GUIDANCE NOTE - OHSU G25 (2001)

GUIDANCE ON THE HANDLING, STORAGE AND TRANSPORTATION OF LIQUID NITROGEN

INDEX:

1. Introduction
2. Properties
3. Hazards
4. Dewars
5. Handling
6. Filling
7. Use
8. Storage
9. Transport of Dewers
10.Maintenance
11.Action in the event of:

1. Introduction

The objective of this guidance note is to promote the safe use of liquid nitrogen in the University. Liquid nitrogen is used in biological sample preservation, vacuum pump systems, cooling samples in electron microscopes, cryostats for low temperature research and superconducting magnet systems.

2. Properties

Liquid nitrogen has a boiling point of -195.8C
Liquid nitrogen has a freezing point of -210.0C
Volume of expansion to gas (at-15C, 1 atm) = 0.807g/cc

Specific gravity = 0.808 (normal boiling point, 1 atm) = 0.807g/cc
Colourless, odourless liquid similar in appearance to water

3. Hazards

3.1.1 Temperature Related

The extremely low temperature of liquid nitrogen can cause severe burns or frostbite when in contact with the skin. The hazard level is comparable to that of handling boiling water.
Skin can freeze and adhere to liquid nitrogen cooled surfaces causing tearing or removal.
Soft materials e.g rubber and plastics become brittle when cooled by liquid nitrogen and may shatter unexpectedly.
Liquid oxygen may condense in containers of liquid nitrogen or vessels cooled by liquid nitrogen. This may be identified by a blue coloration This can be extremely hazardous because of the pressure rise on the slightest degree of warming above the boiling point of oxygen (-183C) and the possibility of explosion with oxidisable material. However most liquid nitrogen containers are entirely enclosed except for a small neck area and the nitrogen gas issuing from the surface of the liquid forms a barrier which keeps air away from the liquid and prevents oxygen contamination.
Thermal stress damage can be caused to containers because of large, rapid changes of temperature.

3.1.2 Vapour Related

Large volumes of nitrogen gas are evolved from small volumes of liquid nitrogen (approximately 700 times) and this can easily replace normal air in poorly ventilated areas leading to the danger of asphyxiation. Oxygen is necessary to support life and its volume concentration in the atmosphere is normally 21%. At normal atmospheric pressure persons may be exposed to oxygen concentrations of 18% by volume, or even less, without adverse effects. Symptoms of oxygen deprivation, such as increased pulse and rate of breathing, fatigue and abnormal perceptions of response, may be apparent in oxygen concentration of 16%. Permanent brain damage or death may arise from breathing atmospheres containing less than 10% oxygen. initial symptoms will include nausea, vomiting and gasping respiration.
Oxygen condensed into leaking containers can explode on heating following resealing or blockage with ice.
The low temperature of the vapour can cause damage to the softer tissues e.g eyes and lungs but may not affect the skin during short exposure

3.1.3 Damage to equipment

Damage may occur to equipment due to contact with liquid nitrogen.

Condensation of water around electrical cables may result in an electric shock hazard.
The freezing of electric cables may break the insulation and result in a shock hazard.
The formation of ice around inlets and outlets may apply stresses to equipment resulting in damage.
Spilled liquid nitrogen will crack tiles which may create a problem.
Rubber tubing through which liquid nitrogen is being transferred may suddenly crack, releasing the liquid.
Liquid nitrogen should never be poured down sinks as this will crack waste pipes causing potentially dangerous leaks.

4. Dewars

4.1 Types of dewar

Liquid nitrogen is transported in vacuum insulated containers called dewars. There are basically two types of dewar. The first type are dewars that are used for the storage and transport of liquid nitrogen. They are typically narrow necked to facilitate pouring. If suitable they can be fitted with a liquid withdrawal device to allow the liquid to be removed without tipping. The second type of dewar is used for cooling items inserted into it. These are wider necked and sometimes come with storage racks or compartments, which can be removed.

