Transcript SAFETY WITH CRYOGENIC SYSTEMS

SAFETY WITH
CRYOGENIC SYSTEMS
Safety aspects
1. Physiological
2. Suitability of materials and
construction
3. Explosions and flammability
4. Excessive pressure
1. Physiological
Hazard - Frostbite (cold burns)
Safety - Eye and body protection
Safety glasses, face shields, grease free leather
gloves, proper footwear, apron and trouser
clothing.
Clothing – free from grease and oils.
Clothing for hydrogen service - minimum
static electricity build-up.
Clothing for oxygen service – minimum
flammability.
Hazard
Nitrogen Asphyxiation - due to spillage or
leakage, in large vessels that have contained
gaseous or liquid nitrogen, resulting in
dilution of oxygen content
-creates respiratory problem resulting in
unconsciousness;may lead to death
Safety
- use oxygen monitors (acceptable 16-25% )
-fresh air purged prior to entry
-work in teams, with one team member
outside working area.
-safety rope and lifeline attached to person
entering the tank and lifeline continuously
monitored.
Hazard
Excess pure oxygen
-lung damage occurs if oxygen concentration in
air exceeds 60%
-prolonged exposure leads to bronchitis,
pneumonia, lung collapse.
-oxygen enriched air leads to flammability and
explosion hazards.
Safety
-place oxygen detectors or
monitors
-watch for fog or mist
2. Suitability of materials and construction
Brittle fracture :
-Failure of materials at low
temperature under impact.
-occurs very rapidly
resulting in instantaneous
failure.
- failure causes severe damage if the
equipment is under pressure
- release of fluid such as oxygen may result
in fire or explosion
Tests to determine material suitability
Drop weight tests :
NDT (nil ductility transition )temperature
of the material
- varies with material, thickness,hardness
and other properties
Charpy impact test
-low cost test
- specifies minimum allowable impact
value for various materials
- stainless steel, aluminum, copper and
brass have high impact resistance at low
temperatures
Thermal stress
-caused by thermal contraction of
materials
- thermal contraction leads to spillage of
cryogen and possibility of fire or
explosion
- results in differential contraction when
two dissimilar materials are joined
- contraction in long pipes is a serious
problem
- provision is made for change in length by
using bellows, expansion joints, flexible
hose
4. Over pressure
- due to phase change from liquid to gas
- systems containing liquid cryogen reach
bursting pressure very rapidly, if not
relieved and trapped in enclosures
- in uninsulated systems the liquid
vapourises more rapidly
- more the fluid greater the pressure
- liquid and vapour phase exists between
TPT and CPT.
- entrapment of cold liqiuds and gases may
occur in relief valves and burst discs
resulting in overpressure
- vent lines without rain traps may collect
rainwater and freeze closed
Ullage space – vapour filled space above
liquid is provided for safe cryogenic
storage.
- relief valves to be provided,inspected and
maintained periodically
- in event of freeze closing, ice plug is
melted by inserting warm copper rod
Conclusion :
Safety is ensured in cryogenic systems by
following good operating procedures and
practices.
- to select materials and methods of
construction compatible with properties of
cryogens
- eliminate sources of ignition
- provide proper insulation and relief
- eliminate cracks, recesses,pores in the design
and assembly of parts
- provide adequate ventilation
- keep the surrounding clean and free from
contamination
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