Cryogenic safety · 2020-02-20 · material will typically increase by around 20% down to liquid...
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Cryogenic safety Rob Done CEng MIMechE Sample Environment Section Instrument Design Group Rutherford Appleton Laboratory STFC Oxfordshire - UK
Transcript of Cryogenic safety · 2020-02-20 · material will typically increase by around 20% down to liquid...
Cryogenic safety
Rob Done CEng MIMechE
Sample Environment Section
Instrument Design Group
Rutherford Appleton Laboratory
STFC
Oxfordshire - UK
Cryogenic safety
• Health and safety
• Legislation
• Cryogenic hazards
• Asphyxiation
• Risk assessment
Health and safety
Moral
o To prevent people getting hurt
o Minimise the risk that you or anyone else will be injured or
will suffer ill health
Health and safety
Legal
o To comply with the law
o It is not acceptable for organisations to carryout their
activities in such a way that people will get hurt
Health and safety
Financial
o Poor Health & Safety is expensive
Cost of investigation
Injured person away from work
Injury claim
Prosecution by the HSE
Poor public relations
273 K
0 K
373 K
Freezing point of water
Boiling point of water
Absolute zero
Health and safety
Hyperthermia
Hottest temperature on Earth
Normal body temperature
Coldest temperature on Earth
Boiling point of liquefied gas
Hypothermia
343.7 K
313 K
310 K
294 K
183.8 K
120 K
Cryogenics
Humans can live
Lut Desert in Iran
Vostok - Antarctica
Health and Safety
The temperature range of cryogenic technology has
potential life threatening consequences to the human
body.
More and more facilities throughout the World use
cryogenic materials for cooling equipment – increased
likelihood of accidents happening.
Relevant Legislation
There is no specific cryogenic safety legislation.
It is addressed through related general safety requirements:
Relevant Legislation
The Health and Safety at Work Act – 1974
o General occupational health and safety provision
o Enforcement
Relevant Legislation
The management of Health and Safety at Work Regulations –
1999
o Core measures relating to the minimum health and safety
requirements for the workplace
o Risk assessments
Relevant Legislation
The Provision and Use of Work Equipment Regulations (PUWER) –
1998
o Place duties on people and companies who own, operate or
have control over work equipment
o Provision of protective devices and controls
Relevant Legislation
The Pressure Equipment Directive (PED)
o Meet the essential requirements covering the design,
manufacture and testing of pressure vessels
o Vacuum vessels
Relevant Legislation
The Pressure Systems (Safety) Regulations (PSSR) – 2000
o Minimises the risks when working with systems or
equipment which contain a liquid or gas under pressure
o Installation
Relevant Legislation
These regulations address the specific hazards associated
with cryogenic materials, but do not include the many
related hazards:
Lone Working
Extreme cold
Toxicity
Electromagnetic fields
Manual handling
Cryogenic hazards
1. Pressure build-up
Continuous evaporation generates
a gaseous atmosphere and an
increase in pressure inside any
liquid cryogen storage vessel.
If not properly controlled and
released by suitable measures, this
can result in a significant build-up
of pressure.
A pressure relief valve (PRV) of
suitable specification, which has
been registered for statutory
inspection, should be used to
prevent over pressurisation of the
vessel or system.
Cryogenic hazards
2. Expansion Ratio
This is the ratio of the volume of the cryogenic liquid from the boiling
point to normal ambient temperature and atmospheric pressure.
The evaporation of one litre of liquid nitrogen produces
696 litres of gas at NTP.
The evaporation of one litre of liquid helium produces
757 litres of gas at NTP.
Gas volume from liquid Relative Density
Liquid methane 1 : 627 0.555
Liquid nitrogen 1 : 696 0.967
Liquid helium 1 : 757 0.138
Liquid hydrogen 1 : 851 0.069
Liquid oxygen 1 : 860 1.105
Liquid neon 1 : 1438 0.697
Solid carbon dioxide 1 : 554 1.520
Cryogenic hazards
3. Cold contact burns
Liquid or low-temperature gas from cryogenic materials
will produce effects on the skin similar to burns.
Contact with uninsulated pipes etc. will cause contact
burns and may result in the skin freezing to the pipework.
Gases released as the cryogenic liquids vapourise can
permanently damage delicate skin e.g. the eyes.
Cryogenic hazards
4. Embrittlement of materials
When materials are cooled, the Young’s modulus of the
material will typically increase by around 20% down to
liquid helium temperatures.
This will increase the material’s strength and stiffness,
but also the brittleness which could also cause failure of
parts due to the change in this property.