4.2 Dewar construction

All dewars must be suitably constructed for use with liquid nitrogen. Dewars built after 1 July 2001 will be subject to the Carriage of Dangerous Goods (Classification, Packaging and Labelling) 2 Regulations(5) and the Approved Requirements of Transportable Pressure Receptacles (4) {CDGCPL2}

The inner vessel is typically constructed from aluminium with an epoxy glass fibre neck or stainless steel with a stainless steel neck. The outer vessel is typically aluminium, carbon steel or stainless steel.

4.3 Liquid withdrawal devices

Figure 1 shows a diagram of a liquid withdrawal device. Because the dewar is sealed, the gas which boils off due to atmospheric warming raises the pressure in the dewar above atmospheric. These devices must be fitted with a pressure relief device that will prevent the internal pressure exceeding the design limit of the dewar, which must be below 0.5 bar As the pressure in the dewar is greater than atmospheric pressure the gas at the top of the dewar pushes the liquid out via the dip tube.

Liquid withdrawal devices are normally attached to the dewar via a clamping device. The device must have a secondary retaining measure, e.g a retaining wire, to restrain travel in the case of pump failure.

Warning:

Only fit liquid withdrawal devices to dewars that are designed for these devices.

4.4 Labelling

Liquid nitrogen dewars must be clearly and adequately labelled. Figure 2 shows a typical label. The label includes:

Basic safety information
Transport labelling information
Gas supplier contacts

In addition any pressurised dewars should have a unique identification number for insurance purposes.

As a minimum the label must include the statutory labelling requirements to meet the Carriage of Dangerous Goods Classification, Packaging and Labelling) and Use of Transportable Pressure Receptacles Regulations 1996 (5) and basic safety information for users.

The CDGCPL2 Regulations require the label to include:

Product designation, ie NITROGEN, REFRIGERATED LIQUID
Product UN number, ie UN 1977
Product danger sign, ie a green diamond with a cylinder symbol and the number 2 at the bottom

Danger signs should have a side length of at least 100 mm unless the size and/or shape of the dewar make this impractical.

5. Handling

5.1 Manual Handling

Always handle liquid nitrogen carefully. Avoid skin contact with either liquid nitrogen or items cooled by liquid nitrogen as serious burns can occur. Beware of wearing gloves, rings, wrist-bands or bracelets which may trap liquid nitrogen close to the skin.
Keep vessel upright at all times, except when pouring liquid from dewars specifically designed for that purpose.
Delicate tissues, such as those of the eyes, can be damaged by an exposure to liquid nitrogen which is too brief to affect the skin of the face or hands. Therefore, stand well clear of boiling and splashing liquid and its issuing cold gas.
Do not 'walk', roll or drag dewars and always protect the vessel from severe jolting or impact.
Use tongs to withdraw objects immersed in liquid and handle the tongs and the object carefully.
The Manual Handling Regulations 1992 apply to the handling of liquid nitrogen dewars. Irrespective of the size or type of dewar , users must carry out a manual handling assessment of the activities operators are required to perform. In addition a COSHH risk assessment must also be carried out.
Do not attempt to lift or move large, heavy vessels without assistance. Trolleys and tipping trolleys should be considered and are recommended for dewars of 25 litres and above.

5.2 Protective clothing

The following protective clothing, manufactured to an approved standard, must be worn when handling, filling or using dewars:

Eye Protection - Goggles, face visor or glasses with cheek and brow guards

Hand Protection - Non-absorbent, insulated gloves, made from a suitable material such as leather. The gloves should be loose fit for easy removal. Sleeves should cover the ends of the gloves. Alternatively, gloves with an elasticated fitting may be worn. Gauntlet gloves are not recommended because liquid can drop into them.
Protective shoes - Safety shoes with reinforced toe protection are recommended. Only enclosed shoes should be worn.
Body Protection - Overalls, lab coat or similar type of clothing should be worn. They should be made preferably without open pockets where liquid could collect. Trousers should be worn outside boots for the same reason. Where dewars are being carried over uneven ground or on stairs at chest height, then additional splash protection should be considered. A splash resistant apron may be appropriate.

