Health and Safety Policy
Hazardous Substances
UHSP/15/HS/00 Schedule 3.12/01
Hazardous Substances Policy - Control Measures
Enhanced Good Chemical Practice
for Work with Very Low Boiling Point Liquefied
Gases
This document is a schedule from and it should be used in conjunction with University hazardous
Substances Policy. It sets out requirements additional or alternative to Good Chemical Practice for
control of the exceptional risks from cryogenic liquids. Cryogenic liquids are those liquefied gases
which have very low boiling points such as oxygen, nitrogen, argon, etc. Although individual cryogenic
liquids may exhibit particular properties, some of which present quite severe hazards, for example, the
fire risk associated with liquid oxygen, ALL cryogenic liquids present two principal hazards:
a) Very low temperatures and the risk of serious personal injury or material damage;
b) Very high liquid-to-gas expansion ratios and the risk of over pressurisation of holding vessels.
And, with the exception of oxygen, cryogenic liquids present a third principal hazard of oxygen
deficiency and the risk of asphyxiation.
This document contains further information on the properties of cryogenic liquids and guidance and
procedures for meeting the Policy requirements.
Revised November 2001
UHSP/15/HS/00 Schedule 3.12/01
CONTENTS
Introduction
PAGE
1
Requirements for Work with Liquefied Gases
Identification
Containers
Location
Transfer/Dispensing Equipment
Filling Dewars
Transport
Ventilation
Liquid Oxygen
1
1
2
2
2
2
3
3
3
Further Information and Guidance
Some Physical Properties of Cryogenic Liquids
The Low Temperature Hazard
The Asphyxiation Hazard
The Liquid-to-gas Expansion Ratio Hazard
Ice Plug Hazards
Liquid Oxygen
Liquid Oxygen Condensation Hazard
Liquid Gas Condensation and Solidification Hazard
Other Condensate Hazards
Liquid Helium Hazard
Other Hazards
Statutory Examination and Testing
4
4
4
4
5
5
5
6
6
6
6
6
6
Guidance and Procedures
Identification
Containers
Ventilation
Transfer/Dispensing Equipment
Personal Protective Equipment
Filling
Precautions against Boiling and Splashing
Liquid Oxygen
Liquid Helium, Argon and Neon
Transporting Dewars
Storage of Dewars
Maintenance
7
7
7
7
8
8
8
9
10
10
10
11
12
Emergency Action
Spillage
An ice plug forming
First Aid
12
12
13
13
Assessment of Ventilation Requirements
Relationship between Air Change Rate and Gas Concentration
Calculating the Potential Oxygen Depletion in a Room Due to Liquid Gas Filling
and Spillage
Oxygen Depletion Example for Liquid Nitrogen
14
14
15
17
Schedule 3.12
Hazardous Substances Policy - Control Measures
Enhanced Good Chemical Practice
for Work with Very Low Boiling Point Liquefied
Gases
Introduction
Hazards and Risks
Those liquefied gases which have very low boiling points and which, therefore, are very cold are called
cryogenic liquids. The gases which are commonly encountered as cryogenic liquids are those which
are present in the atmosphere, i.e., oxygen, nitrogen, argon, etc. Although individual cryogenic liquids
may exhibit particular properties, some of which present quite severe hazards, for example, the fire
risk associated with liquid oxygen, ALL cryogenic liquids present two principal hazards:
c) Very low temperatures, e.g. -196°C for liquid nitrogen, and the risk of serious personal injury or
material damage;
d) Very high liquid-to-gas expansion ratios (ranging from 1:695 for nitrogen to 1:1440 for neon at 20°)
producing huge volume changes as the liquids vaporise and the risk of over pressurisation of
holding vessels.
And, with the exception of oxygen, cryogenic liquids present a third principal hazard of oxygen
deficiency, caused by a reduction of the proportion of oxygen to other gases in the atmosphere as a
cryogenic liquid vaporises and the risk of asphyxiation.
Containers
In order to minimise evaporation losses at their very low temperatures cryogenic liquids are kept in
vacuum insulated vessels. The smaller vessels called dewars which may contain up to 50 litres of
cryogenic liquid are open (not sealed or pressure not exceeding 0.5 barg) and are of two types.
Dewars used for the storage and transport of liquid are typically narrow necked to facilitate pouring
and to further minimise evaporative losses. Wide necked dewars are used to cool and to store items
immersed in the liquid. Small-scale bulk storage vessels for quantities up to 1000 litres are also
vacuum insulated and of two types called static tanks and transportable containers and are operated
at pressures above 0.5 barg. Bulk storage vessels are equipped with pipework and valve assemblies
for filling and dispensing liquid.
In the light of the exceptional risks from cryogenic liquids, the following
requirements are additional or alternative to Good Chemical Practice.
Requirements for Work with Liquefied Gases
See the section “Guidance and Procedures”, page 7 of this schedule, for further details of the
measures needed to implement these requirements.
Identification
1.
All vessels containing cryogenic liquids must be clearly labelled.
2. Once labelled, a vessel must be used only for the indicated substance, unless it is ascertained
that the vessel is free of the original substance and it is relabelled.
S3.12-1
Schedule 3.12
Containers
3. The correct vessels must be used, designed and constructed in accordance with the relevant
Code1.
4. The outlet of vessels that are not designed to be sealed must be kept free of obstruction (by for
example ice) to prevent pressurisation. Outlets may be looseley covered, but must never be
stoppered.
