5.6. Cryogenes#
5.6.1. Introduction#
A cryogenic liquid is defined as a liquid with a normal boiling point below –150 °C. The most commonly used industrial gases that are transported, handled, and stored in the liquid state at cryogenic temperatures are argon, helium, hydrogen, nitrogen, and oxygen.
There are a number of general precautions and safe practices that must be observed because of the extremely low temperatures and high rates of conversion into gas for all the cryogenic liquids.
Within the TU Delft we mostly use liquid nitrogen and liquid helium in set-ups for cooling. Liquid nitrogen is also being used for freezing of biological samples.
5.6.2. Definitions:#
Dewar flask: open dewar flask smaller than 10 litres Dewar: is a closed vessel designed for holding cryogenic liquids Liquid transport vessel : Is a closed dewar on wheels
5.6.3. Instructions for working with cryogenic liquids#
Prepare a safety report in the LabServant.
If you want to work with more than 10 litres of cryogenic liquids in a new lab / new set-up always contact first your HSE- advisor to discuss safety measures and personal protective equipment.
Make sure you have had the proper instructions from an assigned person of your department and that you have read the procedures.
After you have had the proper training and you have made the safety test you can get access to the set-up.
Contact your HSE-advisor for more information and working procedures
Physical properties of liquid nitrogen and liquid helium
Both helium and nitrogen are a colourless, odourless gas at room temperature.
Liquid nitrogen tends to initially settle on the floor and liquid helium tends to settle at the ceiling.
When nitrogen or helium gas is released in substantial amounts, it displaces oxygen from the air and can cause asphyxiation. Eg. the vaporisation of 1 litre of liquid nitrogen forms almost 700 litres of gas and liquid helium almost 760 litres.
5.6.4. Hazards of liquid nitrogen and liquid helium#
Warning
The cloudy vapor that appears when liquid N2 is exposed to the air is condensed atmospheric moisture. The gas itself is invisible.
Risk |
Explanation |
Precautions |
|---|---|---|
asphyxiation |
When nitrogen gas is released it displaces oxygen from the air resulting in a less oxygen |
* Be aware of dispersals from dewars and the hazard of dispersals in confined spaces such as elevators. |
Risk of burns, blistering and frostbite |
* Liquid nitrogen and liquid helium boils when into contact with warmer objects or materials, which can cause splashing. |
* Avoid direct contact with liquid. |
Risk of burns, blistering and frostbite |
* Liquid nitrogen and liquid helium boils when into contact with warmer objects or materials, which can cause splashing. |
* Avoid direct contact with liquid. |
Oxygen has a higher boiling point than nitrogen (O2 = -183 °C; N2 = -196 °C). This means that atmospheric oxygen can condense if it comes into contact with liquid nitrogen, resulting in local increases in oxygen concentrations. When cooling flammable substances or organic materials, this can create an explosion risk. |
Increased oxygen concentration (above 22%) will give an increased ignition risk and faster combustion. |
Write the precautions in your safety report, because this can be different per set-up. |
5.6.5. Risk of condensing oxygen#
5.6.6. Precautions:#
Hazards of liquid nitrogen (3)
Risk of high pressure caused by warming of liquid nitrogen and liquid helium
Pressure can quickly build up in a sealed unit with risk of rupture of dewar.
The function of the pressure release valve or the vent line can be defeated by the formation of ice.
Precautions:
Do not put cryogenic liquids in closed vessels that cannot withstand the pressure.
The use of house hold thermos flasks is prohibited
5.6.7. Hazards of liquid nitrogen and liquid helium (4)#
Risk of rapid boiling / splashing of liquid nitrogen when inserting objects into liquids
Precautions:
Always perform these operations slowly to minimize boiling and splashing.
Use tongs to withdraw objects immersed in the liquid, and take care where both tongs and object are then placed. (Note that objects soft and pliable at room temperature usually become very hard and brittle at cryogenic temperatures).
5.6.8. Hazards of liquid nitrogen (5)#
Risk of thermal stress caused by freezing of materials
Materials frozen in cryogenic liquids may change characteristics e.g. materials may become brittle and may shatter unexpectedly.
In rapidly cooled down objects, stress will build up due to the thermal shock. This might damage the object and will increase the risk of exploding vials.
Precautions:
Pour liquid nitrogen only into containers suitable for holding liquid nitrogen (i.e. not glassware, no household thermos flasks).
Always place materials into liquid nitrogen slowly. The nitrogen vapor can be used to pre-cool the material.
Containers should be filled slowly to minimize thermal shock.
5.6.9. Hazards of liquid nitrogen and liquid helium (6)#
Personal protection The following personal protection equipment is obligated when handling cryogenic liquids
Face shield: Always use a face shield. Safety glasses won’t give you optimal protection
Gloves: At the very least gloves suitable for handling cryogenic liquids. They will provide short-term protection from accidental contact with the liquid.
Shirt: Arms should not be exposed- wear long-sleeved shirt or laboratory coat.
Trousers: These are to be worn outside the shoes to prevent liquid spilling into shoes. Preferably without cuffs.
Shoes: Enclosed footwear, preferably safety shoes/boots, must be worn whilst handling liquid nitrogen
5.6.10. Decanting liquid nitrogen and/or siphoning liquid helium#
Before using the liquid-nitrogen / liquid helium dispensing system, new employees must be instructed in its operation by an assigned user. Be aware of the existing procedures. The assigned user grants them authorisation in Lab Servant. Only with this authorisation employees can gain access to the dispensing room using their campus card.
