Recommend a thread sealant for this problem?

Hi,
I welded together a low pressure air tank (120psi) for a project. While tightening a 1/4-18 NPT safety relief valve into a coupling, the coupling cracked down one side. Since the coupling is welded to the tank, I can't replace it easily.

I welded the crack closed, but I think the coupling is now distorted. Even with 5 wraps of teflon tape, it still leaks air slowly past the threads on the cracked side of the coupling.

Is there a putty or thread sealant that will stop this kind of leak? It would be nice to be able to remove the relief valve later, but a permenant sealant would be ok if that's what it takes to fix it.

Thanks!

I would not recommend using that tank unless You have the certifications required for welding a pressure vessel. In that case you would already know what to do with the cracked bung. Compressed air can cause violent explosions at considerably less pressure than you are using.

I would not recommend using that tank unless You have the certifications required for welding a pressure vessel. In that case you would already know what to do with the cracked bung. Compressed air can cause violent explosions at considerably less pressure than you are using.

No, I’m not a certified pressure welder. You’re probably not a certified plumbing instructor. That doesn’t mean you’re stupid. However, I do appreciate your concern for my safety.

The tank is well over-built with heavy 6â€x 6â€x 3/8†square tube capped on the ends with 3/8 plate. At 120psi, there’s only a little more than 4,000 lbs. pushing on the ends. The design is copied from a time-tested model currently on the market. I hydro-tested the tank before compressing air in it (no flies on me).

When the coupling cracked, the crack was forced open. This made the inside diameter of the coupling just a hair larger, and possibly slightly oval shaped. I did chase the threads with the tap after welding the crack. I didn’t want the weld to penetrate through to the threads, because I don’t think the tap would even go through that without drilling it out first, and it had to still be ¼-18 NPT when I was done. Besides, cutting threads in welded metal is difficult at best.

The customer is happy even with the slow leak, but I can’t let it go out like that. A two-part epoxy based product sounds like just the thing. I’ll look into JB Weld and Marine-tex.

Thanks!!

crack

If you did not penetrate the crack all the way to the threads, so that they had to be retapped, then there is still a crack at the threads and that is what is leaking.

If you did not penetrate the crack all the way to the threads, so that they had to be retapped, then there is still a crack at the threads and that is what is leaking.

And I'm assuming JB Weld will cover that. Right?

Jb W

It might, you can try it because you have nothing to lose. Either it works or you replace the split coupling.

I would replace the coupling. To much time to try to fix.

I'd throw it in the scrap bin and buy one! Your basic design violates the best practices for design of a pressure vessel. Your welding skills and methods are in doubt definitly not certified. Now your trying to patch cracks with JB Weld... On top of all this you are making this for a customer. Have a read and see what the basics are...

http://en.wikipedia.org/wiki/Pressure_vessel

1) hope you have liability insurance.
2) don't use so much teflon tape, it will break fittings


At best, the relief valve would only blow out and lodge in the poor customer's brain as he's filling the tank through that schrader valve.


Take note, don't get cocky here. Some of us are giving you information which is contrary to what you wish to hear. Don't get defensive to those of us who think and operate by the 'worst case scenario' method. We're not trying to make your life hell, but we feel your approach to the problem at hand is niggardly and exposes the end user to grave physical harm. To fix this correctly would require a great deal of work for you, but that is inevidable in my opinion. You're hoping to make money off a product which is known to be defective, as explained by your comments here. Be aware that a plaintiff's lawyer might find your posts very interesting some day.

You should not be asking about how we sleep at night, but thanks for your concern.

Thanks for the tip on not using too much teflon tape.

The customer knows my skills, has been right along with me as I built it, and has released me from any liability.

I apologize if I sound ungreatful, but I only asked for a thread sealant recommendation, not information on how to live my life.

Thanks!

Not to start a fight, but you are being very very naive!!! Customer relieved you of liability, don't fall into that trap. Consider the time he or someone else decides to have some "fun" with it, puts something stronger than confetti in it and someones eye gets destroyed. The list of possibilities is endless, could even be an oversight that something else ends up in the cannon. Guess who they are going to come back to. That's not even considering the possiblity of a fitting cutting loose.

Unless it was my business, I had sufficient certification and insurance, that thing would go in the trash! Look at it this way, is it worth loosing everything you have worked for in life?

I sleep well at night... I have trade certifications, use approved methods, and have liability insurance...

How about you? What have you got for assets someone may want to own in a settlement?

"he or someone else decides to have some "fun" with it,"

Then I believe he or someone else would be responsible. Don't you?
Like if I sold him a gun, and he shot someone?

Also, because the crack is securely welded, and these are tapered threads, the threads are getting pleanty of bite, just a little deeper in the hole.
The valve is not going to blow out.

Remember this is just a slow leak, losing about 10psi an hour.
It's NOT air gushing past a valve that's wobbling in it's fitting.

Hey BTW, I saw a regulation basketball hold 100psi before bursting on Discovery Channel last night. So you guys think my welds are like rubber?
That's just insulting.

Did you provide sufficient safeguards to keep the device from being misused, warning placards, training manuals, etc? Keep in mind that the $1150 is not all materials and labor, but other overhead items like certification and insurance. Not busting on you, but there is a real world out there. Been in business too long, and had have seen too many horrors to want to put myself at that kind of risk.