5. Vessels must be maintained in good condition. In addition to the checks carried out before filling
and before transportation the appropriate maintenance procedures must be carried out on a
regular basis, or at least at intervals not exceeding six months. Vessels operating at above 0.5
barg are subject to statutory examination in accordance with University Policy.2
Location
6. Vessels must be kept at or above ground level in a secure, adequately ventilated designated level
location away from sources of heat, air intakes, drains and other underground openings, in
accordance with the appropriate BCGA Code of Practice.1
7. Storage areas must be kept clear and access restricted to authorised persons.
Transfer/Dispensing Equipment
8. The correct Transfer/Dispensing Equipment must be used for liquefied gases, particularly if the
cryogenic liquid has a boiling point below that of liquid air and it is contained in a narrow necked
vessel. Do not fit equipment to dewars not specifically designed for the equipment.
9. Liquid withdrawal devices that seal a dewar must be fitted with a pressure relief device that
prevents the pressure in the dewar rising above 0.5 barg.
10. Transfer lines (except small bore helium lines, see para. 31, page S3.12-10) must be fitted with a
pressure relief device so that if liquefied gas becomes trapped as a result of blockage at both ends
excessive pressure build-up may be vented.
11. The delivery end of transfer/dispensing lines must be fitted with a suitable diffuser.
Filling Dewars
12. The filling operation must not be left unattended.
13. The bulk storage vessel must be fitted with appropriate decanting equipment that includes a
device for venting excess gas before it reaches the dewar.
14. Dewars must not be filled whilst a liquid withdrawal device is still in place.
15. Dewars must be in good condition and free of water or ice inside and narrow-necked dewars not
excessively frosted around the neck.
16. Persons involved in the filling must wear the appropriate protective clothing.
17. Dewars must not be over-filled.
18. Do not lean into the area over the top of larger sample storage dewars when the cover is not in
place.
1
Transportable Vacuum Insulated Containers of Not More Than 1000 litres Volume, CP 27, British Compressed Gases
Association, 1994, ISSN 0260-4809
Vacuum Insulated Tanks of Not More Than 1000 litres Volume which are Static Installations at Users Premises, CP 28, British
Compressed Gases Association, 1997, ISSN 0260-4809
The Safe Use of Liquid Nitrogen Dewars up to 50 litres, CP30, British Compressed Gases Association, 2000, ISSN 0260-4809
2
Statutory Inspection, Examination and/or Testing of Specified Equipment:….( UHSP/16/SIET/01)
S3.12-2
Schedule 3.12
Transport
19. If a container of cryogenic liquid, no matter how small, is transported by lift:
•
only a lift authorised by the Budget Centre Health and Safety Co-ordinator must be used;
•
lifts authorised for the transport of liquid gas must be classified to indicate the maximum
amount that may be accompanied by the carrier (the classification to be based on an
assessment to determine the quantity of liquid needed to evaporate to reduce the oxygen
concentration below 18%);
•
the cryogenic liquid must not be accompanied by passengers;
•
the cryogenic liquid must not be accompanied by the carrier if the quantity of liquid exceeds the
amount authorised (bullets one and two, above);
•
the carrier must ensure that others do not enter the lift;
•
the lift must be fitted with an emergency alarm/telephone;
•
the unaccompanied transportation of cryogenic liquids in lifts must be supervised/monitored
outside the lift by a competent person.
20. If cryogenic liquids are to be transported by road vehicle the arrangements must conform with
University Hazardous Substances Policy for transport3. The University's Insurers have imposed
the following further requirements on the carriage of liquid nitrogen by road, and which apply
equally to other cryogenic liquids:
•
There must be a substantial barrier, across the full height and width of the vehicle, separating
the liquid nitrogen from the occupants of the vehicle. In the case of a van with an existing wiremesh screen this should be reinforced with hardboard; if there is no existing screen stronger
material such as 25mm blockboard should be used.
•
Where a van is used at least one window of the cab must be fully open while full containers are
being carried.
•
The vehicle must be clearly marked that it is carrying liquid nitrogen.
N.B. A private motor car is unlikely to meet these requirements.
Ventilation
21. Cryogenic liquids must always be handled and stored in a well-ventilated area.
22. An assessment of the adequacy of ventilation, or of the ventilation requirement must be carried
out for every place where cryogenic liquids are kept or used. (See page S3.12-14)
23. Ventilation is sufficient when the oxygen concentration in the room does not fall below 19.5%
during normal working or storage.
24. The complete spillage of the contents of the largest dewar must not cause the oxygen
concentration to fall below 18%
Liquid Oxygen
25. Liquid oxygen must not be used as a refrigerant.
3
The Carriage of Hazardous Substances by Road, University Hazardous Substances Policy - Transport, UHSP/15/HS/00 Schedule 6
S3.12-3
Schedule 3.12
Further Information and Guidance
Some Physical Properties of Cryogenic Liquids
boiling point
melting point
Liquid-to-gas @ 15°
expansion
@ 20°
ratio
Liquid density kg m-3
Gas density relative to
dry air @ 15°
% (v/v) gas in air
oxygen
nitrogen
argon
neon
krypton
xenon
helium
-183°
-218°
1:842
1:857
-196°
-210°
1:683
1:695
-186°
-189°
1:824
1:838
-246°
-248°
1:1415
1:1440
-169°
-152°
1:689
1:701
-109°
-140°
1:533
1:542
-269°
-272°
1:739
1:752
1142
1.12
807
0.98
1395
1.4
1206
0.7
2415
2.93
2942
4.61
71
0.07
20.9
78
1
0.0015
0.0001
0.00008
0.0005
The Low Temperature Hazard
Risks to People
Cryogenic liquids can have a destructive effect on human tissue, the nature and extent of damage
being dependent on the type of tissue and the extent of exposure. Delicate tissues such as those of
the eyes are particularly at risk and are liable to be damaged even by an exposure to cold gases
which would be too brief to affect other exposed parts of the body. A brief exposure of the respiratory
tract and lungs by inhalation of vapour or cold gas may cause only discomfort in breathing, but
prolonged exposure by inhalation can damage lung and respiratory tract tissue.
