Transcript Safety in Cryogenics - MKS

Safety in Cryogenics
Cryogen Handling Hazards
Bernd Petersen DESY –MKS1
Quellen (u.a):
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Objectives:
at the end of this lesson you should remember the
following hazards
cold burns
embrittlement
asphyxiation
thermal stress
pressure build-up by evaporation
hazards caused by condensation
combustion and explosion
in the handling of cryogens
Outline:
Cryogen Handling Hazards
(restricted to: He,N2,Ar)
Sources of Accidents and Failures
-> Means of Protection and
Protection Measures
Control Quizz
Bernd Petersen DESY –MKS1
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Cryogen Handling Hazards
In the handling of cryogenic liquids
and the operation of low temperature facilities
you have to consider two main risks
Cold Burns
Asphyxiation
- caused by the evaporation of
cryogenic liquids
in closed or badly ventilated areas
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Cryogen Handling Hazards Hazards
Cold Burns
Direct contact with cryogenic liquids and gases as
well as surfaces at cryogenic temperatures can lead to
massive damages of the skin ( cold burns)
or even of tissues
The danger of cold burns is most prominent when
handling open cryogen containers ( LN2 !!!!)
Eyes and mucuous membranes are most at risk
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Cold Burns
Means of protection and preventive measures:
Safety garments which prevent the penetration of cryogenic liquids:
- eye protection
- gloves of insulating and non
combustible material
which can be easily be removed
-high, tight fitting shoes
- trousers without turn-ups which
overlap the shoes
First Aid:
Same procedure as for burns
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Cryogen Handling Hazards
Asphyxiation (oxygen deficiency)
The evaporation of cryogenic liquids in closed or badly
ventilated areas can lead to oxygen deficiency
Due to the fact that most cryogens are odourless and colourless,
this hazard cannot be detected without special equipment
The victim may not even become aware of the oxygen
deficiency !!!
Furthermore, argon and cold nitrogen are heavier than air
and can therefore collect near the floor or in pits
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The symptoms of oxygen deficiency are
( oxygen concentration in % ):
19% - 15% pronounced reduction of reaction speed
15% - 12% deep breaths, fast pulse,
co-ordination difficulties
12% - 10% vertigo,false judgement,
lips slightly blue
10% - 8% nausea, vomiting, unconsciousness
8% - 6% death within 8 minutes,
from 4-8 minutes brain damages
4%
coma within 40 seconds, no breathing, death
Means of protection and preventive measures:
sufficient ventilation of the working place
feed exhaust from transport dewars and from experiments
into a gas recovery system or into the chimney
equip working place with oxygen monitor
enter confined areas, e.g. pits or tanks, only under supervision
and only with portable oxygen monitors
(-> HERA Cryo-Control Room)
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Cryogen Handling Hazards
Asphyxiation (oxygen deficiency)
Example for Helium:
discharge of helium into LHC-tunnel
formation of fog !!!
Cryogen Handling Hazards
Asphyxiation (oxygen deficiency)
O2-concentrations after helium discharge into LHC-tunnel
similar in HERA-tunnel or TTF-tunnel
middle
bottom
top
Cryogen Handling Hazards
Asphyxiation (oxygen deficiency)
protection measures
Evacuation of the area !
Use safety garments !
Control and measure O2 content of surrounding air !
Use independent breathing apparatus if needed !
Helium: most dangerous area ‚on top‘
Cold N2 or Argon: most dangerous ‚at the bottom‘
Sources of Accidents
and Failures
Embrittlement
Low temperature embrittlement
Causes overloaded components to fracture spontaneously rather than
accommodating the stress by plastic deformation
Appropriate steels for low temperature use are listed in the Technical Rules for
Pressure Vessels AD-Merkblatt W10 . (In general, materials with face-centered
cubic (fcc) crystal structure as copper, nickel, certain copper nickel alloys, zircon
and titanium are suitable for cryogenic applications.)
If hydrogen is present during the production of materials or if components come
into contact with hydrogen in operation, then hydrogen embrittlement can occur
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under certain conditions.
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Sources of Accidents and Failures
Thermal Stress
The development of thermal stress is due to the contraction of materials
when cooled down to cryogenic temperatures.
The stress can appear as a transient effect e.g. when cooling down thick
walled components or it can appear as permanent load e.g. in piping. In
both cases the stress can cause damage.
As the expansion coefficient of most materials decreases with temperature,
most contraction takes place above LN2 temperature ( 77K).
