Steel Tank Threads - what happened to the 7/8-14 2A UNF?

[VintageSCUBA]

Hello,

I am new to the board so excuses me if this topic has been gone over before, but I would like the inside info on this question if someone has it.

Early steel SCUBA tanks had a 1/2" tapered pipe thread, and the switch to the 3/4-14 NPSM makes sense to me as an obvious engineering move. In the late 1980's when the high pressure steel tanks made there debut, specifically the "Genesis" or "Sherwood" branded steel tanks the engineers made the decision to switch to the 7/8-14 2A UNF thread form. This is a more modern thread form, with better specifics as to proper tolerances. The 3/4" NPSM thread form is actually a very old thread form that can be traced back to the 1940's and the tolerance on the form is very broad. By definition, the NPSM is a straight pipe thread and sealing is obtained by the mating of the 30° seats of the male and female ends with the help of and "O" ring.

Now, when I say high pressure, I am talking about 3,500 PSI, or about 241 BAR. I understand that outside the United States, "High Pressure" means 300 BAR, or about 4,350 PSI.

The current manufacture of HP steel tanks by Worthington and other manufacturers has them back to the older thread form of the 3/4-14 NPSM. Why does it appear that the more modern thread form, the 7/8-14 UNF has been abandoned?

Note:

1) The 3/4-14 NPSM internal thread form has a major diameter of 1.041, with a pitch diameter of 0.9924 and a minor diameter of 0.9459. So, this may certainly lead to some confusion for the layman as some might assume that a 3/4 in thread would have an outside diameter of 0.750, not 1.041.

2) The 7/8-14 UNF 2A has a major diameter of 0.8734 (...the decimal equivalent of 7/8" is 0.8750) with a pitch diameter of 0.8243 and a minimum minor diameter 0.7980. An obvious point of confusion for many, is that the "larger" dimensionally thread form of 7/8-14 UNF has the smaller actual outside diameter of 0.875! And the smaller dimensional thread form of the 3/4-14 NPSM actually has the larger outside diameter of 1 3/64"!

Now, from what bottom drawer did this idea come from?

 

[DA Aquamaster]

1/2" threaded tanks are a pain to VIP and a major pain to tumble as you have to use smaller media that is harder to get in and out of the smaller valve opening.

Your dimensional description pretty much captures the advantage of the 3/4" thread versus the 7/8 thread genesis tanks - the 3/4" thread offers a "larger" hole to inspect through and to insert or remove tumbling media making it more service freindly in actual real world use.

Plus the 3/4" valves are standard and while 3/4" valve technology may be older and the dimensional standards may not be as precise, it is adequate for 3442 psi tanks and staying with 3/4" valves increases commonality and greatly simplifies logistics, stocking of valves, manifolds, neck o-rings, parts, etc.

Not to stand in the way of progress or anything, but 7/8" threads were a major PITA and I am glad it is a fad that is the next best thing to dead in the scuba industry.

Genesis did not help the cause of the 7/8'' valve any either with some of their wackadoo offerings such as wierd angled valves, offset valves etc.

 

[Luis H]

I totally agree with DA Aqua Master (this is not uncommon).

Since the area of the opening is a square function of the diameter, the small inside diameter difference can make a big difference in servicing and inspecting the tank.

IMHO going to any thread size smaller than the present standard of ¾”-14 NPSM is a bad…bad idea. As a kid working on a dive shop (back in the early 70’s) guess who gets to do the old dirty job of tumbling tanks.

I am down to only a few vintage tanks with a narrow neck and will not touch any ½” npt tanks unless it was given to me…and then only to be polite.

I was given recently an old Sportways with the old straight ½” straight pipe thread, and I kept it because it is cool. I was lucky that I was able to service the valve and it is OK because that is probably one of the hardest valves to find.

 

[WD8CDH]

There is really no technical reason to not use 3/4" NPS for higher pressures. Europe uses an almost identical but not interchangable 25mm thread up to 4500psi.

The 7/8" (.875-14UNF) isn't just used for steel tanks. It is common for aluminum composite 4500psi fire pack tanks and up to 5000psi life raft inflation tanks.

1.125-12UNF and 2.0-12 UNF are used at 3600psi.

I have also seen 1.3125-12 used to 4500psi.

It seems that .875-14UNF is used in SCUBA more for intentional non interchangability rather than technical limits.

 

[Luis H]

My understanding is that the major reason for the 7/8” thread was to make the higher pressure tanks not compatible with valves originally intended for lower pressures.

I can’t think of any structural reason not to use the ¾”-14 NPSM threads at higher pressures. The worst case maximum pressure thrust (assuming the major diameter of 1.041” and a 8333 psi hydro test pressure, for a 5000psi tank) is only 7092 pounds.

I don’t know the rated load for a ¾”-14 NPSM thread fitting, but I am very comfortable saying that this kind of load is not an issue for the ¾”-14 NPSM, even without doing any structural analysis.

 

[Luis H]

Europe uses an almost identical but not interchangable 25mm thread up to 4500psi.

That is the only scary part about the ¾”-14 NPS valve threads. I have read that it is possible to thread the 25 mm European valve on a US tank and it will actually seal and hold pressure to a point. Since the threads are not fully engaged it will then shear the tops of the threads and the failure is catastrophic, with no warning. Both the valve and the tank can become missiles.

It seems that the lucky thing is that European valves look for the most part very different to the valves we have here in the US and Canada. Where it has been an issue (I read) was in Australia, where they might have imported from both Europe and the US. For the most part, the Aussies seem to have very strict safety regulations so it is probably not a big issue.

 

[VintageSCUBA]

Everyone's comments are appreciated. I really don't have anything against the 3/4" thread, I own steel tanks with 1/2, 3/4 and the 7/8 threads, all purchased over my current 32 year tenure of diving. It's just that when I purchased my Genesis tanks in 1989 I was just getting started with "technical" diving, both cave diving and wreck diving off the coast of New Jersey and New York. The HP steel tanks and DIN valves where being touted by the gear manufactures and Dive Shop Owners as the best thing since that little pull tab on beer cans.

I must say that I do believe that the DIN connection is far superior to the post and yoke setup.

I guess my real point is that I really have absolutely no love or respect for gear manufactures, they have their bottom line as the Holy Grail and don't give a hoot for their customers. I am just very suspicious and careful now when I am considering new SCUBA gear purchases.

Try having any 5 - 6 year old decompression computer repaired or serviced. They make you feel like your asking for service on a 1959 Ford Edsel! "Oh, ...that unit has been discontinued, ...that model has been dropped from our current inventory, or; replaced by,..." ...or maybe the company has changed ownership 2 to 3 times in the last 5 years. Then they say to you, "...but we will give you a $50.00 credit on our new $1,500.00 whiz-bang Super Galaxy Deluxe model, but, ...I don't have any right now, there on back order but for $1,500.00 down I'll hold one for you when they come in."

 

[vshearer]

You guys are making me feel really bad I just picked up a set of double HP100's (PST),a Dive-Rite Manifold and bands as a set but they have the 7/8-UNF thread. Now I feel like I wasted $300......

 

[DA Aquamaster]

For $300 I'd live with a set of HP doubles.

 

[Bratface]

Yes, for $300 you did good (Southern for good deal)