Continued exposure of the skin to cryogenic liquids or to very cold atmospheres can result in frostbite
and whilst this is painful as freezing commences, much more pain is suffered during thawing when the
damage also becomes more obvious.
Contact with uninsulated pipes or vessels carrying cryogenic liquids can cause unprotected or
inadequately protected skin to stick to the very cold surface as a result of the freezing of moisture. A
subsequent attempt to prise the affected part free could cause flesh to be torn. This is also a potential
risk if wet clothing is worn whilst working with cryogenic liquids.
Working in an environment which has been cooled by the presence of cryogenic liquids can give rise
to the condition of hypothermia if insufficient protection is taken against the cold. Hypothermia is a
potential hazard at air temperatures below about 10°C, and is indicated by a general slowing down of
mental and physical capability and activity.
Risks to Materials/Equipment
Many materials are not suitable for low temperature work with cryogenic liquids. For example, at
cryogenic liquid temperatures carbon steel loses ductility, becomes very brittle, its toughness is
considerably reduced and it becomes prone to brittle failure at relatively low levels of stress or shock
loading. Similar effects are observed in other materials including rubbers, and synthetic polymers and
composites. Even if the material does not undergo a ductile to britle transition, repeated thermal
cycling may result in stress cracking and ultimately fatigue failure. Clearly, materials for work with
cryogenic liquids must be selected with care. Moreover, care should also be taken to prevent a
potential spillage from contacting exposed structural components of buildings etc. and, for example,
the tyres of motor vehicles.
The Asphyxiation Hazard
Cryogenic liquids gradually vaporise, bringing about an increase in the proportions of the particular
gases present in the atmosphere compared with the normal levels. When cryogenic liquids other than
oxygen vaporise, the proportion of oxygen present in the atmosphere will also change, being reduced
as the amount(s) of other gas(es) present increase. This change presents the risk of asphyxia, the
onset of which begins immediately the oxygen content of the air falls below 20.9%, though it is at first
hardly noticeable. As the proportion of oxygen present diminishes the breathing and pulse rates
increase, clear thought becomes more difficult and muscular co-ordination begins to fail. As the level
of oxygen falls below 14%, these symptoms worsen, with little muscular effort producing rapid fatigue
and with ready loss of emotional control. When the level of oxygen falls to below 10% muscular
movement may become impossible, though, up to this stage the victim may have been totally unaware
S3.12-4
Schedule 3.12
of his predicament. By the time the victim is unable to move, death is very close and, as there is no
pain and the victim is largely unaware of what is happening, death may be inevitable - even if
resuscitation is possible, brain damage may already have occurred.
The foregoing describes the onset of gradual asphyxia. However, sudden exposure to an oxygen-free
atmosphere can cause immediate unconsciousness, and death within minutes. Such a situation could
arise near the ground and especially in pits and trenches as cold gases such as nitrogen, argon,
krypton and xenon are considerably heavier than air at ambient conditions and, therefore, sink to
accumulate at low level.
The Liquid-to-gas Expansion Ratio Hazard
In the absence of external or further cooling, and with boiling points which are well below room
temperature, the temperatures of cryogenic liquids will be those of their respective boiling points. As a
consequence of this, any heat input into a cryogenic liquid will initially cause vaporisation and the
liquid will remain at its boiling point until vaporisation is complete. The expansion which accompanies
the vaporisation of a cryogenic liquid produces a huge volume change as liquid turns to gas. If the
cryogenic liquid were kept in a sealed vessel, vaporisation would cause a pressure build-up that would
be liable to burst the vessel. The risk of explosion of a vessel holding a cryogenic liquid may also
arise from the condensation and freezing of moisture from the atmosphere in the mouth of a vessel or
end of a pipeline, etc.
The risks arising from vaporisation are greatest with liquid helium which has the lowest latent heat of
vaporisation (less than 1/50th that of liquid nitrogen). Liquid helium evaporates rapidly when heated or
when liquid is first transferred into warm or partially cooled equipment. A relatively large increase in
the rate of boiling may also arise from only a very limited deterioration in the insulating system of a
storage vessel. Pressure relief devices for liquid helium systems must therefore be of adequate
capacity to release the large quantities of vapour that may result.
A quench of a superconducting magnet will result in the boil off of all the liquid helium that surrounds it
and the production of a very cold gas cloud that rises to the ceiling.
Ice Plug Hazards
Ice plugs formed as a result of the condensation and freezing of moisture from the atmosphere in the
mouth of a vessel or end of a pipeline, etc may be ejected at high velocity due to pressure build up.
This can result in serious injury. In the worst case, ice plugs can build up sufficient pressure in the
neck of a dewar to cause catastrophic failure of the dewar, which could result in serious injury, even
fatalities. In order to prevent ice plugs forming, protective caps should be fitted and in good condition
and the dewar should be fully emptied after 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 insulating bung in place. If possible the dewar should be sited in a sheltered
but well ventilated location, eg under a canopy.
Liquid Oxygen
Although oxygen is essential to support common combustion reactions, it cannot burn on its own.
However, oxygen-enriched atmospheres allow fires to burn much more fiercely. Moreover, in such an
atmosphere, combustible materials are much more readily ignited, on heating alone, and well below
their normal ignition temperatures. Materials which are saturated with oxygen are also much more
readily combustible and may even be spontaneously combustible in this condition. The ignition of a
material which has been contaminated with liquid oxygen may occur some time after the initial contact.
Mixtures of liquid oxygen with certain substances, especially fuels and organic matter, are so reactive
that many of these combinations are sensitive and violent explosives. Examples of these materials
are: grease, oil, wood, cotton wool, coal, steel wool. Insulating materials for use with liquid oxygen or
liquid air must be carefully chosen and combustible types such as expanded polystyrene excluded.
