Re: [AMMRL] pipes for helium gas

Dear Zdeněk,

We use 1 and 1/4 inch stainless steel pipes for the helium gas transfer from NMR magnets to a large airbag. The pipe length is 35 meters, the airbag volume is 32m3 or 1000 ft3. Our experience states that:

1. At normal boiloff the pressure inside the system is 0.2 mbar, this is the half weight of the airbag per its area. We continuously measure that pressure with a sensitive digital pressure meter that works in the range -30..+30 mbar.

2. When the NMR magnet is refilled with liquid helium, the pressure in the pipe system increases. To avoid overpressure of the magnets, we use 20mbar check valve that flushes helium to the air and some self-made alarm system that produce sound if that occurs. Initially we used a 6 mbar check valve, but replaced it with 20 mbar after one year of regular helium refills. The alarm system with 6 mbar check valve produced sound at the beginning of every helium transfer. Additionally, the system produced the sound in these cases: (i) if the magnet is full and liquid helium goes from the magnet to the pipe system, (ii) if the dewar with LHe is empty and the warm helium gas goes to the magnet and (iii) when we refill pumped Av800 Bruker magnet which is closest to the check valve. The last point was the reason to replace the 6 mbar check valve with a 20 mbar one. Bruker allows overpressure 1 PSI = 70 mbar, so  we are inside allowed overpressure range for Bruker magnets.

4. The check valves on exhaust of each magnet were kept when the pipe system was installed. Hence the overpressure above does not produce helium back flow to other magnets if one magnet is refilled.

3. Stainless steel pipe is covered with snow and frost 6-10 meters apart from the magnets during helium refill. The temperature inside the stainless steel pipe at a distance of 2 meters from the magnet goes down to -160 degrees celsius while helium refill is in progress. We use a radiator (2 meter length) to increase the temperature on the piping path from magnet to airbag to ensure positive helium temperature in the airbag. Actually the 0.5 meter of radiator is enough to heat the helium up to room temperature. Are you sure that PE (polyethylene) would be Ok at that temperature? Nonetheless, normal boiloff (between refills) produces room-temperature helium, and we use silicon pipes to pass that helium e.g. though atmospheric pressure device.

4. When the farthest magnet is refilled (35 meters apart airbag), the alarm system produces its alarms with ~5 second delay. That is the time that is required for excessive pressure to go through the pipe.

5. Your airbag volume (500L) makes me very doubtful. This bag will be filled by evaporation of less than one liter of liquid helium (1L of LHe at 4.2K produces 750 liters of helium gas at room temperature). That bag may be used only if you have a very powerful helium compressor that will pump all the helium that evaporates while helium refills. We have a compressor that pumps 8.3 m3/hour and it can't pump instantly all the helium that evaporates during helium refills (20-40 m3/hour in our practice, we refill ~250 liters of LHe per one refill of one or two magnets). We pump evaporated helium while refill is in progress and additionally 4 hours after refill is finished.

6. If you would prefer a powerful compressor, you should be ready to stop helium transfer instantly if any problem occurs with compressor or lasers that control airbag status (see previous reports in this chart a few days before). While the system was designed, I considered that a more powerful compressor is sufficiently more expensive than an airbag of large size and now we successfully use the large airbag.

Maxim