Steel Tank and Damaged Threads?

[Rand]

I was putting together some X7 100cu ft. steel tanks with thermo pro valves and one of the valves did not want to seat properly. I got to within a penny's width of where it should sit on the tank neck and then the valve got very difficult to turn. I removed it and observed that the threads on the valve showed signs of wear. I tried several times with the same result.

I was using a wrench to tighten, but stopped with I felt the unusual resistance.

The wear to the valve threads is at the first couple of threads that start catching the threads on the tank neck, so this section is well inside the tank. The valve appears to be level and is threaded on properly.

I haven't had a chance to get it filled to see if 1. it can hold pressure in this position and 2. if I have damaged the valve and/or tank necessitating repair/replacement.

I was wondering if I could file down the worn threads and still use the valve.

I'm taking it to the LDS this week, but would like to know what to expect if anyone has any insight to offer. Thanks.

 

[crpntr133]

Doesn't make sense that it would be those threads since it makes it almost all the way in. If the trouble was the first couple threads I don't think it would make it that far. Do you have the right O-ring?

 

[XS Scuba]

Hey Rand:
Don't try to fill the cylinder until the valve is fully seated. Good idea to take it in to your LDS. Most likely the issue can be resolved by chasing the threads with the correct tap. If you still have problems give me a call. BTW, I'm traveling this week and will be back in the office following Labor Day.

I'm taking it to the LDS this week, but would like to know what to expect if anyone has any insight to offer. Thanks.

 

[DA Aquamaster]

It sounds like the threads are not fully cut near the bottom of the neck. As suggested above, chasing it with a tap should correct the problem.

Stop trying to install the valve - the softer brass will give way to the steel in the tank so you'll just end up screwing up the valve. Do not file anything off the valve threads. You'll end up with a valve that will not have enough thread to thread contact to be safe.

 

[oxyhacker]

Did you try another valve (gently), just to isolate whether the problem was with the valve or tank threads, or both?

Technically, you should not use a thread cutting tap on tank neck threads, since removing metal from the tank is against DOT regs. You should use a thread chaser which pushes the metal back where it belongs without removing it. However, you'll have to look long and hard to find one in 3/4-14 NPSM and it will be expensive if you do. That being the case, many divers and shops will give a nervous look around to make sure no one is watching, then run a tap down the neck. Since tank threads are required to be designed to withstand 10 times the normal tank pressure there is fair safety margin, but you really don't want to be removing any more metal that absolutely necessary. This is one case where sharper is not better, and an old dull tap actually better than a new one, since it will tend to reform the threads rather than recut them.

Hopefully the shop will have a tap so you don't have to buy one, but if you do, a 3/4-14 NPSM (often abbreviated to NPS) tap can be had from most industrial suppliers for $20-40.

 

[Muad'dib]

Are you 100% sure you got the right valve for these tanks
M25 does somewhat fit"s 3/4 threads,but it wont go all the way.
Please don't fool around with tanks when you're not 100% sure.
I've seen one go 30meters.100ft up into the air,and it was NOT funny.

 

[Rand]

The shop checked it out today and the problem was that the tank threads were not cut all the way down. Even though I did not buy the tank from them, the shop called XS and explained the problem. XS offered to send them the tap to correct the problem (saving me the shipping charges).

The valves were shipped along with the tanks, directly from the manufacturer, so I'm fairly certain they are the correct ones. Plus, out of the six tanks I purchased, only that one had an issue.

I saved a few bucks by ordering from outside the shop, but it would have been a little less hassle to get them from the shop and have them assembled correctly the first time. Plus, it's not convienient to drive halfway across the country if you need some personal attention to handled an issue. Lesson learned.

 

[oxyhacker]

I find this a little wierd. Tapping the neck is part of the manufacturing process, since it involves metal removal and could (if badly done) effect the strength of the neck and threads, which are critical to the safety of the tank. And the tank is supposed to, as I understand it, be hydroed after the manufacturing process is complete, to insure that everything went as planned. Come to think of it, how did they ever hydro it if the threads were incorrectly formed?

So you or the shop shouldn't be finishing up the job of manufacturing the tank. If the tank wasn't made righ, then they ought to give you a new one, or at least get it back to the factory so they can be sure the job is done right. Sending a tap so you or the shop can finis the job is a bit like sending you a hammer if the tank was dented. Sure it'll probably turn out just fine, but it isn't how it is supposed to be done.

I may be overreacting, and would be interested in hearing Mike's comments on this.

The shop checked it out today and the problem was that the tank threads were not cut all the way down. Even though I did not buy the tank from them, the shop called XS and explained the problem. XS offered to send them the tap to correct the problem (saving me the shipping charges).

 

[WD8CDH]

Possably the threads on the valve were a little damaged and when threaded into the tank gummed up the tank threads. If this is true, chasing the tank threads is a very valid fix.

 

[DennisW]

Sounds to me like XS Scuba did the right thing and you have a great LDS. They went beyond what was necessary for you. See, folks there are some great LDS out there.