First; put down that hammer!
Recently several incidents involving off-gassing liquid nitrogen cryogenic cylinders have occurred across campus. Each case involved a valve that had not been fully closed, which allowed a small amount of nitrogen to off-gas. Over time, this caused a buildup of ice in and around the valve, freezing it in the ON position. The ice prevented lab personnel from closing the valve, which resulted in a gradual loss of the cylinder's entire contents.
Until now, it was assumed that once a valve froze, the tank must be allowed to purge or it could rupture. However, Prax Air now recommends pouring warm water over the liquid nitrogen tank's valve to melt the ice, thus freeing the valve so it can be shut off.
Note: Precautions should be taken when attempting this method. If liquid nitrogen is being discharged from the valve (not just off gassing) or the cylinder is located in a small room or one with poor ventilation, do not attempt this method. Also, do not attempt to move an off gassing cylinder; allow it to purge until empty.