6. Filling

6.1 Fill Procedure

Dewar filling must be carried out by properly trained personnel wearing appropriate protective clothing. The filling procedure must include the essential elements listed below.

6.1.1 Pre-fill checks

Check that the supply vessel is in an appropriate location and at the correct operating pressure as stated on the vessel. If the pressure is too high ensure that someone trained to do so vents the tank.
Check that the dewar is labelled for liquid nitrogen service. Do not fill a dewar which is labelled for another product.
Check that the filling equipment is clean and free from damage. Do not attempt to use blocked or damaged filling equipment.
Ensure that the dewar is not fitted with a liquid withdrawal device. Initiating the fill with the device in place may lead to overfilling or over pressurisation of the dewar.
Do not fill the dewar if you suspect that:
there is water inside
there is ice inside
there is excessive frosting around the neck

6.1.2 Filling

Purge the hose to clear any excess atmospheric moisture or dust.
Insert the fill hose into the dewar and ensure it is secure.
Initiate the fill slowly by cracking open the fill-valve
When the dewar has cooled the fill-valve can be opened to establish a steady flow of liquid.
For dewars with neck tubes, stop the fill when the liquid reaches the bottom of the neck.
For dewars that do not have neck tubes, stop the fill when the liquid reaches the required level, which should be a level below that which the insulating bung will reach when placed onto the dewar after filling.
When the dewar is full, replace the protective cap. If the cap rattles, this is evidence that the dewar is overfilled and liquid is boiling at a greater rate than is normal.
If fitting a liquid withdrawal device, fit it immediately after the fill, ensuring that the dewar has not been overfilled.
Check that the labelling has not been damaged by liquid spills during the fill. Replace if necessary.

7. Use

7.1 Outdoor use

When using dewars outdoors there is an increased risk of ice plugs forming in the neck due to condensation of atmospheric moisture or rain freezing on the neck. It is essential that, except when pouring or handling the storage racks, the cap is kept on the dewar. It is also essential that the cap be in good condition with the bung in place. If possible the dewar should be sited in a sheltered but well ventilated location, eg under a canopy.

7.2 Indoor use

When using dewars indoors there is an increased risk of creating an asphyxiation hazard.

7.2.1 The use of passenger lifts when transporting dewars

Transporting dewars containing liquid nitrogen in an occupied lift is hazardous and should be avoided whenever possible. The main hazards are liquid splashing or boiling liquid vaporising into the lift, creating an oxygen- deficient atmosphere.

Therefore, where possible, a goods lift should be used for transporting dewars.

Spillage of liquid nitrogen can cause embrittlement and subsequent failure of certain materials , eg carbon steel. If liquid nitrogen is spilled onto a lift floor, the lift should subsequently be checked for mechanical damage.

Before a dewar containing liquid nitrogen is transported in any lift that can carry persons, a detailed risk assessment in accordance with the Management of Health and Safety Regulations and the Confined Space Regulations must be carried out and emergency procedures established.

Where the use of passenger lifts cannot be avoided, one or more of the following should be adopted:

Dewars must only be filled to 90% of the net capacity to reduce the risk of spillage.
Dewars fitted with liquid withdrawal devices must be vented to less than half the relief-valve set pressure.
Only an operator who has received suitable instruction or training should be allowed in the lift during the transportation of dewars containing liquid nitrogen.
The operator should have a fully functional oxygen depletion monitor that will warn the operator when the oxygen level has depleted to 19.5%, allowing immediate evacuation from the lift before a dangerous level is reached.
The operator must have control of the lift to enable immediate evacuation at the next available floor, in the event of escape of the product.
The lift must be fitted with an emergency alarm/telephone.
If the lift is equipped with an extraction fan it should be switched on before the operator takes the dewar into the lift.
Do not transport in a lift a dewar that is venting gas.
Do not vent dewars whilst in a lift.
Do not transport a leaking or defective dewar in a lift.
Do not transport an overfilled dewar in a lift.