There is a potential for hazards to be created
directly by the people carrying out the task –
this is known as the ‘human factor’
Cryogenic hazards
5. The human factor
Asphyxiation
• Asphyxiation due to the creation
of an oxygen depletion
atmosphere (ODA) is by far the
biggest cause of fatalities when
handling cryogenic fluids
• Each year an average of 20 deaths
occur in Europe involving people
entering an ODA
• Between 1992 and 2002 in the
USA, 85 nitrogen asphyxiation
incidents were recorded, in which
80 people were killed
Asphyxiation – Oxygen
• The only gas that supports life
• Approximately 21% concentration of oxygen in
the air we breathe
• Cognitive skills such as thinking and decision
making are impaired when this concentration
falls only slightly below this value
• These effects are not noticeable to the affected
individual Good night out or ODA?
Asphyxiation – Oxygen
• The working environment is hazardous as soon
as the oxygen concentration inhaled is less
than 18%
• With no oxygen present, inhalation of only 1-2
breaths of nitrogen or other inert gas will
cause sudden loss of consciousness and can
cause death
Asphyxiation – Causes of ODA
• Evaporation of liquefied gases
• 50 litres of liquid nitrogen warming
up would result in 35m3 – enough to
fill approximately 18 telephone
boxes.
Asphyxiation – Causes of ODA
• Confined spaces
Substantially enclosed
Hazardous substances
Lack of oxygen
• Inadequate ventilation
No supply of fresh air
No local exhaust ventilation
Asphyxiation – Ventilation
• Adequate cross-ventilation must
be provided where cryogenic
liquids are used or stored
• For rooms above ground level
with no special ventilation
openings, ventilation will
provide typically one air change
per hour
• With well sealed window (e.g.
double glazing), this will be less
• Consideration must be given to
the use of oxygen level sensors
and alarm systems in areas
where the ventilation is poor
Risk assessment
• A risk assessment, with oxygen
concentration calculations
should be carried our for all
areas where the potential for an
ODA is possible:
Confined space
Insufficient ventilation
Storage of cryogenic liquids
Processing of cryogenic liquids
Risk assessment
• Release of cryogenic gases
will happen due to:
General evaporation of the
cryogenic liquids
Catastrophic failure of a
storage vessel
• Two separate oxygen
concentration calculations
are required:
Normal evaporation of
cryogen
Sudden release of cryogen
from a storage vessel
Normal evaporation of cryogen
nV
LCionconcentratGas
Where:
L = gas release (m3/h)
V = room volume (m3)
n = air changes per hour
Normal evaporation of cryogen
1000
periodTime
ratioExpansionVrNLreleasedGas
Where:
N = Number of Dewars
Vr = Volume released per Dewar
Vr is often known as the Dewar manufacturer quoted
evaporation rate – typically 2.5% of the liquid capacity per
day for new storage vessels
Prudent to double this value to allow for the deterioration of
the Dewar insulation over time
Note: the factor of 1000 is used when the volume of release
is expressed in litres
Normal evaporation of cryogen
Where:
Vw = volume of workplace (obtained
from room dimensions; length, width
and height)
Vi = volume of objects/equipment
within the workplace (e.g. sample
storage vessels – cupboards)
Note: if the height is greater than 2m,
then the height should be taken as 2m
for the purposes of the calculation
This is a little greater than a ‘typical’
person’s height.
ViVwV
Robert Wadlow
2.72 m (8ft 11.1in)
2.0 m
Normal evaporation of cryogen
100
C100
21ionconcentratOxygen
18% - 19.5% May affect physical and intellectual performance without
person’s knowledge.
15% - 18% Decreased ability to work strenuously. May impair co-ordination
and may induce symptoms in persons with coronary, pulmonary,
or circulatory problems.
12% - 15% Respiration deeper, increased pulse rate, and impaired co-
ordination, perception and judgement.
10% - 12% Further increase in rate and depth of respiration, further
increase in pulse rate, performance failure, giddiness, poor
judgement, blue lips.
8% - 10% Mental failure, nausea, vomiting, fainting, ashen face, blue lips.
6% - 8% Loss of consciousness within a few minutes, resuscitation
possible if carried out immediately.
0% - 6% Loss of consciousness almost immediately, death ensues, brain
damage, even if rescued.
Sudden release of cryogen
Where:
VD = Capacity of the largest Dewar (litres)
V = Room volume (m3)
fg = Gas factor = 696 for nitrogen
1000
fVVVoxygenofVolume
gD
o
V
V100CionconcentratOxygen oox
Have you been attentive?
Quotation
When anyone asks me how I can best
describe my experiences of nearly forty
years at sea, I merely say uneventful.
I have never been in an accident of any
sort worth speaking about.
I have seen but one vessel in distress
in all my years at sea.
I cannot imagine any condition which
could cause a ship to founder.
Modern shipbuilding has gone beyond
that.
Captain EJ Smith
(Captain of the Titanic)
Do not think because an accident hasn’t happened to you
that it can’t happen ~ Safety saying, circa 1900s
