CLEAPSS advice on handling liquid nitrogen in schools

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15 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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CLEAPSS advice on handling liquid nitrogen in schools

(
http://www.cleapss.org.uk/
)



This is an extract from Section 11 of the CLEAPSS Laboratory Handbook
which is supplied to
all member secondary schools and col
leges (but not to primary schools) on the CLEAPSS Science
Publications CD
-
ROM and on paper.

m


Liquid nitrogen


The use of liquid nitrogen allows exciting and thought
-
provoking demonstrations but
there are significant hazards, the risks from which must be
controlled. They are:



asphyxiation in oxygen
-
deficient atmospheres;



fire in oxygen
-
enriched atmospheres;



cold burns, frost bite & hypothermia from the intense cold;



over
-
pressurisation from the large volume expansion of the liquid;



manual
-
handling accident
s if using large (25 litre) volumes.

There are also risks associated with the transport of liquid nitrogen.

m

Asphyxiation in
oxygen
-
deficient
atmospheres

Small amounts (not exceeding 2 litres) of liquid nitrogen can be safely used in school
laboratories,

without risk of asphyxiation. It would be prudent to ensure good
ventilation. However, if liquid nitrogen is kept in preparation rooms, stores and other
confined spaces, these must be ventilated as much as possible. In the event of a spill in
a confined s
pace, the room should be evacuated. If liquid nitro
gen has been kept in a
confined space, when retrieving it, a second person should be present in case of
asphyxia.

m

Fire in oxygen
-
enriched
atmospheres

The boiling point of oxygen is
-
183

°C, higher than

that of nitrogen,
-
196

°C. Liquid
nitrogen may condense liquid oxygen from the surrounding air. Subst
ances will burn
more fiercely and ignite. Provided that the quantities of liquid nitrogen are small (eg, 2
litres) there is not much risk of significant
oxygen enrichment. Never leave a test tube,
or any other container surrounded by liquid nitrogen, for long periods.

m

Cold burns, frost
bite and hypo
-
thermia from the
intense cold

When handling liquid nitrogen, eye protection (preferably goggles or a face

shield)
must be worn as well as non
-
absorbent, leather gloves. Shoes and gloves need to be
easily removable but open
-
toed shoes should
not

be worn. Demonstrators need to be
vigilant about the possibility of splashes of liquid nitrogen becoming entrap
ped
in
clothing, etc.

m

Over
-
pressuris
-
ation from the
large volume
expansion of

the liquid

2 litres of liquid nitrogen will become 1366 litres of gaseous nitrogen. In an enclosed
vessel this will give rise to a huge increase in pressure. Because liquid nitro
gen is cold,
it will condense any moisture in the atmosphere, possibly forming an ice plug which
may seal an open vessel, causing the pressure to build up. To our knowledge this has
caused at least two explosions in schools/colleges. Liquid nitrogen must b
e
transported, kept and used only in a vented Dewar flask, specially designed for
cryogenic work. Do
not
, under any circumstances, use ord
inary vacuum flasks.
(Suitable small Dewars are available for less than £100 (2005 prices), eg, from
Camlab.

m

Trans
port of
liquid nitrogen

Under the
Transportable Pressure Vessels Regulations
, liquid nitrogen must only be
transported in vessels which are suitable for this purpose (see above). Open vessels are
not

suitable, because of the risk of splashing and spills.

I
f liquid nitrogen is transported by road, the
Carriage of Dangerous Goods and Use of
Transportable Pressure Equipment Regulations

require that the driver shall have had
special training. The BCGA Code of Practice CP30
The Safe Use of Liquid Nitrogen
Dewars

up to 50 litres
(British Compressed Gases Association, 2000) states that
Dewars must be transported separately from driver or passeng
ers. Flat
-
back pick
-
ups,
trailers or vehicles fitted with a separating bulkhead should be used. These two
requirements me
an it is unlikely that schools will be able to obtain free supplies from a
friendly source
unless

within walking distance or unless the supplier is able to
deliver. Hospitals, food
-
processing companies, universities and even some doctors’
surgeries use liq
uid nitrogen, so this may be possible.

If, exceptionally, small amounts of liquid nitrogen are transported by road,

it is
important to remember that small
-
capacity Dewars have a relative low base area. Thus
there is a distinct risk of the Dewar falling ove
r, eg, if the driver has to brake suddenly.
This could lead to a major spill of liquid nitrogen, increasing greatly the risk of
asphyxiation. Thus the Dewar must be transported in such a way that it cannot fall
over or spill, for example, by standing it in

a large, deep cardboard or plastic box filled
with crumpled newspaper or similar packing material.

m

Particular risks
when using large
volumes of liquid
nitrogen

Rather than acquiring a small amount of liquid nitrogen from a local user, a school can
pay
to have it delivered, but in that case it is likely to be in 25 litre Dewars. A special
stand will be needed and can usually be hired at the same time as the Dewar.

Because of the bulk and weight of a 25 litre Dewar, a risk assessment will be required
unde
r the
Manual Handling Regulations.

Trolleys should be used to move the Dewar
around. A tipping stand can be used to dispense the liquid nitro
gen into smaller
containers but two people will be required to carry the Dewar and mount it in the
stand. If lifts

have to be used, the Dewar should
not

be accompan
ied in the lift. The
larger volume of liquid nitrogen increases the asphyxiation risk. Slow evaporation in a
store room or other confined space could cause problems. Ensure good ventilation.
When retrievin
g the liquid nitrogen, a second person should be at the door in case of
asphyxia.

First aid for minor
exposures to
liquid nitrogen

The aim of treatment is to raise the temperature of the affected part slowly back to
normal.



Move the victim to a warm room

(22

°C) if possible.



Loosen any restrictive clothing. Do not remove clothing that is stuck to the body
until thawed thoroughly.



Place the affected area in tepid water or flush the affected area with cop
ious
quantities of tepid water for half an hour unti
l the skin changes from pale yellow
through blue to pink or red. Do not use hot water or any other form of direct
heat such as room heaters.



Keep the patient warm and at rest.



Cover the affected part with a bulky, dry, sterile dressing. Do not apply too
ti
ghtly so that it restricts blood circulation.



Send the patient to a hospital emergency department. Ensure that the amb
-
ulance crew is advised of the nature of the accident and the treatment provided
so far.