The transportation of dewars in lifts containing liquid nitrogen should be supervised/monitored outside the lift by a competent person who is aware of the potential hazards and of the action to be taken in an emergency.

Additional local rules may be applied

7.2.2 Stairs and doorways

Stairs present an increased tripping hazard, which may lead to a nitrogen spillage. Where possible, avoid carrying dewars up stairs or steps. If negotiation of stairs is unavoidable:

two people should carry the dewar
ensure that access to the stairway is restricted, other than to the operators

7.2.3 Ice plugs

If ice plugs form they may be ejected at high velocity due to pressure build up. This can result in serious injury. In order to prevent ice plugs forming:

always ensure that the dewar is fully emptied after use
always fit protective caps and ensure that the caps are in good condition

Further information on how to deal with an ice plug can be found under Section 11.3

7.2.4 Ventilation

Cold liquid nitrogen is heavier than air and will accumulate at low level. Users should, therefore, take extra precautions in basement rooms, rooms with ventilation at high level and rooms with pits, ducts of trenches in the floor where nitrogen can be trapped. Consideration must be given to the use of oxygen monitors where ventilation is poor. Where possible, the use of such rooms should be avoided.

A room must be sufficiently ventilated for the two normal conditions listed below so as not to cause a reduction in oxygen concentration below 19.5%;

the normal evaporation of all dewars and liquid nitrogen containers within the room
the filling losses from filling the largest dewar from a warm condition.

Additionally, the complete spillage of the contents of the largest dewar must not cause the oxygen concentration to fall below 18%. A formula for calculating the oxygen concentration following the complete spillage of the largest dewar is shown below:

Resulting % oxygen concentration = 100 x Vo / Vr

where Vo = the volume of air, m³

Vr = the room volume, m³

Vo can be calculated as follows:

Vo = 0.21 Vr - Vd x Fg

1000

where:

Vd = dewar capacity

Vr = room volume

Fg = gas factor. This is 683 for nitrogen

0.21 = the normal concentration of oxygen in air

8. Storage

8.1 Storage of full or part-full dewars

Full or part-full dewars should be stored in designated areas that meet the same criteria as dewars in use. There must be adequate ventilation. Dewars should be stored in a dry area, sheltered from rain. The caps should always be fitted in storage. Dewars must be securely stored to prevent access by unauthorised personnel.

If storage rooms have forced ventilation an alarm to indicate its failure is recommended. If storage rooms have reduced ventilation when unoccupied then an alarm to indicate oxygen deficiency is recommended. Alarms should be fitted outside the room so that operators are aware of the hazard before entering the room.

8.2 Storage of empty dewars

Always ensure that a dewar is completely emptied before putting into storage. Dewars must be emptied in a safe, well ventilated area. If possible allow the dewar to warm to ambient temperature. Always store the dewar with the dust cap in place and store dewars in a dry place.

9. Transport of dewars

9.1 Recommendations for transporting dewars

Dewars must be fitted with caps to prevent the ingress of moisture, whether full or empty. Caps must not be secured down unless securing methods are integral to the manufacturer's design of the cap.

Dewars should be checked for damage before transportation and must be transported separately from the driver or passengers. Flat- back vehicles, vehicles with a separating bulkhead that gas cannot leak through should be considered. Where this is not possible, the driver must wear a personal monitor. Dewars must not be transported in cars.

Dewars must be adequately secured during transportation to prevent spillage or mechanical damage.

Dewars must be checked for adequate signage before being transported by road.

Drivers must be adequately trained in the handling of dewars and the properties of liquid nitrogen.

9.2 Regulatory requirements

Dewars filled on site and supplied to another School/Department on another site must comply with the Chemicals (Hazards Information and Packaging) Regulations 1994. In addition to the labelling requirements given in Section 4.4, the CHIP Regulations require the recipients of liquid nitrogen dewars to be in possession of a gas safety data sheet, which is available from the gas supplier.

