Ice Formation Safety Tips
Ultra Low Loss Cryostats offer considerable advantages over higher loss units in terms of operating expenses and minimized service requirements. However, they bring with them the potential for hazards not found in higher loss rate systems.
In particular, ultra low loss cryostats are more susceptible to atmospheric conditions than higher loss systems. It is not unusual for cryostats with helium loss rates below 50 cc/hr to experience internal pressure below atmospheric pressure when a high pressure system moves into the area quickly. If this happens, a partial vacuum will be formed in the helium chamber of the cryostat. It is not unusual to see helium boil-off completely stop for a period of hours under such conditions.
If the cryostat system is properly sealed, vacuum in the helium/magnet chamber will have no adverse effects. However, if seals on the system are not completely air tight, air and water can be drawn into the system resulting in considerable ice formation. Not only can this ice cause problems with electrical connectors located in the cryogen service stacks, but if enough ice is allowed to form, it can completely seal the helium chamber. If this happens, the chamber will overpressurize and the magnet will quench. A magnet quench inside a chamber plugged with ice could (and has been known to) rupture the helium chamber. This causes considerable damage to the system and can be dangerous to operating personnel.
All low loss systems eventually accumulate some ice within them. Typically, ice formed during helium transfer falls harmlessly to the bottom of the cryostat. But the system’s operators should be aware of the hazards of ice and should make every effort to minimize it.
The steps outlined below describe proper procedures for insuring a well sealed system. It is highly recommended that these procedures be followed each time a liquid helium transfer is made or the system is opened for any reason. This will insure minimum ice is generated within the system and minimized risk to the system’s operators.
SEALING PROCEDURE TO BE FOLLOWED AT THE COMPLETION OF EACH HELIUM TRANSFER:
1. Minimize the time the system is open to the atmosphere during liquid helium transfer
This is the most likely time for air and water to be drawn into the system.
2. Immediately following helium transfer, seal all valves and fittings
Carefully check all hoses, pop-off valves, etc. for leaks by wiping soapy water over all connections and watching for bubbles to appear.
3. Monitor off-gas from the system using an appropriate gas flow meter
Helium flow should begin within 24 hours after sealing the system. If gas flow through the flow meter is not detected within 24 hours, recheck all seals carefully and determine whether a High Pressure Atmospheric System has moved through your area during the past 24 hours. If one has, monitor the flow for another 24 hours.
4. Periodically check the flow meter on the system to see that helium gas is flowing out of the system
If gas flow is not detected for more than 24 hours, recheck all fittings as described above. If no off gas is generated within another 24 hours, CONTACT THE FACTORY IMMEDIATELY!