Any known issues w/NPT vs. NPSM threaded tanks?

[Sea View]

I have two steel Pipe threaded tanks (NPT) a sweet 32 pony (an old O2 bottle at 1800) and a standard 72 at 2400. I brought both of these down to my new LDS for a fill; they also were in need of VIP. The LDS was very concerned the tanks were not 3/4-inch NPSM and failed the VIP because of the thread type - not the condition of the tank or the valves; they said it was a DOT violation.

As far as I understand there are two common pipe thread versions that have been used on Scuba tanks, the tapered National Pipe Thread (NPT) and the straight National Standard Free-Fitting Straight Mechanical Pipe Thread (NPSM). Some older tanks had the NPT styles which are no longer used and all newer models all have NPSMs.

I have used these tanks for many years and are still in great shape - just passed hydro 2yrs ago (Through another LDS I stopped going to some time back - too much sales pressure - actually surprised they didn't try sell me new tanks at the time)

Has anyone else found the same issue w/ NPT type threaded tanks & valves?
I would have thought they would have been kicked back at the last hydro if there was a real DOT violation or safety concern with that style thread.

I don't know if there is a real safety issue or the LDS is being overly conservative, they also have stopped filling any Luxfer AL80s older than 1990 - regardless if the pass the fracture testing (which they offer - but won't fill even if they pass - strange).

I guess I'll have to go back to the old LDS for an attempt at VIP for these 2 NPTs to save them.

 

[kidspot]

I've got a 72 with NPT threads in my LDS for Hydro at the moment, they did a "pre" vis to make sure it was worth getting a hydro and didn't say anything about the valve... of course I don't think this actually helps you any, but I'd assume if it was a DOT violation they would have needed to fail it...

Aloha, Tim

 

[Sea View]

I've got a 72 with NPT threads in my LDS for Hydro at the moment, they did a "pre" vis to make sure it was worth getting a hydro and didn't say anything about the valve... of course I don't think this actually helps you any, but I'd assume if it was a DOT violation they would have needed to fail it...

Aloha, Tim

Hey Tim Thanks for the Reply, I want to believe that is the case also, they should have failed it or brought up the DOT concern last year during the hydro.
I'll drop it of with the guys who did it last time and se if I can get it VIP'd.

JC

 

[Lead_carrier]

I don't know if there is a real safety issue or the LDS is being overly conservative, they also have stopped filling any Luxfer AL80s older than 1990 - regardless if the pass the fracture testing (which they offer - but won't fill even if they pass - strange).

I guess I'll have to go back to the old LDS for an attempt at VIP for these 2 NPTs to save them.

I haven't run in to the issue with the NPT tanks, but I have with the older aluminum tanks. DON'T take one older than 1990 to Florida. They will not fill them, but in the same breath they will grab the 2400 psi steel, ask how many burst disk you have installed so they can take them to 3500 psi. ( they told me it was safe because the do it in Europe.)

 

[Whitelightnin]

The NPT threads are not a DOT violation. You probally ran into someone who wasn't sure & just made up the DOT thing. I have several of the tanks with the NPT threads & still use them. Also, I am a DOT licensed hydrotester as well as a scuba shop owner.

James

 

[Sea View]

James, thank you - I was hoping someone in the business would comment on this, I was surprised that they got through Hydro if they Violated DOT spec.- I thought something was fishy. I'm starting to think that if it's not new or they didn't sell it they won't touch it. Thanks for confirming the DOT violation concern!
Maybe it's a bad idea to drop off my double hose w/these guys for a tune up
JC

The NPT threads are not a DOT violation. You probally ran into someone who wasn't sure & just made up the DOT thing. I have several of the tanks with the NPT threads & still use them. Also, I am a DOT licensed hydrotester as well as a scuba shop owner.

James

 

[Sea View]

I stopped using my AL80's, some time ago - too bad they are still in very good condition. I have 2 Luxfers circa '83 and A USD '79 (don't know who made those - the stamp is USD, nothing else ) I think they're destined to become wind chimes.

I've heard the stories on the steel tanks, the cavers are pushing their LP steels to 20-25% over the operational rating because of their faith in the 4000PSI @10,000 cycle testing.
Personally I'd stop short of 3000, it's not the steel tank I'm all that concerned about, it's the yoke on my regs (Old Scubapro, Rated at 3K) - I don't want the 1st stage punching through the back of my head.

I haven't run in to the issue with the NPT tanks, but I have with the older aluminum tanks. DON'T take one older than 1990 to Florida. They will not fill them, but in the same breath they will grab the 2400 psi steel, ask how many burst disk you have installed so they can take them to 3500 psi. ( they told me it was safe because the do it in Europe.)