What pressure does liquid nitrogen exert during storage, and how is it safely managed?

Liquid nitrogen storage pressure depends on the vessel design and its intended application. Open dewars used in most IVF laboratory and sample storage applications operate at or near atmospheric pressure. The LN2 sits in direct contact with atmospheric air through the open neck, and evaporation occurs at a controlled rate determined by the quality of the vacuum insulation.

Closed, pressurised liquid nitrogen vessels used for dispensing and transfer applications are a different category entirely. These operate at working pressures typically between 5 and 10 bar (approximately 70 to 145 psi), controlled by a pressure building coil and regulated by a pressure relief valve. The pressure relief valve is not an optional safety feature – it is a critical component. If a pressure relief valve fails or is obstructed, internal pressure builds until either the relief device activates at a higher threshold or, in a worst-case scenario, the vessel ruptures.

In the UK, pressurised cryogenic vessels are subject to the Pressure Systems Safety Regulations 2000 (PSSR), which require a written scheme of examination, regular inspection by a competent person, and maintenance records. Facilities using pressurised LN2 dewars for dispensing or bulk transfer must ensure these regulatory requirements are met and documented.

A specific pressure hazard relevant to standard open laboratory dewars is ice plug formation. When moisture in the air freezes in the neck of a dewar, it can form a partial or complete blockage. Behind an ice plug, nitrogen gas pressure builds as the liquid continues to evaporate. The plug can be expelled at high velocity or, in more severe cases, the accumulated pressure can rupture the inner vessel. BCGA Code of Practice CP30 specifically addresses ice plug management and instructs users to immediately vacate the area if an ice plug is discovered.

Never attempt to remove an ice plug from a liquid nitrogen dewar by force. The correct response is to move the dewar to an open, well-ventilated outdoor area and allow the plug to clear naturally as the vessel warms.

For storage vessels with proven pressure management and safety performance, view Cryolab’s range at cryolab.co.uk/product-category/storage-vessels/.