Cryogenic Tank Boil-Off Rate and Hold Time

The boil-off is the rated normal evaporation rate applied to the contents: an 11,000 gallon liquid nitrogen tank at 0.25% a day loses 27.5 gallons every day, used or not, which is 825 gallons a month. On a site drawing less than that, boil-off exceeds use and the tank loses more than the process takes. That is the argument for tank sizing running counter to intuition, because a LARGER tank has less surface area per unit volume and so a lower percentage boil-off -- two half-size tanks lose more than one full-size tank holding the same total. The hold time is the second half and the intuition there is backwards too. With the vent closed the boil-off has nowhere to go, so it pressurizes the vapour space, and the time to reach the relief setting is proportional to how much vapour space there is: that tank at a 20% vapour space holds about 22 hours from 30 to 75 psig, and at a 60% vapour space it holds three times as long. A tank at 90% full is not a tank with more margin -- it has less. The hold time is a screening estimate from the ideal gas law at the entered vapour temperature; it holds that temperature constant and ignores liquid stratification, the saturated coupling between liquid and vapour, and heat going into warming the liquid, so the manufacturer's published hold-time data governs. The rated evaporation rate is itself measured under defined conditions at a stated fill and pressure, and real boil-off varies with fill, ambient temperature, and solar exposure. A tank boiling off at many times its rating -- 2% a day where 0.25 is rated -- has lost the vacuum in its annulus, which frost or sweating on the outer jacket confirms; that is a tank problem rather than a fitting leak, and a vessel that is no longer insulated vents continuously. It does not size relief devices, which must also cover the fire case, and does not address the pressure building coil, the economizer, or withdrawal behaviour. It does not address the asphyxiation hazard in confined or poorly ventilated spaces, which is the leading cause of cryogenic fatalities, or the cold burn and embrittlement hazards. CGA standards, NFPA 55, and the tank manufacturer govern.

boil-off = tank contents x the rated normal evaporation rate, with the implied heat leak that boil-off mass times the latent heat of vaporization; hold time = the vapour mass needed to raise the vapour space from its current pressure to the relief setting, from the ideal gas law at the entered vapour temperature, divided by the boil-off rate.

A boil-off and hold-time estimate from a rated evaporation rate the user supplies. The normal evaporation rate is a manufacturer figure measured under defined conditions at a stated fill and pressure, and real boil-off varies with fill level, ambient temperature, solar exposure, and the condition of the vacuum jacket. The hold time is a SCREENING estimate: it holds the vapour temperature constant and ignores liquid stratification, the saturated liquid-vapour coupling, and heat going into warming the liquid. It does not size relief devices, which must cover the fire case as well as normal boil-off, and does not address the pressure building coil, the economizer, or withdrawal behaviour.

One percentage and one ideal-gas mass; no manufacturer performance table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: tank_volume_gal, ner_pct_per_day, liquid_density_lb_gal, latent_heat_btu_lb, vapour_space_pct, current_pressure_psig, relief_pressure_psig, molecular_weight, vapour_temp_r, alt_vapour_space_pct, withdrawal_gal_day, boil_off_gal_day, boil_off_lb_day, vapour_space_ft3, hold_time_hr, alt_hold_time_hr

• The evaporation rate is entered it is a rated figure at a stated fill and pressure, not a field measurementthe tank manufacturer's data
• Hold time is a screening estimate constant vapour temperature, no liquid stratification or saturated couplingthe manufacturer's published hold-time data
• Not relief sizing relief devices must also cover the fire caseCGA standards and NFPA 55