A 9.4 Tesla superconducting magnet, used for mass spectroscopy in a campus laboratory recently suffered a catastrophic failure. The incident was apparently caused by over-pressurization and failure of the liquid helium (LHe) chamber. Although there were no injuries because the incident occurred during off-hours, the potential for significant injury due to the venting of LHe into the facility was present. There was also significant damage to equipment associated with the magnet.
A magnet achieves superconductivity (zero resistance to electrical current) when it is bathed in LHe. If for some reason the magnetic coil starts to resist the electrical current, it heats up, causing an explosive expansion of the LHe. This expansion of gas is vented through a large bore vent, sealed by a membrane called a "rupture disk". This process of explosive venting is known as a "quench".
Several days prior to the explosion, this unit quenched during off-work hours. The helium chamber was open to the air for several hours before staff discovered that the magnet had quenched. They placed a stainless steel flange closing the vent opening in order to prevent additional air from entering the chamber (under the direction of the magnet's manufacturer). Apparently, before the vent was closed, enough moist air entered the chamber and iced over the secondary vent that handled the normal boil-off from the helium chamber. When the magnet was put back into service with a non-yielding steel flange in place of the "rupture disk", and the normal vent hole closed due to ice, the off-gassing helium had nowhere to go and expanded until the vessel ruptured.
- After a quench event, users should suspect the possibility that the LHe vent or quench vent device might be blocked. This can be checked by putting a flow meter on the vent to measure helium output. If there is no measurable output, there may be a blockage and the potential for a catastrophic pressure buildup. Clear any potential blockage per the manufacturer's recommendations.
- For superconducting magnets, when a quench occurs, do not reseal the quench vent with a non-pressure rated relief material. Check with the manufacturer for the safe bursting pressure material (in this case it was 5 psi), and install only the proper material.