It does look nicely done, but that does not relieve liability. And, it is a pressure vessel.

Not to torture you - but I have designed and built many machines, and in this case would have scavenged a used compressor tank or bought a new surplus one ["Surplus Center" in Omaha] for the job.... Okay lets say the ports were wrong, I know you need a big outlet.

I would have put 1/2" or 3/4" end caps on that tubing and PRIOR to welding it up, would have drilled and tapped the 3 ports that you appear to have welded on. Or to have drilled holes, and then inserted flanged nipples through from the inside and brazed them in place. Neater, cleaner, not likely to blow out and lodge in some kids forehead. Better yet to have drilled and tapped ONE outlet and to have tee'ed for the others.

Nice silly machine [what the hell do you do with that?] and I doubt that it will ever cause a death, but I would mix some machining and more subtle welding into it. I would NEVER weld on a coupler as a port. Many of the import couplers have very weird metallurgy and you will have unknown stresses and future break points. You got yours early, as a gift. Brazing would be more predictable. But remember coupler are couplers, not tank outlets!

Did you preheat this structure while welding and cooling down? That will make a big difference in its integrity.

For what its worth dept. : I have a pressure vessel MFG date 1916, its steel with bronze cast end caps, maybe 150 gallon. The end caps are brazed on, and it will loose about 5psi per month. Never saw anything like it, but man, what a great piece of work. Dont make them like that anymore.

All that said, I would only sell that if I was dirt poor or had BIG insurance for pressure vessel construction [fat chance!]

“Not to torture you - but I have designed and built many machines, and in this case would have scavenged a used compressor tank or bought a new surplus one ["Surplus Center" in Omaha] for the job.... Okay lets say the ports were wrong, I know you need a big outlet.

I would have put 1/2" or 3/4" end caps on that tubing and PRIOR to welding it up, would have drilled and tapped the 3 ports that you appear to have welded on. Or to have drilled holes, and then inserted flanged nipples through from the inside and brazed them in place. Neater, cleaner, not likely to blow out and lodge in some kids forehead. Better yet to have drilled and tapped ONE outlet and to have tee'ed for the others.

Nice silly machine [what the hell do you do with that?] and I doubt that it will ever cause a death, but I would mix some machining and more subtle welding into it. I would NEVER weld on a coupler as a port. Many of the import couplers have very weird metallurgy and you will have unknown stresses and future break points. You got yours early, as a gift. Brazing would be more predictable. But remember coupler are couplers, not tank outlets!

Did you preheat this structure while welding and cooling down? That will make a big difference in its integrity.

For what its worth dept. : I have a pressure vessel MFG date 1916, its steel with bronze cast end caps, maybe 150 gallon. The end caps are brazed on, and it will loose about 5psi per month. Never saw anything like it, but man, what a great piece of work. Dont make them like that anymore.

All that said, I would only sell that if I was dirt poor or had BIG insurance for pressure vessel construction [fat chance!]”




Thanks Raucina,
You made some good points. I did try to locate a pressure vessel that would work for this application. You are correct that the ports are wrong, and I did need a big outlet.
I needed a 1” output from the tank to the solenoid valve. I looked into paintball-gun tanks, etc. but nothing had that large an opening. Trying to modify one of those would be more dangerous than anything.

I decided the end caps were too thin (5/16”) to tap with NPT threads. If I had it to do again I would take your suggestion and use ½” end caps and tap directly into them.

The “couplers” I used are actually reducers. The fat ends don’t fit through the holes. As you suggested, they were inserted from the back and welded to the inside of the cap before the cap was welded to the tank. They couldn’t come out even if they were not welded. I agree, brazing would have been better for this job, but I wasn’t sure the strength would be enough.

If you’ll notice, all the valves are below the top of the frame, so they are less likely to get broken off. A single “T” would not allow that.

This was a big chunk of cold steel, so yes I had to pre-heat before welding.

Did anyone read my post that I saw a basketball hold 100psi before bursting on Discovery Channel? I'm sure no part of my tank is weaker than a basketball.

Thanks!

Tick tick tick...

BOOM

phupper said:

Did anyone read my post that I saw a basketball hold 100psi before bursting on Discovery Channel? I'm sure no part of my tank is weaker than a basketball.

Click to expand...

Yup, I read it! The one major difference between your tank, and a basketball is basketballs won't throw shrapnel for hundreds of feet. Also, if you read the wiki link I posted earlier, the basket ball is the ideal shape for a pressure vessel. The wiki article states, the optimum shape for a pressure vessel would be a sphere.

Psst... Wanna see what happens when a pressure vessel blows?


or,
http://www.doli.state.mn.us/pdf/tenn_boiler_pictures.pdf
Yea I know your tank isn't that big, but the sudden release of energy from a pressure vessel isn't pretty even on a small scale.

Now enough of this... ___

"but the sudden release of energy from a pressure vessel isn't pretty even on a small scale"

It is when it's full of CONFETTI!!

Yea I've seen that Mythbusters episode. I might point out that they had to go more than 2X the rated pressure to get liftoff. My tank is only ever filled to less than half the pressure I think it would withstand, which is why it can be square.

Thanks!