Dewars transported by road must comply with the Carriage of Dangerous Goods, (Classification, Packaging and Labelling) and the Use of Transportable Pressure Receptacles Regulations 1996. For dewars built before 1 July 2001, the requirements are that dewars are labelled as described in Section 4.4. For dewars built after 1 July 2001, see Section 4.2

Under the Pressure Systems Safety Regulations 2000, dewars must be inspected by an insurance inspector every 12 months and the result of the inspection must be included on the University's database of pressure systems.

10. Maintenance

In addition to the checks carried out before filling and before transportation the following should be carried out on a regular basis, or at least at intervals not exceeding six months.

Empty the dewar in a safe area and allow it to warm naturally to atmospheric temperature.
Check that the cap is in good condition and, if not, replace it.
Check the neck for twisting or damage
Check the dewar for mechanical damage.
Ensure that the dewar is free from dirt and contaminants, including any insulating bungs which may have detached from the caps and fallen into the dewar.
If the dewar is contaminated, wash the dewar out with warm water. Ensure that the dewar is completely dry.
Check that the liquid withdrawal device is in good condition. If the retaining wire, securing collar or valves are damaged, then replace the device.
For dewars fitted with liquid withdrawal devices, the relief device should be replaced at a suitable frequency, not exceeding ten years.

11. Action in the event of:

11.1 Large Spillage

Evacuate all personnel from the area likely to be affected by the liquid and the evolved nitrogen gas.
Pay particular attention to pits, basements, cellars and stairwells because the cold gas will collect in those areas. Try to prevent the gas flowing along the ground into such areas by closing doors.
Take appropriate action to ensure that the ventilation system does not spread the nitrogen to other areas.
Open exterior doors and windows to encourage evaporation of the liquid and the safe dispersal of the gas.
Allow the liquid to evaporate naturally
The evolved gas will be very cold and will create a cloud of condensed water vapour restricting visibility. Do not allow anyone to enter this cloud.
Do not allow anyone to enter the area until it is sure that the nitrogen gas has all dispersed and that the air is safe to breathe. If in doubt an oxygen monitor should be used to check oxygen levels.

11.2 Small spillage

In the event of a small spillage of liquid nitrogen the area should be well ventilated and the area evacuated.

11.3 An ice plug forming

If an ice plug forms there is danger that:

It will detach at high velocity when the dewar pressure rises
It will cause sufficient pressure build up in the dewar to cause it to rupture.

Extreme caution must be exercised if an ice plug is found. All personnel, except those required to deal with the incident, should be evacuated from the area.

The recommended method of dealing with the plug is to insert a copper tube into the neck and blow warm nitrogen gas onto the blockage.

Ensure that the dewar is completely sandbagged before approaching it. Extreme caution should be taken when inserting the copper tube. The tube should be inserted into the neck without making contact with the ice blockage. The gas supply should be set up so that the defrosting process can be initiated in a remote or protected position. Once the defrost process has been initiated the operator should retire to a safe place whilst the blockage is being cleared.

Ensure that the dewar is examined by a competent person before returning it to service.

11.4 Burns due to liquid nitrogen

Flush the affected areas of the skin with copious amounts of tepid water, but do not apply any form of direct heat. The casualty should be moved to a warm place ( about 22 C). If first aid is not available immediately, arrangements should be made for the patient to be transported to hospital.

While waiting for transport:

Loosen any restrictive clothing
Place the affected part in tepid water, or run tepid water over it, until the skin changes from pale yellow through blue to pink or red.
Protect frozen parts with dry, sterile dressings. Do not apply tightly so as to cause restriction of blood circulation.
Keep the patient warm and at rest
Ensure that the ambulance crew or the hospital is advised of the accident and first aid treatment already given.
Smoking and alcoholic beverages reduce the blood supply to the affected part and should be avoided