S3.12-5
Schedule 3.12
Liquid Oxygen Condensation Hazard
The fire and explosion hazards of liquid oxygen can also arise indirectly. The boiling points of
nitrogen, neon and helium are so low that the liquids and any surfaces cooled by them to these
temperatures are liable to condense out liquid oxygen or oxygen enriched liquid air from the
atmosphere, and if this is allowed to continue for some time the concentration of liquid or gaseous
oxygen can accumulate to levels which present the fire and explosion hazards detailed above.
Oxygen-enriched air can also result if liquid air is left to evaporate in a dewar, since the lower boiling
liquid nitrogen will evaporate off first.
Cryogenic liquids are normally handled in insulated vessels, often with narrow necks in which the gas
acts as a barrier against oxygen contamination. However, oxygen contamination can arise in a widemouthed vessel which is not closed. Oxygen can also condense out in vacuum systems to which cold
traps are applied before pumping down.
Liquid Gas Condensation and Solidification Hazard
As its boiling point is the lowest of these cryogenic liquids, liquid helium is able to cause the
condensation and solidification of all the other gases which come into contact with it or with surfaces
cooled by it. Liquid nitrogen can also condense out and solidify liquid argon in a system containing
argon gas. However, solid argon has a very low vapour pressure at the boiling point of liquid nitrogen
and, as a consequence, once argon has solidified it can take a very long time to fully vaporise, even at
high vacuum. If the vessel containing solid argon is subsequently sealed with residual solid argon,
then a rise in temperature of only a few degrees to, say -180°C will produce a large increase in
pressure, and the risk of the vessel exploding.
Other Condensate Hazards
If liquid oxygen and other cryogenic liquid vessels are completely emptied from the vessel, there is a
risk of an accumulation of hydrocarbon gases condensed from the atmosphere or of water freezing
from moisture condensed out of the atmosphere.
Liquid Helium Hazard
A further hazard arising from the particular properties of helium is that of thermal oscillations. This
phenomenon is liable to occur in tubes which lead to ambient temperature; it takes the form of
pressure surges which arise from the interaction of heat transfer and fluid flow effects. Once
oscillations have started during transfer the process should be stopped. Suggested remedies are to
change the position of the transfer tube relative to the warm internal parts of the receiving vessel or, if
the problem is persistent, to change the volume of the warm space in the receiving vessel by inserting
foam glass plugs.
Other Hazards
The densities of the liquids vary considerably. The very low density of liquid helium allows helium
dewars to be of particularly light internal construction, thus minimising conduction paths and thermal
mass. Helium dewars are therefore much more sensitive to overpressurisation and to mechanical
shocks or of the movement of liquid when tilted, especially when full. This also means that helium
dewars are not sufficiently robust for use with any other cryogenic liquids.
Statutory Examination and Testing
Vessels that operate at above 0.5barg or which are sealed and could become pressurised to above
0.5 barg due to equipment malfunction must meet the relevant statutory requirements for inspection,
examination and approval,4 and relevant University Policies.5
4
The Pressure Systems Safety Regulations 2000, The Carriage of Dangerous Goods (Classification, Packaging and Labelling)
and Use of Transportable Pressure Receptacles Regulations 1996
5
Statutory Inspection, Examination and/or Testing of Specified Equipment:….(UHSP/16/SIET/01), Pressure Systems
(UHSP/10/PS/97)
S3.12-6
Schedule 3.12
Guidance and Procedures
Identification
1.
All vessels containing cryogenic liquids must be clearly labelled:
•
with the proper name of the substance against the background colour indicated below;
•
with the appropriate hazard warning symbol(s);
•
with coloured tape, applied a few centimetres below the top edge, according to the code below.
Liquid air
Liquid oxygen
Liquid nitrogen
Liquid argon
Liquid neon
Liquid helium
GREY
BLACK
GREY/BLACK
TURQUOISE
BROWN/BLACK
BROWN
Containers
2.
The correct vessels must be used:
•
made from materials that are both compatible with the cryogenic liquid and that are able to
withstand the very wide and fairly sudden temperature variations that will obtain;
•
fragile vacuum jacketed vessels must be housed in a metal case or must be completely bound
with adhesive tape to guard against the effects of implosion;
•
vessels which are not open to the atmosphere must be fitted with pressure-relieving devices
vented to a safe area;
•
vessel venting systems must be so designed as to permit the discharge points to be
maintained above 0°C.
•
vessels used for transport by road must meet the appropriate statutory standards of
construction.
•
Vessels must be designed and constructed in accordance with the relevant BCGA Code.
3. Precautions against pressurisation of vessels not designed to be sealed should include:
•
loose covering of outlets to allow gas to escape, but to minimise the formation of ice or
condensation of air;
•
use of the manufacturer’s cap designed for the particular vessel
Such vessels must never be stoppered
Ventilation
4. In all cases the ventilation requirement must be assessed following one or other of the worked
examples in the section “Assessment of Ventilation Requirements” later in this schedule. The
ventilation requirement will depend on a number of factors including the location and its size, the
particular cryogenic liquid, the activities and equipment used. Ventilation can be natural or
provided by mechanical means.
5. Where there is reliance on natural ventilation, an indoor location should have ventilation openings
with a total area of 1% of the ground area. The openings should be positioned diagonally across
the room, the main opening at the highest point of the location for gases lighter than air, or at
ground level for gases heavier than air.
6. For handling (storing, filling, withdrawal, etc). transportable cryogenic vessels with non-flammable,
non-toxic contents in locations above ground level, natural ventilation is generally redarded as
sufficient, provided that the room is large enough and that the outdoor area is not enclosed by
walls etc.
