A Practical guide to liquid nitrogen tanks
Hi,
I get asked questions about liquid nitrogen tanks all the time because their use is so common.
I would like to discuss the following with you:
- The hazards of ln2
- The parts and safety features of ln2 tanks
- Dispensing ln2
- Handling of cryogenic liquids and materials
- Transporting ln2 tanks within a building
- Filling ln2 tanks at a fill station
- Emergency situations
Hazards of ln2
Extreme cold: Ln2 can cause instant severe frostbite to skin and eyes. Ln2 vapor can freeze body tissue. Proper PPE (personal protective equipment) must be worn.Proper PPE includes a lab coat, splash proof goggles, a face shield, and gloves designed for handling of cryogens.In addition to this protective equipment, closed toe shoes and pants without cuffs must be worn. Cuffs can trap liquid against the body.
Embrittlement: Plastics and rubber when subjected to extreme cold can become brittle and shatter.Only transfer liquid nitrogen through hoses designed for ln2 contact.
Boiling: Ln2 must not be dispensed into warm or hot vessels, intense boiling may result causing liquid to spatter everywhere.Boiling may also lead to an overpressurization of a vessel.
Thermal shock: Ln2 may thermally shock vessels leading to failure. Glass vessels may break from rapid cooling, and stainless vessels become increasingly more brittle from cryogenic temperatures.
Asphyxiation: Large spills or leaks can result in a rapid displacement of oxygen, in poorly ventilated areas this can be deadly.Nitrogen is an odorless invisible gas, and your body will not subject any reactions to breathing it until you slip into unconsciousness.This is due to the fact that nitrogen is a large quantity of the air we breathe.Only store tanks in properly ventilated locations with oxygen depletion alarms.
Expansion: Liquid nitrogen can expand up to 700 times when changing from a liquid to a gas. Rapid heating will lead to explosive boiling which can create enough expansion to rupture a system.Ln2 must never be trapped between two valves, or in vessels without a properly installed and rated pressure relief valve.
Samples stored submerged in ln2: Sample tubes submerged in ln2 may have liquid seep into the threads or into the tube itself. Sample tubes may explode after removing from bath from building pressure.
Liquid oxygen: Dewars full of ln2 can become enriched from liquid oxygen from long term use, the liquid nitrogen will display a blue hue when there is condensed oxygen in the mix.
Additionally cold traps can accumulate liquid oxygen, this can be dangerous because cold traps can also contain frozen solvent from a process.If you see liquid oxygen close the fume hood sash, turn off any equipment and exit the lab immediately. Let the trap warm and evaporate overnight before attempting to dismantle the apparatus.
The parts and safety features of ln2 tanks
First and foremostā¦ Download the manual. Tank manufacturers will have a manual that you can request via email, or are downloadable from their webpage.
Typically an ln2 tank is 1.5 meters tall and 50 cm in diameter and contains anywhere between 100-160 liters of liquid.There is an inner vessel that contains the liquid and the outer vessel that is held under vacuum.The vacuum space between acts as a highly efficient thermal barrier.This extreme insulation protects the operator and lowers the evaporation rate of the liquid.Ln2 tanks will have three pressure safety devices consisting of two pressure relief valves and a burst disk.
Rate of evaporation: Typically the evaporation rate is 2-3% each day and 40-60% monthly
Pressure relief valves: There are two pressure relief valves located on the top of the tank these aid in venting excessive pressure from the tank and are the first line in maintaining proper tank pressure.The primary pressure relief valve opens at the lowest pressure and regularly opens to vent excessive gas, a slight hiss is normal in operation and venting is common. The pressure gauge on the tank should be less or equal to that of the setting of the primary pressure relief valve.The secondary pressure relief valve is set at a higher pressure and features a vent tube that is extended to ensure function should the primary fail or become blocked with ice or debris.
Burst disk:The burst disk is located in the bottom of the tank, in the event of overpressurization it will burst and vent the tank, this can be catastrophic and dangerous to people and equipment nearby.If your cylinder vents loudly from the high pressure relief valve and has frost build up around the pressure relief valves leave the room immediately and contact emergency services.
Liquid valve: Used to withdraw liquid from tank and fill tank.This side of the tank will feature a dip tube.
Gas valve: Gas can be withdrawn from this valve for process gas.
