Cryogenic Liquids

Liquids ·

Cryogenic liquids are stored and transported at temperatures below –150 °C. They include liquefied natural gas (LNG at –162 °C), liquid nitrogen (–196 °C), liquid oxygen (–183 °C), liquid argon (–186 °C) and liquid hydrogen (–253 °C). They represent approximately 1% of pump applications globally, but the engineering complexity of cryogenic service far exceeds that of any conventional liquid duty. Material embrittlement, vaporisation management and thermal insulation are the defining challenges.

Common Cryogenic Liquids

  • Liquid natural gas (LNG): –162 °C at atmospheric pressure; density 0.43–0.47 g/cm³; the fastest-growing cryogenic commodity globally
  • Liquid nitrogen (LIN): –196 °C; density 0.81 g/cm³; non-flammable, non-toxic but an asphyxiant by oxygen displacement; the most widely used cryogenic liquid
  • Liquid oxygen (LOX): –183 °C; density 1.14 g/cm³; a powerful oxidiser — all organic materials must be excluded from contact; fire and explosion risk if contaminated
  • Liquid argon (LAR): –186 °C; density 1.40 g/cm³; non-reactive; used in steel manufacturing, semiconductor fabrication and welding
  • Liquid hydrogen (LH₂): –253 °C (only 20 K above absolute zero); density 0.071 g/cm³; extremely flammable and explosive; used in rocket propulsion and emerging hydrogen fuel applications

Key Physical Properties and Challenges

  • Boiling point and vapour pressure: All cryogenic liquids boil at atmospheric pressure at their storage temperature — any heat input causes vaporisation and pressure build-up
  • Density and viscosity: Kinematic viscosity is very low (0.1–0.3 cSt) — similar to solvents; centrifugal pump efficiency is acceptable but NPSH management is critical
  • Expansion ratio: LN₂ expands 694:1 on vaporisation from liquid to gas at atmospheric pressure — a small liquid leak can displace all oxygen from an enclosed space
  • Material embrittlement: Standard carbon steel and many aluminium alloys become brittle at cryogenic temperatures and fail without warning — only specific cryogenic-grade materials are acceptable

Material Embrittlement

The ductile-to-brittle transition (DBT) occurs in ferritic steels at temperatures below approximately –40 °C. At cryogenic temperatures, standard carbon steel and ferritic stainless steel are completely unsuitable. Materials that retain adequate ductility and toughness at cryogenic temperature include:

  • Austenitic stainless steel (304, 316, 310): Excellent toughness at all cryogenic temperatures — the standard material for LN₂, LOX and LAR service
  • Aluminium alloys (5083, 6061): Toughness increases at low temperature — widely used in LNG storage tanks and LNG pump construction
  • 9% nickel steel: High nickel content preserves toughness to –196 °C — used for large LNG storage tank shells
  • Inconel 718: For liquid hydrogen service at –253 °C — maintains ductility to near absolute zero
  • Copper and copper alloys: FCC crystal structure — good cryogenic toughness; used in LN₂ and LOX piping

Pump Design for Cryogenic Service

  • Thermosiphon or submerged pump: The pump is submerged inside the cryogenic storage vessel — no suction piping, minimum heat ingress, no NPSH problem
  • Vertical turbine design: Pump bowl submerged in liquid; motor above at ambient temperature; long shaft with low-conductivity material to limit heat conduction
  • Cool-down procedure: Pump must be gradually cooled with cryogenic liquid before being brought to full speed — thermal shock from sudden cool-down causes seal and impeller damage
  • Insulation: Vacuum-jacketed casing and piping (vacuum insulation panels or perlite-filled annulus) are essential to limit boil-off and maintain stable liquid conditions at the pump inlet
  • Bearing and seal materials: Graphite-filled PTFE bearings run on the cryogenic liquid itself — no external lubrication required or possible at these temperatures

Safety Requirements

  • Oxygen-enriched atmospheres — from LOX leakage or LN₂ cooling of surrounding air below its dew point — dramatically increase fire and explosion risk of any combustible material
  • Liquid hydrogen requires strict ATEX Zone 1 protection plus special hydrogen-rated components — hydrogen embrittles many steels
  • All personnel must be trained in asphyxiation hazards and the use of oxygen monitors in enclosed cryogenic pump areas
  • Pressure relief valves sized for maximum vaporisation rate must be installed on all closed sections of cryogenic piping

Applications

  • LNG import terminals — ship unloading, storage tank transfer and vaporisation feed pumps
  • LNG fuelling stations for trucks, ships and locomotives
  • Liquid nitrogen and liquid oxygen production in air separation plants
  • Industrial gas distribution — filling of cryogenic road tankers and portable dewars
  • Aerospace — LOX and LH₂ fuelling of rockets and spacecraft
  • Medical gas supply — liquid oxygen for hospital breathing systems
  • Semiconductor manufacturing — LN₂ for wafer cooling and LAr for plasma etching