vessel.
Safe Diving,
rx7diver
Akimbo,
One thing a lot of younger folks don't realize is that the 1/2" tapered thread valves were put on tanks, and they stayed in place until the next hydro, five years later. This was before we had yearly visual requirements for scuba cylinders (you know, rusty tanks exploding, etc.), so these valves were very practical at that time. Now, when they have to be dismantled each year, it becomes a real pain, especially with those doubles. I have one set of cylinders, my PJ tanks, which still have the 1/2 inch valve, and I do the work on them myself as the LDS doesn't really know how to handle them. The photos are all of 1/2" tapered thread cylinders for the USAF doubles, and it appears that my friend, "Pierre" was using a 3/4 inch valve opening with a reducer (this was in 1963).
SeaRat
One thing a lot of younger folks don't realize is that the 1/2" tapered thread valves were put on tanks, and they stayed in place until the next hydro, five years later. This was before we had yearly visual requirements for scuba cylinders (you know, rusty tanks exploding, etc.), so these valves were very practical at that time. Now, when they have to be dismantled each year, it becomes a real pain, especially with those doubles. I have one set of cylinders, my PJ tanks, which still have the 1/2 inch valve, and I do the work on them myself as the LDS doesn't really know how to handle them. The photos are all of 1/2" tapered thread cylinders for the USAF doubles, and it appears that my friend, "Pierre" was using a 3/4 inch valve opening with a reducer (this was in 1963).
SeaRat
Attachments
What Akimbo said above is correct.
There are technical (structural) reasons why the pressure cylinders are regulated, but the valves just have to meet CGA standards (but are not tightly controlled).
The valve is basically an almost solid block of brass that is just machined and does not require any heat treatment, etc. Due to structural geometry, the material strength of the valve is not particularly critical. Even the threads have a reasonably high safety factor. There is more than enough threads to carry the pressure trust.
Catastrophic valve failure are normally only caused by either large impact shearing the valve off, or similar kind of accidental load/ impact that break the valve or part of the valve off. It can be very exciting when this happens (and can cause substantial damage or injury), but it is not normally as catastrophic as a cylinder rupture.
Pressure vessels are thin wall (even the aluminum cylinders fall under thin wall pressure vessel classification) and require relatively high strength material. All DOT cylinders (that I am familiar with) are made from heat treated alloys.
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