S3.12-7
Schedule 3.12
7. Ventilation is sufficient when the oxygen concentration in the room does not fall below 19.5% as a
result of:
•
the normal evaporation of all dewars and cryogenic liquid containers within the room; and
•
the filling losses from filling the largest dewar from a warm condition
8. The complete spillage of the contents of the largest dewar must not cause the oxygen
concentration to fall below 18%
Transfer/Dispensing Equipment
9. If the cryogenic liquid has a boiling point below that of liquid air (i.e. below -196°) and it is to be
dispensed from a narrow-necked vessel, then a vacuum-jacketed syphon must be used in order to
avoid the formation of a plug of solid air in the neck of the vessel which might occur during
pouring.
10. Valved dispensers must be used for the transfer of cryogenic liquid from 50 litre or larger
containers. The upper part of the dispenser body must form a seal with the neck of the container.
11. If the storage container is pressurised for sustained withdrawal, this must be done with dry, oil-free
gas of the same type as the liquid or with a dry, oil free inert gas. The pressure employed must be
the minimum necessary to discharge the liquid.
12. The fill connections on bulk storage vessels must not be modified or exchanged. The connections
may vary according to the gas in order to prevent the wrong fluid being dispensed into a container.
13. A suitable filling funnel must be used with the top partly covered to prevent splashing when
transferring cryogenic liquids from one small vessel to another.
Personal Protective Equipment
14. When handling cryogenic liquids, eye protection must be worn to give impact protection against
jets of liquid or gas, ice plugs and failed equipmentand to prevent eye contact from splashes. Full
face protection is recommended when dispensing cryogenic liquids.
15. Suitable non-absorbant, insulated protective gloves must be worn when handling anything which
is or which may have been in contact with cryogenic liquids. The gloves should be loose fit for
easy removal. Sleeves should cover the ends of the gloves. Gauntlet gloves are not
recommended because liquid can drip into them.
NB Gloves DO NOT give adequate protection when immersed in cryogenic
liquid.
16. Protective clothing, overalls or similar, should be worn and preferably not have external pockets or
turnups where liquid could collect. Trousers must be worn to cover high-top shoes or outside
boots for the same reason. Where dewars are being carried over uneven ground or on stairs at
chest height, the user should consider additional splash protection. A splash resistant apron may
be appropriate.
17. Objects which are being inserted into or withdrawn from cryogenic liquids must be handled with
long tongs or similar devices.
Filling
18. Carry out the following pre-fill checks:
•
Check that the bulk storage supply vessel is in an appropriate location.
•
Check that the bulk storage supply vessel is fitted with appropriate decanting equipment that
includes a device for venting excess gas before it reaches the dewar. Where the bulk storage
vessel operates at above 1.5 barg, the decant-valve on the vessel should be a slow opening
type, eg a globe-valve, not a ball-valve.
S3.12-8
Schedule 3.12
•
Check that the bulk storage supply vessel is at the correct operating pressure. If the pressure
is too high ensure that someone trained to do so vents the tank.
•
Check that the dewar is labelled for the particular cryogenic liquid. Do not fill a dewar which is
labelled for another product.
•
Check that the filling equipment is clean, free from damage and complete (e.g., the diffuser is
in place on the discharge end of transfer/dispensing tubing). Do not attempt to use blocked or
damaged filling equipment.
•
Ensure that the dewar is not fitted with a liquid withdrawal device. If the dewar is fitted with a
liquid withdrawal device this must be removed to avoid over filling or over pressurisation of the
dewar. Before removing the device ensure that the dewar is vented to atmospheric pressure
by opening the vent-valve fully and ensuring that the pressure gauge is reading zero.
•
Check that the dewar is in good condition. Ensure that there is no neck damage or twisting.
Ensure that the insulating bung under the protective cap has not detached. If it has, fit a new
cap before filling. If the bung has fallen into the dewar then it must be removed. Do not fill a
dewar which is damaged or has the bung inside.
19. The filling procedure should be modelled on the following:
•
Purge the hose to clear any excess atmospheric moisture or dust. This can be done by
securing the hose and cracking the decant valve slightly for a short period. Close the valve as
soon as frosting appears.
•
Insert the fill hose into the dewar and ensure it is secure.
•
Initiate the fill slowly by cracking open the fill-valve. If the dewar has warmed the liquid will boil
and turn to gas immediately on contact.
•
When the dewar has cooled the fill-valve can be opened to establish a steady flow of liquid. If
liquid is spitting back out of the dewar then the flow should be reduced.
•
For dewars with neck tubes, stop the fill when the liquid reaches the bottom of the neck. The
"sound" of the fill will change, indicating that it has happened. Do not fill past the bottom of the
neck.
•
For dewars that do not have neck tubes, stop the fill when the liquid reaches the required level,
which shall be a level below that which the insulating bung will reach when placed onto the
dewar after filling. Never overfill a dewar.
•
When the dewar is full, replace the protective cap. If the cap rattles, this is evidence that the
dewar is over filled and liquid is boiling at a greater rate than is normal. Leave the dewar in the
open air until there is no excessive boiling.
•
If fitting a liquid withdrawal device, fit it immediately after the fill, ensuring that the dewar has
not been overfilled. (Rapid gas boiling should indicate overfilling). Check the pressure
indicator on the device to ensure the pressure rise has stabilised at 0.1-0.2 barg- (2-3 psig). If
the pressure is rising towards 0.5 barg, open the vent-valve on the device and reduce the
pressure. Check the pressure indicator again and repeat the venting cycle as many times as is
necessary to obtain a steady pressure reading. Inability to achieve a steady pressure reading
is an indication of loss of vacuum from the insulating jacket. Test that the liquid line is clear of
ice blockage by operating the liquid valve momentarily, allowing liquid to issue out.
•
After filling, isolate fill hoses and vent before disconnection.
•
Check that the labelling has not been damaged by liquid spills during the fill. Replace if
necessary.
Precautions against Boiling and Splashing
20. Containers should be filled slowly (as boiling and splashing always occur when a warm container
is being filled; slow-filling will also minimise thermal shocks to the container, and avoid a too rapid
S3.12-9
Schedule 3.12
build up of pressure. Boiling and splashing will also occur when warm objects are immersed in
cryogenic liquids).
