During the many safety audits we conduct it is commonplace to find a user of liquid nitrogen putting himself and others in considerable danger by allowing the opportunity for liquid nitrogen to be trapped during his / her application process.
Liquid nitrogen is commonly delivered to an application by use of a stainless steel armor cased cryogenic transfer hose, commonly four feet long. One end is connected to the liquid nitrogen vessel and the other end to the equipment requiring the liquid nitrogen. These systems are often automated so there is a solenoid valve on the equipment end that calls for liquid nitrogen when the system needs it and shuts off the supply when the system is satisfied. This sounds pretty straight forward and would work fine if it were not for the presence of one fault, the human factor.
If the system were running as previously described, all would be well. However, adding the human factor introduces the opportunity for a margin of error. If someone were to shut of the valve on the liquid nitrogen vessel while the solenoid valve was closed they would have trapped liquid nitrogen between the two valves. Liquid nitrogen expands at approximately 700 times its own volume and does so very quickly.
The end result would be that your four foot cryogenic transfer hose has become a four foot pipe bomb. These accidents are rare but the potential for a catastrophic accident is present and steps to prevent it should always be taken.
The installation of a safety relief valve is required to relieve any unanticipated pressure build up in the transfer hose. The safety relief should not be placed in the direct line of the cryogenic flow as it would freeze and be rendered useless. An appropriate safety relief valve should be constructed so as to remove the relief valve from the cryogenic flow where it could remain operative. For more information on this subject please contact us.