Easily the most commonly asked questions about 4L-DOT class cryogenic liquid cylinders relate to pressure ratings. This can be a bit confusing since the data plate itself does not tell you directly what pressure relief should be installed on the liquid cylinder.
Yesterday's question came from a client who asked about an application where their client is using low pressure (4L-100) liquid nitrogen vessels with 22psi reliefs. 4L-100 are low pressure cryogenic liquid cylinders and are typically for liquid-only applications. “Liquid only” vessels are built with no pressure builder or pressure builder circuit. The absence of pressure building and gas use coils help the vessels hold product longer (all other things equal) than VGLs since they have less heat sinks.
The question was- “Can a pressure builder be added to a Liquid Only 4L-100?” It seems the end user has a need to build a few pounds of pressure behind the liquid to push it out continuously. While technically possible to make some type of PB loop on a vessel designed for liquid nitrogen only this would be a non-spec modification and would never really work right. The more effective solution to meet this need is to use a VGL which already has a PB loop and fit it with a 22psi pressure relief. The PB loop would then only be cracked open for a brief moment and closed as needed. Otherwise the PB will cause the unit to build pressure above 22psi continuously and vent until empty.
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