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Sources of Accidents and Failures
Thermal Stress
Thermal
Expansion of
some solids
Plastics > 2%
Steel 0,3%
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77K
Sources of Accidents and Failures
Pressure Build-up by Evaporation
Cryogen liquids do expand by a factor of 500 to 1500 when evaporated and
warmed up to room temperature (300 K)
(Helium: 2K l -> 300K d
factor= 900)
-> significant pressure build-up in a closed container
Possible reasons for an elevated heat input are:
fast cooldown of components or cryogenic installations,
large heat production within the object to be cooled ( e.g. quench) ,
loss of insulating vacuum,
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thermoacoustic oscillations
Comparison (ideal) Gas
e.g. Helium T > 30K
Pressure x Volume = const x Temperature
at Pressure = CONST
-> Volume = const/CONST x Temperature
-> double Temperatur ->double Volume
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Sources of Accidents and Failures
Pressure Build-up by Evaporation
Large air leak into the wave guide of an insert
-> Evaporation of about 2.6 kg/s mass flow through safety valve
Heat input caused by
the break down of
Insulation vacuum:
40kW / m2 without MLI
6kW/ m2 with MLI
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Sources of Accidents and Failure
Wave guide Air Leak :13.06.2002
Level ???
Temperature
Pressure
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Further mechanisms which can lead to a pressure
increase:
Boiling retardation
In very clean vessels boiling may not start but at
temperatures above the boiling point. In this case
the boiling can be from violent to explosion like.
Boiling retardation can be prevented by
introducing porous material as boiling nuclei.
Stratifikation
If the cryogen in a large tank is not disturbed for
some time, a temperature stratification may occur.
The stratification causes a larger pessure rise than
expected due to the elevated temperature in the
liquid surface layer.
The release of cryopumped gas
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Sources of Accidents and Failures
Preventive Measures
Against Pressure Build-up
Redundancy
i.e. more safety devices than
required
double safety devices
Diversity
i.e. safety devices based on different
mechanisms
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Sources of Accidents and Failures
Preventive Measures against PressureBuild-up
Release Flap
Safety Valve
Caution in the
Vicinity
Of
Release devices !
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Sources of Accidents and Failures
Condensation of Gases
Purposeful use of condensation in cryo-pumps and cryo-traps
Danger: Freeze-up of exhaust pipes
of dewars by air and/or air moisture. The plugging causes pressure build-up.
Preventive Measures:
- Only use dewars with separated eshaust and safety valve lines
- Do not leave dewars open to atmosphere
-Equip exhaust lines to atmosphere with check valves ( nonreturn)
-Perform a leaktest before evacuating cryogen baths with emphasis on leaks by which
air might be sucked into the cold part of the apparatus
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Sources of Accidents and Failures
Condensation of Gases
LN2 exhaust line: danger of freeze-up
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Unwanted condensation or freezing can also lead to:
- Mechanical damage
By ice accretion on valves, turbines etc.,
By dripping condensate at cryogenic temperatures or
By water condensate
- Pressure build-up by desorption
- Formation of explosive mixtures
By condensation of air or oxygen on combustible liquids
or materials flammable up to explosive mixtures can be
created:
in the insulation material of transfer lines, in a liquid hydrogen dewar
on the active charchoal of a cryogenic adsorber
Important ! – Air condensate is a strong promoter of combustion as it can have
an oxygen concentration of up to 50%. This is due to the higher boiling point of
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oxygen
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Phase equilibrium diagram of a N2 / O2 solution
21% O2 79%N2 (air)
Boiling temperature O2
82K
Boiling
temperature N2
50% O2
100% N2
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100% O2
Sources of Accidents and failures : Combustion and Explosion Hazard
Oxygen
Liquid or gaseous oxygen can
-strongly promote combustion processes,
- reduce the ignition temperature,
- accumulate in combustible materials ( especially also in clothes)
which will burn from violent to explosion like when ignited.
Ozone
A safety risk is posed by the production of ozone in systems which contain oxygen
and which are exposed to gamma or neutron radiation.
An explosive amount of ozone cam already be created from the oxygen impurities
in a liquid nitrogen dewar.
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Preventive measures against
condensation and its
consequences:
Purge and evacuate all equipment
thoroughly before operation.
Operate installations at slight overpressure
in order to avoid impurities from entering
through leaks.
Whereever possible – do use vacuum
insulation.
Otherways- do use incombustible
insulation material.
Damage caused by He-Purifier
Explosion
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General safety measures
all work in cryogenic areas has to be checked an approved by
the MKS cryo-operators in charge
inform the MKS cryo-operators about any problems
follow the operating instructions ( ‚Betriebsanweisungen‘)
Objectives:
at the end of this lesson you should now remember the
following hazards
cold burns
embrittlement
asphyxiation
thermal stress
pressure build-up by evaporation
hazards caused by condensation
combustion and explosion
in the handling of cryogens