21. When an open-ended pipe is to be inserted into a cryogenic liquid the warm end must be closed
off until the cold end has cooled down to the temperature of the liquid. (without this precaution a
stream of liquid and cold gas is liable to issue from the warm end of the pipe.)
Liquid Oxygen
22. Naked flames, smoking and other sources of ignition must be prohibited in areas where liquid
oxygen is stored or handled. Clothing that has been contaminated with liquid oxygen must be
removed and aired for at least an hour away from sources of ignition.
23. Oil, grease, combustible materials and flammable substances must be kept well away from liquid
oxygen.
24. Containers of liquid oxygen and other cryogenic liquids must not be completely emptied, but they
should occasionally be left with a residue, allowed to warm up to ambient temperature and purged
with dry, oil-free nitrogen before refilling.
25. Equipment in which liquid oxygen has been used must be purged with dry, oil-free nitrogen or oilfree air before repairs are carried out.
26. Liquid nitrogen traps on vacuum systems must be filled with liquid nitrogen only after pumping
down, so as to prevent the condensation of oxygen. Air must not be re-admitted to a system until
the liquid nitrogen reservoir has been removed, otherwise there is a danger of re-admitted air
condensing oxygen onto organic residues in the trap.
27. Only in exceptional circumstances may liquid oxygen be used as a refrigerant. If the material to
be cooled is held in a glass container, then this must be housed in a liquid-tight copper or brass
container so that in the event of a glass breakage the material being cooled cannot come into
contact with the liquid oxygen.
Liquid Helium, Argon and Neon
28. If it is necessary to use liquid nitrogen to cool a vessel containing argon, the vessel must not be
sealed until it has been held at a higher temperature, say -78°C, for at least 30 minutes.
29. Liquid helium must be stored and handled under positive pressure or in closed systems to prevent
the infiltration and solidification of air or other gases.
30. Only the specially designed containers must be used to store and transport liquid helium.
31. A vacuum insulated line must be used for the transfer of liquid helium. However, it may not be
appropriate to fit pressure relief valves to small bore (i.e. 1mm-4mm dia.) vacuum cased helium
transfer lines without introducing a heat leak, and hence disrupting or stopping the transfer of
liquid.
32. If work with liquid helium is contemplated, advice must be sought from the Health and Safety Unit.
33. Because of the possibility of liquid nitrogen, helium or neon condensing out oxygen from the air, all
surfaces which are to be exposed to these liquid gases must also be to oxygen-clean standards.
Transporting Dewars
34. Dewars, whether full or empty, must always be covered, preferably fitted with manufacturer’s caps
to prevent the ingress of moisture.
• Only caps designed for use with the dewar type should be used.
•
Caps must not be secured down unless securing methods are integral to the manufacturer's
design of the cap.
•
If caps come loose during road transportation a chain or wire my be fitted but in such a manner
that the cap is kept in place but not sealed as the dewar must be free to vent.
S3.12-10
Schedule 3.12
35. Do not transport a full damaged dewar or a full dewar that has lost vacuum.
36. Dewars must be adequately secured during transportation to prevent spillage or mechanical
damage.
37. Dewars must be checked for adequate labelling before being transported by road.
38. Where possible, avoid carrying dewars up stairs or steps. Stairs present an increased tripping
hazard, which may lead to spillage. If the negotiation of stairs is unavoidable:
• two people are recommended for carrying the dewar
• consider the installation of a stair lift where practical
• ensure that access to the stairway is restricted, other than to the operator
• consider additional body protection against spills, e.g. a carrying apron.
39. Use a barrier system to ensure that others do not enter a lift containing dewars:
•
the barrier should be inside the lift and extend across the complete door opening;
•
the barrier should preferably be of an extendable/retractable type, fixed permanently to one
side of the door frame;
•
the barriers system should include a sign displaying the following details:
Sign colours
DANGER:
Grey = YELLOW
DO NOT ENTER:
Grey = RED
Both signs, black and
white as shown
DANGER
DO NOT ENTER
Liquid Gas in
transit
Storage of Dewars
40. Full or part full dewars should be stored in designated areas:
• with adequate ventilation;
•
that are dry or sheltered from rain;
•
secure to prevent access by unauthorised personnel;
•
caps shall always be fitted in storage.
41. 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 situated outside the room so that operators are aware of the
hazard before entering the room.
42. Empty dewars should be stored in a dry area, but before putting into store:
• ensure that a dewar is completely empty.
•
If possible allow the dewar to warm to ambient temperature.
•
Always store the dewar with the dust cap in place.
S3.12-11
Schedule 3.12
43. Handle empty dewars as full. It may be that they still have some residual content.
Maintenance
44. In addition to the checks carried out on dewars 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. If the neck is in any way damaged the dewar should
not be used.
•
Check the dewar for mechanical damage. Do not use the dewar if damage is found to the
support trunnion for the tipping trolley, the dewar stand, the dewar wheels or the dewar wall.
(Minor dents and scratches are acceptable, but excessive corrosion or dents that are severe
enough to have caused damage to the inner wall are not).*
•
Ensure that the dewar is free of 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. If a detergent is used,
ensure that the dewar is thoroughly rinsed. Ensure that the dewar is completely dried.
•
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.
* If a repair is carried out on a dewar it must be done to the original manufacturing standard.
45. In addition to the statutory examination, more frequent checks to maintain bulk storage vessels in
safe condition should include, as appropriate;
• Check the pressure build and gas use regulators for correct settings and function;
•
Check valves for smooth operation;
•
Check condition and function of the liquid level and pressure indicators;
•
Leak test the vessel and all valves, fittings and pipework using an inert gas or the working
medium;
•
Check the condition and content of all labels
•
Carry out remedial action.