Pressure building circuit: A tubing system that evaporates liquid nitrogen to a gas in order to pressurize the headspace of the tank. This is typically used to build pressure to dispense liquid.It is comprised of one way valves, a regulator, and a manually actuated valve.The pressure may take several minutes to build once you have opened the pressure building circuit.
Economizer circuit: Allows gas from the headspace which would be normally be vented through the primary pressure relief valve to be expelled through the tank along with the liquid or gas being currently dispensed.This can only be used during the operation of the tank.
Liquid level gauge: liquid level gauges are usually the float style gauges, they allow the operator to see the liquid level in the tank in real time.
Warning********
Never ubstruct, block, tape, or modify these pressure relief devices.
Do not store tanks in direct sunlight or areas subject to intense temperature.
Always have adequate ventilation and low o2 alarms.
If a tank is dented or damaged it must be professionally inspected to ensure safety.
Never attempt to work on or maintenance a tank with ln2 in it, only empty tanks may be serviced.
Never tamper with any safety devices or protective covers.
If you own the tank it is your responsibility to maintain it and keep it in good operational condition.
Unexpected leaks from o rings and valves is a sign that maintenance is needed.
Always consult the tank manufacturer for the safe operating parameters of your tank.
Tanks should always be used and stored in an upright position.
Always store tanks in well ventilated areas.
Dispensing Ln2
Always wear PPE and appropriate dress when handling cryogens.*****
Consult the manufacturerās manual and become familiar with the piping circuit on your tank before dispensing liquid.
- Dress out in your PPE.
- Connect a manufacturer approved hose to the liquid valve of the tank.
- Place hose in a container to be filled or attach to process.
- Open the liquid valve slowly, if no liquid exits the tank you will need to use the pressure building loop to make some headspace pressure to push the liquid out of the tank.
- Dispense the liquid slowly to avoid splashing, do not dispense into warm containers/ dewars, they may thermally shock and cause the liquid boil violent.
- Do not dispense liquid at high pressures, liquid will splatter.
- Close the valve on the tank once the desired amount of liquid has been dispensed.
Filling a Ln2 tank at a fill station
Many large research facilities have ln2 filling stations that you may need to use from time to time.When filling a tank always follow all posted procedures posted.
- Weigh in your tank and tare the scale to 0.
- Dress down in your PPE and appropriate clothing.
- Attach the liquid and gas lines to the tank with the labeled hoses.
- Open the gas valve
- Open the liquid valve
- Turn on the fill station and push the fill button.
- Monitor the liquid level in the tank via a float gauge.
- When the fill is complete turn off the fill station.
- Shut the liquid valve
- Shut the gas valve
- Disconnect the lines from the tank, you may need a heat gun to melt the ice.
- Place the tank back on the scale and record the weight.
- Document the weight and check to see if it meets manufacture fill requirements.
Warning*****
Never trap liquid between two valves.
Never stand in front of the fill ports of the filling station or tank.
Hoses may contain pressurized liquid use extreme care when removing them.
Always double check your valves.
Never leave a filling tank unattended.
Transportation of tanks indoors
Tanks should be transported by two people down ramps, or over rough corridors.Proper PPE must be worn at all times. Tanks should not be transported in public walkways or main hallways.If a tank is to be sent up via elevator it should be sent up a freight elevator. Proper procedure is to send the tank unaccompanied up to someone waiting on that floor.
Emergency situations
In the event of a large spill, leak, venting situation, or low o2 alarm immediately exit the building.
If your tank is venting aggressively from the secondary pressure relief valve and has large ice formation leave the building, this is a sign that the tank is becoming dangerously overpressurized.Contact emergency services immediately.
If your tank is damaged or leaking contact emergency services and exit the building.
Proper PPE will go a long way in preventing easily avoidable injuries, good awareness of the process and tubing circuits is vital to staying safe.
It is good practice to teach all employees to meetup in a safe area after an alarm or emergency event.Have an exit route planned ahead of time that everyone is familiar with and perform a head count after an event to ensure that everyone has exited the building.
Low oxygen environments can present dangers rescue personnel, extreme care must be taken to ensure that the rescuer doesnāt become a victim. When in doubt check your o2 levels when entering these environments. Forced ventilation may be used to displace the nitrogen gas.
