10-04-2017, 08:09 PM
Prepared by:MAHESH.H.M.
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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
References :
1. Cryogenic Engineering by Thomas.M. Flynn 2. Cryogenic Safety Manual by Physics Division, Argonne National Laboratory