Emergency Action
Spillage
•
Evacuate all personnel from the area likely to be affected by the liquid and the evolved gas.
•
Try to prevent the gas flowing along the ground into pits, basements, cellars and stairwells by
closing doors. The cold gas will collect in those areas..
•
Take appropriate action to ensure that the ventilation system does not spread the gas to other
areas.
•
Open exterior doors and windows to encourage evaporation of the liquid and safe dispersal of the
gas.
•
Allow the liquid to evaporate naturally.
S3.12-12
Schedule 3.12
•
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 you are sure that the gas has all dispersed and that the
air is safe to breathe. If in doubt, use an oxygen monitor to check oxygen levels.
An ice plug forming
If an ice plug forms there is a 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 the minimum number
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. Compressed air is not recommended as it contains moisture.
Ensure that the dewar is completely sandbagged before approaching it. Extreme caution should be
taken when inserting the copper tube. Insert the tube 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 has been initiated the operator can retire to a safe place whilst
the blockage is being cleared.
The pressure build up may have damaged the inner wall of the dewar. Ensure that the dewar is
examined by a competent person before returning it to service.
For advice in dealing with an ice blockage, contact your gas supplier or dewar manufacturer.
First Aid
Contact with Cryogenic Liquids
Contact between cryogenic liquids and eyes or skin should be treated immediately by flooding the
affected area with large quantities of cold water, followed by the application of cold compresses.
Never use dry heat.
If the skin is blistered or the eyes are affected, medical attention must be obtained as soon as
possible.
Treatment of Cold Contact Burns
Cold contact burns will require medical attention as soon as is practicable. Frozen tissues are
painless and appear waxy with a pallid discoloration. During thawing, frozen tissues become painful,
swollen, and are very prone to infection when thawed. Thawing should be induced slowly with the aim
of completion after arrival at hospital or of medical attention.
Until medical attention is available, the following first aid measures may be employed.
•
Remove any clothing that may constrict the blood circulation to the frozen area. Clothing which
has stuck to frozen tissue must not be removed until completely thawed.
•
Do not permit smoking or the consumption of alcoholic drinks as these will decrease blood flow to
the frozen tissue.
•
Thawing is commenced by placing the affected area in a tepid water bath. Never use dry heat
as this and wet temperatures above about 45°C may cause further burns.
S3.12-13
Schedule 3.12
•
A massive exposure to cryogenic liquid which has caused the general body temperature to be
depressed, will require re-warming by total immersion in a bath.
•
Precautions against shock must be taken following any accident involving cryogenic liquids.
•
If the thawing of frozen tissue is complete before the arrival of medical attention or before arrival at
hospital the affected area must be well-covered with dry sterile dressings.
Inhalation of Inert Gases
Dizziness or loss of consciousness while working with cryogenic liquids must be treated by moving the
affected person immediately to a well ventilated area. Artificial respiration and treatment for shock
should be given as necessary.
Assessment of Ventilation Requirements
The following guidance is based on that provided in British Compressed Gases Association CP30.
The rate at which a room is ventilated is usually expressed in the number of air changes per hour.
In locations above ground level with no special ventilation openings, natural ventilation provides
typically 1 change per hour. However, a lower value will apply where windows are sealed with tight
seals. For underground rooms with small windows 0.4 changes per hour is assumed.
Mechanical ventilation is considered necessary where the ventilation requirement is for more than 2
changes per hour.
Relationship between Air Change Rate and Gas Concentration
In typical situations, assuming a certain leakage rate from the vessel and a homogenous distribution of
gas:
Ct =
(
L
1 − e −nt
VR * n
)
where:
Ct
Lg
VR
n
t
*
= gas concentration
= gas release m3/hr
= room volume m3
= air changes per hour
= time in hours
= multiply
For long periods (t tending to infinity) Ct approximates to:
C∞ =
Lg
VR * n
Where C∞ = increase in gas concentration after a long period.
With the increase in leaked gas concentration there will be a concomitant reduction in oxygen
concentration. Thus if the normal oxygen concentration is about 21%, the reduced oxygen
concentration as a result of liquid gas evaporative loss approximates to:
C∞ * 0.21*100%
or
S3.12-14
Schedule 3.12
C ox =
Lg ∗ 0.21 ∗ 100
VR * n
%
Thus, in order to ensure oxygen concentration does not fall below 19.5%, the necessary air change
rate can be estimated as follows:
Cox = 19.5 =
n=
n=
Lg ∗ 0.21 ∗100
VR * n
Lg ∗ 0.21 ∗100
VR * 19.5
1.08 ∗ Lg
VR
If the manufacturer supplies leakage rate in terms of the amount of liquid lost, then the gas release
rate Lg may be calculated from:
Lg =
f g * Ll
24 *1000
= m3 / h
Where:
= liquid loss in litres per day
= Liquid to gas expansion ratio.
Ll
fg
Calculating the Potential Oxygen Depletion in a Room
Due to Liquid Gas Filling and Spillage
Five cases are considered:
a) Evaporative loss from the storage tank,
b) filling losses which always occur when a dewar is being filled,
c) spillage of the contents of the dewar and
d) the 'next worst case' where the entire contents of the vessel are lost to the room immediately
after the dewar is filled
e) the “worst case” where there is catastrophic failure of the full storage tank.
a) Evaporative loss from storage tank
In order to allow for a deterioration in the insulation performance over the life of the tank it is prudent to
double the manufacturer’s quoted evaporation rate.
Reduced oxygen concentration, C ox =
where:
Lg
VR
n
*
= gas release m3/hr
= room volume m3
= air changes per hour
= multiply
S3.12-15
2 ∗ Lg ∗ 0.21 ∗ 100
VR * n
%
Schedule 3.12
b) Filling
A value of 10% of the volume of the product in the dewar is used to estimate the losses to atmosphere
during filling.
