Cryogenic Liquid Cylinder Testing- NER Intro

NER is a TLA (Three Letter Acronym) for Normal Evaporation Rate. This is the expected boil off rate of cryogenic liquids due to heat incursion and associated temperature rise in liquid nitrogen, argon, or oxygen above their boiling points. As the liquids go above the boiling point they (by definition) turn to gas. As gas accumulates in the vessel’s head space pressure builds. If the gas is not used as product the pressure will build to the point that the vessel vents off. Excessive venting is an indication that a vessel is not performing properly and might have a weakened vacuum allowing greater heat incursion.

The release of molecules through venting reduces the weight of the vessel. So- the easiest way to measure NER is by weighing the vessel over a period of several days. Conducting a fully controlled NER is difficult to accomplish in most production environments as it requires fully stabilizing the liquid, not moving the vessel during testing, and taking readings at nearly the same time over a period of days along with having a scale capable of measuring nearly 1000 that is not needed for other purposes.

Tomorrow, we’ll detail steps for a full NER test. In the meantime, it is important to pressurize your vessel and check all plumbing and fitting connections for leaks with an approved solution prior to starting the NER. You should also use the leak test on the outlet of all connections to assure that none of the valves are leaking.