Liquid Nitrogen Fills
by Deryck Webb
Filling cryostats with LN2 is routine for most NMR facility managers and it becomes a ritual that is seldom thought of or altered. However, it is important to remind ourselves of the dangers inherent to handling cryogens. This article will offer the reader a perspective on how LN2 fills are done here at NANUC. I will outline my usual procedures and the precautions I take to avoid the hazards posed by LN2.
The most important piece of equipment you would like to survive the filling procedure is yourself! In a Varian NMR News from February 2002 there was a very interesting article on the dangers of LN2.
Consider two situations: a quench filling the room with helium, and a simple nitrogen fill in a magnet pit:
1. The quench could cause someone working on a magnet to receive a cryo
burn and fall to the ground, possibly sustaining injury. As long as the
room is big enough, the helium might rise to the upper part of the room,
leaving only the injuries caused by the fall or initial burn.
2. Carelessness while nitrogen filling produces a nitrogen rich atmosphere,
which in turn causes disorientation followed by unconsciousness. Falling to
the ground now puts you in an even higher concentration of nitrogen. Within
a very short time you will be dead.
-- John Breslin, Varian U.K.
To avoid the dangers of a low O2 area I now wear an O2 sensor as recommended by both Varian and Oxford. For more information go to www.crowcon.com. Gloves and safety glasses should also be worn to guard against cryoburns.
Because of its very nature, the most important thing when working with a cryogen is maintaining control. The extremely cold temperature can freeze human flesh very rapidly. When spilled on a surface the liquid tends to cover it completely cooling a large area.
Venting into an insulated container will control the overflow and not allow LN2 to disturb sensitive seals.
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When filling a magnet, I always orient the dewar so that the liquid nozzle is pointing away from the cryostat. If there is a break at the nozzle, or it is blown off, the LN2 or gas isn't blowing directly onto the cryostat. Also, to limit LN2 spillage I vent the cryostat into a sufficiently thick Styrofoam box. The operator is still able to visually see when the cryostat is full, but the LN2 isn't spraying out the vent port uncontrollably.
Before initiating the fill, the transfer lines are purged with N2 gas to remove any O2 or water. With a self-pressurizing dewar the filling pressure is controlled with a main flow valve. To minimize internal stresses as material is cooled, flow should be started slowly and then maintained at about 1L/min. In cases where N2 gas is used to pressurize the dewar there are two valves: one allowing the pressurizing N2 gas(5psi) to flow into the dewar; another allowing LN2 to flow out.
With both valves closed on the LN2 dewar, the N2 gas is connected to the vent port and the liquid port is connected to the cryostat. The liquid valve is open, but there will be little or no flow as the dewar is not pressurized. While supporting the filling tube to ensure no kinks, the vent valve, which controls the pressurizing N2 gas, is slowly opened in stages. The LN2 will begin to flow gradually freezing the filling tube. When the filling tube is frozen and stable the vent valve can slowly be opened fully so that the LN2 flows at 5psi.
(Click to enlarge)
When I approach 0.03-0.05% of the fill limit, I slow down the transfer to avoid excess LN2 venting. When liquid is seen from the vent tube, the liquid valve is closed and in the case of a manually pressurized dewar the pressure is released. The tube is allowed to warm for about 4 to 5 minutes and then removed. Excess rocking of the magnet is avoided by grasping the stack and countering the pressure applied when putting the heat exchangers into place. The transfer dewar is stored in a cool, dry room with safety valves open to allow venting of excess pressure.
It is easy to become lackadaisical when using LN2. This article is meant to refresh one's understanding of the dangers involved in the common place ritual of filling the cryostat with LN2. Simple adjustments to some procedures can help avoid devastating accidents and injuries.
We all like to keep our magnet rooms and magnets tidy, but one common problem is water stains and accumulated dust around areas which see condensation. To limit the amount of frost which builds up on filling tubes and vents I use 3/4" rubber insulation. I purchased mine at a refrigeration supplier, but they are available from Oxford as well:
There is still a little condensation on the insulation, but it does not accumulate and drip off. The transfer and vent tubes will need a little more time to thaw after the transfer, but if you leave the insulation off the dewar port area it will thaw quickly and can be removed after a couple of minutes.
Deryck Webb is the NMR technologist at NANUC, and can be contacted at firstname.lastname@example.org.