VO = 0.21[VR −[
0.1 * VD * f g
1000
]]
where:
0.1
VR
VD
fg
0.21
*
= 10% volume loss during filling
= room volume, m3
= dewar capacity, litres
= Liquid to gas expansion ratio.
= The normal concentration of oxygen in air, 21%
= multiply
Resulting oxygen concentration, C ox =
where:
Vo
VR
*
100 * Vo
VR
= the volume of oxygen, m3
= room volume, m3
= multiply
c) Spillage
For the spillage of the entire contents of a dewar:
VO = 0.21[V R −[
VD * f g
1000
]]
Resulting oxygen concentration, C ox =
100 * Vo
VR
where the symbols are as before.
d) Filling and spillage together
The 'next worst case', where the entire contents of a dewar are lost to the room immediately after
filling, equivalent to 110% of vessel contents to allow for the 10% filling losses prior to spillage:
VO = 0.21[VR −[
1.1 * VD * f g
1000
]]
where:
1.1
VR
VD
fg
0.21
*
=110% volume loss during filling and spillage
= room volume, m3
= dewar capacity, litres
= Liquid to gas expansion ratio.
= The normal concentration of oxygen in air, 21%
= multiply
Resulting oxygen concentration, C ox =
where:
Vo
VR
= the volume of oxygen, m3
= room volume, m3
S3.12-16
100 * Vo
VR
Schedule 3.12
e) Catastrophic failure of Storage Tank
For the release of the entire contents of a tank:
VO = 0.21[V R −[
VD * f g
1000
]]
Resulting oxygen concentration, C ox =
100 * Vo
VR
where the symbols are as before (except, VD.=tank capacity)
Oxygen Depletion Example for Liquid Nitrogen
Example:
A basement room contains two 25 litre and three 10 litre dewars.
Room dimension: 7 x 8 x 2.5 metres =140m3,
5 litre dewar: loses 0.2 litres per day through evaporation
10 litre dewar: loses 0.15 litres per day through evaporation
(dewar manufacturers' quoted evaporation rates).
Normal Evaporation Losses
Evaporation is a continuous process, hence the increase in nitrogen concentration (Ct) can be
calculated over a long period using:
Ct =
L
VR * n
where:
Ct
L
VR
n
*
= gas concentration
= gas evaporation rate m3/hr
= room volume m3
= air changes per hour
=multiply
Whilst manufacturers will quote the evaporation rate for their dewar, it is prudent to double it when
calculating the rate of nitrogen release. L. This allows for a deterioration in the insulation
performance over the life of the dewar. The nitrogen gas factor of 683 at 15° has to be used to
calculate the volume of gaseous nitrogen released through evaporation, as the dewar manufacturer's
figures relate to the volume of liquid nitrogen lost.
Thus:
L=
2 * 683 * (2 * 0.2 + 3 * 0.15)
= 0.048m 3 / h
24 * 1000
Assume there is an average of 0.4 air changes per hour in the room. The nitrogen concentration
increase is, therefore:
Ct =
0.048
= 0.001 = 0.1%
140 * 0.4
Air already contains 78% nitrogen; thus, in this case, evaporation from the five dewars in the
circumstances described would reduce the oxygen concentration by some 0.02%. This is because air
contains 21% oxygen, so the oxygen depletion can be approximated as 0.1% x 0.21 = 0.02%,
S3.12-17
Schedule 3.12
In this example, normal nitrogen evaporation from the dewars has only a small effect in increasing the
nitrogen concentration, and thus reducing the oxygen concentration, in the room. If. however, far
more dewars were stored in the same room used in the above example, or if a much smaller room
was used for the five dewars mentioned, then the nitrogen concentration would increase by a much
higher larger factor. If Ct in such a case was calculated to be 0.05 (i.e. 5%), then forced ventilation
would be recommended, since this would reduce the oxygen concentration in the room by 1%, which
is at the level where the safety margin has been virtually used up.
Losses due to Filling
First calculate the volume of oxygen in the room, VO.
Using:
VO = 0.21[VR −[
0.1 * VD * f g
1000
]]
The same dewars and room size are used (140m3), but here the largest nitrogen release is during the
filling of the largest (25 litre) dewar and again the nitrogen factor of 683 must be used to convert liquid
to gaseous nitrogen.
Thus:
VO = 0.21[140 −[
0.1 * 25 * 683
] ] = 29.04m 3
1000
The resulting oxygen concentration in the room (Cox) can then be calculated: Using:
C ox =
100 * Vo
VR
Thus:
C ox =
100 * 29.04
= 20.7%
140
Clearly, this is acceptable. As a guide it is recommended that the combined effect of normal
evaporation and filling processes should give rise to alarm if the oxygen level falls to 19.5%.
Losses Due to Filling and Spillage
Following the same process as above, calculate the volume of oxygen in the room VO) as a result of
the spillage of the entire contents following filling.
Using:
VO = 0.21[VR −[
1.1 * VD * f g
1000
]]
Again we have a 140m3 room and again the largest release is from the 25 litre dewar.
Thus:
VO = 0.21[140 −[
1.1 * 25 * 683
] ] = 2505m 3
1000
Then calculate the resulting room oxygen concentration (Cox) after the spillage:
S3.12-18
Schedule 3.12
Using:
C ox =
100 * Vo
VR
Thus:
C ox =
100 * 25.5
= 18.2%
140
This is just above the level (set at 18%) at which oxygen monitors are usually set to give an
emergency alarm, leading to immediate evacuation.
In this example, it is recommended an oxygen monitor be fitted with two levels of alarm:
•
19.5% should lead to urgent investigation and corrective action
•
18.0% should cause immediate evacuation - assuming that this level results from spillage.
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30.10.01
S3.12-19