The TU clothing regulations apply when decanting liquid nitrogen and liquid helium. The prescribed personal protective equipment – face mask, cryogenic gloves, lab coat – must be worn at all times.
To prevent oxygen enrichment in open Dewar flasks, they must first be completely emptied before refilling with liquid nitrogen.
5.6.11. Working with cryogenic liquids during normal office hours (07.30-17.00)#
Transporting liquid nitrogen or liquid helium
The elevator must be blocked for all other transport (using the campus) when transporting cryogenic liquids.
Entering the elevator during transport is prohibited.
Avoid any contact with cryogenic liquids during transport. Helium can only be transported in a helium dewar transport vessel (use a certified dewar vessel). Ideally, a closed Dewar flask or one with a handle should be used for liquid nitrogen.
When moving open Dewar flasks with liquid nitrogen, if at all possible use one with a narrow neck.
Adhere to the TU clothing regulations Working with cryogenic liquids during normal office hours (07.30-17.00)
5.6.12. Working with liquid nitrogen or liquid helium#
Make sure that the risk of asphyxiation from leaking dewars in the laboratory has been assessed and that any additional preventive measures have been taken (for example, the installation of oxygen deficiency monitors). Make sure that you have enough ventilation.
Make sure that the risks of working with liquid nitrogen and/or liquid helium and the precautions taken to mitigate them are described in a safety report, safety assessment sheet and/or working instructions.
When working with open Dewar flasks, if at all possible use one with a narrow neck.
Adhere to the TU clothing regulations and wear the required personal protective equipment.
5.6.13. Working with cryogenic liquids during normal office hours (07.30-17.00)#
Outside normal office hours working with cryogenic liquids is not allowed (TU policy). Under certain circumstances it is inevitable to work with cryogenics. Then the following rules apply. Outside normal office hours, a second knowledgeable person must be at shouting distance during all operations involving cryogenic liquids. Using cryogenic liquids in experiments
Decanting liquid nitrogen and/or siphoning liquid helium is only allowed by authorised persons.
Large Dewar flasks should only be filled and transported to the laboratory during normal working hours. If an exception to this rule is necessary, enter into an agreement with your HSE-advisor.
5.6.14. Working with cryogenic liquids outside normal office hours (17.00-07.30)#
Working with liquid nitrogen and/or liquid helium
The safety report of working with liquid nitrogen and/or liquid helium outside normal office hours must be approved by the HSE advisor, the Area Supervisor and the relevant head of department.
When working with open Dewar flasks, if at all possible use one with a narrow neck.
Adhere to the TU clothing regulations and wear the required personal protective equipment.
5.6.15. Working with cryogenic liquids outside normal office hours (17.00-07.30)#
Handling of Dewars
Be aware of existing procedures in your building.
If there are uncertainties or deviations, please immediately notify a technician!
Make sure there is an emergency procedure.
Ensure floor surfaces are free of obstructions and appropriate for moving Dewars.
Ensure all parts of the Dewar (wheels, handles, etc.) are in proper functioning condition.
Use proper hoses and / or fitting siphons which can withstand cryogenic liquids
Cryogenic liquids, larger volumes more than 1 liter, must never be stored in areas other than the approved
Departmental storage facilities (ventilation, detection).
Safe limit in an unventilated space: Calculate the room volume in m3 and the max volume of nitrogen in m3 (1 liter of LN2 or LHe corresponds respectivaly with a volume of 0.7 m3 and 0.76 m3 gas @ 21 °C and 1 atm). The ratio of gas volume/room volume should be <0.15.
5.6.16. Elevator procedure for transporting cryogenic liquids#
You can call the goods lift to any floor using an authorised campus card.
Hold the campus card up to the card scanner in the hall. If the card has been scanned successfully, the word Voorkeur / Service (Preference) will appear on the display screen.
You then have to present the campus card at the lift cage within 30 seconds. Once you have done this, the words TU Transport will appear on the display screens on all floors. The lift will go to the correct floor and wait there for the next action. If you do not present the campus card at the lift cage within 30 seconds, the lift will resume normal service.You can now proceed to load the lift.
Choose the floor you require in the lift cage.
Get out of the lift cage and hold the campus card up to the scanner in the hall. The lift will go to the floor you have selected and will wait there with its doors closed.
You can open the lift doors on the selected floor by holding the campus card up to the scanner in the hall on that floor.
You can now proceed to unload the lift. When you have finished unloading the lift, you can return it to normal service by holding the campus card up to the scanner in the lift cage. The display screen will now indicate the present floor. N.B. If you do not return the lift to normal service, it cannot be used by anyone else. Repeat steps 4 to 6 if you need to transport several loads
5.6.17. In case of an emergency#
In case of burns with cryogenic liquids immediately rinse with water for at least 15 min.
In case of an emergency directly call the internal alarm number. This number is available 24/7 (so also during the weekend). First aid people will quickly be at the place of the emergency and can directly help during office hours. Outside office hours you have to wait for professional help.
When there is a leakage in an elevator, close elevator and directly call the internal alarm number .
In case of a big spillage of liquid nitrogen, directly evacuate the area and call the internal alarm number