Hydrotesting of custom pressure vessel
Hydrotesting of custom pressure vessel
(OP)
I am working on a project where we have a custom pressure vessel that is made with 1in DOM tube with some end caps brazed onto the ends. The intention is to pressurize this up to 9ksi with nitrogen gas for a test we are doing. I had no involvement with the design of the vessel; however, I am not trusting of the brazed end caps for such high pressure. I had two of these vessels hydro tested today and both of them lost pressure and began leaking at 8ksi. Further inspection does not reveal any leaks or visible cracks in the vessel. Is it possible this leak/loss of pressure was solely due to the threads? Unfortunately, it is an NPT thread. After the test failed at 8ksi, we were able to retest it up to 8ksi before it started leaking again. Being new to hydro-testing, is this a common result? Is the ability to re-pressurize to a high pressure a sign that there is a bad connection or would this be an indicator of a crack in the material or end cap braze?





RE: Hydrotesting of custom pressure vessel
I assume that the brazed end caps then have NPT threads machined in them?
If you can re-pressurize to around the same pressure again before leaks occur it's a fair bet that the threaded connections are leaking.
RE: Hydrotesting of custom pressure vessel
Where are these threaded components? What is their rating and size?
Why is NPT a bad thing?
Can you draw out and describe each item (valves, pressure source, gauges etc)
Can you test with soap solution like this http://www.wintonproducts.com/id21.htm
If not why not?
It might be a failure eof the item or maybe a failure eof your test equipment. Can you try pressure testing something you know is good for the pressure to see?
I think you're right to be nervous of brazed end caps - they could detach and become projectiles - so I hope this has been considered in your test environment. 9000 psi sounds like a lot to me for such items.
why can't this be tested using water or oil / hydraulic fluid?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Hydrotesting of custom pressure vessel
I say unfortunately NPT because of the designed to leak situation that is going on there.
Thanks for all the advice.
RE: Hydrotesting of custom pressure vessel
Brazing doesn't mean much either way, I've seen braze joints at higher pressures work well - depends on the design of the joint.
"Could it be not using enough teflon on the npt fittings. " Could be. Could also be too much. Did you use any pipe sealant (Loctite or dope) along with the ptfe tape?
Could also be that your pump can't make it to 9 ksi, and starts leaking back at 8k. Did you test the pump with a dead-headed gauge first? When was the gauge last calibrated?
RE: Hydrotesting of custom pressure vessel
Good point problem with the tools, not the product!
Jeff
Pipe Stress Analysis Engineer
www.xceed-eng.com
RE: Hydrotesting of custom pressure vessel
RE: Hydrotesting of custom pressure vessel
You can buy hand pumps that will pump to 10-12 ksi with hydraulic oil, see McMaster-Carr. For less than $500 your company would suddenly have hydro testing equipment.
Yes, use sealant.
RE: Hydrotesting of custom pressure vessel
RE: Hydrotesting of custom pressure vessel
Pressurizing the DUT with a gas in one of these test fixtures would be a very bad idea in my opinion.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Hydrotesting of custom pressure vessel
"On one end the end cap was drilled and tapped to NPT. "
What thickness are we talking about for the screwed part of the connection? Can't be more than 5mm?
I think the fact it has held 8000 psi without blowing out is a near miracle.
Have you got a drawing / sketch / photo?
I think you really need to re-think your connection into this minute piece of pipe.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Hydrotesting of custom pressure vessel
Please remember that there are other connections for valves and pressure gauge in the application. In case they are not leak free you will loose pressure anyhow.
Good luck.
RE: Hydrotesting of custom pressure vessel
That's a little sketch. The end caps are much thicker than the tube. There is about 0.5in of NPT thread in the end cap. I recognize the poor design in this situation; unfortunately for me, my only responsibility is to test these vessels and have them fabricated. I appreciate all the advice, good stuff. The actual pressure system I have to pressurize these vessels was designed to use coned and threaded connections and is rated at least 2.5 times greater than the pressure I will be using. The main purpose of testing these vessels was originally to ensure that those brazed end caps can hold that pressure. The numbers I have crunched for the vessel itself suggests that we will be close to yield but about 10ksi under. I am going to try some additional sealant tomorrow and retest this sucker. I will update the results.
RE: Hydrotesting of custom pressure vessel
Good luck.
RE: Hydrotesting of custom pressure vessel
Search "hydraulic hand pumps" in McMaster. Sounds like you may already have one.
RE: Hydrotesting of custom pressure vessel
Does this look like a typical failure?
So to investigate this water situation, and since the water in the chamber was released when the previous vessel burst, we tested the second unit without the water jacket. In this instance, there were no pressure fluctuations and we were able to keep it at 9.5ksi for about 10 minutes without pressure loss. I am not familiar with the water jacket testing so I have no idea what the issue was or if the burst pressure was legitimate since the vessel itself should be good up to 11ksi before yield. I can only wager that there was some sort of issue with the test apparatus and/or due to the use of such a small vessel in a large cylinder test chamber.
I also used 2 different thread sealant methods, one being liquid sealant and the other being mil-spec ptfe tape. Both sealed the vessel and there were no visible leaks around this connection when observed. The main purpose was to ensure the brazed end caps would hold and they did (quite incredibly). So its been an interesting situation, thanks for all the help. If I had access to a better testing facility, this likely would have been avoided completely; however, gotta make due with what's available.
RE: Hydrotesting of custom pressure vessel
You better go back to the 'designers' and beat them on their heads, telling them they should not put you into such a danger...
prex
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RE: Hydrotesting of custom pressure vessel
What the pressure fluctuations are I can't work out.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Hydrotesting of custom pressure vessel