piping seal testing issue

[rcmac21]

I have a large piping test loop, 4" sch.40 SS304 that holds 20 gallon of fluid. I am using a very dangerous fluid and can not afford a leak. I first pressure test and have no leaks. Then heat piping to 200C and then vacuum test. I can pull a 1950 microns and when I disconnect vacuum I do not hold, I increase to 3500 microns. My issue is that I can not find my leak. I pressure test agian after it cools and no apparant leak. I am looking for advise on testing method. One more piece of information, I have entire piping loop wrapped in trace heaters and ceramic insulation, so testing for leaks is somewhat difficult. Attached is drawing of loop. My leak issue is critical as the fluid auto-ignites at 125C (when exposed to oxygen) and can not be permitted to come into any contact with water.

 

[Duwe6]

Use Nitrogen with 5% Hydrogen as your tracer gas. H2 is easy to detect in small quantities - so easy that I have seen successful leak detection done with the Safety Dept's air monitor. H2 leak will alarm the "Lower Explosice Level" detector of that air monitor.

After vacuuming out the system and heating it, carefully pressure your pipe circuit to it's full Design Pressure with the 5% H2 gas and start 'sniffing'. When the alarm goes off, then use soapy water to look for bubbles of leaking H2 in the area you are getting the alarm.

Unless your molecule you are trying to keep in your pipe loop is very tiny, the H2 will leak out 100's if not 1000's of times faster than your molecule. And, unlike helium, it is a renewable resource. When we use up the heilum, it's all gone.

 

[vpl]

"I have entire piping loop wrapped in trace heaters and ceramic insulation,"

It's going to be difficult to find a small leak if it's under the insulation or heaters. If it's critical that the system not leak, you really need to have access to the whole thing so that you can find the leaker. If you can't completely remove the insulation/heaters, then remove it from the most likely spots -- welds, joints, etc. Then follow Duwe6's suggestion.

Patricia Lougheed

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