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

T ests 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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